Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups.  K_0.1[Co(bpy)]_2.9[Fe(CN)₆]₂ ([Cobpy–Fe]), K_0.2[Co(phen)]_2.8[Fe(CN)₆]₂ ([Cophen–Fe]), {[Co(bpy)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 ([Cobpy2–Fe]), and {[Co(phen)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 Cl_0.11 ([Cophen2–Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2′-bipyridine) to the common synthetic protocol of Co–Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2–Fe] and [Cophen2–Fe] adopt a pentanuclear molecular structure, while [Cobpy–Fe] and [Cophen–Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co–Fe Prussian blue analogue (PBA), K_0.1Co_2.9[Fe(CN)₆]₂ ([Co–Fe]). Photocatalytic studies reveal that the activities of [Cobpy–Fe] and [Cophen–Fe] are significantly enhanced compared to those of [Co–Fe], while molecular [Cobpy2–Fe] and [Cophen2–Fe] are inactive toward water oxidation. [Cobpy–Fe] and [Cophen–Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s^–1, respectively, which are ∼50 times higher than that of [Co–Fe] (1.8 × 10^–2 s^–1). The complete inactivity of [Cobpy2–Fe] and [Cophen2–Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O–O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.

Bidentate pyridyl groups are coordinated to the catalytic cobalt sites in a cyanide-based Co−Fe structure to afford well-tuned extended network structures, which exhibit an outstanding photocatalytic performance compared to the regular Co−Fe PBA.

Building an Iron Chromophore Incorporating Prussian Blue Analogue for Photoelectrochemical Water Oxidation

The replacement of traditional ruthenium-based photosensitizers with low-cost and abundant iron analogs is a key step for the advancement of scalable and sustainable dye-sensitized water splitting cells. In this proof-of-concept study, a pyridinium ligand coordinated pentacyanoferrate(II) chromophore is used to construct a cyanide-based CoFe extended bulk framework, in which the iron photosensitizer units are connected to cobalt water oxidation catalytic sites through cyanide linkers. The iron-sensitized photoanode exhibits exceptional stability for at least 5 h at pH 7 and features its photosensitizing ability with an incident photon-to-current conversion capacity up to 500 nm with nanosecond scale excited state lifetime. Ultrafast transient absorption and computational studies reveal that iron and cobalt sites mutually support each other for charge separation via short bridging cyanide groups and for injection to the semiconductor in our proof-of-concept photoelectrochemical device. The reorganization of the excited states due to the mixing of electronic states of metal-based orbitals subsequently tailor the electron transfer cascade during the photoelectrochemical process. This breakthrough in chromophore-catalyst assemblies will spark interest in dye-sensitization with robust bulk systems for photoconversion applications.

How to Build Prussian Blue Based Water Oxidation Catalytic Assemblies: Common Trends and Strategies

Prussian blue (PB) and its analogues (PBAs) have at least a three-century-long history in coordination chemistry. Recently, cobalt-based PBAs have been acknowledged as efficient and robust water oxidation catalysts. Given the flexibility in their synthesis, the structure and morphology of cobalt-based PBAs have been modified for enhanced catalytic activity under electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions. Here, in this review, the work on cobalt-based PBAs is presented in four sections: i) electrocatalytic water oxidation with bare PBAs, ii) photocatalytic processes in the presence of a photosensitizer (PS), iii) photoelectrochemical water oxidation by coupling PBAs to proper semiconductors (SCs), and iv) the utilization of PBA-PS assemblies coated on SCs for the dye-sensitized photoelectrochemical water oxidation. This review will guide readers through the structure and catalytic activity relationship in cobalt-based PBAs by describing the role of each structural component. Furthermore, this review aims to provide insight into common strategies to enhance the catalytic activity of PBAs.

Precious Metal-Free Photocatalytic Water Oxidation by a Layered Double Hydroxide-Prussian Blue Analogue Hybrid Assembly

The development of earth-abundant photocatalytic assemblies has been one of the bottlenecks for the advancement of scalable water splitting cells. In this study, a ZnCr layered double hydroxide and a CoFe Prussian blue analogue are combined to afford an earth-abundant photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). Compared to bare ZnCr-LDH, the SC-WOC hybrid assembly exhibits a threefold enhancement in photocatalytic activity, which is maintained for 6 h under photocatalytic conditions at pH 7. The band energy diagram was extracted from optical and electrochemical studies to elucidate the origin of the enhanced photocatalytic performance. This study marks a straightforward pathway to develop low-cost and precious metal-free assemblies for visible light-driven water oxidation.

