Hydrothermally synthesized Iron Phosphate Hydroxide thin film electrocatalyst for electrochemical water splitting

Electrocatalysts for Hydrogen Production - Catalyst Design Strategies Based on Crystal Structures of Multimetal Oxides

Solid-state crystalline multimetal oxides have been widely studied for their applications as electrode materials, e. g., in fuel cells, water electrolyzer, lithium-ion batteries, and metal-air batteries. Particularly, hydrogen production via water electrolysis is considered a key technology for realizing a sustainable circular carbon society. Enhancing the activity of electrocatalysts is a critical challenge for improving the efficiency of the water electrolysis technology. Thus, strategies for designing prominent catalyst materials have drawn considerable attention. Our group has been conducting research aimed at establishing comprehensive design strategies for the rational and rapid development of highly active catalysts. In this Personal Account, we present our research efforts on enhancing the activity of cost-efficient nonprecious metal-based oxide catalysts for the oxygen evolution reaction at the anode of water electrolysis. Specifically, we propose design strategies based on crystal structures of multimetal oxides and demonstrate how these strategies have contributed to the development of highly active electrocatalysts.

BaRuO3 coated Ti plate as an efficient and stable electro-catalyst for water splitting reaction in alkaline medium

Water splitting using an electrochemical device to produce hydrogen fuel is a green and economic approach to solve the energy and environmental crisis. The realistic design of durable electro-catalysts and their synthesis using a simplistic technique is a great challenge to produce hydrogen by water electrolysis. Herein, we report a stable highly active barium ruthenium oxide (BRO) electro-catalysts over Ti plate using a soft chemical method at low temperature. The synthesized material shows facile hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER) in alkaline medium with over-potentials of 195 mV and 300 mV, respectively at a current density of 10 mA cm-2. The excellent stability lasts for at least 24 h without any degradation for both the HER and OER at the current density of 10 mA cm-2, inferring the practical applications of the material toward production of green hydrogen energy. Certainly, the synthesized catalyst is capable adequately for the overall water splitting at a cell voltage of 1.60 V at a current density of 10 mA cm-2 with an impressive stability for at least 24 h, showing a minimum loss of potential. Thus the present work contributes to the rational design of stable and efficient electro-catalysts for overall water splitting reaction in alkaline media.

Cobalt-Doped Iron Phosphate Crystal on Stainless Steel Mesh for Corrosion-Resistant Oxygen Evolution Catalyst

We report an oxygen evolution reaction (OER) catalyst prepared by the incorporation of cobalt-doped iron phosphate on stainless steel mesh (SSM) through a one-step hydrothermal method. Compared to the catalytic property of bare SSM, our OER catalyst (0.84-CoFePi) showed a 42% improvement in current density at the potential of 1.9 V vs. RHE, and the onset potential was decreased by 26.5 mV. Furthermore, the loss in current density of bulk electrolysis after 12 h in 1 M KOH (pH 14) solution and 0.0441 wt% H2SO4 (pH ≈ 3) containing 0.1 M NaCl solution was negligible (3.1% and 3.2%, respectively). Moreover, our cobalt-doped iron phosphate on SSM exhibits the dramatic improvement in corrosion resistance to a basic, mild acidic solution and chloride ions compared to bare SSM.

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

A rational regulation of the solar water splitting reaction pathway by adjusting the surface composition and phase structure of catalysts is a substantial approach to ameliorate the sluggish reaction kinetics and improve the energy conversion efficiency. In this study, we demonstrate a nanocrystalline iron pyrophosphate (Fe4(P2O7)3, FePy)-regulated hybrid overlayer with amorphous iron phosphate (FePO4, FePi) on the surface of metal oxide nanostructure with boosted photoelectrochemical (PEC) water oxidation. By manipulating the facile electrochemical surface treatment followed by the phosphating process, nanocrystalline FePy is localized in the FePi amorphous overlayer to form a heterogeneous hybrid structure. The FePy-regulated hybrid overlayer (FePy@FePi) results in significantly enhanced PEC performance with long-term durability. Compared with the homogeneous FePi amorphous overlayer, FePy@FePi can improve the charge transfer efficiency more significantly, from 60% of FePi to 79% of FePy@FePi. Our density-functional theory calculations reveal that the coexistence of FePi and FePy phases on the surface of metal oxide results in much better oxygen evolution reaction kinetics, where the FePi was found to have a typical down-hill reaction for the conversion from OH* to O2, while FePy has a low free energy for the formation of OH*.

Cobalt and iron phosphates with modulated compositions and phases as efficient electrocatalysts for alkaline seawater oxidation

An electrocatalyst which is suitable for use in both fresh water and real seawater electrolysis is very uncommon. In this work, we have developed a series of iron-tuned cobalt phosphates and cobalt-tuned iron phosphate solid solutions as electrocatalysts exhibiting excellent OER activities not only in freshwater but also in alkaline real seawater with a faradaic efficiency of 95%.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Morphological and Elemental Investigations on Co–Fe–B–O Thin Films Deposited by Pulsed Laser Deposition for Alkaline Water Oxidation: Charge Exchange Efficiency as the Prevailing Factor in Comparison with the Adsorption Process

Abstract

Mixed transition-metals oxide electrocatalysts have shown huge potential for electrochemical water oxidation due to their earth abundance, low cost and excellent electrocatalytic activity. Here we present Co–Fe–B–O coatings as oxygen evolution catalyst synthesized by Pulsed Laser Deposition (PLD) which provided flexibility to investigate the effect of morphology and structural transformation on the catalytic activity. As an unusual behaviour, nanomorphology of 3D-urchin-like particles assembled with crystallized CoFe2O4 nanowires, acquiring high surface area, displayed inferior performance as compared to core–shell particles with partially crystalline shell containing boron. The best electrochemical activity towards water oxidation in alkaline medium with an overpotential of 315 mV at 10 mA/cm2 along with a Tafel slope of 31.5 mV/dec was recorded with core–shell particle morphology. Systematic comparison with control samples highlighted the role of all the elements, with Co being the active element, boron prevents the complete oxidation of Co to form Co3+ active species (CoOOH), while Fe assists in reducing Co3+ to Co2+ so that these species are regenerated in the successive cycles. Thorough observation of results also indicates that the activity of the active sites play a dominating role in determining the performance of the electrocatalyst over the number of adsorption sites. The synthesized Co–Fe–B–O coatings displayed good stability and recyclability thereby showcasing potential for industrial applications.

Graphic Abstract