Acid-stable oxygen-evolving catalysts: progress in non-precious material engineering and scalability barriers.

As a highly promising clean energy source to replace fossil fuels in the 21st century, hydrogen energy has garnered considerable attention, with water electrolysis emerging as a key hydrogen production technology. The development of highly active and stable non-precious metal-based catalysts for the hydrogen evolution reaction (HER) is crucial for achieving efficient and low-cost hydrogen production through electrolysis. Recently, heterostructure composite catalysts comprising two or more non-precious metals have demonstrated outstanding catalytic performance. First, we introduced the basic mechanism of the HER and, based on the reported HER theory, discussed the essence of constructing heterostructures to improve the catalytic activity of non-noble metal-based catalysts, that is, the coupling effect between components effectively regulates the electronic structure and the position of d-band centers. Then three catalytic effects of non-precious metal-based heterogeneous catalysts are described: synergistic effect, electron transfer effect and support effect. Lastly, we emphasized the potential of non-precious metal-based heterogeneous catalysts to replace precious metal-based catalysts, and summarized the future prospects and challenges.

Developing efficient and economical electrocatalysts for acidic oxygen evolution reaction (OER) is essential for proton exchange membrane water electrolyzers (PEMWE). Cobalt oxides are considered promising non-precious OER catalysts due to their high activities. However, the severe dissolution of Co atoms in acid media leads to the collapse of crystal structure, which impedes their application in PEMWE. Here, we report that introducing acid-resistant Ir single atoms into the lattice of spinel cobalt oxides can significantly suppress the Co dissolution and keep them highly stable during the acidic OER process. Combining theoretical and experimental studies, we reveal that the stabilizing effect induced by Ir heteroatoms exhibits a strong dependence on the distance of adjacent Ir single atoms, where the OER stability of cobalt oxides continuously improves with decreasing the distance. When the distance reduces to about 0.6 nm, the spinel cobalt oxides present no obvious degradation over a 60-h stability test for acidic OER, suggesting potential for practical applications.

