Low crossover, high conductivity proton exchange membrane with boron nitride coating

Abstract

Described herein are proton exchange membranes (PEMs) with improved proton conductivity and reduced crossover. The PEMs described herein include a boron nitride coating that imparts crossover blocking properties to hinder a permeation of hydrogen gas across the PEMs. Additionally, depositing the boron nitride coating on a surface of a PEM can generate one or more nitrogen-containing functional groups in the PEM. The nitrogen-containing functional groups can increase proton conductivity. Accordingly, the boron nitride coating of the PEMs described herein can result in increased proton conductivity and decreased crossover. Also described herein are methods of making and using the PEMs described.

Description

Attorney Docket No.: 093331-1516558 (8320 AKI)LOW CROSSOVER, HIGH CONDUCTIVITY PROTON EXCHANGE MEMBRANE WITH BORON NITRIDE COATINGSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under Grant no. W91 INF- 19-2-0269 awarded by the Army Research Office. The government has certain rights in the invention.CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of and priority to U.S. Provisional Application No. 63 / 689,968, filed on September 3, 2024, which is hereby incorporated by reference in its entirety.FIELD

[0003] This application is in the field of polymer electrolyte membrane materials. More specifically, and without limitation, this application relates to polymer electrolyte membranes including a nanocrystalline boron nitride coating, methods of making such membranes, and devices or systems incorporating such membranes, for example membrane-electrode assemblies, polymer-electrolyte-membrane fuel cells, or electrochemical membrane reactors.BACKGROUND

[0004] Some electrochemical devices depend on the flow of protons, or the flow of both protons and electrons, through a proton conducting material (e.g., such as a membrane). Accordingly, materials which conduct protons, or both protons and electrons, have applications as electrolytes or electrodes in a number of electrochemical devices including fuel cells, hydrogen pumps, supercapacitors, sensors, hydrogen separation membranes and membrane reactors.

[0005] One application for proton conducting materials is in fuel cells. Fuel cells are under development as an alternative to fossil fuels for power generation because of their higher efficiency and the lower level of pollutants produced from their operation. The fuel cells can use a fuel gas, such as hydrogen, to generate a flow of electricity. Operating efficiency of the fuel cells is dependent on one or more of proton conductivity, hydration of the proton conducting materials, operational pressure, and operational temperature.

[0006] Current fuel cells are facing challenges associated with gas crossover, such as hydrogen crossover, through the proton conducting material. The gas crossover can decrease Faraday efficiency of the fuel cells and contribute to degradation of the proton conducting material. In particular, conventional fuel cell technology suffers from a tradeoff between proton conductivityAttorney Docket No.: 093331-1516558 (8320 AKI) and fuel crossover. Thicker proton conducting materials have been explored to mitigate crossover effects but have also resulted in a reduction of proton conductivity.SUMMARY

[0007] Described herein are embodiments and examples of proton exchange membranes for use in fuel cells and methods for making and using the proton exchange membranes. The proton exchange membranes described herein include a boron nitride coating that provides beneficial performance characteristics to the fuel cells incorporating the proton exchange membranes.

[0008] Provided herein is a proton exchange membrane (PEM) including a perfluorosulfonic acid membrane structure having a surface and a structure and a boron nitride coating deposited on the surface using laser deposition, where the laser deposition is configured to form one or more nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure. In some examples, the nitrogen-containing functional groups include amine groups. In some examples, the nitrogen-containing functional groups are positioned between the surface and about 30 nm deep of the perfluorosulfonic acid membrane structure. In some examples, the perfluorosulfonic acid membrane structure includes a perfluorosulfonic acid or a polytetrafluoroethylene copolymer. In some examples, the boron nitride coating has a thickness between about 5 nm to about 15 nm. In some examples, proton conductance of the proton exchange membrane is between 30 mS / cm to 70 mS / cm at a temperature ranging from about 30 °C to about 80 °C. In some examples, hydrogen crossover current of the proton exchange membrane is less than 5 mA / cm2at a temperature ranging from about 30 °C to about 80 °C.

[0009] Also provided herein is a membrane-electrode assembly including an anode, a cathode, and a proton exchange membrane between the anode and cathode. The proton exchange membrane includes a perfluorosulfonic acid membrane having a surface and a structure and a boron nitride coating deposited on the surface using laser deposition, where the laser deposition is configured to form one or more nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure. In some examples, the nitrogen-containing functional groups include amine groups. In some examples, the perfluorosulfonic acid membrane structure includes a perfluorosulfonic acid or a polytetrafluoroethylene copolymer. In some examples, the boron nitride coating has a thickness between about 5 nm to about 15 nm. In some examples, proton conductance of the proton exchange membrane is between 30 mS / cm to 70 mS / cm at a temperature ranging from about 30 °C to about 80 °C. In some examples, hydrogen crossover current of the proton exchange membrane is less than 5 mA / cm2at a temperature ranging from about 30 °C to about 80 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0010] Also provided herein is a fuel cell including a first bipolar plate including a first set of channels to provide gas flow, a second bipolar plate comprising a second set of channels to provide gas flow, and the membrane-electrode assembly described herein positioned between the first bipolar plate and the second bipolar plate. In some examples, the membrane-electrode assembly exhibits a power density between about 600 mW / cm2to about 1000 mW / cm2at a temperature ranging from about 30 °C to about 80 °C.

[0011] Also provided herein is a method of producing the proton exchange membrane described herein. The method includes providing a perfluorosulfonic acid membrane structure having a surface and a structure and directly depositing a boron nitride coating on the perfluorosulfonic acid membrane surface using laser deposition, where the laser deposition is configured to form one or more nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure. In some examples, the laser deposition is pulsed laser deposition (PLD). In some examples, the nitrogen-containing functional groups include amine groups. In some examples, the perfluorosulfonic acid membrane structure includes a perfluorosulfonic acid or a polytetrafluoroethylene copolymer. In some examples, the boron nitride coating has a thickness between about 5 nm to about 15 nm.

[0012] Without wishing to be bound by any particular theory, there can be discussion herein of beliefs or understandings of underlying principles relating to the invention. It is recognized that regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.

[0013] These and other aspects, objects and embodiments will become more apparent when read with the detailed description and figures that follow.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 provides a schematic illustration of an example fuel cell including a proton exchange membrane with a boron nitride coating according to some embodiments of the present disclosure.

[0015] FIG. 2 provides a flowchart of an example process of producing a proton exchange membrane according to some embodiments of the present disclosure.

[0016] FIG. 3 provides a schematic of a pulsed laser deposition (PLD) process.

[0017] FIG. 4 provides an image of transparent 10 nm BN coating onto a PF SA based membrane.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0018] FIG. 5 provides an XPS plot of intensity (counts per second (cps)) versus binding energy (eV) for Bls.

[0019] FIG. 6 provides an XPS plot of intensity (cps) versus binding energy (eV) for Nls.

[0020] FIG. 7 provides a TEM image showing nanocrystalline BN nanosheets from a 10 nm deposition of BN onto a polymer surface.

[0021] FIG. 8 provides a microscopy image indicating nanocrystalline structure of BN with (0002) plane.

[0022] FIG. 9 provides an SEM image of a bare PF SA membrane surface.

[0023] FIG. 10 provides an SEM image of a PF SA membrane surface after 10 nm BN deposition.

[0024] FIG. 11 provides a cross-section SEM image of a plasma-exposed PFSA based membrane surface after 10 nm BN deposition. The SEM image shows micron scale mounds on a top exposed surface caused by energetic deposition and plasma exposure.

[0025] FIG. 12 provides a photograph of a 1 cm2pressed cell.

[0026] FIG. 13 provides a plot of membrane conductance (mS / cm) versus temperature (°C) for baseline and modified membranes.

[0027] FIG. 14 provides a plot of potential (V) versus current (A) at a temperature of 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C.

[0028] FIG. 15 provides a plot of system resistance (mOHm-cm2) versus temperature (°C).

[0029] FIG. 16 provides an AFM image of 10 nm BN on Si.

[0030] FIG. 17 provides a Phase Contrast Image of 10 nm BN on Si.

[0031] FIG. 18 provides an AFM image of 2 nm BN on Si.

[0032] FIG. 19 provides a Phase Contrast Image of 2 nm BN on Si.

[0033] FIG. 20 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 30 °C.

[0034] FIG. 21 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 40 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0035] FIG. 22 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 50 °C.

[0036] FIG. 23 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 60 °C.

[0037] FIG. 24 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 70 °C.

[0038] FIG. 25 provides a plot of cell potential (V) and current density (mA / cm2) for baseline and modified membranes at 80 °C.

[0039] FIG. 26 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 30 °C.

[0040] FIG. 27 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 40 °C.

[0041] FIG. 28 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 50 °C.

[0042] FIG. 29 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 60 °C.

[0043] FIG. 30 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 70 °C.

[0044] FIG. 31 provides a plot of cell resistance (mOhm-cm2) versus cell potential (V) for baseline and modified membranes at 80 °C.

[0045] FIG. 32 provides a plot of crossover current (mA / cm2) versus temperature (°C) for baseline and modified membranes.

[0046] FIG. 33 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 30 °C.

[0047] FIG. 34 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 40 °C.

[0048] FIG. 35 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 50 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0049] FIG. 36 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 60 °C.

[0050] FIG. 37 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 70 °C.

[0051] FIG. 38 provides a plot of crossover current (mA / cm2) versus voltage for baseline and modified membranes at 80 °C.

[0052] FIG. 39 provides a plot of change in proton transport (%) versus temperature (°C) for modified membranes.

[0053] FIG. 40 provides a plot of change in crossover (%) versus temperature (°C) for modified membranes.

[0054] FIG. 41 provides a plot of conductivity / crossover ratio (mS / cm) / (mA / cm2) versus BN thickness at a temperature of 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C.

[0055] FIG. 42 provides a plot of open circuit voltage (OCV) (V) versus temperature (in °C, 100% relative humidity) for baseline and modified membranes.

[0056] FIG. 43 provides Arrhenius fits of conductivity for baseline and modified membranes.

[0057] FIG. 44 provides Arrhenius fits of crossover values for baseline and modified membranes.

[0058] FIG. 45 provides a plot of proton transport activation energies (J / mol) for baseline and modified membranes.

[0059] FIG. 46 provides a plot of H2 transport activation energies (J / mol) for baseline and modified membranes.

[0060] FIG. 47 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 30 °C.

[0061] FIG. 48 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 40 °C.

[0062] FIG. 49 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 50 °C.

[0063] FIG. 50 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 60 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0064] FIG. 51 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 70 °C.

[0065] FIG. 52 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 80 °C.

