An integrated electrode and flow field material and a method to manufacture such a material

Abstract

The present invention relates to methods and techniques for fabricating porous electrodes, and more particularly relates to utilizing phase inversion techniques during fabrication. The porous electrodes can be used, for example, in redox flow batteries, electrolyzers for water splitting, electrochemical synthesis, or CO2 reduction, among other uses. Resulting electrodes, batteries, and other systems are also covered by the present disclosure.

Description

[0001] Title: An integrated electrode and flow field material and a method to manufacture such a material.

[0002] Description:

[0003] The present invention relates to methods and techniques for fabricating porous electrodes, and more particularly relates to utilizing phase inversion techniques during fabrication. The porous electrodes can be used, for example, in redox flow batteries, electrolyzers for water splitting, electrochemical synthesis, or CO2 reduction, among other uses. Resulting electrodes, batteries, and other systems are also covered by the present disclosure.

[0004] Redox flow batteries (RFBs) have attracted significant attention in recent years due to their suitable characteristics for long-duration stationary energy storage applications. However, the wider adoption of RFB systems is still limited by the capital cost, to which the reactor stack is a large contributor. Further cost reduction can be achieved by reducing kinetic, mass-transfer, and ohmic losses to improve the stack performance. Porous electrodes are key components within the battery stack, providing active sites for electrochemical reactions, and transport pathways for active species. Commercially available carbon-fiber electrode architectures include carbon paper, carbon felt, and carbon cloth. They have a highly porous structure, but their relatively low surface area and low hierarchical fiber orientation increase both electrochemical and pumping losses.

[0005] European patent application EP 4 104 228 in the name of the present inventors relates to a method of fabricating a porous electrode, comprising a step of exposing a polymer solution to a first solvent, the polymer solution comprising a first polymer and a second polymer, and subsequently exposing the polymer solution to a second solvent, the second solvent being effective to induce phase inversion such that the first polymer of the polymer solution is separated from each of the second polymer of the polymer solution, the first solvent, and the second solvent, the first polymer being porous and forming a porous membrane. Such a method further comprises a step of performing one or more post-treatment actions to the porous membrane, wherein the one or more post-treatment actions comprises crosslinking the porous membrane, wherein the one or more post-treatment actions comprises one of carbonization of the porous membrane or graphitization of the porous membrane. US 2015 / 295247 relates to electrodes suitable for use in a redox flow battery, the electrode comprising a porous electrode having a plurality of perforations, the perforations ranging in diameter from 100 pm to 10 cm. The method disclosed herein comprises forming a plurality of perforations in a porous electrode, the holes ranging in diameter from 100 pm to 10 cm. Examples of forming such perforations include use of a laser, punching, milling, drilling, electrical discharge machining, cutting, or templating.

[0006] WO 02 / 43937 relates to a method of making a product with a micro to nano sized structure using a mould having a corresponding structure at a mould surface in which a fluid containing a casting material is brought into contact with the mould surface wherein the fluid is subjected to a treatment to induce phase separation therein, wherein the casting material is at least partially solidified on the mould surface and the resulting product is released from the mould surface. The fluid is only partially solidified by means of induced phase separation on the mould, wherein the partially solidified product is released from the mould for further processing, including further phase separation. The phase separation process is a vapour, liquid, or thermal induced phase separation process, a colloid, pressure, or reaction induced phase separation process, or the phase separation process is non-solvent induced.

[0007] US 2021 / 257630 relates to a method of fabricating a porous electrode, comprising the following steps of exposing a polymer solution to a first solvent, the polymer solution comprising a first polymer and a second polymer; and subsequently exposing the polymer solution to a second solvent, the second solvent being effective to induce phase inversion such that the first polymer of the polymer solution is separated from each of the second polymer of the polymer solution, the first solvent, and the second solvent, the first polymer being porous and forming a porous membrane. Such method further comprises performing one or more post-treatment actions to the porous membrane, namely removing the porous membrane from the second solvent, drying the porous membrane, thermally stabilizing the porous membrane, and one of carbonizing or graphitizing the porous membrane.

