Unlocking the Potential of Cobalt-Based Metal-Organic Frameworks for Alkaline Oxygen Evolution and One-Pot Synthesis of Styrene Carbonate from Styrene and Carbon Dioxide

This work presents the synthesis of a Co(II) MOF, IITG-7, with a 5-(((1H-imidazole-2-yl)methyl)amino)isophthalic acid linker using a solvothermal method. The activated MOF IITG-7a demonstrates excellent performance for the oxygen evolution reaction (OER) in an alkaline medium, requiring only 350 mV overpotential to achieve a 10 mA/cm2 current density, along with a minimal Tafel slope of 55.2 mV/dec and a charge transfer resistance of 3.5 Ω. Additionally, IITG-7a was successfully applied as a catalyst for the direct synthesis of styrene carbonate (SC) from styrene and CO2, achieving 100% conversion in the presence of tert-butyl hydroperoxide (TBHP) and tetrabutylammonium bromide (TBAB) at ambient pressure. The catalyst also shows nearly 90% selectivity for SC in reactions with styrene derivatives and retains its catalytic efficiency for up to three cycles. These results highlight IITG-7a as a promising catalyst for both the OER and CO2 conversion.

Graphitized Carbon-Supported Co@Co3O4 for Ozone Decomposition over the Entire Humidity Range

Ground-level ozone (O3) pollution has emerged as a significant concern due to its detrimental effects on human health and the ecosystem. Catalytic removal of O3 has proven to be the most efficient and cost-effective method. However, its practical application faces substantial challenges, particularly in relation to its effectiveness across the entire humidity range. Herein, we proposed a novel strategy termed "dual active sites" by employing graphitized carbon-loaded core-shell cobalt catalysts (Co@Co3O4-C). Co@Co3O4-C was synthesized via the pyrolysis of a Co-organic ligand as the precursor. By utilizing this approach, we achieved a nearly constant 100% working efficiency of the Co@Co3O4-C catalyst for catalyzing O3 decomposition across the entire humidity range. Physicochemical characterization coupled with density functional theory calculations elucidates that the presence of encapsulated metallic Co nanoparticles enhances the reactivity of the cobalt oxide capping layer. Additionally, the interface carbon atom, strongly influenced by adjacent metallic Co nuclei, functions as a secondary active site for the decomposition of O3 decomposition. The utilization of dual active sites effectively mitigates the competitive adsorption of H2O molecules, thus isolating them for adsorption in the cobalt oxide capping layer. This optimized configuration allows for the decomposition of O3 without interference from moisture. Furthermore, O3 decomposition monolithic catalysts were synthesized using a material extrusion-based three-dimensional (3D) printing technology, which demonstrated a low pressure drop and exceptional mechanical strength. This work provides a "dual active site" strategy for the O3 decomposition reaction, realizing O3 catalytic decomposition over the entire humidity range.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

Research on engineered electrocatalysts for efficient water splitting: a comprehensive review

Water electrolysis plays an interesting role toward hydrogen generation for overcoming global environmental crisis and solving the energy storage problem. However, there is still a deficiency of efficient electrocatalysts to overcome sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Great efforts have been employed to produce potential catalysts with low overpotential, rapid kinetics, and excellent stability for HER and OER. At present, hydrogen economy is driven by electrocatalysts with excellent characteristics; thus, systematic design strategy has become the driving force to exploit earth-abundant transition metal-based electrocatalysts toward H₂ economy. In this review, the recent progress on newer materials including metals, alloys, and transition metal oxides (manganese oxides, cobalt oxides, nickel oxides, PBA-derived metal oxides, and metal complexes) as photocatalysts/electrocatalysts has been overviewed together with some methodologies for efficient water splitting. Metal-organic framework (MOF)-based electrocatalysts have been highly exploited owing to their interesting functionalities. The photovoltaic-electrocatalytic (PV-EC) process focused on harvesting high solar-to-hydrogen efficiency (STH) among various solar energy conversion as well as storage systems. Electrocatalysts/photocatalysts with high efficiency have become an urgent need for overall water splitting. Also, cutting-edge achievements in the fabrication of electrocatalysts along with theoretical consideration have been discussed.

