An in situ surface-enhanced Raman spectroscopy system for real-time monitoring of electrocatalytic three-phase interfaces

Abstract

An in-situ surface enhanced Raman spectroscopy system for real-time monitoring of the electrocatalytic three-phase interface belongs to the field of electrocatalytic characterization, and can realize accurate and convenient observation of the changes of the three-phase interface during the electrocatalytic reaction, and explore the deep mechanism of the reaction. It includes a Raman spectrometer, a peristaltic pump, an in-situ flow cell, an electrolyte bottle, a gas bottle, an electrochemical workstation and a data recording computer. The in-situ flow cell, the peristaltic pump and the electrolyte bottle are connected with a rubber tube to realize the circulation of electrolyte, the in-situ flow cell is connected to the gas bottle to pass the gas, and the electrode clamps of the electrochemical workstation are respectively clamped on the three electrodes of the in-situ flow cell. During the test, the in-situ flow cell is placed under the Raman spectrometer laser, and the test parameters are adjusted and the data is recorded by the computer. The present invention provides a simple and effective platform for exploring the path and mechanism of electrocatalytic reactions.

Description

Technical Field

[0001] The present invention belongs to the field of electrocatalytic characterization, and specifically relates to an in-situ surface enhanced spectroscopy system for real-time detection of an electrocatalytic three-phase interface, which can accurately and conveniently observe the changes in the three-phase interface during an electrocatalytic reaction and explore the deep mechanism of the reaction. Background Art

[0002] Surface Plasmon Resonance (SPR) refers to the phenomenon that under the excitation of incident light of a certain wavelength, free electrons on the surface of nano-metal films such as gold, silver, and copper will produce cooperative resonance and propagate parallel to the metal surface. Surface-Enhanced Raman Scattering (SERS) technology is a vibrational spectroscopy technology that uses SPR to enhance the Raman scattering of analytes. It has ultra-sensitive detection properties and is currently widely used in fields such as materials science.

[0003] The interfaces of various phases in the electrocatalytic process generally undergo obvious changes during the reaction. Such surface changes are often related to the underlying mechanism. Monitoring the interfaces of the electrocatalytic process, especially real-time monitoring, can provide an effective platform for mechanism research. However, it is difficult for non-in-situ SERS research to reveal the real-time characteristics of the reaction process. Traditional in-situ SERS research is limited by electrode characteristics, and its exploration is mostly focused on the solid-liquid phase. On the other hand, ordinary metal electrodes are insensitive to some Raman bands, so the monitoring bands are also quite limited, and it is difficult to conduct research across multiple bands. Therefore, by using mesoporous gold gas diffusion electrodes and specially designed in-situ Raman reaction cells, we can achieve real-time monitoring of the in-situ SERS system, which is of great significance in current related research. Summary of the invention

[0004] We have invented a system that can realize in situ surface-enhanced spectroscopy for real-time detection of electrocatalytic three-phase interfaces. This new technology provides a simple and effective platform for exploring the pathways and mechanisms of electrocatalytic reactions.

[0005] The technical solution of the present invention includes the following contents:

[0006] (1) This system includes a Raman spectrometer, a peristaltic pump, an in-situ flow cell, an electrolyte bottle, a gas bottle, an electrochemical workstation, and a data recording computer. The electrolyte and gas depend on the system. The in-situ flow cell, the peristaltic pump, and the electrolyte bottle are connected with a rubber tube to circulate the electrolyte. The in-situ flow cell is connected to the gas bottle to pass the gas. The electrode clamps of the electrochemical workstation are respectively clamped on the three electrodes of the in-situ flow cell. During the test, the in-situ flow cell is placed under the Raman spectrometer laser, and the test parameters are adjusted and the data are recorded by the computer, such as Figure 4325nm, 532nm, and 785nm laser sources represent ultraviolet, visible light, and near-infrared SPR excitation conditions, respectively, and can be used in a wide wavenumber range (50-4000cm -1 ) to detect the three-phase interface of the catalytic material.

