Oxygen Evolution Catalysts Research Articles

Alkali Metal Iridates as Oxygen Evolution Catalysts Via Thermal Transformation of Amorphous Iridium (oxy)hydroxides.

AbstractEfficient water‐splitting is severely limited by the anodic oxygen evolution reaction (OER). Iridium oxides remain one of the only viable catalysts under acidic conditions due to their corrosion resistance. We have previously shown that heat‐treating high‐activity amorphous iridium oxyhydroxide in the presence of residual lithium carbonate leads to the formation of lithium‐layered iridium oxide, suppressing the formation of low‐activity crystalline rutile IrO2. We now report the synthesis of Na‐IrOx and K‐IrOx featuring similarly layered crystalline structures. Electrocatalytic tests confirm Li‐IrOx retains similar electrocatalytic activity to commercial amorphous IrO2 ⋅ 2H2O and with increasing size of the intercalated cation, the activity towards the OER decreases. However, the synthesised electrocatalysts that contain layers show greater stability than crystalline rutile IrO2 and amorphous IrO2 ⋅ 2H2O, suggesting these compounds could be viable alternatives for industrial PEM electrolysers where durability is a key performance measure.

Electrodeposited Manganese Dioxides and Their Composites as Electrocatalysts for Energy Conversion Reactions.

Enhancing the efficiencies of electrochemical reactions for converting renewable energy into clean chemical fuels as well as generating clean energy is critical to achieving carbon neutrality. However, this enhancement can be achieved using materials that are not constrained by resource limitations and those that can be converted into devices in a scalable manner, preferably for industrial applications. This review explores the applications of electrochemically deposited manganese dioxides (MnO2) and their composites as electrochemical catalysts for oxygen evolution (OER) and hydrogen evolution reactions for converting renewable energy into chemical fuels. It also explores their applications as electrochemical catalysts for oxygen reduction reaction (ORR) and bifunctional OER/ORR for the efficient operation of fuel cells and metal-air batteries, respectively. Manganese is the second most abundant transition metal in the Earth's crust, and electrodeposition represents a binder-free and scalable technique for fabricating devices (electrodes). To propose an improved catalyst design, the studies on the electrodeposition mechanism of MnO2as well as the fabrication techniques for MnO2-based nanocomposites accumulated in the development of electrodes for supercapacitors are also included in this review.

Porphyrin-Confined Supported Ultrasmall Ir Clusters as Oxygen Evolution Catalysts for Water Electrolysis.

Metalloporphyrin ligands themselves can participate in the redox process, making them beneficial in promoting the multielectron catalytic process of the oxygen evolution reaction (OER). However, OER catalysts synthesized by traditional chemical strategies face challenges in water electrolysis. We synthesized high-performance and stable alkaline and acidic OER electrocatalysts loaded with ultrasmall iridium clusters by taking advantage of the attraction and confinement of Ir atoms by the Ir-N bonds formed by the porphyrin cavity. The N in the porphyrin cavity forms an Ir-N bond with Ir so that Ir carries a negative charge and attracts Ir atoms to form ultrasmall Ir clusters above the cavity to adjust the electronic structure of the Ir clusters. The resulting catalyst Tpyp-Ir(IrOX) exhibits a small overpotential (242 and 259 mV) at a current density of 10 mA cm-2 in alkaline and acidic conditions and demonstrates good long-term operational stability. In addition, Tpyp-Ir(IrOX) exhibits a higher transition frequency (TOF) (1.69 O2 s-1 at 300 mV) in 1 M KOH, which is 7 times that of Ir/C.

In Situ Unraveling Surface Reconstruction of Ni‐CoP Nanowire for Excellent Alkaline Water Electrolysis