Self-Reconstruction of Fe-Doped Co-Metal-Organic Frameworks Boosted Electrocatalytic Performance for Oxygen Evolution Reaction

Rational design of facile and low-cost efficient electrocatalysts for oxygen evolution reaction (OER) is crucial to solve the energy crisis. Benefiting from in situ self-reconstruction from metal-organic frameworks (MOFs) to (oxy)hydroxides in alkaline electrolytes, MOFs have become alternative OER catalysts. Thus, Fe-doped Co-MOF nanosheets (Co-MOF/Fe) were prepared and utilized straightforwardly as OER electrocatalysts. CoFe-layered bimetallic hydroxides (CoFe-LDHs) with abundant active sites are obtained from in situ conversion of Co-MOF/Fe after etching by the KOH electrolyte, which are generally actual active species. Meanwhile, the introduction of Fe ions will also produce a synergistic effect that greatly improves the electrocatalytic OER performance. The optimized catalyst (Co-MOF/Fe₁₀) shows exceptional OER activity (η₁₀ = 260 mV) and excellent durability over 50 h. The outstanding OER performance of Co-MOF/Fe₁₀ can also be reflected in the two-electrode hydrolyzer (1.57 V at 10 mA cm^-2). This study offers a pathway to probe the catalytic mechanism of MOFs and the rational construction of efficient MOF-derived catalysts.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Gentle one-step co-precipitation to synthesize bimetallic CoCu-MOF immobilized laccase for boosting enzyme stability and Congo red removal

Laccase has received extensive attention in pollutant degradation due to its high efficiency and environmental friendliness, but free laccase has poor stability, easy inactivation, and difficulty in recycling, which limited its application. It was a smart strategy to construct a synergistic system for the efficient adsorption and degradation of pollutants by enzyme immobilization to improve the stability and recyclability of the enzyme. In this study, the materials were synthesized by a one-step co-precipitation method. With Cu-MOF as the main body, Co²⁺ was introduced to construct bimetallic CoCu-MOF as the protective carrier of the enzyme. The enzyme-carrying capacity and enzyme activity of Lac@CoCu-MOF were 2-fold and 3.5-fold higher than those of Lac@Cu-MOF, respectively. Lac@MOF composites had a good protective effect on enzyme in various interfering environments. At pH = 7, free laccase was completely inactivated and Lac@CoCu-MOF maintained 51.76% enzyme activity. In addition, the removal rate of Congo red by Lac@CoCu-MOF reached 90 % in 1 h at pH = 4 % and 95 % in 5 h at pH = 7, and the final TOC mineralization rate reached 86.05 %. After six cycles, the degradation rate of Lac@CoCu-MOF remained above 75 %. Therefore, Lac@CoCu-MOF was constructed with the advantages of enzyme immobilization (enhanced stability and easy operation), material adsorption, and biocatalysis (fast diffusion and high activity), which has great guiding significance for the industrial application of enzyme.

Conductive Co-triazole metal-organic framework exploited as an oxygen evolution electrocatalyst

A Co-triazole metal-organic framework (Co-trz) endowed with electrical conductivity was synthesized effortlessly via a microwave-based method. Providing a high density of catalytic centers with electrically conductive features, as suggested by DFT calculations, the framework exhibited a low overpotential for the oxygen evolution reaction (OER) with good kinetics. A mechanistic reaction pathway was proposed based on monitoring alterations in the oxidation state and local coordination environment of Co centers upon the occurrence of the OER. Due to its performance and its chemical and electrochemical robustness, the framework was highlighted as a promising MOF electrocatalyst for the OER.

Mesoporous K-doped NiCo2O4 derived from a Prussian blue analog: high-yielding synthesis and assessment as oxygen evolution reaction catalyst

The conversion and storage of clean renewable energy can be achieved using water splitting. However, water splitting exhibits sluggish kinetics because of the high overpotentials of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) and should therefore be promoted by OER and/or HER electrocatalysts. As the kinetic barrier of the former reaction exceeds that of the latter, high-performance OER catalysts are highly sought after. Herein, K-doped NiCo₂O₄ (HK-NCO) was hydrothermally prepared from a Prussian blue analog with a metal–organic framework structure and assessed as an OER catalyst. Extensive K doping increased the number of active oxygen vacancies and changed their intrinsic properties (e.g., binding energy), thus increasing conductivity. As a result, HK-NCO exhibited a Tafel slope of 49.9 mV dec⁻¹ and a low overpotential of 292 mV at 10 mA cm⁻², outperforming a commercial OER catalyst (Ir) and thus holding great promise as a component of high-performance electrode materials for metal-oxide batteries and supercapacitors.

OER characteristics of K-doped NiCo₂O₄ catalyst and K doping control through simple hydrothermal synthesis.