[0066] FIG. 53 provides polarization curves (cell potential in volts versus current density in mA / cm2) at the beginning of life for a baseline membrane and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0067] FIG. 54 provides linear sweep voltammetry measurements (crossover current in mA / cm2versus voltage) at the beginning of life for a baseline membrane and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0068] FIG. 55 provides a respective conductivity (mS / cm) to crossover (mA / cm2) ratio for a baseline membrane and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0069] FIG. 56 provides power density curves (power density in W / cm2versus current density in mA / cm2) for a baseline membrane and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0070] FIG. 57 provides cyclic voltammetry curves (y-axis of current density in mA / cm2and x- axis of cell potential in volts) for a baseline membrane and membranes with a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0071] FIG. 58 provides a plot of electrochemical surface area (cm2 / gpt) for baseline and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0072] FIG. 59 provides a plot of cell potential (V) versus current density (mA / cm2) for baseline and modified membranes at 60 °C or 80 °C.

[0073] FIG. 60 provides a plot of power density (W / cm2) versus current density (mA / cm2) for baseline and modified membranes at 60 °C or 80 °C.

[0074] FIG. 61 provides a plot of crossover current (mA / cm2) versus voltage (V) for baseline and modified membranes at 60 °C or 80 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0075] FIG. 62 provides an SEM image of a pristine PFSA based membrane after hot pressing to gas diffusion electrodes (GDE).

[0076] FIG. 63 provides an SEM image of a 10 nm BN - NR-211 membrane after hot pressing to GDE.

[0077] FIG. 64 provides compiled thickness data (membrane thickness in pm versus BN thickness) including post-deposition thickness in pm measured before and after 12 hours of operation for a baseline membrane and modified membranes having a film thickness of 2 nm, 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure.

[0078] FIG. 65 provides accelerated stress test data (OCV degradation in Vt / Vo versus time in hours) for a baseline membrane and modified membranes having a film thickness of 5 nm, 10 nm, or 15 nm tested at 80 °C, 100% relative humidity and 250 kPaabs back-pressure. The tested cells were held at OCV with a temperature of 90 °C and 30% relative humidity for 100 hours of operation.

[0079] FIG. 66 provides ToF-SIMS 3D images of 10 nm BN / Nafion membranes showing a confluent coating of BN on the surface and also deeper implantation of particular nitrogen species up to 35 nm into the film.

[0080] FIG. 67 provides depth profiles of 10 nm BN membranes before or after water treatment of being immersed in a water bath, showing the BN remaining on the surface.

[0081] FIG. 68 provides a plot of water uptake (%) for baseline and modified PFSA based membranes at 30 °C or at 100 °C.

[0082] FIG. 69 provides ToF-SIMS 3D images of B+and Cs2CN+after water treatment.

[0083] FIG. 70 provides ToF-SIMS 3D images of B+, Cs2CN+, and C2F4+.

[0084] FIG. 71 provides a plot of point to point normalization versus depth (nm) for B+and CS2CN+.

[0085] FIG. 72 provides polarization curves (cell potential in volts versus current density in mA / cm2) for a baseline membrane and modified membranes having a film thickness of 15 nm tested at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C.

[0086] FIG. 73 provides crossover current measurements via linear sweep voltammetry (current density in mA / cm2versus voltage) for a baseline membrane and modified membranes having a film thickness of 15 nm tested at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0087] FIG. 74 provides crossover current measurements via linear sweep voltammetry (current density in mA / cm2versus voltage) for a baseline membrane and modified membranes having a film thickness of 15 nm tested at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C.

[0088] FIG. 75 provides membrane selectivity data for a baseline membrane and modified membranes having a film thickness of 10 nm or 15 nm tested at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C. Membrane selectivity is defined as the ratio between conductivity (mS / cm) at 0.6 V to crossover current (mA / cm2).

[0089] FIG. 76 provides membrane selectivity data for a baseline membrane and modified membranes having a film thickness of 10 nm or 15 nm tested at 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C. Membrane selectivity is defined as the ratio between proton flux and hydrogen flux (proton / EE). Flux values were derived using the Faraday relation shown below as Equation 1.

[0090] FIG. 77 provides molecular models of monomers of nitrogen functionalized PFSA based polymers.

[0091] FIG. 78 provides a plot of diffusion coefficients of hydronium (cm2 / s) versus number of NH2 groups in a monomer for unmodified and nitrogen functionalized PFSA based polymers.

[0092] FIG. 79 provides a plot of hydronium mean-squared displacement (MSD) (A2) versus time (ns).

[0093] FIG. 80 provides a plot of water mean-squared displacement (MSD) (A2) versus time (ns).

[0094] FIG. 81 provides a plot of diffusion coefficients of water (cm2 / s) versus number of NH2 groups in a monomer for unmodified and nitrogen functionalized PFSA based polymers.

[0095] FIG. 82 provides a diagram of a PFSA based membrane including a BN layer.

[0096] FIG. 83 provides a respective molecular structure and aerial view of unmodified and modified PFSA based membrane polymer.

[0097] FIG. 84 provides a plot of crossover change (%) versus conductivity change (%) for previous techniques and the BN deposition method described herein.

[0098] FIG. 85 provides a plot of periodic box length (A) versus number of NH2 groups in a monomer for unmodified and nitrogen functionalized PFSA based polymers.Attorney Docket No.: 093331-1516558 (8320 AKI)DETAILED DESCRIPTION

[0099] Described herein are proton exchange membranes (PEMs) with improved proton conductivity and reduced crossover that can be applied in fuel cell applications. Also described herein are methods of making and methods of using the PEMs. The PEMs described herein include a perfluorosulfonic acid (PF SA) membrane with a surface and a structure. Additionally, the PEMs described herein include a boron nitride (BN) coating deposited on the surface of the PFSA membrane using laser deposition. In some examples, the BN coating can function as a crossover blocking layer, impeding a permeation of molecular hydrogen (Eb) that can cause efficiency losses in the PFSA membrane. The laser deposition process can form one or more nitrogen-containing functional groups in the PFSA membrane structure. The laser deposition described herein includes, but is not limited to, pulsed laser deposition. The nitrogen-containing functional groups described herein include, but are not limited to, amine (NH2) groups. In some examples, the nitrogen-containing functional groups can result in an increase in proton conductivity, such as by facilitating a proton hopping mechanism to cause a faster diffusion of hydronium ions (EECE) through the PFSA membrane. Additionally, the PEMs described herein can provide higher current densities than conventional PEMs while operating at lower temperatures, such as less than 80 °C.

[0100] Various examples of the instant PEMs and methods of making and using the same are described in further detail below. In general, the terms and phrases used herein have their art- recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art.Applications Of Proton Exchange Membranes with Boron Nitride Coating

[0101] FIG. 1 provides a schematic illustration of an example fuel cell 100 including a proton exchange membrane (PEM) 105 with a boron nitride coating 110. PEM 105 can be configured to support proton transfer or proton conduction across PEM 105. Additionally, PEM 105 can be electrically insulative (e.g., non-conductive with respect to electrons). PEM 105 can be a pure polymer membrane or a composite membrane. PEM 105 can be prepared using any suitable material. In some examples, a structure of PEM 105 can include a perfluorosulfonic acid or a polytetrafluoroethylene (PTFE) copolymer.

[0102] In some examples, boron nitride coating 110 can be deposited on at least one side of PEM 105. In some aspects, boron nitride coating 110 may be deposited using laser deposition (e.g., pulsed laser deposition). In some examples, the laser deposition can form one or more nitrogen-containing functional groups, such as amine groups, in PEM 105. The nitrogen-Attorney Docket No.: 093331-1516558 (8320 AKI) containing functional groups can be positioned between a surface of PEM 105 and about 30 nm deep of the structure of PEM 105. In some examples, the nitrogen-containing functional groups can be positioned at a depth of 30 nm, 31 nm, 32 nm, 33 nm, 34 nm, 35 nm, 36 nm, 37 nm, 38 nm, 39 nm, or anywhere in between. As shown, boron nitride coating 110 is deposited on a side of PEM 105 contacting an anode 115 of example fuel cell 100. It will be appreciated that in some examples, PEM 105 may include boron nitride coating 110 deposited on more than one side. Boron nitride coating 110 can have any suitable or desired thickness. In some examples, boron nitride coating 110 can have a thickness between about 5 nm to about 15 nm (e.g., from 5 nm to 10 nm, from 10 nm to 15 nm, or anywhere in between). For example, the thickness of boron nitride coating 110 can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, or anywhere in between.

[0103] PEM 105 can be positioned between anode 115 and a cathode 120 to enable protic communication between anode 115 and cathode 120 and can form a membrane electrode assembly (MEA). Anode 115, cathode 120, or a combination thereof may include components such as carbon supports, ionomers, catalysts (e.g., a platinum catalyst), etc. In some examples, the catalyst may be embedded in or provided as a separate layer in contact with anode 115 or cathode 120. In some examples, the MEA may include an anode gas diffusion layer 125 or a cathode gas diffusion layer 130 contacting a respective electrode (e.g., anode 115 or cathode 120) of fuel cell 100. Gas diffusion layers 125, 130 can allow hydrogen gas and / or oxygen gas to diffuse to anode 115 or cathode 120, respectively. Additionally, gas diffusion layers 125, 130 can allow water to diffuse away from anode 115 or cathode 120. In some examples, gas diffusion layers 125, 130 can include a porous material, such as carbon paper or other suitable carbon fibers. The porous material can be coated with a hydrophobic material (e.g., PTFE) to prevent excessive water buildup in gas diffusion layers 125, 130. In some examples, the MEA can include one or more current collectors, such as an anodic current collector in electric communication with anode 115 and a cathodic current collector in electric communication with cathode 120. The current collectors can be made of an electrically conductive material (e.g., copper, aluminum, steel, etc.). The current collectors can be connected to an external circuit, such as to facilitate a transport of electrons.

[0104] As a non-limiting example, fuel cell 100 can be a device used to generate electrical power from a reaction of hydrogen or another suitable fuel with oxygen. Although one MEA is shown in FIG. 1, it will be appreciated that fuel cell 100 may include one or more MEAs that each include at least anode 115, cathode 120, and PEM 105. A fuel source (e.g., hydrogen) can be provided to anode 115 of the MEA. In some examples, the hydrogen can undergo a hydrogenAttorney Docket No.: 093331-1516558 (8320 AKI) oxidation reaction (Equation 1) to form protons (e.g., hydronium) and electrons. In some examples, unreacted hydrogen can exit an anode side of fuel cell 100.H2^2H +2e" (Equation 1)The electrons formed using the hydrogen oxidation reaction can travel to cathode 120 (e.g., via an external circuit), thereby generating a current output of fuel cell 100. Additionally, an oxygen source (e.g., ambient air) can be provided to cathode 120 to react with the protons permeating through PEM 105 and the electrons that have traveled to cathode 120. PEM 105 can be a semipermeable membrane that can conduct protons while acting as a reactant barrier (e.g., with respect to gases such as hydrogen or oxygen). At cathode 120, the oxygen can undergo an oxygen reduction reaction (Equation 2) to form water molecules. In some examples, water and excess air can exit a cathode side of fuel cell 100.1 / 2 O2+2H +2e ^H2O (Equation 2)

[0105] In some examples, as shown in FIG. 1, fuel cell 100 can include an anode bipolar plate 135 and a cathode bipolar plate 140. For example, bipolar plates 135, 140 may be positioned in contact with a respective gas diffusion layer (e.g., anode gas diffusion layer 125 or cathode gas diffusion layer 130). As non-limiting examples, bipolar plates 135, 140 can be made from a metal (e.g., titanium, stainless steel, etc.), a carbon structure (e.g., graphite), or a composite material. In some examples, bipolar plates 135, 140 can include a flow field or a set of channels 145, 150 to facilitate gas flow with respect to the MEA. In general, bipolar plates 135, 140 can facilitate supplying the fuel source and oxygen source to fuel cell 100. Additionally, bipolar plates 135, 140 can manage water flow in fuel cell 100 and heat produced by fuel cell 100. In some examples, bipolar plates 135, 140 can function as structural support or as a backing medium for stacking fuel cell 100. To connect multiple MEAs in series, the MEAs can be positioned in a stack (e.g., on top of each other) to increase an output voltage.

