Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution,Oxygen Evolution Reactions,and Full Water Splitting

The sustainable and scalable production of hydrogen through hydrogen evolution reaction (HER) and oxygen through oxygen evolution reaction (OER) in water splitting demands efficient and robust electrocatalysts. Currently, state‐of‐the‐art electrocatalysts of Pt and IrO₂/RuO₂ exhibit the benchmark catalytic activity toward HER and OER, respectively. However, expanding their practical application is hindered by their exorbitant price and scarcity. Therefore, the development of alternative effective electrocatalysts for water splitting is crucial. In the last few decades, substantial effort has been devoted to the development of alternative HER/OER and water splitting catalysts based on various transition metals (including Fe, Co, Ni, Mo, and atomic Pt) which show promising catalytic activities and durability. In this review, after a brief introduction and basic mechanism of HER/OER, the authors systematically discuss the recent progress in design, synthesis, and application of single atom and cluster‐based HER/OER and water splitting catalysts. Moreover, the crucial factors that can tune the activity of catalysts toward HER/OER and water splitting such as morphology, crystal defects, hybridization of metals with nonmetals, heteroatom doping, alloying, and formation of metals inside graphitic layered materials are discussed. Finally, the existing challenges and future perspectives for improving the performance of electrocatalysts for water splitting are addressed.     

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

Eutectic‐Derived Mesoporous Ni‐Fe‐O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting

The development of efficient and abundant water oxidation catalysts is essential for the large‐scale storage of renewable energy in the form of hydrogen fuel via electrolytic water splitting, but still remains challenging. Based upon eutectic reaction and dealloying inheritance effect, herein, novel Ni‐Fe‐O‐based composite with a unique mesoporous nanowire network structure is designed and synthesized. The composite exhibits exceptionally low overpotential (10 mA cm^?2 at an overpotential of 244 mV), low Tafel slope (39 mV dec^?1), and superior long‐term stability (remains 10 mA cm^?2 for over 60 h without degradation) toward oxygen evolution reaction (OER) in 1 m KOH. Moreover, an alkaline water electrolyzer is constructed with the Ni‐Fe‐O composite as catalyst for both anode and cathode. This electrolyzer displays superior electrolysis performance (affording 10 mA cm^?2 at 1.64 V) and long‐term durability. The remarkable features of the catalyst lie in its unique mesoporous nanowire network architecture and the synergistic effect of the metal core and the active metal oxide, giving rise to the strikingly enhanced active surface area, accelerated electron/ion transport, and further promoted reaction kinetics of OER.     

Recent Advancement of p‐ and d‐Block Elements,Single Atoms,and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Solar‐assisted photoelectrochemical (PEC) water splitting to produce hydrogen energy is considered the most promising solution for clean, green, and renewable sources of energy. For scaled production of hydrogen and oxygen, highly active, robust, and cost‐effective PEC electrodes are required. However, most of the available semiconductors as a PEC electrodes have poor light absorption, material degradation, charge separation, and transportability, which result in very low efficiency for photo‐water splitting. Generally, a promising photoelectrode is obtained when the surface of the semiconductor is modified/decorated with a suitable co‐catalyst because it increases the light absorbance spectrum and prevents electron–hole recombination during photoelectrode reactions. In this regard, numerous p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes have been widely used as semiconductor/co‐catalyst junctions to boost the performances of PEC overall water splitting. This review enumerates the recent progress and applications of p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes for water splitting. The focus is placed on fundamental mechanism, efficiency, cells design, and various aspects that contribute to the large‐scale prototype device. Finally, future perspectives, summary, challenges, and outlook for improving the activity of PEC photoelectrodes toward whole‐cell water splitting are addressed.     

Monolithic Photoassisted Water Splitting Device Using Anodized Ni‐Fe Oxygen Evolution Catalytic Substrate

Large‐scale industrial application of solar‐driven water splitting has called for the development of oxygen evolution reaction (OER) catalysts that deliver high catalytic activity and stability. Here it is shown that an efficient OER catalytic substrate can be developed by roll‐to‐roll fabrication of electrodeposited Ni‐Fe foils, followed by anodization. An amorphous oxyhydroxide layer directly formed on Ni‐Fe foils exhibits high catalytic activity toward water oxidation in 1 m KOH solution, which requires an overpotential of 0.251 V to reach current density of 10 mA cm^–2. The developed catalytic electrode shows the best OER activity among catalysts with film structure. The catalyst also shows prolonged stability at vigorous gas evolution condition for 36 h. To demonstrate the monolithic photoassisted water splitting device, an amorphous silicon solar cell is fabricated on Ni‐Fe catalytic substrate, resulting in lowering OER overpotential under light illumination. This monolithic device is the first demonstration that the OER catalytic substrates and the solar cells are integrated and can be easily applied for industrial scale solar‐driven water electrolysis.     