[0007] (2) Schematic diagram of in-situ Raman testing Figure 1 As shown, the Raman laser irradiates the surface of the gas diffusion electrode through the light window, and the electrode surface is in contact with the liquid phase and the gas phase at the same time, realizing three-item interface monitoring; the light intensity sensor can be used to detect the intensity of the Raman laser; there are gas inlets and outlets at the bottom of the cell body, and the S-shaped channel can increase the gas diffusion rate to avoid the reaction being limited by lack of gas. In conventional Raman monitoring, since the electrode reacts partly on the surface, a large amount of bulk water in the solution will interfere with the observation of the electrode surface and affect the accurate identification of the signal. Therefore, we designed and manufactured a reaction cell with adjustable thickness from the light window to the electrode surface, and the minimum thickness from the light window to the electrode surface can be adjusted to 10μm, thereby effectively reducing the interface interference of the bulk water and realizing the observation of the ordered structure and mass transfer efficiency of the double electric layer on the electrode surface.

[0008] The working electrode is a gas diffusion electrode with transition metal oxide clusters anchored on the surface of three-dimensional mesoporous gold (3D-Au-MO x , M can be Ni, Co, Zr, V, Cu metal; x refers to a cluster formed by metal oxides of different valences), the specific preparation method of this electrode is:

[0009] A metal M coating is deposited on porous carbon paper by magnetron sputtering. Depending on the required thickness of the mesoporous gold layer, 20-100 nm can be deposited. Then, an Au coating is deposited thereon by magnetron sputtering with a thickness of 20 nm to form a bimetallic composite layer. Afterwards, an Au-M alloy is formed by calcination, wherein the volume ratio of Au and M, the specific calcination time and temperature depend on the metal M. For Ni(II) metal, the volume ratio of Au / Ni can be 0.3-0.8, the calcination temperature is 500-700°C, and the calcination time is 15-30 min. The morphology and pore structure of the 3D-Au matrix can be regulated by adjusting the volume ratio, calcination temperature, time and cooling time. After forming the bimetallic alloy film, electrochemical oxidation is used to etch the metal Ni, where the nitric acid concentration is 0.1-0.3M, the applied potential is 0-0.3V vs. RHE, and the etching time is 5-15min to form a mesoporous Au substrate. Changing the anode potential, reaction time, nitric acid concentration, etc. can control the MO on the Au substrate. x The size of the clusters, the distribution density, the precise control of the main valence state of the M element, etc. Finally, the porous carbon paper layer on the back is immersed in polytetrafluoroethylene hydrophobic sol for hydrophobic treatment, which is conducive to gas diffusion.

[0010] Specific preparation parameters of other metals:

[0011] Co(II): Au / Co volume ratio 0.2-0.5, calcination temperature 800-1000°C, calcination time 10-15min, nitric acid concentration 0.1-0.3M, anode potential 0-0.2V, etching time 5-15min.

[0012] Co(III): Au / Co volume ratio 0.2-0.5, calcination temperature 800-1000°C, calcination time 10-15min, nitric acid concentration 0.1-0.3M, anode potential 0-0.3V, etching time 5-15min.

[0013] Zr(IV): Au / Zr volume ratio 0.2-0.6, calcination temperature 600-900°C, calcination time 15-30min, nitric acid concentration 0.1-0.5M, anode potential 0-0.3V, etching time 15-30min.

[0014] V (IV / V): Au / V volume ratio 0.5-0.8, calcination temperature 600-900°C, calcination time 15-30min, nitric acid concentration 0.1-0.3M, anode potential 0-0.5V, etching time 5-15min.

[0015] Cu(III): Au / Cu volume ratio 0.3-1.0, calcination temperature 400-550°C, calcination time 15-30min, nitric acid concentration 0.5-1.0M, anode potential 0.3-0.7V, etching time 15-30min.

[0016] The contact surface area of ​​this three-dimensional material is much larger than that of the two-dimensional material, and the mesopores are conducive to the diffusion of gas on the electrode surface. Compared with ordinary electrodes that can only study the solid-liquid interface, this electrode can achieve the coexistence of gas, solid, and liquid on the electrode surface, and can be used to study the reaction mechanism of CH4 oxidation, CO2 reduction and other reactions on the electrode. In addition, the gold electrode itself has the SPR effect, and obvious Raman signals can be detected from ultraviolet to near infrared, from low wavenumber to high wavenumber, while the traditional electrode signal is weak and difficult to achieve.