Commercialization of proton exchange membrane (PEM) electrolyzers for green hydrogen production have been recently achieved, but with a limited scale of low gigawatts (GW).1 Large-scale and sustainable deployment of PEM electrolyzers will face the challenges of scarcity and high cost of iridium (Ir) used in the anode to catalyze oxygen evolution reaction (OER). A 1 MW PEM electrolyzer stack currently uses ~0.4 kg of Ir based on an Ir loading of 1.5 mg/cm2, which contributes ~$60k cost per stack.2 Moreover, the Ir production has been about 8 tons/year in recent years.3 This can only support an annual production of 5 GW PEM electrolyzer if assuming 25% Ir is available for PEM electrolyzers with the same Ir loading. Therefore, lowering the Ir loading in PEM electrolyzers is urgently needed to meet the rapid expansion of the PEM electrolysis market.Several groups including Plug have developed supported Ir catalysts to lower the Ir loading by a factor of 5 without sacrifice in efficiency.4-8 However, all support used by far is non- or poorly electrically conductive. The conductivity of the electrodes relies solely on the surface IrOx, which sets stringent limits on the catalyst/electrode development, especially with low Ir contents. Here we first argue from fundamental aspects that the catalysts/electrodes with Ir on conductive support can be free of these limits. We further show that platinum (Pt) and titanium diboride (TiB2) powders are feasible candidates as conductive support for Ir-based OER catalysts. We demonstrated a TiB2 supported IrOx (IrOx/TiB2) catalyst synthesized via wet chemistry deposition without post heat treatment combines a mass activity towards OER with high conductivity. Its conductivity of ~30 S/cm2 is comparable to that of Vulcan carbon, and ~105 times that of the counterpart IrOx/W-TiO2 (W-TiO2 represents commercial tungsten doped TiO2 nanoparticles). Meanwhile, the IrOx/TiB2 catalyst shows a mass activity comparable to that of the counterpart IrOx/W-TiO2, twice that of commercial Ir black, and 50 times that of a commercial IrO2/TiO2 catalyst in acidic solution. Durability test showed that the Ir dissolution of the IrOx/TiB2 in acidic solution holding at 2 V for 100 hours is comparable to that of Ir black. Characterization of the IrOx/TiB2 showed small hydrous IrOx nanoparticles (1-2 nm) uniformly distributed on the surface of TiB2 nanoparticles (~58 nm) with an Ir content of ~33±7 wt%. Membrane electrode assembly evaluation on the IrOx/TiB2 catalyst is undergoing. The results will be reported and discussed. References (1) IEA, World Energy Outlook, 2022. https://iea.blob.core.windows.net/assets/830fe099-5530-48f2-a7c1-11f35d510983/WorldEnergyOutlook2022.pdf (accessed 2023-02-12).(2) Mittelsteadt, C. (Invited) Ir Strangelove: Or How I Learned to Stop Worrying and Embrace the PEM. ECS Meeting s 2022, MA2022-01, 1335-1335.(3) Seeking Alpha Home Page. https://seekingalpha.com/article/4399727-sibanye-should-benefit-from-hydrogen-wars-thanks-to-iridium-exposure (accessed 2023-02-12).(4) Böhm, D.; Beetz, M.; Gebauer, C.; Bernt, M.; Schröter, J.; Kornherr, M.; Zoller, F.; Bein, T.; Fattakhova-Rohlfing, D. Highly conductive titania supported iridium oxide nanoparticles with low overall iridium density as OER catalyst for large-scale PEM electrolysis. Applied Materials Today 2021, 24, 101134.(5) Pham, C. V.; Bühler, M.; Knöppel, J.; Bierling, M.; Seeberger, D.; Escalera-López, D.; Mayrhofer, K. J. J.; Cherevko, S.; Thiele, S. IrO2 coated TiO2 core-shell microparticles advance performance of low loading proton exchange membrane water electrolyzers. Appl. Catal. B‐Environ. 2020, 269, 118762.(6) Zhao, S.; Stocks, A.; Rasimick, B.; More, K.; Xu, H. Highly Active, Durable Dispersed Iridium Nanocatalysts for PEM Water Electrolyzers. J. Electrochem. Soc. 2018, 165, F82-F89.(7) Oakton, E.; Lebedev, D.; Povia, M.; Abbott, D. F.; Fabbri, E.; Fedorov, A.; Nachtegaal, M.; Copéret, C.; Schmidt, T. J. IrO2-TiO2: A high-surface-area, active, and stable electrocatalyst for the oxygen evolution reaction. ACS Catal. 2017, 7, 2346-2352.(8) Lewinski, K. A.; van der Vliet, D.; Luopa, S. M. NSTF advances for PEM electrolysis-the effect of alloying on activity of NSTF electrolyzer catalysts and performance of NSTF based PEM electrolyzers. ECS Trans 2015, 69, 893.

The proton exchange membrane (PEM) water electrolyzer has been considered a versatile approach for practical H2 production. However, the oxygen evolution reaction (OER) in acid media with complicated proton‐coupled electron transfer steps possesses sluggish kinetics and high reaction barriers, severely hindering the development of PEM water electrolyzers. Consequently, high‐efficient Ru‐ and Ir‐based catalysts have always been essential to accelerate the OER rate and lower the reaction barrier in PEM water electrolyzer. Therefore, it is very necessary to construct low‐cost catalysts with excellent electrocatalytic performances to replace these noble metal‐based OER electrocatalysts. In this review paper, a detailed discussion towards fundamentally comprehending the reaction mechanisms of OER was conducted. Accordingly, we proposed the principles of designing advanced OER electrocatalysts with enhanced performances and lowered costs. After that, recent developments in designing various acidic OER electrocatalysts were summarized. Meanwhile, the available regulation strategies about noble metals, nonprecious metals, and metal‐free nanomaterials were presented, which are promising for tuning the electronic structures, boosting the electrocatalytic performances, and reducing the costs of electrocatalysts. We also provided the existing challenges and perspectives of various OER electrocatalysts, hoping to promote the development of PEM water electrolyzers.

