Alba Fernández-Sotillo,Paloma Ferreira-Aparicio
Durable corrosion-resistant coating based in graphene oxide for cost-effective fuel cells components
Chemical Engineering,Energy application,Fuel technology
Summary The current imperative transition to environmentally friendly technologies for power generation requires the reduction of their production costs. In the case of proton exchange membrane fuel cells, current collectors, which are usually integrated as flow field plates in the cells, constitute a critical component due to its weight and cost. In this paper, a cost-effective alternative is proposed, which is based in copper as conductive substrate. The protection of this metal in the aggressive media resulting from the operation conditions is the principal challenge. A continuous reduced graphene oxide coating has been developed to avoid corrosion under operation. The results of the protection behavior of this coating under accelerated stress tests performed in a real fuel cell reveal that the cost-effective procedure for copper coating is able to compete with gold-plated nickel collectors and constitute a real alternative to reduce the production costs and the weight of these systems.
Durable corrosion-resistant coating based in graphene oxide for cost-effective fuel cells components

The current imperative transition to environmentally friendly technologies for power generation requires the reduction of their production costs. In the case of proton exchange membrane fuel cells, current collectors, which are usually integrated as flow field plates in the cells, constitute a critical component due to its weight and cost. In this paper, a cost-effective alternative is proposed, which is based in copper as conductive substrate. The protection of this metal in the aggressive media resulting from the operation conditions is the principal challenge. A continuous reduced graphene oxide coating has been developed to avoid corrosion under operation. The results of the protection behavior of this coating under accelerated stress tests performed in a real fuel cell reveal that the cost-effective procedure for copper coating is able to compete with gold-plated nickel collectors and constitute a real alternative to reduce the production costs and the weight of these systems.

Proton exchange membrane fuel cells (PEMFCs) are recently receiving a great deal of attention as result of the current socioeconomic and environmental situation, which is impelling OECD governments to implement effective policies to address the sustainable management of natural resources and foster the use of renewable energies using green hydrogen as energy vector. Building up a hydrogen economy requires a full value chain approach that will allow activating a virtuous cycle of increased supply and demand for hydrogen. Creating economies of scale is critical to reduce costs significantly, but research and development will also be crucial to reduce expenditures and improve their competitiveness. PEMFCs have recently achieved a notable degree of development, but there are still some issues to be solved before this technology can be widely spread. After the high expectations experienced by fuel cells in the 2000s, which failed to materialize, some new factors are currently enabling the sector to regain momentum. The improvements in technology have substantially reduced fuel cells costs and some commercial products are becoming widely available. These circumstances and the strengthened global resolve to mitigate climate change are currently enjoying a resurgence of interest on fuel cell technologies. At this time, large-scale production of fuel cell vehicles has begun, and homes are beginning to be heated and powered by fuel cells. The key difference, since the last emerging period for hydrogen technologies in the 2000s, is that manufacturing scale up and cost decreases are helping now hydrogen and fuel cells to be commercialized in several sectors, from portable electronics and backup power to fork-lifts and trucks., A cost analysis recently published by Thompson et al. suggests several focal points for research and development in the near and long term in order to drive the automotive fuel cell systems cost to competitiveness. They have found that catalyst layers and bipolar plates remain the two single greatest contributors to cost, and constitute the main targets for near-term cost reduction. Flow field plates are critical components contributing approximately to 80% of the weight of the fuel cell, 50% of its volume, and 40% of its cost, posing a barrier to their commercialization. Metals provide great advantages for their manufacture due to their good mechanical properties, high electrical and