Two new Ni/Co-MOFs as electrocatalysts for the oxygen evolution reaction in alkaline electrolytes

Two novel Ni/Co-MOFs electrocatalysts were synthesized. Ni-MOF demonstrated a good activity in electrocatalytic OER performances and short-term electrochemical durability.

Electrospun Cobalt-Incorporated MOF-5 Microfibers as a Promising Electrocatalyst for OER in Alkaline Media

Metal-organic framework (MOF)-based materials have attracted attention in recent times owing to their remarkable properties such as regulatable pore size, high specific surface area, and elasticity in their network topology, geometry, dimension, and chemical functionality. It is believed that the incorporation of a MOF network into a fibrous matrix results in the improvement of the electrocatalytic properties of the material. Herein, we have synthesized a Co-incorporated MOF-5-based fibrous material by a simple wet-chemical method, followed by an electrospinning (ES) process. The as-prepared Co-incorporated MOF-5 microfibers were employed as an electrocatalyst for the oxygen evolution reaction (OER) in 1 M KOH electrolyte. The catalyst demands a lower overpotential of 240 mV to attain a current density of 10 mA/cm² with a lower Tafel slope value of 120 mV/dec along with a charge transfer resistance value of 2.9 Ω from electron impedance spectroscopy (EIS) analysis. From these results, it has been understood that the incorporation of Co metal into the MOF-5 microfibrous network has significantly improved the OER performance, which made them a potential entrant in other energy-related applications also.

Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal-organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided.

Five novel MOFs with various dimensions as efficient catalysts for oxygen evolution reactions

Five novel MOFs with various dimensions designed as high-efficiency electrocatalysts for oxygen evaluation reactions.

2 D Hybrid of Ni-LDH Chips on Carbon Nanosheets as Cathode of Zinc-Air Battery for Electrocatalytic Conversion of O2 into H2 O2

It remains great challenge to develop precious-metal-free electrocatalysts to implement high-activity electrochemical conversion of O₂ into value-added hydroperoxide species (HO₂ ^- ), which are vulnerable when exposed to various transition-metal-based catalysts. A strategy based on steric hindrance and layered nickel-based layered double hydroxide (Ni-LDH) induction has been developed for one-pot inlaying high-density ultrathin 2 D Ni-LDH chips on in situ-grown carbon nanosheets (Ni-LDH C/CNSs). The resulting material exhibits high electrocatalytic selectivity with a faradaic efficiency up to 95 % for oxygen reduction into peroxide and attains a fairly high mass activity of approximately 22.2 A g^-1 , outperforming most metal-based catalysts reported previously. Systematic studies demonstrate that the greatly increased defect concentration at Ni edge sites of Ni-LDH chips results in more active sites, which contributes a favorable thermodynamically neutral adsorption of OOH* and adsorbed H₂ O₂ molecules relatively weakly. Additionally, the modified CNSs effectively suppress H₂ O₂ decomposition and avoid O-O bond cleavage to produce H₂ O by steric effects. The synergistic effect of CNSs and Ni-LDH chips therefore leads to high activity and high selectivity in a two-electron pathway. A proof-of-concept zinc-air fuel cell is proposed and set up to demonstrate the feasibility of green synthesis of peroxide, generating an impressive H₂ O₂ production rate of 5239.67 mmol h^-1  g_cat. ^-1 .

Preparation of a graphene-phosphorene composite by pressure quenching and its ferromagnetism

A graphene-phosphorene composite was prepared by quenching the pressure in a LECO high-temperature and high-pressure hydrothermal reaction device. The results suggest that the graphene-phosphorene composite is a layered sandwich structure and shows strong ferromagnetism due to the P-C and P-O-C bonds formed between the graphene and phosphorene.

An efficient polymer coating for highly acid-stable zeolitic imidazolate frameworks based composite sponges

Zeolitic imidazolate frameworks (ZIFs) possess tremendous potential in various adsorption and catalysis areas for their particular structures. However, the dispersibility and acid stability of ZIFs are important issues hindering their applications. To address these challenges, a transparent polydimethysiloxane (PDMS) coating was constructed to heterogeneously anchor the Cu doped ZIF-67 (Cu/ZIF-67) nanoparticles on melamine sponge surface, achieving a PDMS-coated ZIF three-dimensional composite sponge. Thus PDMS coating could also effectively protect ZIFs from acid damage to prolong the service life of photocatalyticity. It was demonstrated that the composite sponges were able to repeatedly (over 40 cycles) degrade Sudan I dyes with remarkable photocatalytic efficiency (>97%). More importantly, the ion impenetrability of PDMS coating made the ZIFs based composite a longer term catalytic life than unprotected Cu/ZIF-67 under acid condition. Incidentally, due to the introduction of rough ZIFs and hydrophobic PDMS coating, the obtained sponge also exhibits super-hydrophobicity (158.5°), great compressibility and excellent oil/acid water separation performance. We expect that such a polymer coating strategy could act as a novel inspiration for extending the applications and life span of ZIF-based composites.