Green hydrogen production is importance in the upcoming hydrogen economy era. Proton exchange membrane (PEM) water electrolysis is one of the most important technology to produce the green hydrogen that requires only water and extra electricity supplied from renewable energy. Main component, i.e., membrane electrode assembly (MEA), is a key part consisting of polymer electrolyte membrane and two electrodes which are oxygen evolution reaction (OER) electrode and hydrogen evolution (HER) electrode. Both electrode should be coated on the surface of membranes for better performance and mass production. Catalyst layers for OER and HER electrodes are composed of an electrocatalyst (Pt/C) and proton conducting ionomer. In the previous literature, proton conducting ionomer directly affects their performance and durability. Thus, the optimized design of the microstructure of the catalyst layers is essential. In this study, for the reduction of hydrogen production cost, highly dispersed ionomers were introduced to develop the optimized catalyst layers for OER and HER. The effect of dispersing solvents for ionomers on the performance and durability of catalyst layers was mainly investigated. Developed ionomer dispersions showed higher performance and durability in PEM water electrolysis, which result was made by the electrochemical characterization such as I-V polarization, voltage increasing rate during durability test, and so on as well as the microscopic characterization such as SEM and TEM were carried out to evaluate the effect of ionomer dispersions on the performance and durability of HER electrode in PEM water electrolysis. Acknowledgments This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT ＆ Future Planning (NRF-2019M3E6A1063677) and by 2022 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

Magnetically modified electrocatalysts for oxygen evolution reaction in proton exchange membrane (PEM) water electrolyzers

Low temperature water electrolysis represents one of the critical technologies in distributed hydrogen production. It produces clean hydrogen with fast response time and works well when coupled with renewable but intermittent power sources such as wind and solar. Low temperature electrolysis can be operated using either proton exchange or alkaline membrane electrolyte. Compared to alkaline electrolyzer, proton exchange membrane (PEM) electrolyzer offers advantages of significantly higher current density (x5 improvement) and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential. Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the PGM materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve, however, limit the broad implementation of PEM electrolyzer in the renewable energy landscape. Low-cost transition metal based catalysts are known to be active toward OER in alkaline electrolyte but not in acid. Furthermore, traditional catalyst support such as porous carbon cannot sustain the oxidative potential before being oxidized to CO2. Argonne National Laboratory has recently designed and synthesized a new class of PGM-free OER catalyst for PEM electrolyzer. The new catalysts are consisted of highly porous yet stable transition metal composite derived from the metal-organic-frameworks (MOFs). The new catalysts are also integrated into a porous nano-network electrode architecture to improve the conductivity, mass transport and durability against oxidative corrosion. Two catalyst series, ANL-Cat-A and ANL-Cat-B, were developed and investigated. The OER catalyst activity and durability were first measured by the catalytic layer coated over rotating disk electrode (RDE) method or carbon paper in half-cell containing strongly acidic media. Very promising OER activities were achieved. For example, the half-cell OER current density as the function of the polarization potential of a representative ANL-Cat-A (catalyst loading of 2 mg/cm2) was compared with that of Ir-black (catalyst loading of 0.2 mg/cm2). ANL-Cat-A achieved an OER potential of 1.584 V vs. RHE at the current density of 10 mA/cm2, which is only 29 mV higher than that of Ir black benchmark. The catalyst durability was measured through the multiple potential cycling from the voltage of 1.2 V to 2.0 V (vs. RHE) in the acidic electrolyte. The percentage of current density retention against the initial value was measured as the gauge for stability. Both ANL catalysts demonstrated excellent activity and durability over most of PGM-free catalysts in acidic medium. For example, one ANL-Cat-A catalyst retained 90% and 80% current densities at 1.8 V and 2.0 V after 2,000 voltage cycles, respectively. In contrast, the percentage of the current density retention for Ir black was dramatically decreased after only 1000 voltage cycles. Several catalysts from ANL-Cat-A and –B series were also integrated into membrane electrode assemblies and tested in PEM electrolyzer at Giner Inc. under operating condition (60 °C and ambient pressure). Important processing parameters, such as anode catalyst loading, ionomer-to-catalyst ratio, pretreatment and application methods, have been systematically studied. Several MEAs demonstrated OER current density > 200 mA/cm2 at 1.8 V. This work collaborates with DOE HydroGen Consortium in computational modeling, surface property characterization and advanced electron microscopic imaging. Acknowledgement: This work is supported by U. S. Department of Energy, Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials, an U.S. Department of Energy Office of Science User Facility, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.