[0008] EP 3 868 930 relates to a method of manufacturing a porous carbon fiber including the steps of miscibilizing a carbonizable resin and an eliminable resin to give a resin mixture, molding the resin mixture in miscibilized state into a fibrous form and separating the resin mixture into multiple phases, removing the eliminable resin from the phase-separated resin mixture, carbonizing the resin mixture through heating to obtain a porous carbon fiber, and polishing the surface of the porous carbon fiber. Such a porous carbon fiber is used in a fluid separation membrane as a support thereof.

[0009] WO 2017 / 086609 relates to a method for manufacturing a porous carbon structure, comprising a homogeneous solution preparation step for dissolving an intrinsically microporous polymer in a good solvent, a precursor preparation step for using the homogeneous solution to prepare a precursor, a porous structure formation step for using the precursor to form a porous structure through non-solvent induced phase separation, and a carbonization step for carbonizing the porous structure.

[0010] There is still a need to develop a low-cost porous electrode that meets the requirements of high active surface area and facile electrolyte transport to enhance battery performance and system efficiency.

[0011] Non-solvent induced phase separation (NIPS) is a manufacturing route in the field of membrane technology, which is widely used to fabricate asymmetric membranes, as shown in the above discussed European patent application EP 4 104 228. The microstructure of NIPS electrodes is largely influenced and controlled by the solvent / non-solvent exchange rate at the interface between a polymer solution and a coagulation bath. When the phase exchange process is slow, segregation is delayed. This type of segregation generally results in a sponge-like, relatively dense structure. This sponge-like structure has the advantage of small pore size and high surface area, but also has poor hydraulic permeability and high mass-transfer losses.

[0012] An object of the present invention is to provide a method for manufacturing a porous electrode that tackles the complex challenge of providing simultaneously high surface area for electrochemical reactions, facile mass transfer in the porous electrode, and a low pressure drop required to pump through the liquid electrolyte.

[0013] Another object of the present invention is to provide porous electrode having mechanical robustness in the electrochemical stack.

[0014] The present invention thus relates to a method for manufacturing a conductive polymer-derived carbon scaffold, comprising the following steps:

[0015] -providing a polymer solution,

[0016] -applying the polymer solution on a mold provided with micro-patterns, -contacting the mold containing the polymer solution with a non-solvent for the formation of a porous and solid scaffold,

[0017] - exposing the scaffold to a thermal treatment thereby forming the conductive polymer-derived carbon scaffold, wherein the conductive carbon scaffold contains a plurality of porous sizes and geometries.

[0018] On basis of the above method one or more objects are achieved. The present inventors thus found a method to imprint micro-patterned flow fields directly into the electrode architecture during NIPS processing to improve mass transfer properties. According to the present method NIPS polymer films were cast onto molds with specific micro-patterns, and the resulting micro-channels can be preserved in the electrodes after carbonization.

[0019] The present conductive carbon scaffold is designed as a carbonized porous electrode for use in electrochemical flow systems, such as redox flow batteries. The micro-patterning serves is designed to guide electrolyte flow through the electrode material during battery operation. The patterns are designed to increase electrolyte utilization, enhance the mass transfer rate, and reduce the pressure drop within the cell. In traditional redox flow batteries, electrolyte distribution is managed by a separate flow plate with an engraved flow field. The present design combines the functions of the electrode and flow plate into one component by incorporating micropatterns directly onto the carbonized porous electrode and such integration simplifies the battery design. The micro-patterns are the macroscopic channels on the NIPS electrode obtained from the designed molds with patterns. The size of the macroscopic channels are in a range of 200 micrometres to 5 millimetres. Micro-patterned NIPS electrode intrinsically includes pores within a range of 5 nm to 50 micrometres, and also includes macroscopic channels on the surface within a range of 200 micrometres to 5 millimetres.

