Iridium‐Based Multimetallic Porous Hollow Nanocrystals for Efficient Overall‐Water‐Splitting Catalysis

The development of active and durable bifunctional electrocatalysts for overall water splitting is mandatory for renewable energy conversion. This study reports a general method for controllable synthesis of a class of IrM (M = Co, Ni, CoNi) multimetallic porous hollow nanocrystals (PHNCs), through etching Ir‐based, multimetallic, solid nanocrystals using Fe³⁺ ions, as catalysts for boosting overall water splitting. The Ir‐based multimetallic PHNCs show transition‐metal‐dependent bifunctional electrocatalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic electrolyte, with IrCo and IrCoNi PHNCs being the best for HER and OER, respectively. First‐principles calculations reveal a ligand effect, induced by alloying Ir with 3d transition metals, can weaken the adsorption energy of oxygen intermediates, which is the key to realizing much‐enhanced OER activity. The IrCoNi PHNCs are highly efficient in overall‐water‐splitting catalysis by showing a low cell voltage of only 1.56 V at a current density of 2 mA cm^?2, and only 8 mV of polarization‐curve shift after a 1000‐cycle durability test in 0.5 m H₂SO₄ solution. This work highlights a potentially powerful strategy toward the general synthesis of novel, multimetallic, PHNCs as highly active and durable bifunctional electrocatalysts for high‐performance electrochemical overall‐water‐splitting devices.     

Recent Advances in Conjugated Polymers for Visible-Light-Driven Water Splitting

With the ambition of solving the challenges of the shortage of fossil fuels and their associated environmental pollution, visible-light-driven splitting of water into hydrogen and oxygen using semiconductor photocatalysts has emerged as a promising technology to provide environmentally friendly energy vectors. Among the current library of developed photocatalysts, organic conjugated polymers present unique advantages of sufficient light-absorption efficiency, excellent stability, tunable electronic properties, and economic applicability. As a class of rising photocatalysts, organic conjugated polymers offer high flexibility in tuning the framework of the backbone and porosity to fulfill the requirements for photocatalytic applications. In the past decade, significant progress has been made in visible-light-driven water splitting employing organic conjugated polymers. The recent development of the structural design principles of organic conjugated polymers (including linear, crosslinked, and supramolecular self-assembled polymers) toward efficient photocatalytic hydrogen evolution, oxygen evolution, and overall water splitting is described, thus providing a comprehensive reference for the field. Finally, current challenges and perspectives are also discussed.     

NiMoFe and NiMoFeP as Complementary Electrocatalysts for Efficient Overall Water Splitting and Their Application in PV‐Electrolysis with STH 12.3%

Complementary water splitting electrocatalysts used simultaneously in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can simplify water splitting systems. Herein, earth‐abundant NiMoFe (NMF) and phosphorized NiMoFeP (NMFP) are synthesized as complementary overall water splitting (OWS) catalysts. First, NMF is tested as both the HER and OER promoter, which exhibits low overpotentials of 68 (HER) and 337 mV (OER). A quaternary NMFP is then prepared by simple phosphorization of NMF, which shows a much lower OER overpotential of 286 mV. The enhanced OER activity is attributed to the unique surface/core structure of NMFP. The surface phosphate acts as a proton transport mediator and expedites the rate‐determining step. With the application of OER potential, the NMFP surface is composed of Ni(OH)₂ and FeOOH, active sites for OER, but the inner core consists of Ni, Mo, and Fe metals, serving as a conductive electron pathway. OWS with NMF‐NMFP requires an applied voltage of 1.452 V to generate 10 mA cm^?2, which is one of the lowest values among OWS results with transition‐metal‐based electrocatalysts. Furthermore, the catalysts are combined with tandem perovskite solar cells for photovoltaic (PV)‐electrolysis, producing a high solar‐to‐hydrogen (STH) conversion efficiency of 12.3%.     