[0106] In some examples, a proton conductance of PEM 105 can be between 30 mS / cm to 70 mS / cm (e.g., from 30 mS / cm to 40 mS / cm, from 40 mS / cm to 50 mS / cm, from 50 mS / cm to 60 mS / cm, from 60 mS / cm to 70 mS / cm), or anywhere in between. The proton conductance of PEM 105 can be provided at a temperature ranging from about 30 °C to about 80 °C (e.g., from 30 °C to 40 °C, from 40 °C to 50 °C, from 50 °C to 60 °C, from 60 °C to 70 °C, from 70 °C to 80 °C, or anywhere in between). For example, the PEM may function as described herein at a temperature of 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, or anywhere in between.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0107] In some examples, hydrogen crossover current of PEM 105 is less than 5 mA / cm2at a temperature ranging from about 30 °C to about 80 °C (e.g., 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, or anywhere in between). For example, the hydrogen crossover current of PEM 105 can be from 0 mA / cm2to 1 mA / cm2, from 1 mA / cm2to 2 mA / cm2, from 2 mA / cm2to 3 mA / cm2, from 3 mA / cm2to 4 mA / cm2, from 4 mA / cm2to 4.99 mA / cm2, or anywhere in between.

[0108] In some examples, the MEA can exhibit a power density between about 600 mW / cm2to about 1000 mW / cm2at a temperature ranging from about 30 °C to about 80 °C (e.g., 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, or anywhere in between). For example, the power density of the MEA can range from 600 mW / cm2to 700 mW / cm2, from 700 mW / cm2to 800 mW / cm2, from 800 mW / cm2to 900 mW / cm2, from 900 mW / cm2to 1000 mW / cm2, or anywhere in between.Methods of Producing Proton Exchange Membranes with Boron Nitride Coating

[0109] Provided herein are also methods of making a proton exchange membrane (PEM) as described herein. FIG. 2 provides a schematic illustration of an example of a method 200 of making the PEM 105 that includes a boron nitride coating 110. Certain aspects of FIG. 2 are described with reference to components of FIG. 1.

[0110] At block 205, a perfluorosulfonic (PFSA) membrane 105 is provided. As described above with respect to FIG. 1, PFSA membrane 105 can be formed from PFSA polymers or other suitable polymers. In some aspects, PFSA membrane 105 can be referred to as a PEM or as a PFSA based membrane. PFSA membrane 105 can have a surface and a structure (e.g., a three-dimensional structure having a length, width, and height).[OHl] At block 210, boron nitride coating 110 is directly deposited on the surface of PFSA membrane 105. Boron nitride coating 110 can be deposited on PFSA membrane 105 using any suitable or desired techniques, such as deposition techniques to produce a thin film on a substrate. In some examples, laser deposition is performed to deposit boron nitride coating 110. The laser deposition can form one or more nitrogen-containing functional groups in the structure of PFSA membrane 105. In some examples, the laser deposition is a physical vapor deposition, such as pulsed laser deposition. For example, a pulsed laser beam can be focused within a vacuum chamber to contact a target including a material (e.g., boron nitride) to be deposited, thereby vaporizing the material and forming a plasma plume that can be deposited as a coating on a substrate in the vacuum chamber. In some examples, the deposition of boron nitride coating 110 can be performed at a relatively low temperature (e.g., less than 30 °C).Attorney Docket No.: 093331-1516558 (8320 AKI)

[0112] In some examples, preparing a fuel cell (e.g., fuel cell 100 of FIG. 1) can involve assembling a membrane electrode assembly including PF SA membrane 105 with boron nitride coating 110 and electrodes (e.g., anode 115 and cathode 120 of FIG. 1). The fuel cell additionally can include gas diffusion layers (e.g., anode gas diffusion layer 125 and cathode gas diffusion layer 130 of FIG. 1). In some examples, the electrodes or the gas diffusion layers can be attached to PF SA membrane 105 via hot press. For example, the hot press may be heated to a temperature of about 120 °C and positioned to apply pressure to couple the electrodes or gas diffusion layers to PFSA membrane 105.

[0113] The invention may be further understood by the following non-limiting examples.EXAMPLE 1: NANOCRYSTALLINE BORON NITRIDE COATING FOR HIGH CONDUCTIVITY, LOW TEMPERATURE PROTON EXCHANGE MEMBRANE FUEL CELLS

[0114] Hydrogen fuel cells based on proton exchange membrane (PEM) technology are promising as an alternative to fossil fuel-based energy. Conventional proton exchange membrane fuel cell (PEMFC) technology is operated at fully humidified conditions in a narrow temperature range (e.g., approximately 80 °C) to maintain sufficient proton conductivity and power output, which necessitates a high cost of operation. Described herein is a scalable, room temperature coating of ultrathin boron nitride (BN) deposited via pulsed laser deposition (PLD) that simultaneously increases conductivity of perfluorosulfonic acid (PFSA) based membranes while decreasing crossover (e.g., hydrogen crossover). Remarkably, BN coated membranes show a 20% increase in performance at current operational conditions (1.485 A / cm2@ 0.6 V) and a 20% increase in power density (0.965 W / cm2) while exhibiting a maximum crossover current decrease of 32% (3.58 mA / cm2) relative to conventional membranes. Furthermore, the BN coated membranes demonstrate a reduction of operational temperatures to as low as 60 °C without performance impact, thereby affording substantial reduction of the PEMFC operational cost. These observations are practically relevant for the development of next generation PEM technology by enabling more scalable and cost-effective high performance fuel cell stacks.

[0115] Hydrogen is poised to become a critical component of a global decarbonized energy portfolio, with the potential to revolutionize sectors such as transportation and shipping. These applications use PEMFC technology to facilitate the usage of hydrogen as fuel. However, PEMs suffer from a number of efficiency concerns that prevent commercial large-scale adoption- namely in current densities and power densities at operational voltages (approximately 0.6 V to 0.7 V) and parasitic crossover effects. Current PEM technology suffers from a fundamental tradeoff between proton conductivity and hydrogen fuel crossover. To accommodate for crossover losses,Attorney Docket No.: 093331-1516558 (8320 AKI) conventional membranes used in PEM technology (e.g., with a typical thickness of 25 pm) must thus be made thicker. However, this increase in thickness comes at the expense of reducing proton conductivity, as tortuosity of water channels formed in the membrane increase a path length of the H+ion’s motion from an anode of the PEMFC to a cathode of the PEMFC. Crossover occurs by diffusion of hydrogen from one electrode to another electrode, creating mixed potentials at the electrodes, leading to large losses in open circuit voltage (OCV). Further, diffused gases may interact with one another, the polymer membrane, the platinum catalyst, or a combination thereof. In doing so, various PEM degradation pathways can be exacerbated, thereby limiting the lifetime (e.g., an operational lifetime) of modern fuel cells. As a result of reaction stochiometry and lower driving forces for oxygen crossover, hydrogen crossover can exhibit a greater impact on Faradaic efficiency. Reducing hydrogen crossover therefore can correspond to a more impactful route for extending PEM lifetimes. Hydrogen permeates from nanopores in the PEM that allow for water transport (and thus proton transport) as well as throughout the rest of the membrane’s body due to diffusion effects caused by the concentration gradient from anode to cathode. As such, the mechanisms of hydrogen crossover are twofold; one by gas permeation through a polymer material of the PEM by thermally activated jumps, and another by motion through the same water channels that protons travel through, in the form of Stokes’ drag of dissolved gas. Hydrogen crossover and proton transport through PEMs are both affected by several common factors such as pressure, temperature, and cell compression. To accommodate for crossover losses, conventional membranes (e.g., Nafion™NR-211) are typically made thicker for practical application, such as having a thickness of 25 pm. However, this comes at an expense of proton conductivity due to increased resistance to H+ion diffusion from anode to cathode.

[0116] Several additives to PEM based technologies have been explored in the literature to increase durability or efficiency by reducing crossover, such as recombination layers, 2D materialbased crossover mitigation layers, and radical scavengers. The addition of recombination layers can be prohibitively expensive due to using precious metal catalysts, despite efficacy in mitigating crossover without impacting conductivity. Amjadi et al. illustrated the use of SiCh nanoparticles to increase the conductivity of conventional membranes (183 pm thick) at low relative humidities (RH), while lowering the crossover. However, the Si nanoparticles impede water channels at high RH states, lowering the conductivity of the membrane and losing the performance increase seen at low RH. In other words, at higher relative humidities, this performance increase was lost, and the conductivity was negatively impacted, which was attributed to the difficulty of water transport in the high relative humidity state with Si nanoparticles impeding the channels. Many of the other approaches previously studied, particularly the transferred 2D material barrier layers, drasticallyAttorney Docket No.: 093331-1516558 (8320 AKI) reduce the conductivity of the PEM, a result of the similar interruption of water transport through the membrane in these cases, while successfully reducing both the diffusive and drag induced crossover. Including 2D material layers, such as h-BN and graphene, in the polymer membrane has been shown to increase membrane durability through a reduction of fuel crossover. The tradeoff can include decreased proton conductivity resulting from interrupted water transport channels. Recent studies show this conductivity / crossover tradeoff can be mitigated through controlled introduction of opres in the 2D material layer. Further, coating the anode side of the PEM has the advantage of increasing hydrogen utilization while also maintaining proton conductivity. 2D materials, particularly transferred h-BN and graphene sheets, have been shown to decrease crossover but simultaneously decrease the conductivity, and thus the maximum current output.