The surface reconstruction behavior of transition metal phosphides precursors is considered as an important method to prepare efficient oxygen evolution catalysts, but there are still significant challenges in guiding catalyst design at the atomic scale. Here, the CoP nanowire with excellent water splitting performance and stability is used as a catalytic model to study the reconstruction process. Obvious double redox signals and valence evolution behavior of the Co site are observed, corresponding to Co2+/Co3+ and Co3+/Co4+ caused by auto‐oxidation process. Importantly, the in situ Raman spectrum exhibits the vibration signal of Co–OH in the non‐Faradaic potential interval for oxygen evolution reaction, which is considered the initial step in reconstruction process. Density functional theory and ab initio molecular dynamics are used to elucidate this process at the atomic scale: First, OH− exhibits a lower adsorption energy barrier and proton desorption energy barrier at the configuration surface, which proposes the formation of a single oxygen (–O) group. Under a higher –O group coverage, the Co–P bond is destroyed along with the POx groups. Subsequently, lower P vacancy formation energy confirm that the Ni‐CoP configuration can fast transform into a highly active phase. Based on the optimized reconstruction behavior and rate‐limiting barrier, the Ni‐CoP nanowire exhibit an excellent overpotential of 1.59 V at 10 mA cm−2 for overall water splitting, which demonstrates low degradation (2.62%) during the 100 mA cm−2 for 100 h. This work provide systematic insights into the atomic‐level reconstruction mechanism of transition metal phosphides, which benefit further design of water splitting catalysts.

Porous and B‐site Substituted Y2[Mn0.2Ru0.8]2O7 Pyrochlore for Boosting Acidic Water Oxidation Activity and Stability

Boosting the reaction stability without sacrificing the activity and cost is extremely important but full of challenging for the RuO2‐based oxygen evolution catalysts. Herein, porous and B‐site substituted Y2[Mn0.2Ru0.8]2O7 (p‐Y2[Mn0.2Ru0.8]2O7) pyrochlore toward oxygen evolution reaction is innovatively synthesized. The formed meso/macro‐porous structure can increase the specific surface area and corresponding active sites, meanwhile, Mn‐substitution can modulate the electronic structure, stabilize the morphology, and reduce the dosage of Ru species. Excitingly, the p‐Y2[Mn0.2Ru0.8]2O7 performs 50 h stable operation, significantly outperforming the commercial RuO2(CM) counterpart with less than 2 h life. Furthermore, the required overpotential to achieve 10 mA cm‐2 is only 266 mV, accompanied with favorable reaction kinetics and catalyst utilization.

Nanoparticles of CoFeZn Supported on N-Doped Carbon as Bifunctional Catalysts for Oxygen Reduction and Oxygen Evolution

Nanoparticles of CoFeZn Supported on N-Doped Carbon as Bifunctional Catalysts for Oxygen Reduction and Oxygen Evolution

TiO2 modified NiFe-LDH as photoanode to achieve high efficiency photochemical water oxidation reaction

With the semiconductor modified oxygen evolution (OER) catalyst as the photoanode, the solar energy can be fully utilized, and the synergistic reaction makes the OER catalyst play a great role in the photochemical (PEC) water decomposition. Herein, we adopted a one-step hydrothermal method to introduce TiO2 to form heterojunction as photoanode in a well-known NiFe-LDH electrocatalyst, which contributes to carrier separation and transfer efficiency, thereby speeding up the water oxidation reaction kinetics. Benefiting from the TiO2/Ni4Fe6-LDH heterojunctions, the TiO2/ Ni4Fe6-LDH shows an overpotential of 371 mV @30 mA cm−2 and a Tafel slope of 131.36 mV/dec under illumination, which is better than Ni4Fe6-LDH and TiO2. The comparison experiment of oxygen evolution efficiency showed that the polarization current density of TiO2/Ni4Fe6-LDH reached 12 mA cm−2 under the illumination voltage of 1.72 V, which was twice the current density under the non-illumination condition. Surface electronic structure analysis shows that these excellent OER performance are largely due to the introduction of TiO2, which not only enables Ni4Fe6-LDH surface to have more oxygen vacancy, increase photoanode conductivity and accelerate reaction electron conduction, but also accelerates hole capture and enhances OER reaction kinetics. This synergistic effect between semiconductor and OER catalyst provides more ideas for realizing efficient and stable photoelectrochemical water oxidation.