Ultrasensitive Iron‐Triggered Nanosized Fe–CoOOH Integrated with Graphene for Highly Efficient Oxygen Evolution

Effectively active oxygen evolution reaction (OER) electrocatalysts are highly desired for water splitting. Herein, the design and fabrication of nanometer‐sized Fe‐modulated CoOOH nanoparticles by a novel conversion tailoring strategy is reported for the first time and these nanoparticles are assembled on graphene matrix to construct 2D nanohybrids (Fe? CoOOH/G) with ultrasmall particles and finely modulated local electronic structure of Co cations. The Fe components are capable of tailoring and converting the micrometer‐sized sheets into nanometer‐sized particles, indicative of ultrasensitive Fe‐triggered behavior. The as‐made Fe? CoOOH/G features highly exposed edge active sites, well‐defined porous structure, and finely modulated electron structure, together with effectively interconnected conducting networks endowed by graphene. Density functional theory calculations have revealed that the Fe dopants in the Fe? CoOOH nanoparticles have an enhanced adsorption capability toward the oxygenated intermediates involved in OER process, thus facilitating the whole catalytic reactions. Benefiting from these integrated characteristics, the as‐made Fe? CoOOH/G nanohybrids as an oxygen evolution electrocatalyst can deliver a low overpotential of 330 mV at 10 mA cm^?2 and excellent electrochemical durability in alkaline medium. This strategy provides an effective, durable, and nonprecious‐metal electrocatalyst for water splitting.     

Hybrid Photoelectrochemical Water Splitting Systems: From Interface Design to System Assembly

Photoelectrochemical (PEC) water splitting has attracted increasing attention due to its potential to mitigate energy and environmental issues. Hybrid PEC systems containing semiconductor photosensitizers and molecular catalysts are reported to be highly active and stable for water splitting with great potential for facilitating clean fuels production. In this review, following a showcasing of the fundamental details of hybrid PEC systems for water splitting, semiconductor/molecular catalyst interface designs are highlighted, with a focus on interfacial physicochemical interactions and binding, and interfacial energetics and dynamics for efficient charge transfer. Recent advances in hybrid system assemblies for PEC water splitting are also briefly introduced. Finally, future challenges and directions in the field of hybrid PEC water splitting for solar energy conversion are reviewed. The current review provides state‐of‐the‐art strategies for optimized interface design for creating highly active and stable PEC water splitting assemblies.     

Ni‐Metalloid (B,Si, P,As, and Te) Alloys as Water Oxidation Electrocatalysts

Breakthroughs toward effective water‐splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni‐metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface‐oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (d_Ni–Ni). The activation energy E_a scales with d_Ni–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species.     

3D Nanostructures for the Next Generation of High‐Performance Nanodevices for Electrochemical Energy Conversion and Storage

Among the different nanostructures that have been demonstrated as promising materials for various applications, 3D nanostructures have attracted significant attention as building blocks for constructing high‐performance nanodevices. Particularly over the last decade, considerable research efforts have been devoted to designing, fabricating, and evaluating 3D nanostructures as electrodes for electrochemical energy conversion and storage devices. Although remarkable progress has been achieved, the performance of electrochemical energy devices based on 3D nanostructures in terms of energy conversion efficiency, energy storage capability, and device reliability still needs to be significantly improved to meet the requirements for practical applications. Rather than simply outlining and comparing different 3D nanostructures, this article systematically summarizes the general advantages as well as the existing and future challenges of 3D nanostructures for electrochemical energy conversion and storage, focusing on photoelectrochemical water splitting, photoelectrocatalytic solar‐to‐fuels conversion from nitrogen and carbon dioxide, rechargeable metal‐ion batteries, and supercapacitors. A comprehensive understanding of these advantages and challenges shall provide valuable guidelines and enlightenments to facilitate the further development of 3D nanostructured materials, and contribute to the achieving more efficient energy conversion and storage technologies toward a sustainable energy future.     