[0117] A previous study by Yoon et al. showed a 42.8% decrease in crossover via a chemical vapor deposition (CVD) grown trilayer h-BN coating by using a transfer method but exhibited a similar 42% decrease in current output as well. While many 2D materials can conduct protons intrinsically, the conductivity is enhanced through defect engineering by growth methods such as CVD. However, CVD transferred material, while more proton conductive than pristine exfoliated flakes due to the higher density of vacancies and grain boundaries, is still not sufficiently conductive to have a lower effect on conductivity than crossover. Additionally, 2D material films grown by CVD are often grown at high temperatures, incompatible with polymer membranes, and must be transferred to the membrane after growth. Accordingly, direct deposition on polymer membranes can be impossible. Instead, the 2D material films are typically transferred to the membrane after growth, limiting scalability and cost-effective integration. Using ammonium persulfate (APS) copper etchant has recently been demonstrated to lower Nafion’s intrinsic proton conductive performance. As a result, alternative deposition or fabrication methods exhibiting high throughput and avoiding the use of degradative chemicals are preferred.

[0118] In this regard, the techniques described here include a scalable pulsed laser deposition (PLD) growth method for direct deposition of boron nitride (BN) with atomic control directly onto ionomer membranes at room temperature. Using a pulsed laser deposition (PLD) growth method stands out as a relatively unexplored yet potentially scalable method for direct deposition of 2D materials with atomic control directly onto ionomer membranes at room temperature. Particularly, boron nitride (BN) grown at room temperature by the PLD method has been shown to result in atomically thin layers on flat sapphire substrates. These films are also observed to exhibit nanometer-sized grains, suggesting that the increased presence of grain boundaries and other defects can allow water and proton conducting channels to form, resolving a key limiting factor in previous studies using the 2D material barrier layer approach. Direct deposition also can avoidAttorney Docket No.: 093331-1516558 (8320 AKI) using harmful chemicals by forgoing low throughput transfer methods. The description provided herein demonstrates that a confluent BN layer can serve as an effective hydrogen diffusion barrier.Plasma Assisted Surface Modification of Proton Exchange Membranes

[0119] FIG. 3 depicts a process used to deposit BN films directly onto a surface of a PF SA based membrane (25.4 micrometers thick), at room temperature, as described by Biswas et al. the thickness of the deposited layer can be correlated to the number of laser shots to the target in the PLD process, as calibrated on a silicon substrate. Additional description is provided in the Methods section below. The PLD process of BN is known to produce many reactive radical species in the energy range of 10 eV to 100 eV in the plasma plume, such as B+, N* and N+alongside BN. FIG. 4 depicts an image of the BN film deposited on the PF SA based membrane, where the dark contrast on top of the transparent PF SA based membrane is the deposited BN layer. Upon XPS deconvolution of the surface, seen in FIG. 5 and FIG. 6, it is observable that both the B Is spectra and N Is spectra show multiple bonding states on the membrane surface. While the presence of B-N is indeed observed on the surface, 58.3 atomic % (at%) of the nitrogen signal is attributed to chemical modification of the membrane surface (N-C bonds), likely a result of N+or N* reacting with the polymeric surface of the membrane. The additional formation of B-N bonds suggests the formation of a continuous BN layer on the surface of the polymer is plausible. The additional formation of a BN layer on the surface at room temperature suggests the potential for this layer to act as a crossover mitigation layer, while the nitrogen functionalization by C- NH2 bonding of the surface could enhance the conductivity of the membrane surface, allowing for faster proton transport from catalyst to electrolyte.

[0120] 10 nm of BN was deposited onto a polymethyl methacrylate (PMMA) surface to mimic the deposition of BN onto a polymer, and then transferred to a Cu TEM grid for characterization. In contrast to PMMA, Nafion could not be dissolved to image the BN. The TEM images illustrate the presence of many nanometer-sized flakes, as seen in FIG. 7 and FIG. 8. These flakes, owing to their nanocrystalline nature, allow for the transport of protons and water through the abundance of grain boundaries formed in a layer of the material. The inset diffraction pattern in FIG. 8 validates the polycrystallinity of the BN film. As shown in FIG. 8, the flakes are further observed to exhibit a d-spacing of 0.33 nm, correlating to the (0002) plane of hexagonal BN. This spacing is consistent with the previous characterization of the room temperature PLD BN layers deposited on silicon.

[0121] SEM images of the surface (FIG. 9 and FIG. 10) show marked differences between a bare PF SA based membrane and a 10 nm BN deposited layer on the PF SA based membrane. The bare PFSA based membrane is observed to have no obvious distinguishing features, appearing as aAttorney Docket No.: 093331-1516558 (8320 AKI) homogeneous membrane; the 10 nm BN layer, however, alters the surface of the membrane. Additionally, as shown, the 10 nm BN layer forms a continuous film, though contrast variations indicate spatial inhomogeneity within the BN layer. The implications of this non-uniform structure and the presence of multiple bonding states are addressed below. A number of defects are now seen on the surface of the PFSA based membrane, seen to be craters caused by the deposition of energetic particles in the plasma plume. This morphological change is seen in the cross section (FIG. 11) of the 10 nm BN / membrane layer as well, clearly illustrating mounds caused by energetic deposition. This surface roughening has also been observed in direct plasma treatment of PFSA based membranes, with some literature suggesting an increase in conductivity with plasma treatment and spray coated electrodes, while Ramdutt et al. reported a loss of conductivity due to weakened adhesion between the gas diffusion electrodes (GDE) and the membrane. The stark difference in seemingly conflicting results is attributed to Cho et. al’s use of a barrier layer (spray coated carbon ink) that acted as a protective layer to maintain the adhesion between the GDE and the membrane. As described herein, it was discovered that the PLD process deposits a layer of BN to act as that protective layer, while still enabling the surface modification.Crossover and Conductivity Across a Temperature Range

[0122] Membrane electrode assembly (MEA) packages were constructed via hot press from the BN-deposited membrane (FIG. 12) and tested in a commercially obtained fuel cell test system (see Methods section below). FIG. 13, FIG. 32, FIG. 39, FIG. 40, and FIG. 41 illustrates the fuel cell testing data at 100% relative humidity (RH) and 150 kPa backpressure at various temperatures. The conductivity data at 0.6 V obtained from the current interrupt method of fuel cell conductivity determination is shown in FIG. 13 after the system resistances (obtained at each temperature by pressing gas diffusion electrodes (GDEs) to the bipolar plates and electrical wiring and extracting the resistance from IV curves) have been subtracted (FIG. 13, FIG. 14, and FIG. 15). The thickness of the layer is correlated to the number of laser shots to the target in the PLD process, as calibrated on a silicon substrate (FIG. 16, FIG. 17, FIG. 18, and FIG. 19). FIG. 16 and FIG. 17 show the 10 nm film’s height profile from a scratch test and the resulting phase contrast image, showing a conformal 10 nm film. Interestingly, the 2 nm BN layer leads to a minor reduction to no change to the baseline conductivity, attributed to the presence of an incomplete layer of BN leading to heterogeneous adhesion of the catalyst to the membrane (as discussed below with respect to the time-of-flight secondary ion mass spectrometry (ToF SIMS) data). The 2 nm layer atomic force microscopy (AFM) is seen in FIG. 18 and FIG. 19 showing 1-2 nm island growth, with phase contrast AFM imaging showing additional extremely thin islands that have yet to coalesce into a film, leading to a heterogeneous membrane surface. This incomplete adhesion isAttorney Docket No.: 093331-1516558 (8320 AKI) also suggested due to the 2 nm membrane’s inability to reach lower potentials, an indication of a mass transport issue likely caused by a lack of catalyst supply of protons to facilitate low potentials and associated high currents. As the nominal thickness of BN increases, a rise in conductivity is seen across all temperatures. These measurements were conducted in coordination with polarization curves (FIG. 20, FIG. 21, FIG. 22, FIG. 23, FIG. 24, and FIG. 25), clearly showing the increase in outputted current as compared to the baseline. Although FIG. 13 focuses on the conductivity of the membrane at 0.6 V, an industrially relevant potential of operation, the increase in conductivity (and thus decrease in resistance / hydrogen evolution reaction (HFR)) holds across the full potential range (FIG. 26, FIG. 27, FIG. 28, FIG. 29, FIG. 30, and FIG. 31).

[0123] The increased current densities and lower conductivities at lower temperatures open the door to lower temperature operation of PEMFC technology, and operation at higher power. Interestingly, the highest conductivity of the PFSA based membrane is observed at 70 °C (48.7 mS / cm, in agreement with Kusoglu et al.); the use of a 5 nm BN layer in conjunction at 50 °C is higher (49.3 mS / cm). With a 15 nm layer, a maximum conductivity described in Example 1 is observed at 60 °C (69.4 mS / cm), suggesting a 42.7% increase in maximum conductivities; this translates to a 12.7% increase in current output at 0.6 V whilst still at a lower operating temperature.

[0124] FIG. 32 illustrates the crossover characteristics of the fuel cell as obtained by linear sweep voltammetry (LSV) (See FIG. 33, FIG. 34, FIG. 35, FIG. 36, FIG. 37, and FIG. 38 for full LSV graphs). It is observed that at 80 °C and 100% RH, the 2 nm layer did not have any statistically significant difference, and exhibited high variability, from the baseline crossover, and arguably had a detrimental effect on the crossover at certain temperatures - this is again attributed to the incompleteness of the 2 nm layer. When increased to a 5 nm layer, the crossover current reduces from 5.19 mA / cm2± 0.10 mA / cm2to 4.48 mA / cm2± 0.26 mA / cm2. With a 10 nm and 15 nm layer, the crossover reduces to 4.48 mA / cm2± 0.16 mA / cm2and 3.48 mA / cm2± 0.17 mA / cm2. This data suggests that the conductivity of the membrane has simultaneously been enhanced, while the crossover has been reduced. This observed phenomenon breaks the traditional relationship between conductivity and crossover.

[0125] Another lens of observation is that of the relative transport difference for protons and molecular hydrogen. FIG. 39 and FIG. 40 reflect this view of the conductivity and crossover relationship, showing the maximum relative increase of the conductivity by 17.9% to 35.5% with a 15 nm layer, depending on the temperature of operation. Meanwhile, for the same layer, the crossover decreases between 28.8% and 37.1% depending on temperature. It is interesting to noteAttorney Docket No.: 093331-1516558 (8320 AKI) that at 30 °C, the 5 nm layer looks to disadvantage the MEA’s crossover by nearly 22% when seen at 0.4 V; this is the result of a large internal short (approximately 630 mOhm-cm2short, from the slope of the LSV sweep) in all three membranes tested, resulting in a positive sloping LSV curve that was unable to be mitigated, leading to a higher than baseline value at 0.4 V (FIG. 33, FIG. 34, FIG. 35, FIG. 36, FIG. 37, and FIG. 38). If using an alternative method of LSV determination, by extrapolating the line to the intercept of the y-axis, the value of 1.91 mA / cm2is obtained, representing a potential 14.5% reduction in crossover in the absence of the short, as seen in FIG. 40. This short consistently disappeared after ramping to 50 °C. The presence of internal shorts is observed in the 2 and 5 nm layers, which then disappears with the more confluent 10 and 15 nm layers. Despite this isolated data point, the crossover is seen to be reduced significantly because of the BN layer. By taking the ratio between conductivity and crossover, as in FIG. 41, the 15 nm BN layer leads to a 2.3 times more selective membrane at 30 °C, and a 1.88 times more selective membrane at 80 °C.