thermal conductivities, low gas permeability, easy machinability, and low price. Thin metal plates of reduced weight and volume reduce the disadvantage of their higher density. The main drawback of using these materials is their corrosion resistance in the acidic and humid fuel cell environment, with temperatures around 80°C. Most metallic materials are not able to withstand those conditions, and their corrosion releases ions that contaminate not only the membrane but also the catalyst layer and lead to a reduction in cell performance and, in the worst case, failure. Metals such as gold, which resist the aggressive PEMFC environment, are expensive and difficult to access. Other alternatives form a passive layer after surface corrosion that reduces electrical conductivity and increases the resistance between the bipolar plate and the diffusion layer. This effect is responsible for about 59% of fuel cell losses. The greatest challenge for metallic materials is finding an inexpensive material with a good corrosion resistance and low contact resistance. The solution to this can be found by modifying the composition and thickness of a passive layer or by using conductive and corrosion-resistant coatings, which are currently the most promising and most studied methods. The development of metal plates with anti-corrosion carbon coatings is one of the most extended approaches.,,, In particular, graphene coatings are considered to have a great potential to produce effective barriers against corrosion on metals, although research on this field requires further efforts. General papers dealing with the corrosion protection properties of graphene on metals are recently increasing,,, but its application to metallic flow field-plates is still very scarce. It is generally accepted that multilayer graphene coatings are needed to achieve an effective corrosion protection, since the presence of small defects in the covering layers causes more damage on the substrate than the complete absence of coating. Thus, in many cases, graphene materials have been mainly applied with polymers in composite coatings., A large number of procedures have been extensively investigated in the literature to synthesize graphene protective coatings using mainly two basic approaches: dry and wet processing. The dry processing methods include chemical vapor deposition, high temperature pyrolysis of organic molecules, rapid thermal annealing, or powder spray (electrostatic powder coating and plasma spray coating). Among the wet processing techniques, electrophoretic deposition, solution spray, dip coating, spin coating, drop casting, vacuum filtration, and brushing are the most used methods. In particular, some graphene oxide solution processing procedures have shown to be of interest for producing continuous films on the surface of oxidizable metals, which promote the self-assembly of graphene oxide platelets on their surface and their subsequent reduction., In previous studies, we have shown the ability of copper ions to induce the ordered growth of large free-standing reduced graphene oxide films, even in the absence of a support beneath it, and have analyzed the mechanism of this process. On the basis of this strategy, an easy and cost-effective solution processing procedure has been developed, which has been protected under patent ES2893957(B2). In this study, the developed procedure has been applied to produce protective barriers against corrosion on copper-expanded microgrids. The resulting samples have been evaluated in corrosion tests and have been finally tested as current collectors in a real fuel cell as proof of concept. The cell performance, its durability and a postmortem analysis of the cell components, as well as the membrane electrode assembly are presented in comparison with the results obtained using gold-coated nickel microgrids in equivalent cells. The presented results show that this procedure can be used as a cost-effective alternative to more expensive coatings, which are usually applied in bipolar plates, and allows using copper as conductive substrate for PEMFCs current collectors.

Scanning electron microscopy (SEM) images of the cross-section of the coated substrate allow the detailed examination of the coating structure (Figure 1). The stacking of rGO self-assembled layers by means of the previously mentioned solution processing procedure provides an effective barrier all around the copper grid that protects the substrate from the external environment. This barrier is formed by a “mille-feuille” structure, which limits the permeation of liquid and gases through its structure. The protective coating has an approximated thickness between 1.0 and 2.0 μm, as shown in Figure 1D.