[0020] According to the above discussed WO 02 / 43937 wherein polymeric structures with micro-structures are created by casting a polymer solution onto a designed mold, wherein, after phase separation, the polymer is solidified and released from the mold, resulting in a product with micro-structures, the present method process extends beyond this phase because, after the phase separation, the present polymer undergoes a series of additional steps, namely drying and carbonizing, resulting in carbonized porous electrodes rather than a polymer product. The term “conductive polymer-derived carbon scaffold” refers to a scaffold wherein the polymer has undergone a thermal treatment and converted to a carbonaceous component. Before the thermal treatment, e.g. carbonization, the product as such can be seen as a polymer scaffold. Typically such a scaffold is not conductive but there are options to make them conductive readily, like by adding conductive particles on the NIPS solution. It is then, after carbonization step(s), when the polymer scaffold becomes carbonaceous and thus conductive. Thermal treatment is the process involving the application of heat to materials to change their properties. To obtain carbon through thermal treatment from organic material, the temperature range is generally between 600-1600°C under inert atmosphere. The organic materials decompose to form carbon scaffolds.

[0021] In an example the carbonization process involves one or more steps within the temperature range of 950 °C to 1650 °C. The temperature ramp rate can vary between 2 °C / min and 10 °C / min, and the duration at the target temperature can range from 40 to 60 minutes. In a specific embodiment of the present method the carbonization process for manufacturing a conductive carbon scaffold comprises two temperature steps, i.e. 850 °C for 40 minutes, followed by 1050 °C for an additional 40 minutes, with a temperature ramp rate of 5 °C / min.

[0022] In an example the polymer solution comprises a pore forming agent, a carbon- forming polymer, and a solvent.

[0023] In an example the non-solvent is an aqueous solution. The term “non-solvent” means a substance incapable of dissolving a given component of a solution. For instance, in the present invention, the polymer polyacrylonitrile (PAN) is dissolved in dimethylformamide (DMF) solvent. PAN is not soluble in water or aqueous solution, so water or aqueous solution is the “non-solvent” in this case. A non-solvent induced phase separation (NIPS) is a process used primarily in polymer science. It involves the separation of a polymer solution into distinct phases upon the addition of a nonsolvent.

[0024] In an example the thermal treatment comprises at least one or more steps of drying, oxidation in air at an elevated temperature, carbonization, or graphitization. The present inventors found that by controlling the gas atmosphere during the carbonization or graphitization phase the electrode can be provided with very specific properties such as a higher heteroatom content, higher hydrophilicity or higher surface area, all properties relevant to improve the performance of the battery. The carbonization process is to provide conductive carbon scaffold under inert atmosphere (e.g., N2). During this process, the atmosphere can be adjusted by controlling the oxygen level in the oven to obtain carbon scaffold with different properties. For instance, If the oxygen proportion is less than 0.5%, the product with higher carbon yield and higher homogeneity. If the oxygen proportion is 0.5-2%, the product with higher heteroatom content, higher hydrophilicity, and higher surface area can be obtained. If the oxygen proportion is more than 2%, the carbonization process may fail, and the product may be burned away.

[0025] In an example the polymer solution further comprises electrocatalytic particles.

[0026] In an example macroscopic elements in the structure of the conductive carbon scaffold have larger sizes than the pores in the conductive carbon scaffold.

[0027] In an example the pores in the conductive carbon scaffold are in a range of 5 nm to 50 micrometres.

[0028] In an example the size of the macroscopic channels are in a range of 200 micrometres to 5 millimetres.

[0029] In an example the length of the macroscopic channels is between 1 and 100% of the thickness of the conductive carbon scaffold.

[0030] In an example the conductive carbon scaffold contains a plurality of pore sizes and pore size distributions, especially containing pore size gradients (across the thickness) of multimodal pore size distributions.

[0031] In an example the geometries include circular, square, and rectangular shapes of multiple sizes and orientations.

[0032] The present invention also relates to the use of a conductive carbon scaffold obtained according to a method as discussed above or a conductive carbon scaffold as discussed above in a redox flow battery as an electrode-flow field component.

[0033] The present invention also relates to the use of a conductive carbon scaffold obtained according to a method as discussed above or a conductive carbon scaffold as discussed above in a flow cell as an integrated flow field-gas diffusion layer.

