Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Controlling the terminal layer atom of InTe for enhanced electrochemical oxygen evolution reaction and hydrogen evolution reaction performance

Herein, we report the method of molecular-beam-epitaxial growth (MBE) for precisely regulating the terminal surface with different exposed atoms on indium telluride (InTe) and studied the electrocatalytic performances toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The improved performances result from the exposed In or Te atoms cluster, which affects the conductivity and active sites. This work provides insights into the comprehensive electrochemical attributes of layered indium chalcogenides and exhibits a new route for catalyst synthesis.

Selective Ethylene Glycol Oxidation to Formate on Nickel Selenide with Simultaneous Evolution of Hydrogen

There is an urgent need for cost-effective strategies to produce hydrogen from renewable net-zero carbon sources using renewable energies. In this context, the electrochemical hydrogen evolution reaction can be boosted by replacing the oxygen evolution reaction with the oxidation of small organic molecules, such as ethylene glycol (EG). EG is a particularly interesting organic liquid with two hydroxyl groups that can be transformed into a variety of C1 and C2 chemicals, depending on the catalyst and reaction conditions. Here, a catalyst is demonstrated for the selective EG oxidation reaction (EGOR) to formate on nickel selenide. The catalyst nanoparticle (NP) morphology and crystallographic phase are tuned to maximize its performance. The optimized NiS electrocatalyst requires just 1.395 V to drive a current density of 50 mA cm^-2 in 1 m potassium hydroxide (KOH) and 1 m EG. A combination of in situ electrochemical infrared absorption spectroscopy (IRAS) to monitor the electrocatalytic process and ex situ analysis of the electrolyte composition shows the main EGOR product is formate, with a Faradaic efficiency above 80%. Additionally, C2 chemicals such as glycolate and oxalate are detected and quantified as minor products. Density functional theory (DFT) calculations of the reaction process show the glycol-to-oxalate pathway to be favored via the glycolate formation, where the CC bond is broken and further electro-oxidized to formate.

Multilayered Molybdate Microflowers Fabricated by One-Pot Reaction for Efficient Water Splitting

The development of high-performance, low-cost and rapid-production bifunctional electrocatalysts towards overall water splitting still poses huge challenges. Herein, the authors utilize a facile hydrothermal method to synthesize a novel structure of Co-doped ammonium lanthanum molybdate on Ni foams (Co-ALMO@NF) as self-supported electrocatalysts. Owing to large active surfaces, lattice defect and conductive channel for rapid charge transport, Co-ALMO@NF exhibits good electrocatalytic performances which requires only 349/341 mV to achieve a high current density of 600 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Besides, a low cell voltage of 1.52 V is required to reach the current density of 10 mA cm^-2 in alkaline medium along with an excellent long-term stability for two-electrode configurations. Density functional theory calculations are performed to reveal the reaction mechanism on Co-ALMO@NF, which shows that the Mo site is the most favorable ones for HER, while the introduction of Co is beneficial to reduce the adsorption intensity on the surface of Co-ALMO@NF, thus accelerating OER process. This work highlighted the importance of the structural design for self-supporting electrocatalysts.

Self-Reconstructed Spinel Surface Structure Enabling the Long-Term Stable Hydrogen Evolution Reaction/Oxygen Evolution Reaction Efficiency of FeCoNiRu High-Entropy Alloyed Electrocatalyst

High catalytic efficiency and long-term stability are two main components for the performance assessment of an electrocatalyst. Previous attention has been paid more to efficiency other than stability. The present work is focused on the study of the stability processed on the FeCoNiRu high-entropy alloy (HEA) in correlation with its catalytic efficiency. This catalyst has demonstrated not only performing the simultaneous hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with high efficiency but also sustaining long-term stability upon HER and OER. The study reveals that the outstanding stability is attributed to the spinel oxide surface layer developed during evolution reactions. The spinel structure preserves the active sites that are inherited from the HEA's intrinsic structure. This work will provide an insightful direction/pathway for the design and manufacturing activities of other metallic electrocatalysts and a benchmark for the assessment of their efficiency-stability relationship.

Significantly Stabilizing Hydrogen Evolution Reaction Induced by Nb-Doping Pt/Co(OH)2 Nanosheets

High stability and efficiency of electrocatalysts are crucial for hydrogen evolution reaction (HER) toward water splitting in an alkaline media. Herein, a novel nano-Pt/Nb-doped Co(OH)₂ (Pt/NbCo(OH)₂ ) nanosheet is designed and synthesized using water-bath treatment and solvothermal reduction approaches. With nano-Pt uniformly anchored onto NbCo(OH)₂ nanosheet, the synthesized Pt/NbCo(OH)₂ shows outstanding electrocatalytic performances for alkaline HER, achieving a high stability for at least 33 h, a high mass activity of 0.65 mA µg^-1 Pt, and a good catalytic activity with a low overpotential of 112 mV at 10 mA cm^-2 . Both experimental and theoretical results prove that Nb-doping significantly optimizes the hydrogen adsorption free energy to accelerate the Heyrovsky step for HER, and boosts the adsorption of H₂ O, which further enhances the water activation. This study provides a new design methodology for the Nb-doped electrocatalysts in an alkaline HER field by facile and green way.