{Co4O4} Cubanes in a conducting polymer matrix as bio-inspired molecular oxygen evolution catalysts

Exploration of efficient molecular water oxidation catalysts for long-term application remains a key challenge for the conversion of renewable energy sources into fuels. Cuboidal {Co4O4} complexes keep attracting interest as molecular water oxidation catalysts as they combine features of both heterogeneous and homogeneous catalysis with bio-inspired motifs. However, the application of many cluster-based catalysts for the oxygen evolution reaction still requires new stabilization strategies. Drawing inspiration from the stabilizing effects of natural polymers, we introduce a conductive polymer-hybrid approach to covalently immobilize {Co4O4} cubane oxo clusters as oxygen evolution catalysts. Polypyrrole is applied as an efficient p-type conducting polymer that promotes hole transfer during the oxygen evolution reaction, resulting in higher turnover frequency compared to the pristine {Co4O4} oxo cluster and heterogeneous Co-oxide benchmarks. The asymmetric coordination of {Co4O4} not only mitigates catalyst decomposition pathways, but also increases the catalytic efficiency by exposing a directed cofacial dihydroxide motif during catalysis.

Probing the structural evolution of cobalt hydroxide in electrochemical water splitting.

Transition-metal hydroxides are a category of earth-abundant and stable electrocatalysts for energy storage and conversion devices involving sluggish oxygen evolution reaction (OER). Understanding dynamic evolution at the solid/liquid interface of transition-metal hydroxides is crucial for the rational design of high-performance OER electrocatalysts. This study implemented in situ electrochemical atomic force microscopy (EC-AFM) to directly image the dynamic structural evolution of α-Co(OH)2 platelets during the electrochemical OER. Reconstruction of α-Co(OH)2, accompanied by ion insertion from the electrolyte, results in a volume expansion, which can be well confirmed from the X-ray diffraction patterns. Our findings are important for the fundamental understanding of transition-metal hydroxide oxygen-evolution catalysts, illustrating the dynamic nature of Co-based nanostructures under electrochemical conditions.

Tethering Cobalt Ions to BiVO4 Surface via Robust Organic Bifunctional Linker for Efficient Photoelectrochemical Water Splitting.

In the quest for efficient and stable oxygen evolution catalysts (OECs) for photoelectrochemical water splitting, the surface modification of BiVO4 is a crucial step. In this study, a novel and robust OEC, based on 3-(bis(pyridin-2-ylmethyl) amino) propanoic acid bifunctional linker known as dipicolyl alanine acid (DPAA) and cobalt ions, is prepared and fully characterized. The DPAA is anchored to the surface of BiVO4 and utilized to tether cobalt ions. The Co-DPAA/BiVO4 photoanode exhibits remarkable stability and efficiency toward photoelectrochemical water oxidation. Specifically, it showed anodic photocurrent increase of 7.1, 5.0, 3.0, and 1.3-fold at 1.23VRHE as compared to pristine BiVO4, DPAA/BiVO4, Co-BiVO4, and Co-Pi/BiVO4 photoanodes, respectively. The photoelectrochemical and IMPS studies revealed that the Co-DPAA/BiVO4 photoanode exhibits a longer transient decay time for surface-trapped holes, higher charge transfer kinetics, and charge separation efficiency compared to Co-Pi/BiVO4 and pristine BiVO4 photoelectrodes. This indicates that the Co-DPAA effectively reduces surface recombination and facilitates charge transfer. Moreover, at 1.23VRHE, the Co-DPAA/BiVO4 photoanode achieved a faradic efficiency of 92% for oxygen evolution reaction and could retain a turnover frequency of 3.65s-1. The- exhibited effeciency is higher than most of the efficient molecular oxygen evolution catalyst based on Ru.

Preparation of nickel-manganese based bimetallic hydroxide nanosheets for enhanced electrocatalytic oxygen evolution reaction

Abstract This paper uses a common one-step hydrothermal method to prepare NiMn-LDH/NF (Layered Double Hydroxide, LDH) oxygen evolution catalyst with outstanding performance. The NiMn-LDH grows into a nanosheet array structure on nickel foam (NF) and it has a big surface area and exceptional ion transport function that could accelerate the diffusion rate of electrocatalytic products. It attempted to modulate the molar proportion of Ni and Mn to explore the oxygen evolution reaction (OER) performance of the NiMn-LDH/NF catalysts. It was found that when Ni:Mn=4:1 (molar ratio, hereinafter), the nanosheets grew more densely and had better OER performance and stability. The electrochemical test results show that the Ni4Mn-LDH/NF catalyst exhibits an overvoltage of 341 mV at a current density of 10 mA cm−2, and the Tafel slope is only 98.99 mV dec−1.