[0034] The present invention also relates to the use of a conductive carbon scaffold obtained according to a method as discussed above or a conductive carbon scaffold as discussed above in a CO2 reduction electrolyzer as a gas diffusion electrode with structured channels for product removal. The present invention also relates to the use of a conductive carbon scaffold obtained according to a method as discussed above or a conductive carbon scaffold as discussed above in as a flow through electrode for capacitive deionization cells.

[0035] In an example uniform electrodes are manufactured using NIPS. For example, a viscous mixture of polyacrylonitrile (PAN) and pore forming polyvinylpyrrolidone (PVP) can be dissolved in a solvent. Non-limiting examples of solvent include N,N- dimethylformamide (DMF), dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP), Dimethyl Sulfoxide (DMSO), Polar Clean(trademark) (methyl-5-(dimethyl amino)-2- methyl-5-oxopentanoate, N,N-dimethylacetamide, TEP (triethyl phosphate), and Tetrahydrofuran (THF), among others. The mixture can be fully mixed after heating and casted in a mold provided with micro-patterns. The casted mixture can subsequently be immersed in a non-solvent, e.g., a water bath, to initiate phase separation into polymer-rich and polymer-lean regions through solvent / non-solvent exchange. During immersion and the solvent / non-solvent exchange, the water-soluble PVP can leach into solution, leaving behind an insoluble porous PAN scaffold. The phase separated material can then be dried and exposed to thermal stabilization and carbonization.

[0036] The present invention thus relates to a method for the manufacturing of novel electrode structures by using moulds with a certain geometrical patterns that acts as the negative of the resulting electrode structure. These moulds define the macroscopic geometry of the electrode structure. In the present method polymer phase inversion is used to create the small-scale porosity. By implementing this material in an electrochemical device, the manufacturing complexity and costs are reduced. The material is set to replace complex arrangements of flow fields and electrodes by using a single layer.

[0037] The present invention is based on a combination of NIPS (or similar solutionbased methods) with the micromolding as discussed above. The present inventors found that complex geometries are difficult to produce with the traditional methods (e.g. perforation) but that with the present method specific and complex geometries, such as circular, square, and rectangular shapes of multiple sizes and orientations, are possible. Thus, the present invention is to be seen in the combination of NIPS (or variations of NIPS) with the micromolding (or more generally, different geometrical frameworks) to make porous electrodes that can be used in a range of applications as discussed above.

[0038] In addition to that, a benefit of the present method is that the shape can be controlled as a function of the thickness of the scaffold. Thus, there is no need to perforate the entire thickness, but channels can be engineered with a preselected thickness (between 0 to 100% of the thickness).

[0039] The present method also enables the manufacture of complex 3D geometries that have been shown to provide improved performance in redox flow batteries. A non- exhaustive list of examples include bioinspired, lung-like geometries, fractal designs, and structures with varying geometries as a function of thickness.

[0040] In a specific embodiment of the present invention templating materials are added to the mold or to the casting solution that can be eliminated after casting by a thermal or chemical treatment. According to such a method electrodes with complex 3D structures can be obtained which structures are not possible with traditional methods.

[0041] The present inventors also found that with the present invention simplification of stack design by using bipolar plates with simple inlets and outlets can be achieved. The 3D geometry with the imprinted channels on the electrode as discussed above provide the function of electrode and flow field channels simultaneously.

[0042] The present inventors also found that the application of the present materials to redox flow batteries provide the function of electrode and flow field, alleviating pressure losses and increase surface area and mass transfer rates.

[0043] The present inventors also found that the application of the present materials to low temperature fuel cells (or reversible fuel cells) enables advanced water management strategies (e.g. removal of liquid water, transport of gas reactants).

[0044] Moreover, the application of the present materials to water electrolyzers (alkaline or acidic) improve the multiphase flows of gas and liquid.

[0045] The microstructure of NIPS can be controlled by adjusting the phase inversion conditions, and the electrochemical performance of the micro-patterned design on NIPS electrodes is strongly influenced by the NIPS microstructure.