The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO2/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.

Membraneless Electrolyzers for Low-Cost Hydrogen Production in a Renewable Energy Future

Hydrogen, as a clean energy carrier, is of great potential to be an alternative fuel in the future. Proton exchange membrane (PEM) water electrolysis is hailed as the most desired technology for high purity hydrogen production and self-consistent with volatility of renewable energies, has ignited much attention in the past decades based on the high current density, greater energy efficiency, small mass-volume characteristic, easy handling and maintenance. To date, substantial efforts have been devoted to the development of advanced electrocatalysts to improve electrolytic efficiency and reduce the cost of PEM electrolyser. In this review, we firstly compare the alkaline water electrolysis (AWE), solid oxide electrolysis (SOE), and PEM water electrolysis and highlight the advantages of PEM water electrolysis. Furthermore, we summarize the recent progress in PEM water electrolysis including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts in the acidic electrolyte. We also introduce other PEM cell components (including membrane electrode assembly, current collector, and bipolar plate). Finally, the current challenges and an outlook for the future development of PEM water electrolysis technology for application in future hydrogen production are provided.

Proton exchange membrane (PEM) water electrolysis has become increasingly attractive due to the penetration of renewable energy (e.g, solar and wind). Hydrogen production from PEM water electrolysis is advantageous over other technologies due to its simple and clean nature. Membrane and electrode assemblies (MEAs) of PEM electrolyzers typically use iridium (Ir) as an anode catalyst and Pt as a cathode catalyst. Performance and durability of the MEAs play an essential role for the cost and viable commercialization of PEM water electrolysis. However, unlike the well-established MEA benchmarks of PEM fuel cells, the performance and durability of PEM electrolyzer MEAs have not been thoroughly studied. The objective of this work is to establish benchmark MEA performance and durability for PEM water electrolysis. For this purpose, a series of oxygen evolution reaction (OER) catalysts, which includes commercial Ir black and various Ir nanostructures, has been evaluated under test protocols established at Giner Inc. These approaches include high-voltage hold (>1.8 V), accelerated stress test (e.g., voltage cycling from 1.4 to 2.0 V), and constant low-current operations. The polarization curves of the MEAs will be obtained after each test. The morphology and structure of MEAs after durability tests will be characterized to correlate to their performance and durability. The established performance and durability may provide metrics and guidance to the community of PEM water electrolysis. Acknowledgement: The financial support is from the Department of Energy under the Contract Grant DE-SC0007471.