[0017] (3) Figure 2They are schematic diagrams of the powder electrode in-situ cell A and the self-supporting electrode in-situ cell B, with electrolyte inlets and outlets on both sides; the light window is placed in the middle, and the disc area under the light window can be loaded with electrode materials, among which the surface of the disc area of ​​the powder electrode in-situ cell is glassy carbon, which can load powder materials; the self-supporting electrode in-situ cell can be loaded with sheet electrodes, and then one side of the conductive glue is glued to the electrode, and one side extends out a metal strip that can be connected to the electrochemical workstation. The two adopt a three-electrode system as a whole, one side of the electrode extends into the inside of the in-situ cell, and the external metal rod can be connected to the electrochemical workstation. The working electrode is a gas diffusion electrode, with an Ag / AgCl electrode as the reference electrode, and a platinum wire (alkaline system) or a graphite rod (acidic system) as the counter electrode.

[0018] Figure 3 Based on two two-phase Raman cells, we designed and improved a customized three-phase in-situ Raman cell for three-phase system, which circulates liquid and gas at the same time. On the basis of the two-phase Raman cell, an additional gas channel is added, in which the gas diffusion electrode is placed at the bottom, the electrolyte can be circulated on it, and the gas can be introduced below it, forming a three-electrode system as a whole. The Raman laser shines on the surface of the material through the light window to achieve simultaneous in-situ monitoring of gas, liquid and solid. For different electrocatalytic conditions, these in-situ reaction cells can add other components, such as gas guide grooves, temperature and light intensity sensors, etc., to respectively achieve gas flow rate regulation and real-time monitoring of SPR effect.

[0019] (4) The exploration of reaction mechanism mainly focuses on the low, medium and high wave number ranges, and the analysis of active sites of oxidation / reduction reaction (low wave number region):

[0020] In the low wave number region (100-1200cm -1 ) can reveal the dynamic evolution of its oxidation active sites and excited active species. By comparing the peak intensity and other information under the same conditions, a qualitative assessment of the local concentration and oxidation capacity of the active sites can be achieved; M(OH) x Taking the MOOH active site as an example, the ratio of its stretching vibration and bending vibration peak intensities can be used as a measurement scale to analyze the order of the oxidation active site (the higher the ratio, the more ordered).

[0021] Similarly, this system can be used to study the reduction active sites, such as 3D-Au-CuO x The solid-liquid interface of the electrocatalytic reduction of CO2. It not only reveals the chemical state of the catalyst surface, but also the appearance and disappearance of chemically adsorbed intermediates associated with CO2 reduction. For the two key liquid products, formic acid and ethanol, the active sites are CuO x, but its key intermediates are *CO and *OCHO*, and their double layer structures gradually tend to be ordered as the reduction potential increases. According to the results of different potentials and reaction times, it is possible to analyze and observe the role of different key intermediates in the formation of ethanol and formic acid and their reaction with CuO x interaction situation.

[0022] (5) Analysis of adsorption intermediates (medium wave number region):

[0023] The characterization of adsorption intermediates is mainly focused on the mid-wave number region (1300-2800 cm -1 ), which focuses on information including oxygen source species (CO3 2- / OH - ), the generation / disappearance of single-carbon intermediates (*CO / *HCHO3, etc.), and multi-carbon intermediates (*C2H6O / *C3H8O, etc.), changes in local concentrations, and possible adsorption sites are used to analyze the active sites and reaction processes of partial oxidation and intermediate transformation / coupling at the interface; based on the types of adsorbed intermediates that appear and changes in concentration, the possible reaction pathways and rate-determining elementary reactions are inferred.

[0024] (6) Double electric layer mass transfer structure analysis (high wave number region):

[0025] High wave number area (2900-4000cm -1 ) can be used to explore the order of the double-layer structure and the change in the interfacial electric field strength during the electrocatalytic process. By analyzing the hydrogen bond structure of interfacial water molecules (four-coordinate hydrogen bonds, two-coordinate hydrogen bonds, and cation hydrate hydrogen bonds), determining the proportion of ordered cation hydrate hydrogen bonds, and combining the AC impedance spectrum to determine the adsorption resistance (Rad) and charge transfer resistance (Rct) of the reaction intermediates, the quantitative relationship between the order of the double-layer structure and its mass transfer characteristics can be revealed; in addition, further analysis of the Stark shift effect of the three interfacial water molecule hydrogen bond structures under different SPR conditions can determine their influence on the interfacial electric field strength.