A Robust Nonprecious CuFe Composite as a Highly Efficient Bifunctional Catalyst for Overall Electrochemical Water Splitting

To generate hydrogen, which is a clean energy carrier, a combination of electrolysis and renewable energy sources is desirable. In particular, for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in electrolysis, it is necessary to develop nonprecious, efficient, and durable catalysts. A robust nonprecious copper–iron (CuFe) bimetallic composite is reported that can be used as a highly efficient bifunctional catalyst for overall water splitting in an alkaline medium. The catalyst exhibits outstanding OER and HER activity, and very low OER and HER overpotentials (218 and 158 mV, respectively) are necessary to attain a current density of 10 mA cm^?2. When used in a two‐electrode water electrolyzer system for overall water splitting, it not only achieves high durability (even at a very high current density of 100 mA cm^?2) but also reduces the potential required to split water into oxygen and hydrogen at 10 mA cm^?2 to 1.64 V for 100 h of continuous operation.     

Synergism of Geometric Construction and Electronic Regulation: 3D Se‐(NiCo)Sx/(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt‐based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru‐based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large‐scale applications. This paper describes the construction of the ultrathin defect‐enriched 3D Se‐(NiCo)S_x/(OH)_x nanosheets for overall water splitting through a facile Se‐induced hydrothermal treatment. Via Se‐induced fabrication, highly efficient Se‐(NiCo)S_x/(OH)_x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as‐prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm^?2 for OER and HER, respectively. Moreover, an overall water‐splitting device delivers a current density of 10 mA cm^?2 for ≈66 h without obvious degradation.     

Recent Progress in Cobalt‐Based Heterogeneous Catalysts for Electrochemical Water Splitting

Water electrolysis is considered as the most promising technology for hydrogen production. Much research has been devoted to developing efficient electrocatalysts for hydrogen production via the hydrogen evolution reaction (HER) and oxygen production via the oxygen evolution reaction (OER). The optimum electrocatalysts can drive down the energy costs needed for water splitting via lowering the overpotential. A number of cobalt (Co)‐based materials have been developed over past years as non‐noble‐metal heterogeneous electrocatalysts for HER and OER. Recent progress in this field is summarized here, especially highlighting several important bifunctional catalysts. Various approaches to improve or optimize the electrocatalysts are introduced. Finally, the current existing challenges and the future working directions for enhancing the performance of Co‐implicated electrocatalysts are proposed.     

Transition‐Metal‐Based Electrocatalysts as Cocatalysts for Photoelectrochemical Water Splitting: A Mini Review

Converting solar energy into hydrogen via photoelectrochemical (PEC) water splitting is one of the most promising approaches for a sustainable energy supply. Highly active, cost‐effective, and robust photoelectrodes are undoubtedly crucial for the PEC technology. To achieve this goal, transition‐metal‐based electrocatalysts have been widely used as cocatalysts to improve the performance of PEC cells for water splitting. Herein, this Review summarizes the recent progresses of the design, synthesis, and application of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting. Mo, Ni, Co‐based electrocatalysts for the hydrogen evolution reaction (HER) and Co, Ni, Fe‐based electrocatalysts for the oxygen evolution reaction (OER) are emphasized as cocatalysts for efficient PEC HER and OER, respectively. Particularly, some most efficient and robust photoelectrode systems with record photocurrent density or durability for the half reactions of HER and OER are highlighted and discussed. In addition, the self‐biased PEC devices with high solar‐to‐hydrogen efficiency based on earth‐abundant materials are also addressed. Finally, this Review is concluded with a summary and remarks on some challenges and opportunities for the further development of transition‐metal‐based electrocatalysts as cocatalysts for PEC water splitting.     