The protection ability against corrosion of the developed coating on copper microgrids has been evaluated by applying linear sweep voltammetry protocols to the samples immersed in 0.5 M H2SO4 electrolyte. The bare substrate, which has been tested as blank, becomes completely oxidized and dissolved at voltages above 0.6 VSHE reaching a high corrosion current density as shown in Figure 2. On the other hand, the coated sample shifts its corrosion potential by 0.5 V up to 0.160 VSHE and reduces it corrosion current density by almost two orders of magnitude to 7.7 · 10−7 A cm−2. It can be observed the appearance of a small peak in the anodic branch of the Tafel curve at ca. 0.44 VSHE due to the formation of oxidized copper species in small extent. This oxidation process taking place in the coated sample can be assigned to a surface copper oxidation induced and limited by the presence of residual hydroxyl groups remaining in the inner layers of the rGO coating. The extent and reversibility of this process has been analyzed by cyclic voltammetry tests in two different voltage ranges, between −0.5 and 0.2 VSHE, and in a wider interval between −0.5 and 1.2 VSHE. The obtained results are presented in Figure 3. The CV profile in the shorter range (Figure 3A) is indicative of the electrical capacitance of the double layer in the rGO-electrolyte interphase during the charging/discharging process of the sample. By increasing the voltage sweep interval in the CV experiment up to 1.2 VSHE, it can be observed a constant profile, which is stabilized after the first cycle (Figure 3B). The anodic and cathodic charges in each cycle are indicative of faradaic processes of formation of oxidized copper species, Cu+ at ca. 0.32 VSHE and Cu2+ at ca. 0.70 VSHE, and their reduction to Cu0 at ca. −0.05 VSHE that do not involve the release of copper ions from the sample to the electrolyte medium. These ions are retained inside the conductive coating of rGO. The sample was evaluated at higher working electrode voltages of 1.6 VSHE and 2.0 VSHE (Figure 4A) after testing the stability of the coating up to 1.2 VSHE (Figure 4B). Before and after being submitted to the LSV tests, CV analyses were performed in a short voltage range between 0.0 and 0.2 VSHE (Figure 4C) and in the range between 0.0 and 1.2 VSHE (Figure 4D). The coated sample showed good stability after increasing the working electrode voltages up to 1.6 VSHE, without significant increase of the cathodic current density. Then, the applied bias was increased up to 2.0 VSHE. Figure 4A shows the results obtained in these LSV tests as compared with the blank (a bare copper microgrid). When the working electrode potential was increased above 1.7 VSHE, a considerable increase of the anodic current density was observed. The oxidation peak observed seems to be limited by diffusion. A comparison of the CV profiles obtained in the 0.0–0.2 VSHE range (Figure 4C) reveals a great change in the electric double-layer capacitance during the charging/discharging processes of the samples before and after being submitted to a larger 2.0 VSHE bias. This effect can be assigned to the increase of the interlayer spacing between successive rGO layers covering the substrate, which become probably hydrated and oxidized at potentials above 1.7 VSHE. Cyclic voltammetry profiles in the 0.0–1.2 VSHE range obtained before and after the enlargement of the interlayer spacing between stacked rGO sheets (Figure 4D) reveal a large enhancement of the anodic current density due to copper oxidation, which is not compensated by the cathodic current during the reverse sweep. This profile can be assigned to the release of copper ions to the electrolyte by diffusion in between oxidized rGO sheets with larger interlayer distances. An analysis of the tested samples after being submitted to 1.2 VSHE shows no apparent modification of their morphology, maintaining completely their structural integrity (Figure 5, images a and b). A more detailed examination of its surface has been performed by atomic force microscopy. Figure 5C compiles images in different modes of acquisition. Topographic images indicate a scarce root-mean-square roughness of the sample on the coated strands, which accounts for a value of 26 nm. The amplitude image represents a map of the slope of the sample, and evidences more easily its shape. The phase shift image, obtained in tapping mode, is a map of how the phase of cantilever oscillation is affected by its interaction with the surface. The physical meaning of this signal is complicated; it can give some information about the relative softness/hardness of the sample, and its chemical nature. In our case, the phase shift image probably indicates the presence of small amounts of oxide retained beneath graphene layers and mainly localized along some observed wrinkles. In contrast with these coated samples, which are able to withstand an applied bias up to 1.6 VSHE and maintain their structure without modification, the analysis of hydrated stacked layers with higher interlayer space shows the movement of copper ions in between layers that finally leads to the release of copper cations to the external medium. The examination by SEM of a coated grid, in which the coating is completely hydrated after being submitted to electrochemical corrosion tests, gives some clues about the mechanism of the substrate corrosion and the diffusion of copper ions through the coating. At first sight, the collector grid is locally attacked as shown in image A in Figure 6. SEM micrographs in Figure 6 (images B–E) reveal the presence of branched structures of copper oxides retained in between rGO-stacked layers. Divergent channels are open by hydration of the coating allowing for the oxidation of the substrate and the diffusion of copper cations toward open areas. According to these results, it can be concluded that the developed coating provides an effective protection to the copper substrates under acidic conditions at electrode potentials below 1.7 VSHE. Above that potential, stacked rGO layers become more oxidized and hydrated causing an increase of the interlayer separation. This expanded structure allows for the diffusion of solvated ions between the substrate and the electrolyte favoring copper corrosion, which is limited by the diffusion rate through the open slits or channels in the rGO-layered coating. By taking into account the stability window of the coating up to 1.7 VSHE, this protection meets the initial requirements to be used on copper microgrids to be applied as current collectors for PEMFCs.