[0026] By comparing and referring to relevant databases, we can analyze the specific structural information represented by the Raman peak in the corresponding wavenumber region, and then infer the deep reaction mechanism based on its changes with potential, etc.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) Real-time monitoring of three-phase interface changes can be achieved.

[0029] Conventional Raman spectroscopy is generally non-in-situ and can only monitor the solid-liquid phase, which can be used to explore the surface conditions of materials. However, in the electrochemical catalytic process, the changes in the material surface are often dynamic and related to many factors, especially the applied potential. If you want to explore the changes in the reaction interface during the reaction, ordinary Raman spectroscopy is difficult to achieve. Therefore, the most important advantage of this real-time monitoring spectroscopy system is that it can reveal the reaction mechanism in the electrocatalytic process by observing the three-phase interface in situ.

[0030] (2) High sensitivity, capable of analyzing and detecting trace active sites on the electrode surface, adsorbed intermediates, and double-layer water structure.

[0031] We use Au, which has excellent electrochemical stability and certain catalytic effect on oxidation reactions, as the plasma resonance matrix, and use the dealloying-electrochemical oxidation etching method to accurately prepare a series of three-dimensional mesoporous gold electrode substrates with multi-wavelength SPR effect. Using the multi-wavelength tunable SPR characteristics of mesoporous gold, we construct in-situ surface enhanced Raman spectroscopy, thereby realizing wide-area detection of the electrocatalytic three-phase interface.

[0032] (3) The system has a simple structure, is easy to assemble and use, and has little interference.

[0033] This system only requires a Raman tester, an electrochemical workstation, a peristaltic pump and a flow cell. It is fast to build and has low cost. The structure of the flow cell itself is very simple. After inserting the working electrode and connecting the peristaltic pump and the electrochemical workstation, you can start the Raman spectrum test. The light window of the Raman cell can be freely adjusted in height, which effectively reduces the interference of bulk water on the electrode surface observation.

[0034] (4) High accuracy and wide monitoring range.

[0035] Due to the SPR effect, the spectral resolution is greatly improved, and the peak shape changes of the three-phase interface during the electrocatalytic process can be accurately obtained. The three-dimensional mesoporous gold electrode enables the system to monitor a wide range, from ultraviolet to near infrared, in a wide wave number range (50-4000cm -1 ) for monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of electrolytic cell monitoring, including Raman laser 1, quartz light window (liftable) 2, electrolyte 3, active layer 4, gas diffusion electrode 5, S-shaped channel 6, temperature / light intensity sensor (retractable) 7, gas inlet 8, and gas outlet 9.

[0037] Figure 2It is an in-situ Raman cell, A is a powder electrode in-situ cell, B is a self-supporting electrode in-situ cell, wherein there are a working electrode 10, a reference electrode (Ag / AgCl) 11, a counter electrode (platinum wire) 12, a counter electrode (graphite) 13, an electrolyte inlet 14, and an electrolyte outlet 15.

[0038] Figure 3 It is a three-phase in-situ Raman cell for the three-phase monitoring system. A is an overall diagram of the three-phase in-situ Raman cell, and B is a component decomposition diagram, including a working electrode 10, a reference electrode (Ag / AgCl) 11, a counter electrode (graphite) 13, an electrolyte inlet 14, an electrolyte outlet 15, a gas inlet 16, a gas outlet 17, an optical window 18, and an ion exchange membrane 19.

[0039] Figure 4 It is a schematic diagram of the in-situ monitoring system of the three-phase interface, which includes an electrochemical workstation 20, a Raman spectrometer 21, an in-situ reaction cell 22, a circulating test solution 23, a peristaltic pump 24, and a gas bottle 25.

[0040] Figure 5 Electron microscope images of the mesoporous gold substrate electrodes of this system. Figure a is a 50nm resolution electron microscope image, Figure b is a 20nm resolution electron microscope image, and Figure c is a 2nm resolution electron microscope image.