Iron-promoted rapid self-reconstruction of nickel-based catalysts for efficient oxygen evolution

High-performance oxygen evolution reaction (OER) catalyst is critical for improving the water splitting efficiency. In this work, an efficient self-supported OER catalyst was prepared by electrodeposition of nickel–iron layered double hydroxide nanosheets on titanium mesh substrate (NiFe-LDH/Ti-mesh). This self-supporting design exposes abundant active sites. At 100 mA cm−2, the NiFe-LDH/Ti-mesh shows a low overpotential of 310 mV in 1.0 M KOH. When assembled into an anion exchange membrane water electrolyzer, it can run stably over 200 h. In-situ Raman reveals that the NiFe-LDH/Ti-mesh surface was reconstructed into NiFeOOH active species. In-situ infrared shows that the introduction of Fe promotes the evolution of oxygen intermediates. Theoretical calculations reveal a weakened binding strength of *OH and accelerated formation of *O in OER by the introduction of Fe. This study provides a facile approach for developing self-supported high-performance OER electrocatalyst, and advances the understanding on the origin of NiFe-based catalysts as well.

Sabatier principle based design of high performance FeCoNiMnMoP high entropy electrocatalysis for alkaline water splitting

FeCoNi-based high entropy intermetallic compounds are promising substitutes for noble metal catalysts for hydrogen evolution reaction (HER) and oxygen evolution (OER), while their activity is restricted by the synergistic effect between the elements. In this paper, the theoretical model of FeCoNiMnMoP bifunctional catalyst has been reasonably designed given the volcanic curve of M−H bond energy as a valuable guide for candidate metal elements. Theoretical calculations show that the synergistic effect between multiple sites not only accelerates the electron transfer but also reduces the rate-determining step (RDS) barrier. At the same time, the introduction of Mo also enhances the total density of states and reduces the d-band center, thus optimizing the adsorption and dissociation of intermediates. The experimental results confirm that the theoretical model has excellent catalytic performance. In the alkaline solution, FeCoNiMnMoP‖FeCoNiMnMoP requires only an ultra-low cell voltage of 1.49 V for overall water splitting when the current density is 10 mA cm−2. In general, novel insights on the effects of intermetallic synergies on water decomposition are provided, which is helpful for the design of bifunctional catalysts based on volcanic diagrams.

Impact of Organic Anions on Metal Hydroxide Oxygen Evolution Catalysts.

Structural metamorphosis of metal-organic frameworks (MOFs) eliciting highly active metal-hydroxide catalysts has come to the fore lately, with much promise. However, the role of organic ligands leaching into electrolytes during alkaline hydrolysis remains unclear. Here, we elucidate the influence of organic carboxylate anions on a family of Ni or NiFe-based hydroxide type catalysts during the oxygen evolution reaction. After excluding interfering variables, i.e., electrolyte purity, Ohmic loss, and electrolyte pH, the experimental results indicate that adding organic anions to the electrolyte profoundly impacts the redox potential of the Ni species versus with only a negligible effect on the oxygen evolution activities. In-depth studies demonstrate plausible reasons behind those observations and allude to far-reaching implications in controlling electrocatalysis in MOFs, mainly where compositional modularity entails fine-tuning organic anions.

Recycling Spent Ternary Cathodes to Oxygen Evolution Catalysts for Pure Water Anion-Exchange Membrane Electrolysis.

Recycling spent lithium-ion batteries (LIBs) to efficient water-splitting electrocatalysts is a promising and sustainable technology route for green hydrogen production by renewables. In this work, a fluorinated ternary metal oxide (F-TMO) derived from spent LIBs was successfully converted to a robust water oxidation catalyst for pure water electrolysis by utilizing an anion-exchange membrane. The optimized catalyst delivered a high current density of 3.0 A cm-2 at only 2.56 V and a durability of >300 h at 0.5 A cm-2, surpassing the noble-metal IrO2 catalyst. Such excellent performance benefits from an artificially endowed interface layer on the F-TMO, which renders the exposure of active metal (oxy)hydroxide sites with a stabilized configuration during pure water operation. Compared to other metal oxides (i.e., NiO, Co3O4, MnO2), F-TMO possesses a higher stability number of 2.4 × 106, indicating its strong potential for industrial applications. This work provides a feasible way of recycling waste LIBs to valuable electrocatalysts.