Structure and oxygen vacancy engineered CuCo-layered double oxide nanotube arrays as advanced bifunctional electrocatalysts for overall water splitting

In recent years, as a green renewable energy production technology, electrochemical water splitting has demonstrated high development potential. Many materials have been reported as successful catalysts in the water-splitting field. However, it is still a huge challenge to produce bifunctional electrocatalysts for the efficient and sustainable generation of hydrogen and oxygen simultaneously. Herein, we successfully developed oxygen vacancies abundant CuCo layered double oxide (O_v-CuCo-LDO) hollow nanotube arrays (HNTAs) loaded on nickel foam as advanced electrocatalysts for total water splitting. When the current density was 10 mA cm^-2, the O_v-CuCo-LDO HNTAs exhibited outstanding onset overpotentials of 53.9 and 72.5 mV for the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline medium, respectively, because of the bimetallic synergistic effect between the cobalt and copper and the unique hollow porous structure. In addition, an as-assembled O_v-CuCo-LDO||O_v-CuCo-LDO electrolytic cell showed a small potential of 1.55 V to deliver a current density of 10 mA cm^-2. Moreover, it also showed remarkable durability after long-term overall water splitting for more than 20 h. The research results in this paper are of great interest to practical applications of the water decomposition process, providing clear and in-depth insights into preliminary robust and efficient multifunctional electrocatalysts for overall water splitting.

Multicomponent Metal Oxide- and Metal Hydroxide-Based Electrocatalysts for Alkaline Water Splitting

Developing cost-effective, highly catalytic active, and stable electrocatalysts in alkaline electrolytes is important for the development of highly efficient anion-exchange membrane water electrolysis (AEMWE). To this end, metal oxides/hydroxides have attracted wide research interest for efficient electrocatalysts in water splitting owing to their abundance and tunable electronic properties. It is very challenging to achieve an efficient overall catalytic performance based on single metal oxide/hydroxide-based electrocatalysts due to low charge mobilities and limited stability. This review is mainly focused on the advanced strategies to synthesize the multicomponent metal oxide/hydroxide-based materials that include nanostructure engineering, heterointerface engineering, single-atom catalysts, and chemical modification. The state of the art of metal oxide/hydroxide-based heterostructures with various architectures is extensively discussed. Finally, this review provides the fundamental challenges and perspectives regarding the potential future direction of multicomponent metal oxide/hydroxide-based electrocatalysts.

Nanostructured silicon photocatalysts for solar-driven fuel production

Solar-driven production of fuels such as hydrogen, hydrocarbons, and ammonia using semiconducting photocatalysts has the potential to be a sustainable alternative to current chemical processes. In recent years, silicon (Si) nanostructures have been recognized as a promising photocatalyst for hydrogen generation and organic oxidation reactions owing to its abundance, biocompatibility, and cost. While bulk Si has been studied extensively, on the nanoscale, plenty of opportunities exist to understand and engineer optimally performing Si photocatalysts. This perspective will highlight key results on the use of Si nanostructures for photocatalytic H₂ production, CO₂ reduction via light and heat-driven chemical looping, and current challenges in utilizing it for fuel-forming reactions. A brief guide on how these challenges can be addressed in the future and other unexplored questions that remain in the field are also discussed.

Synergistic coupling of IrNi/Ni(OH)2nanosheets with polypyrrole and iron oxyhydroxide layers for efficient electrochemical overall water splitting

The design of electrocatalysts with excellent activity and stability for overall water splitting is highly desirable, and remains a challenge. Constructing heterojunctions onto the same substrate is beneficial for the integration of a water-splitting reaction. Herein, self-supported IrNi/Ni(OH)₂@PPy and IrNi/Ni(OH)₂@FeOOH are fabricated by coupling polypyrrole (PPy) and iron oxyhydroxide (FeOOH) on IrNi/Ni(OH)₂nanosheets array, respectively. Benefiting from the nanosheet structure, composition, and heterogeneous interface, the as-constructed IrNi/Ni(OH)₂@PPy and IrNi/Ni(OH)₂@FeOOH catalysts can efficiently drive the hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, the electrolyzer consisting of IrNi/Ni(OH)₂@PPy and IrNi/Ni(OH)₂@FeOOH for water splitting requires only a low cell voltage of 1.49 V to deliver 10 mA cm^-2. This study provides a useful strategy for constructing efficient electrocatalysts by synergistic composition modulation and interface engineering.