[0041] Figure 6 This system is used to in situ monitor the data of active sites at the three-phase interface of electrocatalytic carbon dioxide reduction, adsorption intermediates, and electrochemical double layer ordered structure, including low wavenumber test A, medium wavenumber tests B and C, and high wavenumber test D. DETAILED DESCRIPTION

[0042] The technical solution of the present invention is further illustrated and described below through specific implementation modes in combination with the accompanying drawings.

[0043] Example 1

[0044] (1) Preparation of three-dimensional mesoporous gold with copper oxide clusters anchored on the surface (3D-Au-CuO x )Gas diffusion electrode.

[0045] Au and Cu nano-coatings were deposited on porous carbon paper by magnetron sputtering, and Au-Cu alloy / solid solution films (precursors) were formed by calcination in an inert atmosphere. The main Cu component in the alloy film was dissolved and etched by electrochemical oxidation etching process using nitric acid solution as the electrolyte to construct a three-dimensional continuous mesoporous gold matrix with copper oxide clusters anchored on the surface (3D-Au-CuO x ) thin film material; the back of the porous carbon paper is hydrophobized to obtain 3D-Au-CuO x Gas diffusion electrodes, such as Figure 5 shown.

[0046] (2) Fix the working electrode to the bottom of the flow cell, assemble the flow cell, load the Ag / AgCl reference electrode and counter electrode, connect the inlet and outlet of the peristaltic pump, connect the gas cylinder, and turn on the peristaltic pump to circulate the electrolyte for 10 minutes.

[0047] (3) Turn on the Raman spectrometer and electrochemical workstation, place the electrolytic cell on the Raman spectrometer test bench, connect the wires of the electrochemical workstation, and prepare to start the test. First, use cyclic voltammetry to activate the electrode to remove impurities on its surface and stabilize the electrochemical properties of the electrode. The potential is set to 0-1.23V, the scan rate is 50mV / s, and the cycle is 50 times.

[0048] (4) Parameter settings

[0049] The potential range of the oxidation reaction is 1.3-1.8V. Constant current polarization is used during the test, starting from 1.3V, with an interval of 0.5V each time until 1.8V. The electrolyte uses a 1M KOH solution, and the in-situ Raman instrument uses a 532nm laser. The Raman test starts after 30s of polarization each time, and the polarization is turned off after the test. Tests are carried out in the low wavenumber region, medium wavenumber region, and high wavenumber region respectively. After that, by analyzing the Raman spectra at different potentials and different wavenumber regions, the dynamic changes of the three-phase interface during the electrode reaction can be obtained, and in-situ monitoring can be achieved.

[0050] The operation of testing the reduction reaction is similar to that described above. The electrolyte is 0.2M KHCO3 solution, the potential range is -0.2 to -1.3V, constant current polarization is used during the test, and the in-situ Raman instrument uses a 635nm laser. The Raman test is started after each polarization for 30s. After the test, the polarization is turned off and tests are performed in three different wave number regions.

[0051] (5) Taking the carbon dioxide reduction reaction (CO2RR) as an example, 3D-Au-CuO x CO2RR data analysis on Figure 6 )

[0052] By analyzing the data, we know that Cu2O (518cm -1 and 624cm -1 ), while in the medium wave number region, two important intermediates in CO2RR, *CO and *OCHO*, can be found. In the high wave number region, -K + The H2O double layer structure shows that as the voltage increases, the double layer structure becomes more orderly. Combining these data, we can draw conclusions and analyze the reaction path of CO2RR. x *CO intermediates are generated at the bimetallic interface and then migrate to CuO xThe formation and evolution of the *OCHO* intermediate of formic acid occurred entirely on CuO x On the location.

[0053] The above is only a preferred embodiment of the present invention, and does not limit the method scheme of the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the essence of the method of the present invention shall fall within the protection scope of the present invention.