Effect of Catalyst and Electrode Structures on the Operating Current Densities of Liquid Alkaline Water Electrolyzers

Liquid Alkaline Water Electrolyzers (LAWEs) stand out as the technology best-suited for cost-effective hydrogen production. They do not require expensive platinum group metal catalysts and have proven durability afforded by nickel-based electrodes and porous separators. While nickel electrodes and porous separators provide chemical stability in highly alkaline conditions, they do suffer from lower catalytic activity for hydrogen and oxygen evolution reactions (HER and OER), and separators possess higher ohmic resistance due to the absence of ion-conducting groups. Consequently, most LAWEs operate at low current densities ranging from 0.2 to 0.6 A/cm2, resulting in a bulky footprint and comparatively lower energy efficiency due to a reduced hydrogen production rate. To address these challenges, considerable efforts have been directed towards developing thin separators and efficient transition metal-based electrocatalysts for HER and OER in alkaline media. However, the majority of studies related to catalysts are limited to half cells, with only a limited number of catalysts evaluated in relevant LAWE configurations.In this work, we introduce a next-generation Zero-gap LAWE testing capability, benchmarked for performance through H2NEW consortia. Employing this setup, we have evaluated hydrogen and oxygen evolution catalysts under LAWE-relevant conditions on different electrode substrates, each offering a unique electrode structure. Our preliminary findings indicate that Zero-gap LAWEs, when coupled with NiMo cathode and NiFe-type anode catalysts, can operate at current densities exceeding 0.75 A/cm2 at approximately 1.8 V without compromising their short-term durability. In contrast, the benchmark cell exhibited a meager current density of around 0.22 A/cm2. In this presentation, effects of electrode structure, catalyst type, and other relevant parameters related to Zero-gap LAWEs will be discussed in detail. Acknowledgement:This research is supported by the U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office through the Hydrogen from Next-generation Electrolyzers of Water (H2NEW) consortium.

Improvement of Reversal-Tolerance of Pt/Ti4O7-C Catalysts by the Addition of Oxygen Evolution Catalysts