Significant Enhancement of Water Splitting Activity of N‐Carbon Electrocatalyst by Trace Level Co Doping

Replacement of precious metal electrocatalysts with highly active and cost efficient alternatives for complete water splitting at low voltage has attracted a growing attention in recent years. Here, this study reports a carbon‐based composite co‐doped with nitrogen and trace amount of metallic cobalt (1 at%) as a bifunctional electrocatalyst for water splitting at low overpotential and high current density. An excellent electrochemical activity of the newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co‐N_x sites. In the case of carefully optimized sample of this electrocatalyst, 10 mA cm^?2 current density can be achieved for two half reactions in alkaline solutions—hydrogen evolution reaction and oxygen evolution reaction—at low overpotentials of 220 and 350 mV, respectively, which are smaller than those previously reported for nonprecious metal and metal‐free counterparts. Based on the spectroscopic and electrochemical investigations, the newly identified Co‐N_x sites in the carbon framework are responsible for high electrocatalytic activity of the Co,N‐doped carbon. This study indicates that a trace level of the introduced Co into N‐doped carbon can significantly enhance its electrocatalytic activity toward water splitting.     

Nanoarchitectonics for Transition‐Metal‐Sulfide‐Based Electrocatalysts for Water Splitting

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS‐based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water‐splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS‐based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water‐splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.     

Electronic and Structural Engineering of Carbon‐Based Metal‐Free Electrocatalysts for Water Splitting

Since first being reported as possible electrocatalysts to substitute platinum for the oxygen reduction reaction (ORR), carbon‐based metal‐free nanomaterials have been considered a class of promising low‐cost materials for clean and sustainable energy‐conversion reactions. However, beyond the ORR, the development of carbon‐based catalysts for other electrocatalytic reactions is still limited. More importantly, the intrinsic activity of most carbon‐based metal‐free catalysts is inadequate compared to their metal‐based counterparts. To address this challenge, more design strategies are needed in order to improve the overall performance of carbon‐based materials. Herein, using water splitting as an example, some state‐of‐the‐art strategies in promoting carbon‐based nanomaterials are summarized, including graphene, carbon nanotubes, and graphitic‐carbon nitride, as highly active electrocatalysts for hydrogen evolution and oxygen evolution reactions. It is shown that by rationally tuning the electronic and/or physical structure of the carbon nanomaterials, adsorption of reaction intermediates is optimized, consequently improving the apparent electrocatalytic performance. These strategies may facilitate the development in this area and lead to the discovery of advanced carbon‐based nanomaterials for various applications in energy‐conversion processes.     

Nitrogen-Doped Porous Nickel Molybdenum Phosphide Sheets for Efficient Seawater Splitting

Hydrogen is emerging as an alternative clean fuel; however, its dependency on freshwater will be a threat to a sustainable environment. Seawater, an unlimited source, can be an alternative, but its salt-rich nature causes corrosion and introduces several competing reactions, hindering its use. To overcome these, a unique catalyst composed of porous sheets of nitrogen-doped NiMo₃P (N-NiMo₃P) having a sheet size of several microns is designed. The presence of large homogenous pores in the basal plane of these sheets makes them catalytically more active and ensures faster mass transfer. The introduction of N and Ni into MoP significantly tunes the electronic density of Mo, surface chemistry, and metal-non-metal bond lengths, optimizing surface energies, creating new active sites, and increasing electrical conductivity. The presence of metal-nitrogen bonds and surface polyanions increases the stability and improves anti-corrosive properties against chlorine chemistry. Ultimately, the N-NiMo₃P sheets show remarkable performance as it only requires overpotentials of 23 and 35 mV for hydrogen evolution reaction, and it catalyzes full water splitting at 1.52 and 1.55 V to achieve 10 mA cm⁻² in 1 m KOH and seawater, respectively. Hence, structural and compositional control can make catalysts effective in realizing low-cost hydrogen directly from seawater.     

Amorphous Catalysts and Electrochemical Water Splitting: An Untold Story of Harmony

In the near future, sustainable energy conversion and storage will largely depend on the electrochemical splitting of water into hydrogen and oxygen. Perceiving this, countless research works focussing on the fundamentals of electrocatalysis of water splitting and on performance improvements are being reported everyday around the globe. Electrocatalysts of high activity, selectivity, and stability are anticipated as they directly determine energy‐ and cost efficiency of water electrolyzers. Amorphous electrocatalysts with several advantages over crystalline counterparts are found to perform better in electrocatalytic water splitting. There are plenty of studies witnessing performance enhancements in electrocatalysis of water splitting while employing amorphous materials as catalysts. The harmony between the flexibility of amorphous electrocatalysts and electrocatalysis of water splitting (both the oxygen evolution reaction [OER] and the hydrogen evolution reaction [HER]) is one of the untold and unsummarized stories in the field of electrocatalytic water splitting. This Review is devoted to comprehensively discussing the upsurge of amorphous electrocatalysts in electrochemical water splitting. In addition to that, the basics of electrocatalysis of water splitting are also elaborately introduced and the characteristics of a good electrocatalyst for OER and HER are discussed.     