Bimetal-organic framework MIL-53(Co-Fe): an efficient and robust electrocatalyst for the oxygen evolution reaction

Rationally designing high-efficiency catalysts for the oxygen evolution reaction (OER) is extremely important for developing sustainable energy technologies, but remains a major research challenge. In this paper, a Co/Fe-imidazole-based bimetal-organic framework nanosheet array grown on a nickel foam [MIL-53(Co-Fe)/NF] was prepared via a facile solvothermal process. Surprisingly, MIL-53(Co-Fe)/NF shows excellent OER activity with overpotential as low as 262 mV at 100 mA cm^-2, much lower than those of the single metal-based MOFs, and even comparable to that of the precious RuO₂. The results indicate that the synergetic effect of co-doped Fe and Co makes a crucial contribution to the high activity of the bimetal-based MOF catalyst. Additionally, this catalyst also displays outstanding long-term electrochemical durability for at least 80 h.

Performance enhancement of oxygen evolution reaction through incorporating bimetallic electrocatalysts in two-dimensional metal–organic frameworks

We designed and synthesized a series of bimetallic and monometallic 2D metal–organic framework electrocatalysts with excellent stability, discussing their different electrochemical catalysts for oxygen evolution reaction (OER).

In situ conversion of layered double hydroxide arrays into nanoflowers of NixV1−x-MOF as a highly efficient and stable electrocatalyst for the oxygen evolution reaction

The Ni_0.9V_0.1-MOF exhibits superior catalytic OER performance, needing an overpotential of 290 mV at 150 mA cm⁻² in alkaline media.

Full water splitting by a nanoporous CeO2 nanowire array under alkaline conditions

CeO₂ nanowire array was derived from MnO₂–CeO₂/TM via an acid etching strategy, and MnO₂ acts as a pore-forming agent through selective etching with oxalic acid, it shows excellent OER and HER performance.

Cobalt Amide Imidate Imidazolate Frameworks as Highly Active Oxygen Evolution Model Materials

Two imidazolate-based Co-MOFs, IFP-5 and IFP-8 (imidazolate framework Potsdam), with a different peripheral group -R (-Me and -OMe, respectively) have been synthesized by a solvothermal method and tested toward the oxygen evolution reaction (OER). Remarkably, IFP-8 presents much lower overpotentials (319 mV at 10 mA/cm2 and 490 mV at 500 mA/cm2) than IFP-5 toward OER, as confirmed by online gas chromatography measurements (Faradaic yield of O2 > 99%). Moreover, the system is extraordinarily stable during 120 h, even when used as a catalyst toward the overall water splitting reaction without any sign of fatigue. An integrated ex situ spectroscopic study, based on powder X-ray diffraction, extended X-ray absorption fine structure, and attenuated total reflection, allows the identification of the active species and the factors that determine the catalytic activity. Indeed, it was found that the performances are highly affected by the nature of the -R group, because this small change strongly influences the conversion of the initial metal organic framework to the active species. As a consequence, the remarkable activity of IFP-8 can be ascribed to the formation of Co(O)OH phase with a particle size of a few nanometers (3-10 nm) during the electrocatalytic oxygen evolution.

Preparation of an Fe2Ni MOF on nickel foam as an efficient and stable electrocatalyst for the oxygen evolution reaction

Metal-organic frameworks (MOFs) as versatile templates for preparing transition metal compounds has received wide attention. Benefiting from their diversified spatial structure and controllable chemical constituents, they have become a research hotspot in the field of electrocatalytic water splitting. Herein, Fe2Ni-MIL-88B MOF on nickel foam (Fe2Ni MOF/NF) has been prepared through a one-pot method growth process. Compared with Fe MOF/NF and Ni MOF/NF, the interaction between Fe3+ and Ni2+ in Fe2Ni MOF/NF accelerates the electron transfer through the oxygen of the ligand, leading to increased 3d orbital electron density of Ni, which enhances the activity of the oxygen evolution reaction (OER) in alkaline solution. Fe2Ni MOF/NF provides a current density of 10 mA cm-2 at a low overpotential of 222 mV, and its Tafel slope is also very small, reaching 42.39 mV dec-1. The success of the present Fe2Ni MOF/NF catalyst is attributed to the abundant active centers, the bimetallic clusters Fe2Ni-MIL-88B, the positive coupling effect between Ni and Fe metal ions in the MOF, and synergistic effect between the MOF and NF. Besides, Fe2Ni MOF/NF possesses excellent stability over 50 h of continuous operation, providing feasibility for commercial use.