[0046] Typically, a sponge-like structure or finger-shaped structure can be created by adjusting the phase inversion conditions. The sponge-like structure offers advantages of small pore size and high surface area at the cost of poor hydraulic permeability and significant mass-transfer losses. The finger-shaped macro void structure provides high hydraulic permeability and favourable mass transport properties, albeit with a relatively low surface area.

[0047] For example, to generate the sponge-like structure, polymer films along with the aluminium molds are immersed in a DMF bath for 5 seconds to initiate the presoaking process. This step introduce a thin film of DMF solution on the polymer film surface, aimed at decelerating the demixing process during the phase separation.

[0048] Alternatively, during the phase inversion process in the water bath, salts can be added to the bath to slow down the demixing process during phase separation. For example, and a 5 M NaCI solution can be used as the coagulation bath to create sponge-like microstructure.

[0049] Alternatively, the vapor-induced phase separation (VIPS) method can be used to create a sponge-like structure. After casting, polymer films, along with aluminium molds, are placed in a fume hood for 40-60 minutes to initiate phase inversion using the moisture in the air. Following this step, the polymer films are submerged in a water bath to solidify and form the sponge-like structures.

[0050] To create a finger-shaped macro void NIPS microstructure, the polymer films are directly put into the water coagulation bath to initiate a fast phase separation process. A solid scaffold is formed rapidly after immersion in the non-solvent coagulation bath. This type of demixing generally yields a highly porous, finger- shaped, macro void microstructure, with a top dense layer on the surface.

[0051] To further remove the top dense layer of the finger-shaped structure the polymer films are placed in a fume hood for 10-15 minutes after casting process for vapor induced phase separation. This process helps to absorb moisture on the top surface and can effectively prevent the formation of a dense top layer.

[0052] Examples

[0053] To improve the in-plane mass transport properties of the sponge-like NIPS structure, groove patterned electrodes were designed with the combination of flow- through flow field, and the groove patterns were placed perpendicular to the flow direction. This can help to add some turbulence along the flow direction. For the through-plane flow optimization, pillar patterns were designed and combined with interdigitated flow fields, and the pillar patterns were placed in between the inlet and outlet flow channels. This can facilitate the electrolyte through-plane flow within the rib region. These two designs were used to investigate the function and effects of the micro-patterns.

[0054] Thus, groove-patterned electrodes were combined with flow-through flow fields (FTFFs) for in-plane mass transport optimization, and pillar-patterned electrodes were combined with interdigitated flow fields (IDFFs) for through-plane flow optimizations. Two geometric sizes of the micro-patterns were considered in each design.

[0055] The pore sizes and distributions of the micro-patterned electrodes were determined by mercury intrusion porosimetry.

[0056] The hydraulic permeability of the electrodes was evaluated by half-cell pressure drop measurements, and the electrochemical specific surface area was determined through electrochemical impedance spectroscopy under non-faradaic conditions.

[0057] In addition, electrochemical battery tests with symmetric iron RFB (0.5 M Fe27Fe3+, 2 M HCI) cells and vanadium full-cells (1.6 M V active species at 50% state- of-charge). It was found that due to the small pore size (e.g., -1 pm) and narrow pore size distribution of the sponge-like NIPS structure, the main contribution to the total losses in electrodes without micro-patterns in electrochemical performance comes from mass-transfer. Electrodes with groove and pillar patterns can help reduce the pressure drop and improve mass transfer properties. Pillar-patterned electrodes with the combination of IDFF show high overall electrochemical performance with greatly reduced mass transport resistances. At an electrolyte velocity of 10 cm / s, the total resistances for kinetic and mass transfer are less than 0.1 Q cm2in symmetric iron cell tests. Compared to the activated commercial electrode (e.g., 2-layer Freudenberg H23 carbon paper, thermally treated at 450 °C for 12 h), the total resistances for kinetic and mass transfer are reduced by -60%, which shows great potential for adoption in next generation high-performance RFB systems.