Due to the ever-rising need for clean energy solutions, efficient utilizing of renewable sources is one of the most important research tasks. However, power generation from renewable sources is inevitably intermittent, which lowers the efficiency and obstructs practical use. The water electrolyzer has attracted much attention as a promising energy storage system in grid scale to solve the intermittency issue. The electrolyzer is a catalytic energy converter that converts electric energy into chemical energy in the form of hydrogen gas. Hydrogen gas production is also important because it is a clean energy carrier with zero emissions. Proton exchange membrane (PEM) based electrolyzer has better efficiency and produces hydrogen gas in higher pressure with a compact design compared to its alkaline counterpart. PEM water electrolyzer can produce pure hydrogen gas without CO impurity which is catalyst poison of Pt electrocatalyst in PEM fuel cells. Major efficiency loss of the electrolyzer comes from high overpotential at the anode reaction, oxygen evolution reaction (OER). A suitable electrocatalyst is needed to reduce the high overpotential. Ir is the only metal that can withstand the highly corrosive environment of the anode in acidic conditions with fine activity. However, Ir is even scarcer than Pt, thus development of Ir based efficient electrocatalysts is an urgent issue for the commercialization. Herein, we report our experimental results about tuning electrochemical property of Ir catalysts to enhance the performance of PEM water electrolyzer device. Recently, it has been realized that the low cost of 3d metals such as Ni, or Cu can boost the OER activity of Ir oxide. To exploit the synergy, we made several shaped Ir-Ni bimetallic nanoparticles, Ir-Ni TL, Ir-Ni SC, and Ir-Ni LP.[1] The bimetallic nanoparticles exhibited enhanced OER performance in half-cell experiments; especially, Ir-Ni TL which greatly improved activity. However, they could not be applied to the PEM electrolyzer. Although, not only from our group, many other groups have also reported fancy Ir based alloy electrocatalysts with enhanced OER performance, their application to full electrolyzer has not been reported yet. Severe leaching of the secondary metal, Ni or Cu, from particles produce corresponding metal ions in the system. The leached metal ions contaminate PEM, and lower ion conductivity, which is fatal to cell performance. We paid attention to adjusting the morphology of Ir oxide particles itself, considering its potential application to PEM water electrolyzers. As a result, we successfully synthesized one-dimensional ultrathin IrO2 nanoneedles in gram scale.[2] It is known that one-dimensional structured electrocatalysts possess enhanced performance in various electrochemical reactions. The drawback of conventional one-dimensional electrocatalysts is its low surface area where the reaction would take place. By making ultrathin nanoneedles, sufficient surface area was exposed. The diameter of the nanoneedles was about 2 nm, which consists of 6~8 layers of (110) IrO2 atomic planes. Molten salt method was applied to synthesize the nanoneedles, because it was hard to control the heterogeneous nucleation on the Ir surface and the homogeneous nucleation of the Ir nuclei in a solution using a conventional colloidal synthesis method. Moreover, the molten salt method does not require toxic chemicals and is readily scalable to gram scale. At higher temperatures above the melting point of the salt, NaNO3, Ir oxide particles were obtained in the liquid salt. When cysteamine was added together as an organic shaping agent, one-dimensional ultrathin IrO2 nanoneedles were synthesized. NaNO3 salt was used as an oxygen donor to produce oxide nanoparticles as well as a solvent. The aspect ratio of the nanoneedles was controlled by the concentration of the shaping agent. When larger amounts of cysteamine were used, thinner and longer IrO2 nanoneedles were obtained. Obtained ultrathin IrO2 nanoneedles exhibited enhanced OER performance. The longer and thinner the particles, the higher electric conductivity and OER activity were observed. The conductivity was directly measured by the 4-point probe method. Also, the stability was enhanced compared to unshaped IrO2 nanoparticles. Typically, there was an inverse relation between activity and stability for the OER electrocatalysts. The nanoneedles overcame the relation by its unique shape. When the nanoneedles were applied to PEM water electrolyzers, the efficiency and durability were enhanced compared to conventional unshaped counterparts. We studied how morphology control of Ir based nanoparticles could affect OER property and PEM water electrolyzer performance. We believe our experimental findings will be valuable to researchers who are working on the development of OER electrocatalysts or PEM water electrolyzers. [1] J. Lim et al., Chem. Commun. 2016, 52, 5641-5644. [2] J. Lim et al., Adv. Funct. Mater. 2017, 1704796. Figure 1