Co–Ni‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts

One promising approach to hydrogen energy utilization from full water splitting relies on the successful development of earth‐abundant, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, homologous Co–Ni‐based nanotube/nanosheet structures with tunable Co/Ni ratios, including hydroxides and nitrides, are grown on conductive substrates by a cation‐exchanging method to grow hydroxides, followed by anion exchanging to obtain corresponding nitrides. These hydroxide OER catalysts and nitride HER catalysts exhibit low overpotentials, small Tafel slopes, and high current densities, which are attributed to their large electrochemically reactive surface, 1D morphologies for charge conduction, and octahedral coordination states of metal ions for efficient catalytic activities. The homologous Co–Ni‐based nanotube hydroxides and nitrides suggest promising electrocatalysts for full water splitting with high efficiency, good stability, convenient fabrication, and low cost.     

Zn0.35Co0.65O – A Stable and Highly Active Oxygen Evolution Catalyst Formed by Zinc Leaching and Tetrahedral Coordinated Cobalt in Wurtzite Structure

To arrive to sustainable hydrogen‐based energy solutions, the understanding of water‐splitting catalysts plays the most crucial role. Herein, state‐of‐the‐art hypotheses are combined on electrocatalytic active metal sites toward the oxygen evolution reaction (OER) to develop a highly efficient catalyst based on Earth‐abundant cobalt and zinc oxides. The precursor catalyst Zn_0.35Co_0.65O is synthesized via a fast microwave‐assisted approach at low temperatures. Subsequently, it transforms in situ from the wurtzite structure to the layered γ‐Co(O)OH, while most of its zinc leaches out. This material shows outstanding catalytic performance and stability toward the OER in 1 m KOH (overpotential at 10 mA cm^?2 η_initial = 306 mV, η_{98 h} = 318 mV). By comparing the electrochemical results and ex situ analyses to today's literature, clear structure‐activity correlations are able to be identified. The findings suggest that coordinately unsaturated cobalt octahedra on the surface are indeed the active centers for the OER.     

NiCoFe‐Layered Double Hydroxides/N‐Doped Graphene Oxide Array Colloid Composite as an Efficient Bifunctional Catalyst for Oxygen Electrocatalytic Reactions

Ternary NiCoFe‐layered double hydroxide (NiCo^IIIFe‐LDH) with Co³⁺ is grafted on nitrogen‐doped graphene oxide (N‐GO) by an in situ growth route. The array‐like colloid composite of NiCo^IIIFe‐LDH/N‐GO is used as a bifunctional catalyst for both oxygen evolution/reduction reactions (OER/ORR). The NiCo^IIIFe‐LDH/N‐GO array has a 3D open structure with less stacking of LDHs and an enlarged specific surface area. The hierarchical structure design and novel material chemistry endow high activity propelling O₂ redox. By exposing more amounts of Ni and Fe active sites, the NiCo^IIIFe‐LDH/N‐GO illustrates a relatively low onset potential (1.41 V vs reversible hydrogen electrode) in 0.1 mol L^?1 KOH solution under the OER process. Furthermore, by introducing high valence Co³⁺, the onset potential of this material in ORR is 0.88 V. The overvoltage difference is 0.769 V between OER and ORR. The key factors for the excellent bifunctional catalytic performance are believed to be the Co with a high valence, the N‐doping of graphene materials, and the highly exposed Ni and Fe active sites in the array‐like colloid composite. This work further demonstrates the possibility to exploit the application potential of LDHs as OER and ORR bifunctional electrochemical catalysts.     

Multifunctional Single‐Crystallized Carbonate Hydroxides as Highly Efficient Electrocatalyst for Full Water splitting

The controllable synthesis of single‐crystallized iron‐cobalt carbonate hydroxide nanosheets array on 3D conductive Ni foam (FCCH/NF) as a monolithic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional electrocatalyst for full water splitting is described. The results demonstrate that the incorporation of Fe can effectively tune the morphology, composition, electronic structure, and electrochemical active surface area of the electrocatalysts, thus greatly enhancing the intrinsic electrocatalytic activity. The optimal electrocatalyst (F_0.25C₁CH/NF) can deliver 10 and 1000 mA cm^?2 at very small overpotentials of 77 and 256 mV for HER and 228 and 308 mV for OER in 1.0 m KOH without significant interference from gas evolution. The F_0.25C₁CH‐based two‐electrode alkaline water electrolyzer only requires cell voltages of 1.45 and 1.52 V to achieve current densities of 10 and 500 mA cm^?2. The results demonstrate that such fascinating electrocatalytic activity can be ascribed to the increase in the catalytic active surface area, facilitated electron and mass transport properties, and the synergistic interactions because of the incorporation of Fe.     