[0126] Furthermore, another indication of crossover is gleaned from the open circuit voltage (OCV) of the cell (FIG. 42). The 5, 10, and 15 nm layers correlated to an increase in OCV across all temperatures, corroborating the LSV measurements. At 80 °C, the addition of the BN layer shows a potential improvement from 0.965 V for the bare PFSA based membrane to a high of 0.975 V for the 15 nm BN deposited membrane. In contrast, the 2 nm BN MEA exhibited a lower OCV, likely a result of poor contact between the electrode and the heterogeneous surface of the membrane caused by a non-confluent layer. This effect makes portions of the pressed membrane thinner or more compressible than other areas, which can translate to inhomogeneous spatial hydrogen crossover. As a result, this can enable slightly higher crossover (as seen in the LSV averages for 40 °C to 60 °C) and can result in a reduction in OCV. Such a heterogeneous contact also results in reduced catalyst accessibility for the 2 nm layer, as seen through the electrochemically available surface area (ECSA) measurements (FIG. 43, FIG. 44, FIG. 45, FIG. 46, and FIG. 58). This reductive ECSA effect remains, although is more muted, in the 5 nm layer. When a thicker confluent layer was applied, the crossover by LSV was seen to substantially decrease and the OCV was seen to rise, an indication of lowered membrane hydrogen permeation through the membrane. The ECSA values for these thicker 10 nm and 15 nm layers layers too, was seen to not be statistically different to the baseline ECSA value of 46.5 cm2 / gpt, indicating even contact between electrode and membrane.

[0127] The study of proton transport and molecular hydrogen transport can be approximated by Arrhenius fit, as seen in FIG. 47, FIG. 48, FIG. 49, FIG. 50, FIG. 51, and FIG. 52. It is observed that the fit for hydrogen transport is much more amenable to the Arrhenius fit (R2> 90%) than thatAttorney Docket No.: 093331-1516558 (8320 AKI) of the proton transport (60% < R2< 75%), likely due to a myriad of other factors that affect proton transport in a membrane that are not purely thermal effects (e.g., membrane swelling, hydration, catalyst effects). Nevertheless, the Arrhenius fit of proton transport shows that the addition of the BN layer significantly decreases the proton transport energy barrier (by a maximum of 2.46 kJ / mol for the 10 nm BN), while the activation barrier for the BN treated layers increases by nearly 3 kJ / mol in the 10 nm and 15 nm BN membranes.

[0128] FIG. 53, FIG. 54, FIG. 55, FIG. 56, FIG. 57, and FIG. 58 illustrate the electrochemical test data of BN membranes tested at 80 °C, 250 kPaabs, and 100% relative humidity (RH). FIG. 59, FIG. 60, and FIG. 61 illustrate the electrochemical test data of BN membranes tested at 60 °C and 80 °C, 150 kPaabs, and 100% RH. The polarization curves of FIG. 53 demonstrate 2 nm and 5 nm films lead to a reduction in membrane conductivity. For the 2 nm BN layer, the effect on conductivity originates from the larger activation drop compared to the baseline. AFM mapping in FIG. 18 and FIG. 19 shows 1-2 nm island growth in this specimen, with phase contrast AFM imaging showing additional extremely thin islands that have yet to coalesce into a film, leading to a heterogeneous film. The presence of an incomplete layer of BN leads to incomplete adhesion of the catalyst to the membrane. As the thickness of the deposited BN layer increases beyond 2 nm, the film coalesces to form a confluent layer. As previously stated, AFM data of a 10 nm specimen in FIG. 16 and FIG. 17 demonstrates the uniformity of this thicker layer. Interestingly, the 5 nm film does not show an increase in the activation overpotential; although, there is still a significant reduction of current in the operational range of 0.6-0.7 V. A decrease in potential in the ohmic region of the polarization curve corresponds to increased ohmic resistance, which is consistent for 2D materials interfaced with Nafion. The 10 nm and 15 nm samples do not showcase the same activation overpotential and ohmic resistance issues. These specimens demonstrate an increase in current at the operational range of 0.6 V- 0.7 V. As the nominal thickness of BN increases, this rise in conductivity is seen across all temperatures of collected polarization curves using the iR drop method (FIG. 20, FIG. 21, FIG. 22, FIG. 23, FIG. 24, and FIG. 25). The BN flakes, owing to their nanoscale polycrystalline nature, provide an avenue for the transport of protons and water through the abundance of grain boundaries. This alone, though, is possibly insufficient to explain why ohmic resistance decreases. Further characterization is involved to understand this effect (see FIG. 11, FIG. 62, FIG. 63, FIG. 64, FIG. 65, FIG. 66, and FIG. 67).

[0129] FIG. 54 illustrates the crossover characteristics of the fuel cell as obtained by linear sweep voltammetry (LSV). The reduction in hydrogen (H2) permeation is expected for increasing thicknesses of size-selective layers. It is observed that at 80 °C, 100% RH, the 2 nm layer results in a crossover of 4.5 mA / cm2, and thicker depositions reduce the crossover further. When increasedAttorney Docket No.: 093331-1516558 (8320 AKI) to a 5 nm layer, the crossover current reduces from 5.19 ± 0.10 mA / cm2to 4.48 ± 0.26 mA / cm2. With a 10 nm and 15 nm layer, the crossover reduces to 4.10 ± 0.16 mA / cm2and 3.48 ± 0.17 mA / cm2. Again, this relationship holds at all conditions tested, as seen in FIG. 33, FIG. 34, FIG. 35, FIG. 36, FIG. 37m and FIG. 38. Combining these results with the decrease in ohmic resistance exhibited by the 10 nm and 15 nm specimen indicates a break from the traditional tradeoff relationship between conductivity and crossover. The result is drastic improvements in selectivity as shown in FIG. 55, showing an increase in conduct! vity / crossover ratio as deposition thickness increases. By taking the ratio between conductivity and crossover, the 15 nm BN layer leads to a 1.88 times more selective membrane at 80 °C.Fuel Cell Operational Temperature Flexibility

[0130] The conductivity and crossover improvements were expected to result in a higher power density capable MEA. Indeed, as seen in FIG. 47, FIG. 48, FIG. 49, FIG. 50, FIG. 51, FIG. 52, and FIG. 56, the maximal wattages produced by the BN deposited MEAs outpace the baseline PFSA based membrane. At 80°C, 100% RH, the maximum power density of the baseline PFSA based membrane was 811 mW / cm2, in agreement with literature values for the PFSA based membrane, while the 10 nm and 15 nm produced 964 mW / cm2and 965 mW / cm2, respectively. This represents an 18.8% increase in maximum current density as a result of the BN layer. Additionally, across the temperature ranges studied, the 811 mW / cm2for the baseline was the maximum power density observed for that MEA; the 10 nm BN MEA achieved this power density at 60 °C, 20 °C lower than that for the baseline. This represents a substantial decrease in operation energy for these devices, suggesting the BN deposition affords multiple advantages for fuel cell operation. At lower temperatures of 30 °C, the maximal wattage observed for the baseline is a mere 298 mW / cm2, while the 10 nm and 15 nm BN MEAs exhibited power densities of 648 mW / cm2(117% increase) and 749 mW / cm2(151% increase). This creates a much larger operational space for fuel cell operation with the addition of the BN layer, by enabling the use of temperature as a more effective knob or adjustment parameter. The open circuit voltage (OCV) of the cell (FIG. 52) provides another indication of crossover reduction. The 5, 10, and 15 nm layers demonstrate an increase in OCV compared to baseline across all temperatures, corroborating the LSV measurements.

[0131] Example 1 further shows that the BN modified membranes are capable of lower temperature operation than the typical operation temperature of 80 °C. At 60 °C, both the current density of the 10 nm and 15 nm membranes and the maximum power densities match the performance of bare PFSA based membrane at 80 °C. The additional benefit in operation at lower temperatures is the mitigation of the crossover effect with the application of a 15 nm BN layer, aAttorney Docket No.: 093331-1516558 (8320 AKI)PEMFC system can be run at performances matching the 80 °C PFSA based membrane, but at a crossover value of 2.6 mA / cm2, representing a 50.2% decrease in crossover. This work shows the operational flexibility provided by this scalable surface modification. That is, they can be operated at high power density at current operational conditions, or lower temperatures while matching current efficiencies, and affording lower crossover.Fuel Cell Water Channel Regulation and Proton Transport

[0132] This increase in conductivity is unexpected. Additionally, the reduction in ohmic resistance for membranes with thick BN layers is unexpected. The addition of a layer of material (e.g., an additional conformal material layer) is generally expected to lead to an additional resistance to proton transfer, as seen in previous studies of 2D material barrier layers. The commonality in these aforementioned studies is the use of a transfer mechanism for the 2D layer from the growth substrate to the membrane. In our approach, the PLD process directly deposits material onto the PFSA based membrane. In doing so, as discussed above, a number of radical species are introduced to the surface, as shown by the XPS spectra in FIG. 5 and FIG. 6. These radical species change the surface characteristics of the membrane, in conjunction with the existence of the 2D material layer. Chemical interactions between radical species and ionomer chains can have an impact on membrane surface characteristics. These interactions can cause localized damage to the PFSA based membrane surface, which can at least partially account for the observed change in conductivity. However, changing the transport characteristics of such a small fraction of the overall membrane likely cannot account for the full magnitude of the reduction in ohmic resistance.

[0133] To further investigate the change in bulk conductivity, MEAs were fabricated and immediately cross-sectioned by snapping in liquid nitrogen (see Methods section below). As seen in the SEM images of FIG. 11, FIG. 62, and FIG. 63, hot-pressed BN membranes have thinned considerably, especially compared to pressed baseline membranes. The magnitude of change in thickness for each sample is shown before and after operation in FIG. 64. MEAs were also crosssectioned after performing initial tests for 12 hours (break-in, polarization curve testing and LSV testing, and an OCV hold). The baseline membrane thinned from 25.2 pm to 25.0 pm, while the 15 nm BN membrane thinned from 21.4 pm to 19.1 pm, representing a loss of 2.3 pm in operation. Similarly, the 2 nm, 5 nm, and 10 nm membranes lost 1.7 pm, 2.3 pm and 2.1 pm respectively in the 12-hour test. Surface roughening (FIG. 11) has also been observed in direct plasma treatment of PFSA based membranes, with some literature suggesting an increase in conductivity with plasma treatment and spray coated electrodes, while Ramdutt et al. reported a loss of conductivityAttorney Docket No.: 093331-1516558 (8320 AKI) due to weakened adhesion between the gas diffusion electrodes (GDE) and the membrane. These previous observations support the established correlation between BN film thickness and membrane conductivity, demonstrating decreased performance with incomplete thin films and improved performance with well-adhered, conformal thick BN films. The etching of PFSA based membrane cannot be attributed solely to plasma damage, as the exposure time is too low (300 seconds for the 15 nm sample) to etch microns of material. The plasma exposure likely also enhances the compressibility of the membrane at the surface, which is consistent with a decrease in membrane thickness after hot pressing to the GDE and 12 hours of operation.