[0058] Preparation of pristine membrane

[0059] Polyacrylonitrile (PAN) (Mw~ 150,000 Da, Sigma-Aldrich) and polyvinylpyrrolidone (PVP) (Mw~ 1 ,300,000 Da, Sigma-Aldrich) were added in a 100 mL Pyrex bottle and dissolved in dimethylformamide (DMF) (>99.9%, Sigma-Aldrich) solvent. PAN and PVP were selected here as the carbon precursor and pore-foaming agent to obtain ladder-like porous graphitic structure after thermal treatment. The mass ratio of PAN and PVP was selected as 2:3 to ensure balanced pore connection network and surface area. A PTFE magnetic stirring bar was added in the bottle, and the bottle was sealed by parafilm tape to avoid evaporation. It was placed on a hot plate magnetic stirrer at 80 °C and mixed at 120 rpm for >6 h until a clear yellowish solution was obtained. The bottle was placed on a roller bench at room temperature for >12 h to ensure thoroughly homogeneous mixture of the solution and removal of gas bubbles inside solution. The phase separated polymers were sandwiched between ceramic plated with 65g of weight on top of each membrane. The thermal stabilization of the PAN structure was done under air at 270 °C at a ramp of 2 °C / min with natural cooling to room temperature. The stabilized membranes were finally carbonized under nitrogen environment using the following sequence: RT to 850 °C at a ramp of 5 °C / min, hold 850 °C for 40min, 850 °C to 1050 °C at a ramp of 5 °C / min, hold 40 min, cooling down naturally to room temperature. This final carbonization step yielded porous conductive electrodes.

[0060] Flow cell electrochemical testing

[0061] Capacitance measurement:

[0062] An electrochemical battery tests, i.e. Iron (Fe27Fe3+) symmetric cell testing, was investigated using an iron symmetric flow battery with 0.25 M Iron (II) and Iron(lll), respectively, with 2 M HCI solution as supporting electrolyte. To make comparison for both flow-through and interdigitated flow fields (FTFF and IDFF, respectively), linear electrolyte velocity was used and 1 , 2, 5, and 10 cm / s were selected for both flow- through and interdigitated flow fields. It is noted that the overall volumetric flow rate for IDFF under the same linear electrolyte velocity is much higher than for the flow through design. For battery characterization, impedance spectroscopy was employed to decouple the kinetic and mass transport resistances. So, the mass transport impact for different micro-pattern designs can be seen here. The results presented here are all iR-corrected using the observed ohmic resistance.

[0063] After obtaining the electrodes after carbonization, the electrodes were put under SEM to see the microstructure and some shrinkage after carbonization was seen. The shrinkage ratios of the channel or pillar region are about 20-25%. And the shrinkage ratios between the two channels or two pillars are about 50%. So, the micropatterned designs of the original aluminum molds need to consider the shrinkage of electrodes after carbonization. It is also noted that due to the shrinkage effects, some deformation happens within the micro-pattern regions, especially for the groove patterns. This also needs to be considered in the design to keep the electrode integrity and mechanical properties.

[0064] Performance of the electrodes with micro-groove patterns

[0065] To make comparison, a no pattern NIPS electrode with sponge-like structure was used as the benchmark here. For the no pattern electrode structure, due to the high pressure drop, the velocity of 10 cm / s will lead to electrolyte leakage in the used single cell test fixture and was not used for this electrode. The present inventors found that the kinetic resistance is small as a result of the high ECSA, and major resistance comes from the mass transport part. Although an increase of the flow velocity can help to reduce the mass transport resistance, it is still significant even at a high average flow velocity of 5 cm / s.

[0066] The performance of the electrode with large-groove patterns was tested. The groove patterns were placed perpendicular to the flow direction. In the current battery design, there are 4 large grooves in one electrode. In this structure, a velocity of 10 cm / s is possible due to the lower pressure drop. As shown in the polarization curves, the mass transport limitation region happens at higher current densities, which prove that the groove patterns are beneficial to the mass transport.

[0067] The performance of the electrode with small-groove patterns was tested as well. In the current battery design, there are 7 small grooves in one electrode. The mass transport resistance is further reduced through the small-groove patterns.

[0068] Performance of the electrodes with micro-pillar patterns

[0069] In order to determine the performance of the pillar patterns with the combination of IDFF flow plates the no pattern NIPS with sponge-like structure were used as the benchmark. The present inventors found that the IDFF designs help to reduce the mass transport resistance compared to the FT-FF designs. And the velocity of 10 cm / s is possible here without battery leakage.