Highly Active Trimetallic NiFeCr Layered Double Hydroxide Electrocatalysts for Oxygen Evolution Reaction

The development of efficient and robust earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is an ongoing challenge. Here, a novel and stable trimetallic NiFeCr layered double hydroxide (LDH) electrocatalyst for improving OER kinetics is rationally designed and synthesized. Electrochemical testing of a series of trimetallic NiFeCr LDH materials at similar catalyst loading and electrochemical surface area shows that the molar ratio Ni:Fe:Cr = 6:2:1 exhibits the best intrinsic OER catalytic activity compared to other NiFeCr LDH compositions. Furthermore, these nanostructures are directly grown on conductive carbon paper for a high surface area 3D electrode that can achieve a catalytic current density of 25 mA cm^?2 at an overpotential as low as 225 mV and a small Tafel slope of 69 mV dec^?1 in alkaline electrolyte. The optimized NiFeCr catalyst is stable under OER conditions and X‐ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, and elemental analysis confirm the stability of trimetallic NiFeCr LDH after electrochemical testing. Due to the synergistic interactions among the metal centers, trimetallic NiFeCr LDH is significantly more active than NiFe LDH and among the most active OER catalysts to date. This work also presents general strategies to design more efficient metal oxide/hydroxide OER electrocatalysts.     

Graphitic‐Shell Encapsulation of Metal Electrocatalysts for Oxygen Evolution,Oxygen Reduction,and Hydrogen Evolution in Alkaline Solution

Developing highly efficient, cost effective, and environmentally friendly electrocatalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) is of interest for sustainable and clean energy technologies, including metal–air batteries and fuel cells. In this work, the screening of electrocatalytic activities of a series of single metallic iron, cobalt, and nickel nanoparticles and their binary and ternary alloys encapsulated in a graphitic carbon shell toward the OER, ORR, and HER in alkaline media is reported. Synthesis of these compounds proceeds by a two‐step sol–gel and carbothermal reduction procedure. Various ex situ characterizations show that with harsh electrochemical activation, the graphitic shell undergoes an electrochemical exfoliation. The modified electronic properties of the remaining graphene layers prevent their exfoliation, protect the bulk of the metallic cores, and participate in the electrocatalysis. The amount of near‐surface, higher‐oxidation‐state metals in the as‐prepared samples increases with electrochemical cycling, indicating that some metallic nanoparticles are not adequately encased within the graphite shell. Such surface oxide species provide secondary active sites for the electrocatalytic activities. The Ni–Fe binary system gives the most promising results for the OER, and the Co–Fe binary system shows the most promise for the ORR and HER.     

Progress in Developing Metal Oxide Nanomaterials for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting represents an environmentally friendly and sustainable method to obtain hydrogen fuel. Semiconductor materials as the central components in PEC water splitting cells have decisive influences on the device's solar‐to‐hydrogen conversion efficiency. Among semiconductors, metal oxides have received a lot of attention due to their outstanding (photo)‐electrochemical stability, low cost, favorable band edge positions and wide distribution of bandgaps. In the past decades, significant processes have been made in developing metal oxide nanomaterials for PEC water splitting. In this review, the recent progress using metal oxides as photoelectrodes and co‐catalysts for PEC water splitting is summarized. Their performance, limitations and potentials are also discussed. Last, the key challenges and opportunities in the development and implementation of metal oxide nanomaterials for PEC water splitting are discussed.     

Recent Advances and Emerging Trends in Photo‐Electrochemical Solar Energy Conversion

Photo‐electrochemical (PEC) solar energy conversion offers the promise of low‐cost renewable fuel generation from abundant sunlight and water. In this Review, recent developments in photo‐electrochemical water splitting are discussed with respect to this promise. State‐of‐the‐art photo‐electrochemical device performance is put in context with the current understanding of the necessary requirements for cost‐effective solar hydrogen generation (in terms of solar‐to‐hydrogen conversion efficiency and system durability, in particular). Several important studies of photo‐electrochemical hydrogen generation at p‐type photocathodes are highlighted, mostly with protection layers (for enhanced durability), but also a few recent examples where protective layers are not needed. Recent work with the widely studied n‐type BiVO₄ photoanode is detailed, which highlights the needs and necessities for the next big photoanode material yet to be discovered. The emerging new research direction of photo‐electrocatalytic upgrading of biomass substrates toward value‐added chemicals is then discussed, before closing with a commentary on how research on PEC materials remains a worthwhile endeavor.     