[0134] The thinning of the membrane material is the dominant cause of the ohmic resistance reduction for the 10 nm and 15 nm samples as seen in FIG. 53. By lowering the path length for proton transport by thinning the membrane, the ohmic resistance drops while the size selective nature of the BN layer facilitates lower crossover. This combined effect leads to higher current density, lower crossover membranes at beginning of life. However, the consistent ~2 um thinning during membrane operation for all BN membranes indicates the first few PLD shots likely nucleate defects on the polymer backbone of the PFSA based membrane. As mentioned previously, the PLD process implants extraneous B-C and N-C bonds into the polymer, which creates an interphase region. To evaluate the stability of the membrane and impacts of the interphase region, both accelerated stress testing (AST) and depth profiling via time-of-flight secondary ion mass spectroscopy (ToF-SIMS) were performed, as seen in FIG. 65, FIG. 66, and FIG. 67.

[0135] Accelerated stress testing (AST) was conducted using the DOE standard MEA chemical stability OCV hold test at 90 °C and 30% RH on all confluent coverage samples, as seen in FIG. 65. Delamination issues in the 2 nm MEA prevented a continuous hold on the stress test, leaving the 5 nm, 10 nm, and 15 nm MEAs for complete stress tests. The baseline NR-211 degrades at a rate of 2 mV / hr in the linear region and is also afflicted by pinhole formation after 48 hours of operation. The increase in performance at 67 hours is related to temporary changes in testing conditions when switching gas canisters. Relative to the baseline, all the BN MEAs exhibited early mitigation of drastic OCV drops that correlate with the formation of pinholes in the membrane. The 15 nm BN membrane degrades at a rate of 0.23 mV / hr. The 5 nm and 10 nm BN membranes exhibit a degradation rate of 3.7 mV / hr and 3.6 mV / hr respectively. The increase in degradation rate for the 5 nm and 10 nm specimens is attributed to thinning of the membranes, and the degree of crossover mitigation was not enough to offset this effect. When the BN thickness is increased to 15 nm, the effect of thinning on durability is mitigated by improved crossover reduction. This is reinforced by evidence that the 15 nm samples experience thinning similar to the 10 nm sampleAttorney Docket No.: 093331-1516558 (8320 AKI) and a massive drop in crossover to 3.48 mA / cm2. A pinhole opens at 65 hours in the 15 nm AST plot, but its effects are smaller compared to the pinhole in the baseline membrane. This data suggests that while PLD-based direct depositions can be high throughput with beginning of life improvements to performance, the nucleation of defects to the polymer backbone by plasma deposition can work against the crossover blocking layer itself - a thicker layer such as the 15 nm BN can help to stave off these effects due to a stronger decrease in crossover (e.g., approximately 34%).

[0136] FIG. 66, FIG. 67, FIG. 68, and FIG. 69 illustrate the ex-situ characterization techniques for understanding the source of the conductivity increase. Water uptake measurements in liquid deionized (DI) water were conducted at 30 °C and 100 °C for 1 cm2samples of membrane (FIG. 68). It is seen that the BN deposited membranes exhibit a slight increase in water uptake; at 100 °C, the water uptake of the 10 nm BN layer is 66%, a 18.1% increase in water uptake. A similar trend is observed at 30 °C. Given that the technique employed is not changing the properties of the bulk membrane, merely the surface, such an increase in water uptake is attributed to higher water retention as a result of defects and grain boundaries in the BN layer, allowing water to permeate and be retained in the BN layer and the upper surface of the PF SA based membrane, maintaining water channels to enable proton transport. This suggests that a higher performance at lower temperatures can be, at least, partially attributed to this water retention, making the BN layer a regulator of water transport through the membrane. The small grain sizes in the PLD BN further lead to an increase in associated grain boundary densities, which are established to facilitate water transport through these channels through electrostatic effects analogous to grain boundary diffusion, eliminating the water transport issues encountered by large area sheets of 2D materials.

[0137] Another effect that is explored is the potential doping of the surface of the membrane because of the high energy PLD process. Since PFSA based membranes are polymers, the use of an energetic process has the potential to implant species into the polymer and create an interphase region that may contribute to the conductivity of the membrane. To evaluate this region, depth profiling via ToF-SIMS was utilized. The depth profiling in FIG. 66 is consistent with AFM data, confirming the thickness of the deposited 10 nm film. Furthermore, to elucidate the effect of the BN layer in a fuel cell outside a fuel cell environment, a 10 nm BN-deposited membrane was treated in water held at 80 °C for 6 hours and dried in a vacuum oven and examined. ToF-SIMS was performed on these samples, baseline membranes, and 10 nm BN membranes without water treatment. FIG. 67 illustrates the depth profiles of the pristine 10 nm BN-deposited sample and the water treated sample. In the pristine (e.g., non-treated) sample, a 10 nm layer of BN is indeed observed along with an approximately 35 nm interphase region, including largely nitrogen speciesAttorney Docket No.: 093331-1516558 (8320 AKI) with some boron species as well. This data, in combination with the XPS data of FIG. 5 and FIG. 6, indicates that the PF SA based membrane surface may be functionalized with nitrogen and boron. For example, the PFSA based membrane surface can be functionalized with nitrogen containing acidic groups that can facilitate additional proton transport in the same mode as Nafion’s inherent -SO3 groups. The addition of closely spaced functional groups has been established to correlate strongly to protonic conductivity, suggesting this functionalization due to the PLD process enhances conductivity. Additionally, the functional groups can serve as nucleation points for further membrane thinning.

[0138] Interestingly, in the water treated sample, the intensity of the SIMS signal drops significantly for both boron and nitrogen, although noticeably more so for boron. A BN layer is still observed, but the boron signal almost immediately drops out past the 7 nm to 8 nm cutoff. However, the PFSA based membrane signal is increased in this region, suggesting some intermixing of PFSA based membrane and BN, but the presence of a layer is still observed. The nitrogen signal, however, remains strong (albeit not to the same degree as the pristine sample), suggesting that the functionalization of the surface remains. The differential reduction in boron and nitrogen signals following water treatment likely reflects the selective removal of unbonded species rather than degradation of the deposited BN film itself. The substantial loss of boron (approximately one order of magnitude at the surface) compared to the more moderate reduction in nitrogen (approximately 2-fold) suggests these elements experience different chemical environments. Unbonded boron species exhibit high water solubility and preferential removal during treatment while unbonded nitrogen species demonstrate intermediate solubility. Further, the depth profiles signal there are larger amounts of boron immediately on the surface of the membrane, while there are more nitrogen species deeper in the membrane. Nitrogen species within the membrane are likely bonded to the polymer backbone, while boron at the surface exists as loosely bound impurities. The persistence of measurable boron and nitrogen signals after water treatment indicates that a stable, chemically bonded BN film remains intact, while the treatment effectively removes weakly bound impurities and excess precursor materials that accumulated during deposition. This selective removal process eliminates loose precursor species while preserving the crystalline or well-formed BN network structure. In other words, this data suggests the diffusion and intermixing of PFSA based membrane with boron and nitrogen species, a portion of which remain in the membrane and contribute to conductivity, rather than the degradation of the BN. FIG. 66 and FIG. 69 illustrate an areal view of the 2D material - it is interesting to note the depth of diffusion of nitrogen into the membrane as compared to boron. The nitrogen is clearly seen to remain heavily in the membrane after the water treatment, while the boron remains solelyAttorney Docket No.: 093331-1516558 (8320 AKI) at the surface after the treatment, likely in the form of BN grains rather than other boron containing species.

[0139] ToF-SIMS analysis was also conducted on the 2 nm BN sample to evaluate its completeness on the surface of the membrane. As seen in FIG. 70 and FIG. 71, the nitrogen area map shows only a portion of the sample surface containing nitrogen species, while boron is seen to sit on the surface uniformly. This indicates both heterogeneous surface characteristics in terms of morphology, as well as surface characteristics. This uniformity is restored in the thicker deposition layers.Electrochemical Testing at Various Temperatures

[0140] To further investigate the potential benefits of the PLD-grown BN films, electrochemical experiments were performed at various temperatures, the results of which are shown in FIG. 72, FIG. 73, FIG. 74, FIG. 75, and FIG. 76. Due to their previous success, this temperature study focused on the 10 nm and 15 nm BN films compared to the Nafion™ NR-211 baseline membranes. FIG. 72 shows the polarization curves taken at each temperature for the 15 nm films, while the crossover currents determined by LSV are shown in FIG. 73 and FIG. 74. The bare NR- 211 membrane demonstrates a decreasing trend in conductivity as the temperature decreases. Both BN films demonstrate very little dependence on temperature. Proton conductivity is primarily mediated through vehicular transport at high applied potential and high water activity, and is closely related to the membrane’s water absorption characteristics and water content. For the temperatures tested at constant relative humidity, interfacial mass transport at the vapor / polymer interface is the limiting factor for water permeation. When the Nafion™ surface is exposed to water vapor, there exists a 0.2-1.0 nm fluorine-rich layer acting as a barrier to water vapor adsorption, as confirmed by AFM. With the introduction of the BN layer in tandem with extraneous B-C and B-N bonds through interactions with high energy ions, it is likely this barrier to water transport is affected, which has the positive impact of increasing interfacial mass transport. As a result, vehicular transport mediated proton conductivity increases. Further, water transport increases with temperature, which in turn promotes higher proton conductivity. On the other hand, due to the lower thermal energy of water vapor at low temperatures, baseline membranes experience a dependence on temperature. Since water transport is not as heavily inhibited by the presence of a fluorine-rich barrier layer in the BN membranes, temperature is not as significant in determining its proton conductivity characteristics. More research into the effect of PLD on the fluorine-rich barrier layer is involved to verify these conclusions.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0141] While the behavior of proton conductivity proved to be convoluted, hydrogen crossover followed more readily from analysis of LSV measurements. All membranes demonstrate a distinct, predictable trend with temperature. As temperature increases, the barrier to hydrogen crossover is weaker. Combining the conductivity at operating conditions (0.6V) with the crossover values, selectivity formulations are developed, resulting in the conductivity / crossover ratio and the ratio of proton flux to hydrogen flux. Fluxes are derived using the Faraday relation shown in Equation 1 below. ,Selectivity ratios are shown for all membranes at all temperatures in FIG. 75 and FIG. 76, respectively. Deposited membranes demonstrate improved selectivity, especially at lower temperatures where the lack of hydrogen crossover dominates. The interesting difference between bare membranes and BN deposited membranes is the behavior of the flux-based selectivity values. While Nafion™ has a relatively consistent flux selectivity ratio across tested temperatures, the selectivity ratio of BN-coated membranes exhibits a significant decrease at higher temperatures. A plausible explanation for the temperature-dependent flux-based selectivity of the BN-coated membranes arises from thermal effects on the mechanical relationship of the BN-Nafion™ interface. Given the significant mismatch in coefficients of thermal expansion between BN and Nafion™, rising temperature and the subsequent expansion of the Nafion™ membrane generates tensile stress in the BN flakes. Tensile stress in the BN films causes physical changes to the grain boundary network, effectively widening gaps and lowering the tortuosity of the network. These structural changes lowers the ability of the film to resist hydrogen flux. Therefore, accelerated hydrogen crossover, in tandem with stable proton flux, leads to the observed selectivity decrease at elevated temperatures for BN-deposited films. Importantly, the BN-deposited films are still more efficient at all temperatures. This behavior only highlights the profound benefits for low temperature applications.Nitrogen Functionalization