Plasmon‐Enhanced Photoelectrochemical Water Splitting for Efficient Renewable Energy Storage

Photoelectrochemical (PEC) water splitting is a promising approach for producing hydrogen without greenhouse gas emissions. Despite decades of unceasing efforts, the efficiency of PEC devices based on earth‐abundant semiconductors is still limited by their low light absorption, low charge mobility, high charge‐carrier recombination, and reduced diffusion length. Plasmonics has recently emerged as an effective approach for overcoming these limitations, although a full understanding of the involved physical mechanisms remains elusive. Here, the reported plasmonic effects are outlined, such as resonant energy transfer, scattering, hot electron injection, guided modes, and photonic effects, as well as the less investigated catalytic and thermal effects used in PEC water splitting. In each section, the fundamentals are reviewed and the most representative examples are discussed, illustrating possible future developments for achieving improved efficiency of plasmonic photoelectrodes.     

Exceptional Performance of Hierarchical Ni–Fe (hydr)oxide@NiCu Electrocatalysts for Water Splitting

Developing low‐cost bifunctional electrocatalysts with superior activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for the widespread application of the water splitting technique. In this work, using earth‐abundant transition metals (i.e., nickel, iron, and copper), 3D hierarchical nanoarchitectures, consisting of ultrathin Ni–Fe layered‐double‐hydroxide (Ni–Fe LDH) nanosheets or porous Ni–Fe oxides (NiFeO_x) assembled to a metallic NiCu alloy, are delicately constructed. In alkaline solution, the as‐prepared Ni–Fe LDH@NiCu possesses outstanding OER activity, achieving a current density of 10 mA cm^?2 at an overpotential of 218 mV, which is smaller than that of RuO₂ catalyst (249 mV). In contrast, the resulting NiFeO_x@NiCu exhibits better HER activity, yielding a current density of 10 mA cm^?2 at an overpotential of 66 mV, which is slightly higher than that of Pt catalyst (53 mV) but superior to all other transition metal (hydr)oxide‐based electrocatalysts. The remarkable activity of the Ni–Fe LDH@NiCu and NiFeO_x@NiCu is further demonstrated by a 1.5 V solar‐panel‐powered electrolyzer, resulting in current densities of 10 and 50 mA cm^?2 at overpotentials of 293 and 506 mV, respectively. Such performance renders the as‐prepared materials as the best bifunctional electrocatalysts so far.     

Support and Interface Effects in Water‐Splitting Electrocatalysts

Water‐splitting electrolyzers that can convert electricity into storable hydrogen are a fascinating and scalable energy conversion technology for the utilization of renewable energies. To speed up the sluggish hydrogen and oxygen evolution reactions (HER and OER), electrocatalysts are essential for reducing their kinetic energy barriers and eventually improving the energy conversion efficiency. As efficient strategies for modulating the binding ability of water‐splitting intermediates on electrocatalyst surface, the support effect and interface effect are drawing growing attention. Herein, some of the recent research progress on the support and interface effects in HER, OER, and overall water‐splitting electrocatalysts is highlighted. Specifically, the correlation between the electronic interaction of the constituent components and the electrocatalytic water‐splitting performance of electrocatalysts is profoundly discussed, with the aim of advancing the development of highly efficient water‐splitting electrocatalysts, which may eventually replace the noble‐metal‐based electrocatalysts and bring the practically widespread utilization of water‐splitting electrolyzers into a reality.     

Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe₂O₄ thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm⁻² in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.