PEM technology advances are led by periodical targets determined by the main R&D agencies in order to meet the requirements to obtain acceptable performances, good durability, and affordable costs. The 2020 targets established by the US Department of Energy for corrosion current densities in flow field plates for PEMFCs are set at values below 1 μA cm−2., The rGO coating developed by applying the previously mentioned solution processing procedure allows shifting the corrosion potential of the copper substrate from −0.369 VSHE to 0.160 VSHE and decreasing the corrosion current density by two orders of magnitude from 102.57 to 0.77 μA cm−2. It should be noted that the outstanding protection against corrosion of the provided rGO coating is not easy to obtain using other procedures or techniques. A PEMFC was assembled with the new coated copper grids at the anode and the cathode. The polarization and power density curves obtained are presented in Figure 7. These curves correspond to a completely passive operation mode under natural air convection and without any external heating source. This type of cell, which was initially designed to operate with gold-coated nickel grids, is able to operate with the developed rGO-coated copper collectors with an equivalent performance without deactivation (see Figures S1 and S2). Oxidation in current collectors can induce a fast deactivation process in the fuel cell if metal cations are released to the electrodes, since they are finally exchanged with protons in the membrane and reduce its ionic conductivity. Accelerated stress tests that evaluate the cell performance at 0.5 V provide clear evidence of these processes. The applied operation protocol includes, in addition to stationary potentiostatic periods, analyses by electrochemical impedance spectroscopy, and periodical cycling between the open circuit voltage and very low values. Air-breathing operation under passive mode, dry hydrogen feeding, and periodical cycling at extreme voltages every hour force mass transport limitations and submit the cell to very demanding conditions. As shown in Figure 8, the cell with bare copper collector exhibits a fast degradation immediately after its start-up, and after 4 h of operation, its performance has been reduced by 50%. In less than 10 h, bare copper current collectors experience a performance loss of almost 80% of its initial activity. On the other hand, the corrosion protective coating on those current collectors allows for stable operation of the cell for the 15 days of continuous accelerated stress test conducted. Figure 9 provides more information about the deactivation experienced by the Cu//Cu cell. Electrochemical impedance spectra registered during the potentiostatic stages at 0.5 V (Figure 9A) reveal that, after a well-defined arc obtained in the Nyquist plot during the first hour of operation, a continuous enlargement of the impedance is observed with the elapsed time in the following hours. A fast increase of the mass transport resistance in the cells can be envisaged, as well as a modification in the high-frequency contribution that shifts the high-frequency resistance (RHF) of the cell to higher values (inset in Figure 9A). The registered cyclic polarization curves in Figure 9B also reveal the fast deactivation due to kinetic and transport limitations. After 20 h of operation, the performance loss is so exacerbated that it was decided to shut down the cell, disconnect it, and disassemble its components in order to further analyze the causes for deactivation. A very different behavior is observed for the fuel cell assemblies with coated current collectors as shown in Figure 10. Both the frequency response analysis and the polarization curves obtained along 350 h of continuous operation reveal no evident signs of degradation under the same conditions than the previously described cell. A certain increase of the concentration polarization with time is observed in the current-voltage profiles. Polarization resistances at the low current density range due to kinetic losses and at the intermediate current density range due to ohmic losses in the cell are not affected. As shown in the Nyquist plot in Figure 10, the cell resistance is initially somewhat higher, but it becomes slightly reduced with time, causing performance to improve progressively. This trend can be also appreciated in the cyclic polarization curves, which do not show any evidence of hysteresis even after 300 h of operation. It is worthy to note that the RHF notably decreases after an initial activation period due to progressive hydration of the membrane electrode assembly (MEA) (inset in Figure 10A). Electrical conductivity and oxygen concentration are strongly correlated in rGO. In order to assess the degradation effects in the components, a postmortem analysis has been also conducted by SEM. The cross-section analysis of the current collectors indicates the absence