[0142] Molecular dynamics simulations were performed, as described below in the Methods section, to investigate the effect of nitrogen functionalization of the PF SA based membrane polymer on the transport dynamics of water, and hydronium (EECF) ions, i.e., proton transport through the vehicular mechanism. Due to the uncertainty surrounding the structure of nitrogen functionalized PF SA based membrane polymer, various variants were simulated. In FIG. 77, the monomers of these nitrogen functionalized PFSA based membrane polymer variants,Attorney Docket No.: 093331-1516558 (8320 AKI) simulated in this study, are presented. The modified PFSA based membrane polymer monomers are designated as 'N(a,b),' where 'N' represents nitrogen functionalization through the addition of NH2 groups replacing fluorine (F). The parameter 'a' indicates the total number of NH2 groups added to the monomer, while 'b' signifies the number of NH2 groups added to the side chain containing the sulfonic acid group.

[0143] FIG. 78, FIG. 81, FIG. 82, and FIG. 83 illustrate the simulated average diffusion coefficients of hydronium ions and water molecules for both baseline unmodified PFSA based membranes and various nitrogen functionalized PFSA based membranes at the same hydration level (X = 20) and a temperature of 80 °C. The diffusion coefficients for hydronium ions and water are determined through the center of mass mean-squared displacement, as depicted in FIG. 79 and FIG. 80, respectively. Upon examination of the results in FIG. 77, it is evident that the diffusion coefficient of hydronium ions is enhanced by addition of one or more hydrophilic NH2 group to the polymer for moderate levels of nitrogen functionalization. The highest observed hydronium diffusion coefficient, exhibiting a remarkable 100% increase, is associated with the case N(l,0), where one NH2 group is incorporated into the backbone of the monomer chain. Conversely, in the case of N(l, 1), where one NH2 group is added to the side chain, the diffusion coefficient experiences a decrease, albeit still notable a 77% increase. Moreover, these diffusion coefficients exhibit a generalized decreasing trend as the number of NH2 groups within the monomer of the PFSA based membrane polymer increases. However, the specific trend is intricately tied to the positional arrangement of NH2 groups in the monomer. It is noteworthy that even with this increase, the coefficients remain greater than the baseline unmodified PFSA based membrane, especially for moderate levels of nitrogen functionalization.

[0144] As depicted in FIG. 80, the introduction of hydrophilic NH2 groups to the polymer leads to a reduction in the diffusion coefficient of water. This decline can be attributed to the formation of hydrogen bonds between water molecules and NH2 groups. Therefore, as the quantity of NH2 groups increases, there is a corresponding decrease in the diffusion coefficient of water. However, it is noteworthy that an increase in hydrophilic NH2 groups, may lead to an enhancement of water uptake, as observed in the experimental results, potentially resulting in higher diffusion coefficients. It is crucial to emphasize that the diffusion coefficients of both water and hydronium ions are intricately linked to both the quantity of NH2 groups in the side chain and the specific positional arrangement of NH2 groups within the monomer.Attorney Docket No.: 093331-1516558 (8320 AKI)Conclusions

[0145] Systems and methods described herein relate to fast, low-temperature, direct deposition of boron nitride on PFSA based membranes (e.g., Nafion™ NR-211 polymer membranes) for proton exchange membrane fuel cell applications. Performance data indicates increased conductivity and decreased crossover compared to baseline Nafion™ NR-211 polymer membranes. The conductivity enhancement arises from deposition related thinning of the membrane, decreasing the diffusion path for proton transport. Transport through the BN layer occurs via defects in the lattice, including pores and grain boundaries. This is supported by diffraction pattern and high resolution TEM micrographs showcasing the nanoscale polycrystallinity of the deposited BN layer. Based on the available data, the following mechanism of crossover reduction and ionic transport increase is presented in FIG. 82 and FIG. 83. Concurrently, the layer of BN deposited acts as a crossover mitigation barrier by reducing diffusion of EE through the PEM, as shown by LSV and OCV measurements. By combining these effects, more selective, higher power density, and a highly conductive membrane for PEM fuel cell application is developed at beginning of life.

[0146] As seen in FIG. 84, the BN deposition via PLD method demonstrated in this application is the only method reported to date that both improves the crossover and conductivity characteristics of an MEA (e.g., at 80 °C, 250 kPaabs backpressure, with additional benefit seen at lower temperatures), representing a sizeable advancement in the efficiency of this technology. Further, direct deposition, as opposed to the widely used transfer methods, has the potential to advance the fuel cell industry by increasing the scalability of 2D material -based PEMFC crossover barrier layers. Additionally, the techniques described herein may enable low temperature operation of fuel cells, making PEMFC technology operable at a much lower energy cost. In addition, higher wattage cells are developed, moving the technology closer to economical usage for clean energy applications.

[0147] The results described in Example 1 suggest that 2D materials act as highly effective hydrogen barriers and improve fuel cell efficiency in a quick one-step modification to commercial membranes. The plasma induced damage to the membrane represents a challenge to overcome, possibly by utilizing a more gentle or remote plasma compared to the PLD method. The techniques described herein demonstrate the efficacy of a low temperature, direct deposition method for 2D materials on polymers. Future work involves better understanding and mitigating interactions of radical species with the polymer backbone to push forward development of lessAttorney Docket No.: 093331-1516558 (8320 AKI) damaging processes. Exploration of alternative scalable, industrially compatible methods of low temperature deposition are also of interest, such as magnetron sputtering.MethodsBN Thin Film Growth by PLD

[0148] BN thin films were grown on PFSA based membranes (e.g., Nafion™) by using a loadlock assisted PLD (operating with a krypton fluoride (KrF) laser of 248 nm wavelength and a 25 nanosecond (ns) pulse width) growth facility. The base pressure of the main chamber was approximately 8 * 10'9Torr, and the load-lock chamber pressure was 5* 10'8Torr. A commercially available one-inch diameter h-BN target (99.9% metal basis) was used for the growth. Films were grown at room temperature, and under 100 mTorr N2 partial pressure (to compensate the loss of N2 in BN, if any). Films are grown at a 5 Hz repetition rate and with a laser energy of approximately 220 mJ (fluency of approximately 2.2 J / cm2). The target-to- substrate distance during the growth was kept at approximately 50 mm. The laser spot size was approximately 1.5 x 7 mm2. For experiments, approximately 1.5 cm2x 1.5 cm2sized membranes were used as a substrate. The thickness of BN films was calibrated from Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile, thus controlling a number of laser pulses during each growth. From this analysis, the average growth was as found to be approximately 3 nm / minute to 4 nm / minute. All BN films of varying thicknesses were deposited under the same conditions with different deposition times.TOF-SIMS Analysis

[0149] Positive high mass resolution depth profile was performed using a TOF-SIMS NCS instrument, which combines a TOF-SIMS5 instrument (ION-TOF GmbH, Munster, Germany) and an in-situ Scanning Probe Microscope (NanoScan, Switzerland) at Shared Equipment Authority from Rice University.

[0150] The analysis field of view was 150 x 150 pm2(B13 @ 30keV, 0.3pA) with a raster of 256 by 256 along the depth profile. A charge compensation with an electron flood gun was applied during the analysis. An adjustment of the charge effects was operated using a surface potential. The cycle times was fixed to 100 ps (corresponding to m / z = 0-911 atomic mass unit (a.m.u) mass range). The sputtering raster was 500 x 500 pm2(Cs+@ 250eV, 15nA). The beams were operated in non-interlaced mode, alternating 1 analysis cycle and 0.5 seconds per frame of sputtering followed by a pause of 3 seconds for the charge compensation.Attorney Docket No.: 093331-1516558 (8320 AKI)

[0151] The MCsn+(n = 1, 2) depth profiling has been also used for improving the understanding of the data. This is a useful method, mainly applied to quantify the alloys but also to identify any ion compounds. The cesium primary beam is used for sputtering during the depth profile and permits to detect MCs+or MCs2+cluster ions where M is the element of interest combined with one or two Cs atoms. The advantages of following MCs+and MCs2+ions during TOF-SIMS analysis include the reduction of matrix effects and the possibility of detecting the compounds from both electronegative and electropositive elements and compounds.

[0152] All depth profiles have been point-to-point normalized by the total ion intensity and the data have been plotted using a 10-points adjacent averaging. Both normalization and smoothing have permitted a better comparison of the data from the different samples. The depth calibrations have been established using an interface tool in software to identify the different interfaces and based on the estimated thickness of BN layer.Materials Characterization Techniques

[0153] Atomic force microscopy (AFM) was conducted using a Park NX10 atomic force microscope with an NCHR AFM tip (NanoSensors, Watsonville, CA, USA). XPS was conducted with a VersaProbe4 XPS tool (Physical Electronics, Chanhassen, MN, USA). CasaXPS was used for XPS curve fitting- all curves used were Gaussian-Lorentizan (GL30) distributions. SEM images were obtained with an FEI Quanta 650 ESEM.

[0154] Water uptake measurements were obtained by heating water on a heating stage to 30 °C and 100 °C. Samples were left in a dessicator to remain dry in the meantime. Samples were then immersed in water and left at this temperature for 2 hours before being dried for 1 minute at the same temperature and weighed on a scale.

[0155] Cross sections of MEAs were obtained by dipping MEAs into liquid nitrogen for 60 seconds to ensure brittleness. The MEAs were then mechanically snapped through the center point for SEM imaging to prevent mechanical deformation caused by use of a compressive cleaving tool. SEM imaging was conducted with an FEI Quanta 650.

[0156] TEM samples were made by first spin coating 200 nm of polystyrene (PS) onto an Si substrate, upon which 10 nm of BN was grown. This sample was then hot pressed to a Cu grid at 100 Ibf and 120 °C to cause the grid to adhere to the BN / PS / Si stack. This stack was then placed into toluene to dissolve the PS, resulting in Cu grid / BN for imaging and bare Si. It is important to note that only BN nanosheets are seen in the TEM results while ToF-SIMS and AFM indicate fullAttorney Docket No.: 093331-1516558 (8320 AKI) layers of BN. This is likely due to TEM sample preparation effects, and the deposition on the polymer surface is indeed full coverage.