When hydrogen is depleted in a polymer electrolyte fuel cell, the anode potential increases (cell reversal), which involves the oxidation of supported carbon (COR) and causes the growth of Pt particles, resulting in the deactivation of the anode catalyst. The addition of the oxygen evolution (OER) catalysts has been reported to suppress the degradation1. Though it has also been reported that the combination of Ti4O7 with Ir catalysts increased the reversal tolerance2, the combination of Pt/Ti4O7 and Ti4O7-supported OER catalysts has not been reported. We have reported that Pt catalysts supported on Ti4O7-C synthesized by the carbothermal process showed excellent performance3. In order to investigate how the distribution of the OER catalyst in the catalyst layer affects the reversal-tolerance, we compared the performance of the physical mixture of Pt/Ti4O7-C and Ti4O7-supported OER catalysts with the composite catalysts (Pt and OER catalyst supported on Ti4O7-C) in this study. The hydrogen starvation was carried out during the hydrogen pump tests in order to clarify the degradation on HOR. Experimental Preparation of catalysts: 20 wt% Pt/Ti4O7-C (11 wt% C) was prepared as previously reported3. RuO2 catalysts Ti4O7-C or Pt/Ti4O7-C was impregnated with RuCl3 solution, and precipitated at pH 8 with NaOH solution. The residue was washed, and dried at 150°C. 21 wt% RuO2/Ti4O7-C and 19 wt% Pt-2.0 wt% RuO2/Ti4O7-C were used for the physical mixture (abbreviated to catalyst-M) and for the composite catalyst, respectively. IrO2 catalysts Ti4O7-C or Pt/Ti4O7-C was impregnated in IrCl3 solution, and hydrothermally treated at 180°C. Afterwards, the solution was washed and dried at 80°C. 3.8 wt% IrO2 and 8 wt% Pt-3.7 wt% IrO2/Ti4O7-C were used for the physical mixture and the composite catalyst, respectively. Electrochemical measurement: The MEA was composed of Nafion NRE211 as the electrolyte membrane, a commercial heat-treated c-Pt/C (Pt loading: 0.5 mg-Pt/cm2) as the cathode, and the above-mentioned catalyst or a commercially available c-Pt/C (Pt loading: 0.2 mg-Pt/cm2) as the anode. The RuO2 and IrO2 loadings were 0.021- 0.042 mg/cm2 and are indicated in parentheses. The performance was evaluated with IV curves, the hydrogen pump, CV and LSV by flowing H2 gas to the cathode under 80°C fully humidified condition. The hydrogen starvation was performed by switching the anode supply gas to N2 (RH 100%) while the hydrogen pump was carried out at 0.2 A/cm2 for 10 minutes at a cutoff voltage of 2.7 V. After the test, a hydrogen pump, CV, LSV, and an IV test were performed. The second hydrogen starvation test was repeated in the same manner. Results and Discussion As shown in Table 1, the voltage of c-Pt/C quickly increased above 1.7 V, however, a plateau was observed at 1.8 V after the rapid voltage increase during the second hydrogen starvation, which indicated COR. After the second hydrogen starvation, the current at 0.4 V for the LSV increased from 7.1 mA/cm2 before the second test to 47.9 mA/cm2, indicating the damage of the membrane. The HOR activity also deteriorated after the hydrogen starvation. No improvement was observed for Pt/Ti4O7-C and Pt/Ti4O7-C+RuO2/Ti4O7-M (0.025) under this condition, however, the voltage rise to 1.7 V on the RuO2 composite catalyst was suppressed for the first starvation, due to the OER. This effect disappeared for the second hydrogen starvation. On the other hand, the catalyst physically mixed with IrO2/Ti4O7-C showed a voltage plateau from 1.50 to 1.75 V during the hydrogen starvation, indicating a significant improvement, and the increase in the IrO2 loading improved the reversal-tolerance. Furthermore, for the IrO2 composite catalyst, a plateau due to the OER was observed at 1.5 - 1.7 V for the two hydrogen starvation tests. The voltage increase of the hydrogen pump after the tests was also slight and the change in ECSA was small. Since the difference from the physical mixture was clearly observed, it was deduced that the OER catalyst in the vicinity of the Pt catalyst improved the reversal-tolerance.Acknowledgments: This work was supported by NEDO.(1) T. R. Ralph et al., Platinum Metals Rev. , 46, 117 (2002). (2) T. Ioroi et al., J. Power Sources, 450, 227656 (2020). (3) M. Chisaka et al., Chem. Comm. , 57, 12772 (2021). Figure 1

Research and Development Needs to Enable Renewable-Powered Liquid Alkaline Electrolyzers

Low temperature electrolysis (LTE) is the most promising method for fully renewable hydrogen production. However, to compete with current hydrogen production methods (which produce CO2), substantial cost reductions and efficiency improvements are necessary. Of the various LTE technologies, liquid alkaline (LA) electrolysis is the most mature. It has proven large scalability and can operate with all earth-abundant materials. However, most conventional LA electrolyzers are limited to lower current density operation than newer LTE technologies.Most LA electrolyzers operate with uncatalyzed Ni anodes, despite the existence of many highly active catalysts for oxygen evolution in alkaline conditions. This is because catalyzed systems require constant operation, as the catalyst layers degrade and delaminate rapidly upon exposure to the reverse polarization conditions experienced during system shutdown. This limitation prevents the integration of catalyzed electrodes, as even a single unexpected plant shutdown causes significant efficiency and cost losses. Further, this prevents fully catalyzed LA electrolyzers from directly operating with renewable power sources with large power fluctuations. Therefore, the development of reverse-current tolerant catalyst layers is essential to enable high-efficiency renewable energy powered LA electrolyzers.This talk will detail the mechanisms of electrode degradation during electrolyzer system shutdown. Our findings reveal anode catalyst layers fail through a combined electrochemical and mechanical degradation mechanism. I will discuss key differences in the cathode versus anode operating environments that explain the difference in reverse current tolerance. Considering these results, I will highlight priority research and development directions for developing anode and cathode catalysts for renewable-powered LA electrolyzers.