Water electrolyzers and fuel cells can be used to create a closed loop system for space exploration. Electrolyzers allow for reliable self-sustainable generation of hydrogen and oxygen for energy storage, followed by conversion into electrical energy in a fuel cell. A first-order safety concern for water electrolyzer operation is hydrogen crossover. Transport of hydrogen to the oxygen rich anode in proton exchange membrane (PEM) water electrolyzers poses safety concerns when the hydrogen concentration in the anode flow field approaches the hydrogen lower flammability limit (LFL). Hydrogen storage efficiency relies on high hydrogen pressure, leading to pressure-driven hydrogen crossover. Mitigation of hydrogen crossover through research and development of a platinum metal recombination layer has been demonstrated in high performing, durable PEMWEs.1-4 Ouimet4 explored the use of a novel dual recombination layer configuration to mitigate PEM water electrolyzer hydrogen crossover. In addition, the current state of the art for PEM fuel cells and water electrolyzers rely on perfluoro-sulfonated acid (PSFA) based membranes. There are significant challenges facing the use of PSFA-based membranes; namely, environmental contamination and performance limitations. The use of a hydrocarbon membrane allows for the development of a PSFA-free system that shows higher efficiency and durability. Investigation of hydrocarbon membranes pave way for developing a PEM water electrolyzer that will demonstrate improved gas permeability resistance, mechanical strength, and thermal stability.5-8 There is a need for both hydrogen crossover mitigation strategies and durability testing with hydrocarbon membranes.The research outlined in this work is focused on the development of PSFA-free PEM water electrolyzers with low hydrogen crossover. In this work, the dual recombination layer configuration will be incorporated into a hydrocarbon membrane for PEM water electrolysis. Polarization, electrochemical impedance spectroscopy, electrochemical equivalent circuits, distribution of relaxation times, and materials characterization will be used to investigate the cell performance and durability. References G. Mirshekari, R. Ouimet, Z. Zeng, H. Yu, S. Bliznakov, L. Bonville, A. Niedzwiecki, C. Capuano, K. Ayers, and R. Maric, “High-performance and cost-effective membrane electrode assemblies for advanced proton exchange membrane water electrolyzers: Long-term durability assessment,” international journal of hydrogen energy, vol. 46, no. 2, pp. 1526–1539, 2021.Z. Zeng, R. Ouimet, L. Bonville, A. Niedzwiecki, C. Capuano, K. Ayers, A. P. Soleymani, J. Jankovic, H. Yu, G. Mirshekari, et al., “Degradation mechanisms in advanced meas for pem water electrolyzers fabricated by reactive spray deposition technology,” Journal of The Electrochemical Society, vol. 169, no. 5, p. 054536, 2022.A. Martin, D. Abbas, P. Trinke, T. Böhm, M. Bierling, B. Bensmann, S. Thiele, and R. Hanke-Rauschenbach, “Communication—proving the importance of ptinterlayer position in pemwe membranes for the effective reduction of the anodic hydrogen content,” Journal of The Electrochemical Society, vol. 168, no. 9, p. 094509, 2021.R. J. Ouimet, “Catalyst development by a novel fabrication process for energy applications,” University of Connecticut Doctoral Dissertation, 2021.P. Trinke, P. Haug, J. Brauns, B. Bensmann, R. Hanke-Rauschenbach, and T. Turek, “Hydrogen crossover in pem and alkaline water electrolysis: mechanisms, direct comparison and mitigation strategies,” Journal of The Electrochemical Society, vol. 165, no. 7, p. F502, 2018.P. Trinke, B. Bensmann, and R. Hanke-Rauschenbach, “Current density effect on hydrogen permeation in pem water electrolyzers,” International Journal of Hydrogen Energy, vol. 42, no. 21, pp. 14355–14366, 2017.H. Q. Nguyen and B. Shabani, “Proton exchange membrane fuel cells heat recovery opportunities for combined heating/cooling and power applications,” Energy Conversion and Management, vol. 204, p. 112328, 2020.C. Klose, T. Saatkamp, A. Münchinger, L. Bohn, G. Titvinidze, M. Breitwieser, K. D. Kreuer, and S. Vierrath, “All-hydrocarbon mea for pem water electrolysis combining low hydrogen crossover and high efficiency,” Advanced Energy Materials, vol. 10, no. 14, p. 1903995, 2020.