[0157] TEM imaging was carried out using a probe-corrected JEOL neoARM (JEOL, Akishima, Japan) using 200 kV accelerating voltage and a 27 milliradian convergence semiangle. The electron probe was corrected using ASCOR (CEOS GmbH, Heidelberg, Germany) to achieve a coherent flat-phase envelope greater than the convergence angle of 27 milliradians.ME A Fabrication

[0158] MEAs were fabricated by a hot press method described in the literature. 200 nm of 0.2% PFSA based ionomer dispersion (e.g., 0.2% Nafion™ D-2021 ionomer) was spray coated on the surface of the membranes. Spray coating was conducted at 0.15 mL / min, with 1.6 psi of backpressure, at a path speed of 50 mm / s and spacings of 1.5 mm per pass with a 0.75 mm offset. 1 cm2gas diffusion electrodes (GDEs) were cut from Vulcan carbon with 0.3 mg / cm2Pt and used as both anode and cathode electrodes. This stack was gasketed with 5mil PTFE to prevent the formation of pinholes and pressed at 120 °C for 3 minutes with a force of 20 lbf / cm2. Cells were compressed in the cell fixture via torque wrench in a star pattern in increments of 10 Ib-ft until each bolt was at 50 Ib-ft.Fuel Cell Testing Parameters

[0159] All cells were tested in a Scribner 850 Fuel Cell Test System (Scribner Associates, Southern Pines, NC, USA) in a holder equipped with serpentine bipolar plates. All BN membranes were placed with the BN facing the anode, and each sample type was tested 3 times (3 separate samples). A break in process was conducted in which the voltage was cycled at 100% RH between 0.9 V and 0.6 V ten times, at 32% RH between 0.9 V and 0.6 V ten times, and at 40 °C between 0.9 V and 0.6 V five times. Backpressure throughout the experiment was 150 kPa gauge pressure, or 250 kPaabs.

[0160] Polarization curves at all temperatures were collected by setting the cell temperature, anode and cathode gas temperatures equivalent and under galvanostatic control. The anode flowrate was kept at 0.14 liters per minute (1pm) of H2 and the cathode flowrate was set to 0.56 1pm of compressed air. Before each point was obtained, the system was allowed to come to equilibrium by holding at each current for 5 minutes. LSV measurements were obtained by with humidified 0.125 L / min of hydrogen and 0.125 L / min of nitrogen, at a scan rate of 3 mV / sec and waiting 30 minutes to 1 hour for the cell to come to equilibrium. Cyclic voltammetry conditions were by flowing 0.5 L / min of H2 and 0.05 L / min of N2 at 30 °C and a scan rate of 50 mV / sec. TheAttorney Docket No.: 093331-1516558 (8320 AKI) cell potential was staggered between 0.06 V and 1.2 V. ECSA was computed by taking the area under the curve at both the anodic and cathodic scans and subtracting the area contribution from the double layer capacitance and averaging the contributions.Conductivity Calculations

[0161] The calculation of conductivity was obtained by the inverse of the ionic resistance at 0.6 V, as determined by the current interrupt method. The conductivity (mS / cm) was calculated by the following equation: o- = t / (R - A) where t is the thickness of the membrane (cm), A is the area of the MEA active area (1 cm2) and R (mOhm) is the resistance measured at 0.6 V.Modeling Method

[0162] The PF SA based polymer with structure of monomers in FIG. 77 and degree of polymerization of ten are constructed using the Build Polymer module of Materials Studio. The simulation boxes contain 20 polymer chains (NSCh' = 200), 200 hydronium ions (NH3O = 200), and 3800 water molecules, corresponding to a water uptake of 20 ( = 20). The simulation boxes were constructed using the Amorphous Cell module of BIO VIA Materials Studio using condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) forcefield with forcefield assigned charges. The Geometry Optimization was performed on the periodic cell using Forcite module of BIO VIA Materials Studio with polymer consistent forcefield (PCFF) and partial charges from COMPASS forcefield. The non-bonded interactions, the electrostatic and van der Waals interactions, were calculated by using atom-based methods with a cut-off distance of 15 A.

[0163] Classical all-atom molecular dynamics simulations were performed on the input periodic system generated as previously described with respect to Materials Studio. All molecular dynamics simulations were performed using a Large-scale Atomic / Molecular Massively Parallel Simulator(LAMMPS) software package. The Lennard-Jones interactions were computed using 6 / 9 functional form with 10 A cutoff distance. The electrostatic interactions were computed using Coulomb potential with 15 A cutoff distance and Particle-Particle Particle-Mesh (PPPM) method beyond the cutoff distance. The velocity-Verlet algorithm was used with an integration timestep of1 fs. The Nose- Hoover thermostat is used for both NVT and NPT ensemble simulations. TheNVT ensemble can correspond a statistical ensemble used to study material properties under constant particle number N, constant volume V, and a temperature fluctuating around anAttorney Docket No.: 093331-1516558 (8320 AKI) equilibrium value T. Similarly, the NPT ensemble can correspond to a statistical ensemble used to study material properties under constant temperature T, constant pressure P, and constant particle number N. The NPT ensemble can also be referred to as an isothermal-isobaric ensemble. The system is minimized using a conjugate gradient (CG) algorithm. The system is then equilibrated at pressure of 1 bar and 353 K temperature using the NPT ensemble for 10 ns. The simulation box length of the system after equilibration step is presented in FIG. 85. The system is then simulated using the NVT ensemble for a total of 10 ns, and the last 5 ns of the production run are used for analysis.REFERENCES

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[0211] All references throughout this application, for example patent documents, including issued or granted patents or equivalents and patent application publications, and non-patent literature documents or other source material are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference.

[0212] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art, in some cases as ofAttorney Docket No.: 093331-1516558 (8320 AKI) their filing date, and it is intended that this information can be employed herein, if needed, to exclude (for example, to disclaim) specific embodiments that are in the prior art.

[0213] When a group of substituents is disclosed herein, it is understood that all individual members of those groups and all subgroups and classes that can be formed using the substituents are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. As used herein, “and / or” means that one, all, or any combination of items in a list separated by “and / or” are included in the list; for example “1, 2 and / or 3” is equivalent to “1, 2, 3, 1 and 2, 1 and 3, 2 and 3, or 1, 2, and 3”.

[0214] Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of materials are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same material differently. It will be appreciated that methods, device elements, starting materials, and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials, and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure.

[0215] As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of’ excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of’ does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition, in a description of a method, or in a description of elements of a device, is understood to encompass those compositions, methods, or devices consisting essentially of and consisting of the recited components or elements, optionally in addition to other components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element, elements, limitation, or limitations which is not specifically disclosed herein.

[0216] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it isAttorney Docket No.: 093331-1516558 (8320 AKI) recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

Claims

Attorney Docket No.: 093331-1516558 (8320 AKI)WHAT IS CLAIMED IS:

1. A proton exchange membrane, the proton exchange membrane comprising: a perfluorosulfonic acid membrane structure having a surface and a structure; and a boron nitride coating deposited on the surface using laser deposition, wherein the laser deposition is configured to form a plurality of nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure.

2. The proton exchange membrane of claim 1, wherein the plurality of nitrogen-containing functional groups comprises amine groups.

3. The proton exchange membrane of claim 1, wherein the plurality of nitrogen-containing functional groups is positioned between the surface and about 30 nm deep of the perfluorosulfonic acid membrane structure.

4. The proton exchange membrane of claim 1, wherein the perfluorosulfonic acid membrane structure comprises a perfluorosulfonic acid (PFSA) or a polytetrafluoroethylene (PTFE) copolymer.

5. The proton exchange membrane of any one of claims 1-4, wherein the boron nitride coating has a thickness between about 5 nm to about 15 nm.

6. The proton exchange membrane of claim 1, wherein proton conductance of the proton exchange membrane is between 30 mS / cm to 70 mS / cm at a temperature ranging from about 30 °C to about 80 °C.

7. The proton exchange membrane of claim 1, wherein hydrogen crossover current of the proton exchange membrane is less than 5 mA / cm2at a temperature ranging from about 30 °C to about 80 °C.

8. A membrane-electrode assembly, the membrane-electrode assembly comprising: an anode; a cathode; and a proton exchange membrane between the anode and cathode, wherein the proton exchange membrane, comprises: a perfluorosulfonic acid membrane having a surface and a structure; andAttorney Docket No.: 093331-1516558 (8320 AKI) a boron nitride coating deposited on the surface using laser deposition, wherein the laser deposition is configured to form a plurality of nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure.

9. The membrane-electrode assembly of claim 8, wherein the plurality of nitrogen-containing functional groups comprises amine groups.

10. The membrane-electrode assembly of claim 8, wherein the perfluorosulfonic acid membrane structure comprises a perfluorosulfonic acid (PFSA) or a polytetrafluoroethylene (PTFE) copolymer.

11. The membrane-electrode assembly of any one of claims 8-10, wherein the boron nitride coating has a thickness between about 5 nm to about 15 nm.

12. The membrane-electrode assembly of claim 8, wherein proton conductance of the proton exchange membrane is between 30 mS / cm to 70 mS / cm at a temperature ranging from about 30 °C to about 80 °C.

13. The membrane-electrode assembly of claim 8, wherein hydrogen crossover current of the proton exchange membrane is less than 5 mA / cm2at a temperature ranging from about 30 °C to about 80 °C.

14. A fuel cell, the fuel cell comprising: a first bipolar plate comprising a first set of channels to provide gas flow; a second bipolar plate comprising a second set of channels to provide gas flow; and the membrane-electrode assembly of claim 8 positioned between the first bipolar plate and the second bipolar plate.

15. The fuel cell of claim 14, wherein the membrane-electrode assembly exhibits a power density between about 600 mW / cm2to about 1000 mW / cm2at a temperature ranging from about 30 °C to about 80 °C.

16. A method of producing a proton exchange membrane, the method comprising: providing a perfluorosulfonic acid membrane having a surface and a structure; andAttorney Docket No.: 093331-1516558 (8320 AKI) directly depositing a boron nitride coating on the perfluorosulfonic acid membrane surface using laser deposition, wherein the laser deposition is configured to form a plurality of nitrogen-containing functional groups in the perfluorosulfonic acid membrane structure.

17. The method of claim 16, wherein the laser deposition is pulsed laser deposition (PLD).

18. The method of claim 16, wherein the plurality of nitrogen-containing functional groups comprises amine groups.

19. The method of claim 16, wherein the perfluorosulfonic acid membrane structure comprises a perfluorosulfonic acid (PFSA) or a polytetrafluoroethylene (PTFE) copolymer.

20. The method of any one of claims 16-19, wherein the boron nitride coating has a thickness between about 5 nm to about 15 nm.

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