of oxides on the copper grid surfaces (Figure 11, images A–C), as corroborated by energy dispersive X-ray analysis, which showed a surface atomic composition of 100% Cu atoms. The results obtained from the cross-section analysis of the MEA in the central part of the cell are presented in Figure 11D. Specific elemental composition data for Pt, Cu, S, and F across the selected area of the sample in the SEM image of the MEA cross-section reveal the presence of copper particularly at both catalytic layers, in anode and cathode, and at the central area of the electrolyte membrane in lesser extent. Contamination of these zones by copper ions is probably the main origin of the drastic performance loss after just 20 h of operation, which limits the proton conductivity. This short operation time is not enough for the rest of the components to become degraded. The postmortem analysis of the cell tested for 105 h with the rGO-coated current collectors is shown in Figure 12. Micrographs of the coated collector at different magnifications (images A–D in Figure 12) reveal that the rGO coating has a thickness in the range between 0.5 and 1.0 μm and covers completely the grid structure without failures. By comparing this thickness with that previously measured on the as-prepared coated grid, a substantial reduction can be appreciated. This flattening can be attributed to a decrease of the oxygen content in the external layers. Tegou et al. have shown that the resistivity of graphene oxide (GO) films largely depends on their reduction extent and that the electrical conductivity can be increased by restoring sp2 hybridization in the lattice while retaining some oxygen-bearing groups. The resistance decrease has been evidenced in the electrochemical impedance spectra registered during the accelerated stress test previously shown in Figure 10. The analysis of the MEA shows great differences with the previous one tested with the bare copper collectors. A significant degradation of its structure is evident after the long operation time of the cell under very demanding conditions. The compositional mapping in Figure 12 reveals that copper is absent from the MEA cross-section area, Pt at the anode catalytic layer has been sintered and aggregated since its signal can be only appreciated at singular dots in that zone, and the presence of fluorine in the membrane is specially reduced at certain zones near the anode electrode. This last effect can be attributed to the fluoride emission taking place during long-term operation due to certain degradation of the polymer. In general, the accelerated degradation tests induce a thickness reduction of the catalytic layers, especially at the anode electrodes whatever the collector composition and structure, as recently reported., Pt particles are progressively lost by dissolution from the anode catalytic layer and redeposited in other electrode locations. It can be clearly appreciated in the MEA assembled with the coated collectors, whose long-term performance has allowed observing the degradation of the internal components due to their electrochemical performance for hydrogen oxidation. Pt mapping image in Figure 12E shows how Pt at the anode catalyst layer has lost completely its initial structure, and some Pt spots can be observed at a certain distance from the membrane surface. The degradation effects in the MEAs assembled with coated and uncoated current collectors can be more clearly compared in backscattered electron images (Figure 13). The main difference between the evaluated cells remains in a combined effect of their performance stability and the chemical and structural degradation of the electrochemically active components. The cell assembled with bare copper collectors has only been operated for 20 h due to its performance loss. In that short period, the initial current density at 0.5 V has decreased approximately by 80% of its initial value. However, its structure has not experienced significant changes (Figure 13A). The release of copper ions from the current collectors, which migrate to the membrane and the catalytic layers, has propitiated its malfunction and a drastic decay of the electrochemical activity. On the contrary, a dramatic structural change can be appreciated in a cell assembled with the coated current collectors, which has been operated without deactivation for a much longer period. It must be considered that these changes do not provoke alteration or degradation of the observed electrochemical performance during the accelerated stress tests. Even after 350 h of operation under identical conditions, an equivalent cell (CCC2 in Figure 8) does not show any apparent symptom of deactivation. However, the gas diffusion electrode at the anode suffers in these cells a high structural deterioration. It is clear in the image in Figure 13B that carbon fibers from the gas diffusion layer substrate have been consumed during operation.