(Invited) Electrode Transport Resistance Analysis in Proton Exchange Membrane Water Electrolyzers

Decarbonizing sectors that are challenging and economically demanding to electrify, such as heavy-duty ground transportation, aviation, and industries like ammonia, steel, petroleum refining, synthetic fuels, and cement production, is crucial in addressing climate change and environmental pollution. The key strategy for reducing the reliance of these sectors on fossil fuels and cutting emissions involves utilizing hydrogen with low carbon intensity. Proton exchange membrane water electrolyzers (PEMWEs) play a vital role in this transition due to their high efficiency, rapid response time, and capability to electrochemically compress hydrogen. However, despite making encouraging progress, significant improvements in catalyst activity and utilization are necessary to lower loadings to levels that meet ambitious cost targets.One critical aspect of enhancing catalyst performance for achieving high currents at efficient low voltages is minimizing overpotential losses. These losses result from electron, proton, and water transport to the oxygen evolution catalysts. Additionally, there are secondary current-induced loss mechanisms, including membrane and electrode dry-out and ionic contaminant polarization.In this presentation, we will delve into the relevant transport mechanisms and the fundamental limits of reducing these overpotentials through modeling. We have evaluated the transport properties of electrodes that are challenging to directly probe due to their micrometer and sub-micrometer length scales using ultra-high-resolution three-dimensional imaging with X-ray computed tomography (nano-CT) and plasma-focused ion beam cross-sectioning with scanning electron microscopy (pFIB-SEM) for image-based simulations. Subsequently, we will briefly review existing methods for experimentally diagnosing transport limitations, as well as highlight new methods currently under development in our laboratory. These new diagnostics include a galvanostatic intermittent titration technique (GITT) for electrolyzers and a limiting-current diagnostic for PEMWEs. The presentation will conclude by highlighting fresh insights into the significance of two-phase transport, electrode and membrane dry-out, all enabled by these diagnostics and modeling.

Earth-Abundant Metal Oxides as Oxygen Evolution Electrocatalysts in 1 M H2SO4

Water splitting is a very hot research topic as a preferred source for green hydrogen, as a key vector to facilitate a transition into an environmentally friendly and sustainable energy economy in the mid-term. But it is still far from being economically competitive with respect to current hydrogen production technologies based on fossil fuels. One of the major economic hurdles to reduce electrolysis costs is the lack of low-cost catalysts for hydrogen evolution and, more importantly, for oxygen evolution. The water oxidation process is considered the main bottleneck in the advancement of water splitting devices, since it is the rate limiting process, and the main cause of poor long-term performance.Most recent advances in the field of water oxidation catalysis are based on transition metal oxides, operating exclusively in alkaline electrolysis conditions. Unfortunately, these excellent oxygen evolution catalysts in basic pH ( > 13) rapidly lose their activity when the pH is lowered, and are far away from the performance exhibited by noble metals in acid solution. McCrory et al. established in 2013 1a benchmarking protocol to identify the potential of new OER electrocatalysts (and extended in 2015 2 to HER) comparing the initial activity and stability of the materials as a function of pH and showed how no other known catalysts at that time came close to the performance of IrOx-based materials.A simple processing technique using a partially hydrophobic binder has been proved to stabilize Co oxide, resulting in electrodes able to reach high current densities (> 10 mA cm-2) at competitive overpotentials and with long term stability for acidic OER catalysis3. Following this strategy, we have been able to conduct an extensive survey of the activity and stability of monometallic, binary and ternary earth-abundant transition metal oxides during electrocatalytic OER in 1 M H2SO4. Our results confirm the general validity of the strategy in using a partially hydrophobic electrode to confer high stability to common metal oxides under these harsh conditions.[1] C.C.L. McCrory, S. Jung, J.C. Peters, T.F. Jaramillo, Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction, J. Am. Chem. Soc. 135 (2013) 16977–16987.[2] C.C.L. McCrory, S. Jung, I.M. Ferrer, S.M. Chatman, J.C. Peters, T.F. Jaramillo, Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices, J. Am. Chem. Soc. 137 (2015) 4347–4357.[3] J. Yu, F.A. Garcés-Pineda, J. González-Cobos, M. Peña-Díaz, C. Rogero, S. Giménez, M.C. Spadaro, J. Arbiol, S. Barja, J.R. Galán-Mascarós, Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4, Nat. Commun. 13 (2022) 4341. Figure 1