Claims

1. An in-situ surface enhanced Raman spectroscopy system for real-time monitoring of electrocatalytic three-phase interfaces, characterized in that: (1) comprising a Raman spectrometer, a peristaltic pump, an in-situ flow cell, an electrolyte bottle, a gas bottle, an electrochemical workstation and a data recording computer, wherein the in-situ flow cell, the peristaltic pump and the electrolyte bottle are connected by a rubber tube to realize circulating electrolyte, the in-situ flow cell is connected to the gas bottle to introduce gas, the electrode clamps of the electrochemical workstation are respectively clamped with three electrodes of the in-situ flow cell, during testing, the in-situ flow cell is placed under the laser of the Raman spectrometer, the test parameters are adjusted and the data are recorded by the computer, 325 nm, 532 nm and 785 nm laser light sources represent ultraviolet, visible light and near infrared SPR excitation conditions respectively, and the three-phase interface of the catalytic material is detected within 50-4000 cm -1 ; (2) Raman laser is irradiated onto the surface of the gas diffusion electrode through the light window. The electrode surface is in contact with the liquid phase and the gas phase at the same time, realizing three-phase interface monitoring. The light intensity sensor is used to detect the intensity of the Raman laser. The bottom of the cell body has a gas inlet and outlet. A reaction cell with adjustable thickness from the light window to the electrode surface is designed and manufactured. The minimum thickness from the light window to the electrode surface can be adjusted to 10 μm. The working electrode uses a gas diffusion electrode 3D-Au-MOx with transition metal oxide clusters anchored on the surface of three-dimensional mesoporous gold, where M is Ni, Co, Zr, V or Cu metal; x refers to a cluster formed by metal oxides of different valences, and the specific preparation method of this electrode is: A metal M coating is deposited on the porous carbon paper by magnetron sputtering to a thickness of 20-100 nm, and then an Au coating is deposited thereon by magnetron sputtering to a thickness of 20 nm to form a bimetallic composite layer; thereafter, an Au-M alloy is formed by calcination. wherein Ni(II) metal, the volume ratio of Au / Ni is 0.3-0.8, the calcination temperature is 500-700°C, and the calcination time is 15-30min; after forming the bimetallic alloy film, electrochemical oxidation etching of metal Ni is performed, wherein the nitric acid concentration is 0.1-0.3M, the applied potential is 0-0.3V vs.RHE, and the etching is performed for 5-15min to form a mesoporous Au substrate, and the porous carbon paper layer on the back is immersed in polytetrafluoroethylene hydrophobic sol for hydrophobic treatment; Specific preparation parameters of other metals: Co(II): Au / Co volume ratio 0.2-0.5, calcination temperature 800-1000℃, calcination time 10-15min, nitric acid concentration 0.1-0.3M, anode potential 0-0.2V, etching time 5-15min; Co(III): Au / Co volume ratio 0.2-0.5, calcination temperature 800-1000℃, calcination time 10-15min, nitric acid concentration 0.1-0.3M, anode potential 0-0.3V, etching time 5-15min; Zr(IV): Au / Zr volume ratio 0.2-0.6, calcination temperature 600-900°C, calcination time 15-30min, nitric acid concentration 0.1-0.5M, anode potential 0-0.3V, etching time 15-30min; V (IV / V): Au / V volume ratio 0.5-0.8, calcination temperature 600-900°C, calcination time 15-30min, nitric acid concentration 0.1-0.3M, anode potential 0-0.5V, etching time 5-15min; Cu(III): Au / Cu volume ratio 0.3-1.0, calcination temperature 400-550°C, calcination time 15-30min, nitric acid concentration 0.5-1.0M, anode potential 0.3-0.7V, etching time 15-30min.

2. The in-situ surface enhanced Raman spectroscopy system for real-time monitoring of the electrocatalytic three-phase interface according to claim 1, characterized in that: The in-situ flow cell is a powder electrode in-situ cell A or a self-supporting electrode in-situ cell B, both of which have electrolyte inlets and outlets on both sides; a light window is placed in the middle, and the disc area under the light window is loaded with electrode materials, wherein the surface of the disc area of ​​the powder electrode in-situ cell is glassy carbon, loaded with powder materials; the self-supporting electrode in-situ cell is loaded with sheet electrodes, and then one side of the conductive glue is glued to the electrode, and one side extends out a metal strip that can be connected to an electrochemical workstation; the two adopt a three-electrode system as a whole, one side of the electrode extends into the interior of the in-situ cell, the external metal rod is connected to the electrochemical workstation, the working electrode is a gas diffusion electrode, and the Ag / AgCl electrode is used as a reference electrode.

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