The X-ray photoelectron spectroscopy (XPS) data obtained during the depth profiling analysis of the freshly prepared coating (see Figure S3) reveal that a reduced amount of oxygen (<15%), which is not easy to remove, remains at the external layers. However, inner layers near the substrate contain a very low amount of oxygen in their composition (Figure S4), and are expected to be much more conductive due to the reduced amount of oxygen and the presence of copper. To estimate the conductivity of the most external layers of the rGO coating alone, self-standing films were prepared by the evaporation-assembly method using the same graphene oxide dispersion. The obtained GO films were then externally reduced by applying the same procedure than that used for current collectors: i.e. a soft solution processing treatment with an environmentally friendly reagent. This treatment increased substantially the amount of sp2 hybridized carbon in the lattice as determined by XPS (Figure S5). The surface composition analysis of these films shows a higher amount of surface oxygen groups in the reduced graphene oxide sample (27%) than in the copper coating (14.8%). The electric resistance (Rs) of these self-standing films was measured by means of the 4-points probe method. By considering the thickness (t) of the samples as estimated from SEM micrographs of their cross-sections, the conductivity values (σ) were calculated according to the equation: For a 3.00-μm-thick GO film, a conductivity value of 0.15 S m−1 was obtained. After its reduction treatment, its thickness was reduced by 25% to 2.25 μm and its conductivity increased by 5 orders of magnitude up to 3.2 · 104 S ·m−1. Although this conductivity value obtained for the self-standing reduced sample is expected to be lower than that in the coating, it has been found to be substantially superior to those reported for similar films in the literature, which are in the range between 102 and 104 S·m−1.,,,,, Closer values but still below those obtained in the present study have been obtained for casted GO films reduced with hydrohalic acids (102–104 S·m−1), or for hydrazine reduced sheets (9.9 · 103 S·m−1). Only higher values are reported for isolated single particles of GO (107–108 S·m−1) and GO films treated at very high graphitization temperatures (1000°C), such as those prepared by Wang et al. (5.5 · 1 04 S·m −1) or Becerril et al. (103–105 S·m−1), which additionally require harsh reduction treatments (hydrazine vapor and annealing at 400°C under inert gas flow). On the basis of these analyses, a conductivity value above that of graphite, which has been traditionally used for current collection in bipolar plates, is expected for the coating formed on the copper substrates. According to the presented results, the rGO-coated copper collectors constitute an excellent cost-effective alternative to more expensive materials commonly used as current collectors and bipolar plates in PEMFCs. It could be speculated that reiterated temperature variations in a PEMFC such as those taking place during the AST tests (Figure S2) could cause physical damage to coated current collectors as result of the different thermal expansion coefficients of copper and rGO. Since the gap in the thermal expansion coefficient (TEC) between copper (∼17 ppm/K) and graphene (∼−8 ppm/K at room temperature) is considerable, thermomechanical stresses could be introduced during the fabrication process of high-density integration structures, which could be converted into normal and shear stresses in response to temperature variations. In the case of the studied samples, the thermomechanical stress can be disregarded as a factor contributing to damage the integrity of the developed rGO- coating. Different reasons support this asseveration. The first one is the synthesis procedure of the protective rGO layers, which includes some heating stages largely exceeding 100°C and leads to the formation of an effective barrier for corrosion as shown in the results in Figure 2. A second argument takes into consideration the mechanism of formation and stacking of continuous reduced graphene oxide layers. The participation of surface copper ions in the assembly and reduction of GO flakes in solution generates an interfacial gap between the metal surface and the grown films. Thus, the coating is not in intimate contact with the copper substrate, and leaves an interfacial distance with it. This differentiated structure can be clearly appreciated in cross-section images of pristine rGO-coated copper grids as shown in Figure 1 (images B and C) and in Figure S6 (supplemental information). This gap between substrate and wrapping is generated during the initial synthesis process of the coating, which includes several stages for the formation of the rGO films. A detailed analysis of the inner layers generated during the first synthesis stage of the coating assembly indicates that they contain significant amounts of copper in their structure. XPS spectra registered during a depth profiling of a sample submitted to the preliminary step of synthesis of self-assembly and reduction of graphene oxide reveal that the copper content in inner layers increases as the rGO coating is progressively etched by an Ar+ ion beam gun. Figure S4 in supplemental information evidences the presence of transition layers in the wrapping with mixed Cu-rGO composition. The incorporation of graphene into a Cu matrix has been considered an effective strategy for decreasing the TEC and the thermal stress in Cu/rGO composites and structures. Accordingly, the presence of copper-rich rGO layers in the inner part of the “millefeuille”-like coating with plausible intermediate values of TEC can also contribute to reduce the thermomechanical stress in these samples. The protocol applied for the AST causes in the cell significant variations in its temperature when changing the conditions from the open circuit voltage to a high current density demand close to 0.4 A cm−2 (Figure S2). However, the stable performance maintained during the repeated protocol and the postmortem analysis of the cell components verify the excellent behavior of the coating.