A Tannic Acid–Derived N‐, P‐Codoped Carbon‐Supported Iron‐Based Nanocomposite as an Advanced Trifunctional Electrocatalyst for the Overall Water Splitting Cells and Zinc–Air Batteries

Rational design and construction of a multifunctional electrocatalyst featuring with high efficiency and low cost is fundamentally important to realize new energy technologies. Herein, a trifunctional electrocatalyst composed of FeP_x nanoparticles and Fe–N–C moiety supported on the N‐, P‐codoped carbon (NPC) is masterly synthesized by a facile one‐pot pyrolysis of the mixture of tannic acid, ferrous chloride, and sodium hydrogen phosphate. The synergy of each component in the FeP_x/Fe–N–C/NPC catalyst renders high catalytic activities and excellent durability toward both oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The electrocatalytic performance and practicability of the robust FeP_x/Fe–N–C/NPC catalyst are further investigated under the practical operation conditions. Particularly, the overall water splitting cell assembled by the FeP_x/Fe–N–C/NPC catalyst only requires a voltage of 1.58 V to output the benchmark current density of 10 mA cm^?2, which is superior to that of IrO₂–Pt/C‐based cell. Moreover, the FeP_x/Fe–N–C/NPC‐based zinc–air batteries deliver high round‐trip efficiency and remarkable cycling stability, much better than that of Pt/C–IrO₂ pair‐based batteries. This work offers a new strategy to design and synthesize highly effective multifunctional electrocatalysts using cheaper tannic acid derived carbon as support applied in electrochemical energy devices.     

MOF‐Based Transparent Passivation Layer Modified ZnO Nanorod Arrays for Enhanced Photo‐Electrochemical Water Splitting

This study introduces zeolitic imidazolate framework‐8 (ZIF‐8) as the first metal‐organic framework based transparent surface passivation layer for photo‐electrochemical (PEC) water splitting. A significant enhancement for PEC water oxidation is demonstrated based on the in situ seamless coating of ZIF‐8 surface passivation layer on Ni foam (NF) supported ZnO nanorod arrays photoanode. The PEC performance is improved by optimizing the ZIF‐8 thickness and by grafting Ni(OH)₂ nanosheets as synergetic co‐catalyst. With respect to ZnO/NF, the optimized Ni(OH)₂/ZIF‐8/ZnO/NF photoanode exhibits a two times larger photocurrent density of 1.95 mA cm^?2 and also a two times larger incident photon to current conversion efficiency of 40.05% (350 nm) at 1.23 V versus RHE (V_RHE) under AM 1.5 G. The synergetic surface passivation and the co‐catalyst modification contribute to prolonging the charge lifetime, to promoting the charge transfer, and to decreasing the overpotential for water oxidation.     

Seamlessly Conductive 3D Nanoarchitecture of Core–Shell Ni‐Co Nanowire Network for Highly Efficient Oxygen Evolution

Electrochemical splitting of water is an attractive way to produce hydrogen fuel as a clean and renewable energy source. However, a major challenge is to accelerate the sluggish kinetics of the anodic half‐cell reaction where oxygen evolution reaction (OER) takes place. Here, a seamlessly conductive 3D architecture is reported with a carbon‐shelled Ni‐Co nanowire network as a highly efficient OER electrocatalyst. Highly porous and granular Ni‐Co nanowires are first grown on a carbon fiber woven fabric utilizing a cost‐effective hydrothermal method and then conductive carbon shell is coated on the Ni‐Co nanowires via glucose carbonization and annealing processes. The conductive carbon layer surrounding the nanowires is introduced to provide a continuous pathway for facile electron transport throughout the whole of the integrated 3D catalyst. This 3D hierarchical structure provides several synergistic effects and beneficial functions including a large number of active sites, easy accessibility of water, fast electron transport, rapid release of oxygen gas, enhanced electrochemical durability, and stronger structural integrity, resulting in a remarkable OER activity that delivers an overpotential of 302 mV with a Tafel slope of 43.6 mV dec^?1 at a current density of 10 mA cm^?2 in an alkaline medium electrolyte (1 m KOH).