Photocatalytic water oxidation with a Prussian blue modified brown TiO2

A recently emerging visible light-absorbing semiconductor, brown TiO₂ (b-TiO₂), was coupled with a CoFe Prussian blue (PB) analogue to prepare an entirely earth-abundant semiconductor/water oxidation catalyst hybrid assembly. PB/b-TiO₂ exhibits a sevenfold higher photocatalytic water oxidation activity compared to b-TiO₂. An elegant band alignment unified with the optical absorption of b-TiO₂ and excellent electronic dynamics of PB yield a high-performance photocatalytic system.

Enhanced Oxygen Evolution via Electrochemical Water Oxidation using Conducting Polymer and Nanoparticle Composites

Nano-Co₃ O₄ was used for electrocatalytic water oxidation due to its promising features of better performance and low cost. An enhanced electrochemical water oxidation performance of the nanoparticles can be achieved by mixing them with other types of highly conductive nano/micro-structured materials. Conductive polymers would be one of the candidates to achieve this goal. Here, we report our recently developed nano-Co₃ O₄ and polypyrrole composites for enhanced electrochemical water oxidation. We chose polypyrrole as a support of nano-Co₃ O₄ to obtain highly active sites of nano-Co₃ O₄ with high conductivity. Morphological and chemical characterization of the prepared materials were performed using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). After immobilizing them individually on fluorine doped tin oxide (FTO) substrate, their electrocatalytic properties toward water oxidation were investigated. The optimum composite materials showed significantly higher electrocatalytic properties compared to that of pure nano-Co₃ O₄ and polypyrrole. Electrochemical impedance studies indicated that the composite materials possess significantly less electron transfer resistance toward water oxidation reaction compared to that of only polypyrrole or nano-Co₃ O₄ , while the higher double-layer capacitance and polarization resistance values obtained from fitting of the impedance data represent the faster electrode kinetics in the composite electrocatalyst. Due to the synergetic effect, the optimum nano-Co₃ O₄ and polypyrrole composites could be represent a novel and promising material for water oxidation.

Single Open Sites on FeII Ions Stabilized by Coupled Metal Ions in CN-Deficient Prussian Blue Analogues for High Catalytic Activity in the Hydrolysis of Organophosphates

CN-deficient Prussian blue analogues (PBAs), [M^N(H₂O)_x]_y[Fe^II(CN)₅(NH₃)] (M^N = Cu^II, Co^II, or Ga^III), were synthesized and examined as a new class of heterogeneous catalysts for hydrolytic decomposition of organophosphates often used as pesticides. The active species of the CN-deficient PBAs were mainly C-bound Fe^II ions with only single open sites generated by liberation of the NH₃ ligand during the catalytic reactions. [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] showed higher catalytic activity than [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)], although N-bound Cu^II species has been reported as less active than Co^II and Ga^III species in conventional PBAs. IR measurements of a series of the CN-deficient PBAs after the catalytic reactions clarified that a part of the NH₃ ligands remained on [Co^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and that hydrogen phosphate formed as a product strongly adsorbed on the Fe^II ions of [Ga^III(H₂O)][Fe^II(CN)₅(NH₃)]. Hydrogen phosphate also adsorbed, but weakly, on the Fe^II ions of [Cu^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)]. These results suggest that heterogeneous catalysis of the Fe^II ions with single open sites were tuned by the M^N ions through metal-metal interaction.

Electrocatalytic water oxidation reaction promoted by cobalt-Prussian blue and its thermal decomposition product under mild conditions

Water oxidation studies with Co-Prussian blue and Co₃O₄. Figure adapted from ‘Under the Wave off Kanagawa’ (Kanagawa oki nami ura), also known as ‘The Great Wave’, from the series ‘Thirty-six Views of Mount Fuji’ (‘Fugaku sanjūrokkei’) by K. Hokusai.

Coordination polymers as heterogeneous catalysts in hydrogen evolution and oxygen evolution reactions

This article highlights various strategies of designing coordination polymers for catalysing water splitting reactions.