It has been demonstrated that copper microgrids with continuous and durable coatings prepared by an easy and cost-effective solution processing procedure such as that described in ES2893957(B2) are able to operate as wear-resistant current collectors in a PEMFC and constitute a real alternative to other conductive coatings usually based on much more expensive metals. These components, which have much higher durability than those in the state-of-the-art membrane-electrode assemblies, constitute a further step to a more affordable PEMFC technology and can be applicable to other technological areas requiring wear-resistant conductive materials operating under aggressive media.



Further information and requests should be directed to and will be fulfilled by the lead contact, Paloma Ferreira-Aparicio (paloma.ferreira@ciemat.es).

The developed unique materials are available from the authors under the conditions of a signed materials transfer agreement.

A flattened copper microgrid from Dexmet was used as substrate: 3Cu7-100F. The nomenclature of the grid indicates in the first number the original material thickness, then the material symbol, followed by the strand width, and finally the long way of the diamond aperture, expressed in 10−3 inch units. The microgrid was cut in coupons (15 mm × 70 mm) for electrochemical corrosion tests. The surface of the copper substrates was cleaned from moisture in acetone bath (Panreac, QP grade) and sonicated for 15 min (Ultrasons, JP Selecta). The samples were also submitted to a reduction pretreatment applying a solution processing procedure with L-ascorbic acid (Sigma-Aldrich). Typical efficient reductants such as hydrazine were discarded because of its handling and toxicity issues in favor of natural, easy to handle, and environmentally friendly compounds. For the coating synthesis, an aqueous GO commercial dispersion from Graphenea (4 mg mL−1) was diluted to 0.5 mg mL−1. The coating procedure described in ES2893957(B2) was applied to produce continuous reduced graphene oxide (rGO) films on the microgrid surface.

Preliminary corrosion tests were carried out in a three electrode cell on different samples to analyze the protection properties of the developed coatings. The microgrid samples (working electrode) were immersed in 0.5 M H2SO4 electrolyte to simulate the operation conditions of a proton exchange membrane fuel cell. The size of the samples was calculated to immerse in the electrolyte approximately 1 cm2 of copper surface: i.e. a piece of 2 cm2 of the grid has an exposed surface area of copper including both faces close to 1 cm2. A chloride-free reference electrode was used for the experiments: a saturated mercury/mercurous sulfate (Hg/Hg2SO4) reference electrode with a reference potential of 0.71 V referenced to the standard hydrogen electrode (SHE). The counter electrode was a wounded Pt wire. To avoid oxygen interferences, the electrolyte was saturated with inert gas (N2) by bubbling it into the solution before the test. Blanketing was carried out during measurements by feeding the gas on top of the electrolyte. The cell was controlled by a potentiostat/galvanostat (Autolab 302N). Nova 1.10 software was used to create the protocol, collect and treat the data. With this setup, after determining the open circuit potential (OCP) of the samples, they were submitted to a linear sweep voltammetry (LSV) from −0.5 V to 1.2 VSHE at a scan rate of 5 · 10−4 V s−1. After this first test, in which bare copper substrates were completely dissolved in the electrolyte, rGO-coated samples were able to endure those potential conditions. Therefore, they were submitted to a to a new LSV test up to 1.6 VSHE at the same scan rate of 5 · 10−4 V s−1. To check the properties of the coating after being submitted to that high bias, cyclic voltammetry (CV) tests between 0.0 and 0.2 VSHE, and between 0.0 and 1.2 VSHE were registered. By considering that the rGO-coating was still able to protect the copper substrate, the same sample was then submitted to an LSV test up to 2.0 VSHE at the same scan rate of 5 · 10−4 V s−1. To check the properties of the coating after being submitted to that high bias, cyclic voltammetry (CV) tests between 0.0 and 0.2 VSHE, and between 0.0 and 1.2 VSHE were registered.