Synergistic Electrocatalytic Nitrogen Reduction Enabled by Confinement of Nanosized Au Particles onto a Two-Dimensional Ti3C2 Substrate

A N₂ fixation by the electrocatalytic nitrogen reduction reaction from humidified air is regarded to be a critical tool for producing NH₃ and reducing the globally accelerating CO₂ emissions. Notwithstanding great efforts to improve catalyst activity and selectivity, promoting catalyst accessibility to high N₂ concentrations to ensure that active sites fulfill their function should be a promising design direction. Here, Au nanoparticles are firmly anchored through atomic O on the surface of two-dimensional Ti₃C₂ using an ultrasound reduction process. Akin to the conspicuous role of the web in the predatory process of spiders, N₂ adsorption experiments primarily suggest that a Ti₃C₂ "web" is beneficial for extraction of N₂ from air, and embedding high valence-state Au clusters in the Ti₃C₂ "web" strengthens the chemical bonding effect toward N₂ molecules. The high energy of N₂ adsorption on the interface between gold clusters and Ti₃C₂ is the driving force for weakening triple N≡N bonds, and thereby the activation energy barrier is lowered via effective stabilization of N2H* species and destabilizing NH2NH2* under an alternative pathway. With Au loading content of ∼0.94%, Au/Ti₃C₂ exhibits an outstanding average yield of 30.06 μg h^-1 mg^-1 for NH₃ production, with a high Faraday efficiency of 18.34% at -0.2 V.

Enhanced electrocatalytic activity of water oxidation in an alkaline medium via Fe doping in CoS2 nanosheets

Fe–CoS₂/CC exhibits enhanced catalytic OER performance, needing an overpotential of 302 mV at 10 mA cm⁻² in 1.0 M KOH.

A novel FeS–NiS hybrid nanoarray: an efficient and durable electrocatalyst for alkaline water oxidation

A novel FeS–NiS/TM nanosheet array was developed for alkaline OER; this catalyst only requires an overpotential of 260 mV to afford a current density of 10 mA cm⁻² in 1.0 M KOH.

Concentrated-acid triggered superfast generation of porous amorphous cobalt oxide toward efficient water oxidation catalysis in alkaline solution

Amorphous cobalt oxide on carbon cloth (AMO-CoO/CC) was prepared as an excellent water-oxidation catalyst with 50 mV less overpotential at 10 mA cm⁻² than highly-crystallized Co₃O₄ in 1.0 M KOH.

Synthesis of urchin-like Co3O4 spheres for application in oxygen evolution reaction

For oxygen evolution electrocatalysis of water splitting, unique urchin-shaped Co₃O₄ spheres were successfully grown on nickel foam by hydrothermal synthesis of Co(OH)F precursor and subsequent annealing method. The formation process was investigated by the evolution of phase structure and morphology with hydrothermal reaction time. And it can be explained by a 'disks-flowers-urchins' mechanism. Moreover, the Co₃O₄ urchins/NF exhibits considerable catalytic properties. It shows a low overpotential of 308 mV at a current density of 20 mA cm^-2 in alkaline solution. In the meantime, such material has a small Tafel slope of 82.1 mV dec^-1, large electrochemical active surface area and good long-term stability. The obvious promotion of oxygen evolution reaction performance can be attributed to the special morphology and the direct attachment to the substrate, which improve the exposed active sites, lower the internal resistance and accelerate the charge transport. Thus, the Co₃O₄ urchins/NF not only has a great potential promising behavior, but also provides the basis for subsequent performance improvement.

Trace water accelerating the CO2 cycloaddition reaction catalyzed by an indium–organic framework

A unique four-fold interpenetrated In–MOF is utilized as a catalyst in the chemical fixation of CO₂, and the catalytic activity is promoted by trace water.

Nickel–copper bimetal organic framework nanosheets as a highly efficient catalyst for oxygen evolution reaction in alkaline media

NiCu MOF nanosheets on Ni foam (NiCu-MOFNs/NF) exhibit superior catalytic OER performance, needing an overpotential of 309 mV at 100 mA cm⁻² in 1.0 M KOH.