[0070] The performance of the electrode with large pillar patterns was also tested. There are 20 large pillars in the current electrode design. To make the large pillar patterns in between the inlet and outlet channels, IDFF designs with wider rib width were used. So, there are in total 3 intel channels and 2 outlet channels here. The presence of the large pillar patterns can greatly reduce the mass transport resistance. Even at the flow velocity of 1 cm / s, the battery can maintain good electrochemical performance. The performance of the electrode with small pillar patterns was tested as well. There are 48 small pillars in the current electrode design. The present inventors found that that the mass transport resistance is further reduced compared to the large pillars. The total resistance for kinetic and mass transport is less than 0.1 -ohm cm2for the velocity of 10 cm / s here.

[0071] Comparison of sponge-like and finger-like microstructures

[0072] During the phase exchange process, the NIPS microstructure can be controlled under two different conditions. If the phase exchange process is fast, instantaneous demixing will happen. A solid film is formed very rapidly after immersion in the nonsolvent bath. This type of demixing generally shows a highly porous, finger-shaped, macro void structure. Due to the presence of the macro void pores, this structure can provide low pressure drop and good mass transport properties, but relatively low surface area.

[0073] When the phase exchange process is slow, demixing is delayed. It takes much longer for the polymer film to form. A sponge-like, relatively dense structure is obtained. This structure has smaller pore sizes, which contributes to the high active surface area. However, due to the same reason, it also has low permeability and poor mass transport properties.

[0074] Mercury intrusion porosimetry was used to investigate the pore size distribution. The present inventors found that the sponge-like structure has relatively small pore sizes of round 1-2 microns, with narrow pore size distribution. For finger-shaped macro void structure, the pore diameters are around 10-20 microns. The present inventors also measured the porosity of all the sponge-like structure with micro-patterns. And the results are within 72%-76% this region.

[0075] Figure 1 shows a through-plane (top row) and cross-sectional (middle row) SEM images of sponge-like NIPS electrodes with micro-flow fields. The optical photographs of the electrodes with combination of the flow plate are presented in the bottom row. In the present study, the electrodes with micro-groove and micro-pillar patterns are combined with FTFF and IDFF, respectively.

[0076] Figure 2 shows pressure drops (dots) versus linear velocity of the sponge-like NIPS electrodes with (a) micro-groove patterns in FTFF and (b) micro-pillar patterns in IDFF within the single-cell. The fittings (lines) are based on the non-linear Darcy- Forchheimer equation. The fitted permeability (K) results are summarized in (c). (d) Volumetric specific surface area (av) obtained through EIS measurements in 2 M HCI solution. The error bars correspond to a standard deviation (n=2).

[0077] Figure 3 shows the electrochemical performance of the sponge-like NIPS electrodes with micro-grooves in a 0.5 M Fe27Fe3+symmetric cell with FTFF. The performance of NIPS with no pattern is presented for comparison. Nyquist plots obtaining through EIS measurements (dots) and the fittings (lines) at different electrolyte superficial velocities are shown in (a)-(c). IR-corrected polarization curves are shown in (d)-(f). The error bars in polarization curves correspond to a standard deviation (n=2).

[0078] Figure 4 shows the electrochemical performance of the sponge-like NIPS electrodes with micro-pillars in a 0.5 M Fe2+ / Fe3+symmetric cell with IDFF. The performance of NIPS with no pattern is presented for comparison. Nyquist plots obtaining through EIS measurements (dots) and the fittings (lines) at different electrolyte superficial velocities are shown in (a)-(c). IR-corrected polarization curves are shown in (d)-(f). The error bars in polarization curves correspond to a standard deviation (n=2).

[0079] Figure 5 shows the electrochemical performance of sponge-like NIPS electrodes with micro-flow fields in a 1.6 M full-cell vanadium RFB at 50% SoC. Polarization curves are shown in (a), and EIS spectra is shown in (b)-(c). The electrolyte superficial velocity is 5 cm / s.