Amorphous mixed Ir–Mn oxide catalysts for the oxygen evolution reaction in PEM water electrolysis for H2 production

Proton exchange membrane water electrolyzers (PEMWEs) are electrochemical devices which generate hydrogen (H2) gas from water and electrical energy feedstocks. PEMWEs produce H2 renewably and carbon-free when the electricity is from renewable sources, and are a pathway to enable deep decarbonization across multiple industrial and energy sectors[1]. However, commercial deployment of PEMWEs is currently limited to megawatt-scale due to relatively higher H2 production costs and capital costs than hydrocarbon reforming [2]. The higher costs are due in part to the use of significant quantities of expensive materials (Pt and Ir electrocatalysts and perfluorinated ionomers), insufficient operating performance and durability, and high manufacturing costs. Additionally, commercial PEMWEs additionally use high Ir loadings [3] for the oxygen evolution reaction (OER), and the limited abundance of Ir [4] may limit PEMWE annual deployment of those technologies to gigawatt (GW) scale.3M Nanostructured Thin Film (NSTF) PEMWE OER powder catalysts and electrodes are a unique approach to address the cost and Ir utilization barriers noted above. NSTF catalysts [5] are comprised of nm-scale catalyst metal thin films on a high aspect ratio inert support (Fig. A). NSTF Ir OER catalysts enable high efficiency and high durability due to high OER mass activity and intrinsic resistance to dissolution, imparted by the unique agglomerated thin film catalyst structure. NSTF OER electrodes [6] consist of a dispersed matrix of NSTF catalyst powder particles within a perfluorosulfonic acid (PFSA) ionomer binder (Fig. B), which have high catalyst utilization due to the high electronic conductivity of the primary catalyst particles.One of the key challenges associated with development of OER catalysts and electrodes is the lack of qualified accelerated stress tests (ASTs) to enable rapid assessments of durability under conditions relevant for end-use. The challenge is in part magnified by the long lifetime requirements of 80,000 hours and low required decay rates of single microvolts per hour, which traditionally has required long testing times and multiple replicates to obtain needed statistical significance. Additionally, evaluations of durability have often occurred under steady state testing with fixed current densities, which do not reflect anticipated use profiles when integrated with renewables such as wind and solar with significant power production variability over time. Lastly, operation at increased stack power densities is considered a key strategy to reduce stack capital costs and Ir requirements on a gram per kW basis.In this paper, we will report recent work on our durability assessment of NSTF OER powder catalysts and electrodes under aggressive testing protocols with low catalyst loadings relevant for PEM electrolyzers at large scale. Assessments included steady state durability tests, an accelerated stress test, and a protocol intended to simulate integration with a wind variable renewable energy (VRE) load profile. An example of results from the wind VRE protocol are summarized in Figs. C and D. The wind VRE protocol generated by Alia et al. [7] was modified from voltage control to current control and the maximum current density was scaled to 4.5A/cm2. The protocol was applied to a 3M laboratory CCM comprising a 0.20 mg/cm2 of 78wt% Ir/NSTF powder catalyst OER electrode, 0.09 mg/cm2 of 78wt% Pt/NSTF powder hydrogen evolution reaction (HER) electrode, and a 100 micron thick PEM (800EW 3M PFSA). After 500 hours of the wind VRE protocol, the cell performance was essentially unchanged (1mV voltage decrease at 2A/cm2). S. Dept. of Energy “H2@Scale”, https://www.energy.gov/eere/fuelcells/h2scale.S. Dept. of Energy H2USA Model, Current Forecourt Hydrogen Production v. 3.101.Ayers et al., Catalysis Today 262 121-132 (2016).Babic et al., Electrochem. Soc. 164 F387 (2017).Debe et al., ECS Trans. 45(2) 47-68 (2012).Steinbach et al., 2019 U.S. DOE Annual Merit Review, Project ta026.Alia et al., Electrochem. Soc. 166 F1164 (2019). Figure 1