The fuel cell hardware used for the tests has been designed to operate with gold-plated nickel grid current collectors (CCs), which show excellent durability and stability. In order to analyze the performance of the developed coatings, the 3Cu7-100F copper grid was evaluated as substrate for the new samples. Firstly, the grids to test were cut into circular pieces of the adequate size (4 cm diameter circle) to be inserted into a circular metallic frame. The surface of the microgrids was firstly cleaned in an ultrasonic bath and, then, submitted to a reduction treatment to remove grease and oxides from their surface. Several sets of collectors were prepared to be evaluated by pairs. Bare copper, gold-coated copper, gold-coated nickel, and the new coating for copper according to ES2893957 (B2) have been tested. The microgrid CC pairs were directly assembled at the anode and cathode of the cell to analyze their durability. Gold-coating of samples was performed in a sputter coater (EMITECH K500X) with a gold target by applying two cycles of 40 s by each side.

An air-breathing PEM fuel cell, which is configured according to the WO2015025070A1 and WO2020240063A1 patents, has been used for evaluating the developed CCs. Figure S7 shows a scheme of the cell with an exploded view of its components. This proprietary cell has a self-regulating humidity system implemented at the anode chamber that allows free water exchange with the environment through the surface of a gas-tight membrane, and whose behavior under operation has been explained in detail elsewhere. Cells were assembled using each set of CCs with pristine membrane-electrode assemblies (MEAs). Commercial standard gas diffusion electrodes on woven substrates (ELAT GDE LT250EWALTSI, BASF, 0.25 mgPt·cm−2) were used in anode and cathode with a Nafion 212 membrane. Several cells were assembled for analysis and comparison of their properties using bare or coated copper collectors. Once assembled, the cells were tested for leaks at the anode and gas crossover through the membrane with N2 gas. In the absence of leaks, and after flushing with inert gas, H2 was fed under dead-ended anode mode at a 0.2 barg overpressure. The air-breathing fuel cells were operated at ambient conditions without external heating, and the temperature at the cathode was measured as a function of time. A potentiostat/galvanostat (Metrohm Autolab PGSTAT302N) equipped with a booster unit was used for cell control. A test protocol was applied for durability assessment. The cells were submitted to continuous operation applying cycles including: i) polarization curve test, ii) determination of the open circuit voltage (OCV), iii) and potentiostatic stage at 0.5 V for 90 min. During this last period, electrochemical impedance spectroscopy tests were performed each 10 min. Potentiostatic impedance responses were obtained with a frequency response analyzer (FRA) module. The amplitude of the AC signal was set to 10 mV and the frequency sweep range to 2,000–0.1 Hz. After the tests, the cells were disassembled and prepared for postmortem examination.

The coated grid collectors have been analyzed by atomic force (AFM) and scanning electron microscopies (SEM). Some AFM images were obtained in a Nanotec AFM microscope in the non-contact dynamic (tapping) mode. More general analyses were performed by SEM using an Hitachi FE-SEM SU-6600 equiped with a secondary electron detector and a low impedance backscattered electron detector. Freshly prepared samples and portions of the current collectors after disassembly of operated fuel cells have been examined. The degradation of the cell components has been assessed by comparing their long-term performance and the changes in their structure and composition at the end of the accelerated stress tests. Cross-sectional analyses of the MEAs and CCs were carried out by scanning electron microscopy. A metallographic method was applied to examine in detail the cross-section of the membrane electrode assemblies used with the different current collectors. This preparation procedure, which has been described elsewhere, is more tedious than sharp knife preparation, but avoids the problems of lack of reproducibility and the difficulty to perform a planar cut using that technique.