[0080] The present invention thus relates to a method for manufacturing sponge-like NIPS electrodes under slow phase inversion conditions via a non-solvent induced phase separation technique resulting in the integration of micro-flow fields including groove and pillar designs into the electrode architecture. The resulting sponge-like NIPS electrodes feature small pore sizes and a narrow pore size distribution, contributing to a high volumetric specific surface area while presenting challenges in terms of the low permeability. The incorporation of micro-flow fields into the electrode structure significantly enhances electrolyte penetration, leading to a marked reduction in pressure drop and an improvement in mass transfer. Numerical results indicate that both micro-grooves and micro-pillars are effective in enhancing electrolyte mixing, thereby improving local mass transfer within the electrodes compared to the no pattern electrodes. The reduction of total resistance of kinetic and mass transfer underscores the potential of the present sponge-like NIPS electrodes with optimally designed micro-flow fields to markedly enhance the performance of next-generation redox flow batteries reactors.

Claims

CLAIMS1. A method for manufacturing a conductive polymer-derived carbon scaffold, comprising the following steps:-providing a polymer solution,-applying the polymer solution on a mold provided with micro-patterns,-contacting the mold containing the polymer solution with a non-solvent for the formation of a porous and solid scaffold,- exposing the scaffold to a thermal treatment thereby forming the conductive carbon scaffold, wherein the conductive carbon scaffold contains a plurality of porous sizes and geometries.

2. The method according to claim 1 , wherein the polymer solution comprises a pore forming agent, a carbon-forming polymer, and a solvent.

3. The method according to any one or more of claims 1-2, wherein the nonsolvent is an aqueous solution.

4. The method according to any one or more of claims 1-3, wherein the thermal treatment comprises at least one or more steps of drying, oxidation in air at an elevated temperature, carbonization, or graphitization.

5. The method according to any one or more of claims 1-4, wherein the polymer solution further comprises electrocatalytic particles.

6. The method according to any one or more of claims 1-5, wherein the gas atmosphere is controlled during the thermal treatment, e.g. carbonization or graphitization phase, for providing the conductive carbon scaffold with specific properties such as a higher heteroatom content, higher hydrophilicity, or higher surface area7. Conductive carbon scaffold containing a plurality of porous sizes and geometries, wherein macroscopic elements in the structure of the conductive carbon scaffold have larger sizes than the pores in the conductive carbon scaffold.

8. Conductive carbon scaffold according to claim 7, wherein the pores in the conductive carbon scaffold are in a range of 5 nm to 50 micrometres.

9. Conductive carbon scaffold according to any one of claims 7-8, wherein the size of the macroscopic channels are in a range of 200 micrometres to 5 millimetres.

10. Conductive carbon scaffold according to any one of claims 7-9, wherein the length of the macroscopic channels is between 1 and 100% of the thickness of the conductive carbon scaffold.

11. Conductive carbon scaffold according to any one of claims 7-10, containing a plurality of pore sizes and pore size distributions, especially containing pore size gradients (across the thickness) of multimodal pore size distributions.

12. Conductive carbon scaffold according to any one of claims 7-11 , wherein the geometries include circular, square, and rectangular shapes of multiple sizes and orientations.

13. The use of a conductive carbon scaffold obtained according to a method according to any one of claims 1-6 or a conductive carbon scaffold according to any or more of claims 7-12 in a redox flow battery as an electrode-flow field component.

14. The use of a conductive carbon scaffold obtained according to a method according to any one of claims 1-6 or a conductive carbon scaffold according to any or more of claims 7-12 in a flow cell as an integrated flow field-gas diffusion layer.

15. The use of a conductive carbon scaffold obtained according to a method according to any one of claims 1-6 or a conductive carbon scaffold according to any or more of claims 7-12 in a CO2 reduction electrolyzer as a gas diffusion electrode with structured channels for product removal.

16. The use of a conductive carbon scaffold obtained according to a method according to any one of claims 1-6 or a conductive carbon scaffold according to any or more of claims 7-12 as a flow through electrode for capacitive deionization cells.

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