Development of high-efficiency alkaline OER electrodes for hybrid acid-alkali electrolytic H2 generation

The development of high-efficiency oxygen evolution reaction (OER) electrocatalysts is of great importance for electrolytic H₂ generation. In this work, we report in-situ growth of MnCo₂O₄ nanoneedles and NiFeRu layered double hydroxide (LDH) nanosheets on nickel foam (NF) (MnCo₂O₄@NiFeRu-LDH/NF) that can function a highly efficient electrode toward electrocatalysis of OER. Such electrode demands an overpotential of as low as 205 mV to reach 10 mA cm^-2 in alkaline electrolyte and can run stably over 120-hours continuous operation. A hybrid flow acid/alkali electrolyzer is set up by using the Pt/C as the acidic cathode coupling with the MnCo₂O₄@NiFeRu-LDH/NF as the alkaline anode, which only requires an applied voltage of 0.59 V and 0.94 V to attain an electrolytic current density of 10 mA cm^-2 and 100 mA cm^-2, respectively. The present work could push forward the further development of the electricity-saving electrolytic technique for H₂ generation.

Understanding Trends in Electrochemical Methanol Oxidation Reaction Activity on a Single Transition-Metal Atom Embedded in N-Coordinated Graphene Catalysts

The lack of efficient catalysts and research on the mechanism for the methanol oxidation reaction (MOR) impedes the development of direct methanol fuel cells. In this work, based on density functional theory calculations, we systematically investigated the activity trends of electrochemical MOR on a single transition-metal atom embedded in N-coordinated graphene (M@N₄C). By calculating the free energy diagrams of MOR on M@N₄C, Co@N₄C was screened out to be the most effective MOR catalyst with a low limiting potential of 0.41 V due to the unique charge transfers and electronic structures. Importantly, one- and two-dimensional volcano relationships in MOR on M@N₄C catalysts are established based on the d-band center and the Gibbs free energy of ΔG*CH3OH and ΔG_*CO, respectively. In one word, this work provides theoretical guides toward the improved activity of MOR on M@N₄C and hints for the design of active and efficient MOR electrocatalysts.

Synergistic Tuning of CoO/CoP Heterojunction Nanowire Arrays as Efficient Bifunctional Catalysts for Alkaline Overall Water Splitting

Bifunctional electrocatalysts with superior activity and durability are of great importance for electrocatalytic water splitting. Herein, hierarchical structured CoO/CoP heterojunctions are successfully designed as highly efficient and freestanding bifunctional electrocatalysts toward overall water splitting. The unique microstructure combining two-dimensional nanosheets with one-dimensional nanowires enables numerous exposed active sites, shortened ion-diffusion pathways, and enhanced conductivity, significantly improving performance. Such freestanding electrodes achieve high current density over 400 mA cm^-2 at low overpotentials and have exceptional electrocatalytic activity as well as long-term durability for both hydrogen and oxygen evolution reactions under alkaline conditions. Remarkably, a high current density of 20 mA cm^-2 is generated at a low cell voltage of 1.56 V in an alkaline water electrolyzer, originating from synergistic interactions between CoO and CoP exposing active sites and facilitating charge transfer and enhancing kinetics. This work provides new insight into designing low-cost but high-performance bifunctional electrocatalysts for practical water splitting.

Activated FeS2 @NiS2 Core-Shell Structure Boosting Cascade Reaction for Superior Electrocatalytic Oxygen Evolution

Unlike single-step reactions, multi-step reactions can be greatly facilitated only if all the intermediate reactions can be catalyzed simultaneously and progressively. Herein, the theoretical analysis and experiments to illustrate the superiority of the cascade oxygen evolution reaction (OER) are conducted. As different OER intermediate reactions demand Fe_x Ni_1-x OOH with altered Fe/Ni ratios, gradient Fe-doped NiOOH can be an ideal electrocatalyst for the efficient cascade OER in line. Fine controlling of the nucleation sequence of iron and nickel sulfides leads to a FeS₂ @NiS₂ core-shell structure. The activated outward diffusion of Fe dopants results in the gradient Fe/Ni ratios in the Fe_x Ni_1-x OOH shell, where a cascade OER can happen. Electrochemical tests suggest that the FeS₂ @NiS₂ only needs an overpotential of 237 mV to reach the current density of 10 mA cm^-2 , with fast reaction kinetics and good stability.

Ambient Electrosynthesis toward Single-Atom Sites for Electrocatalytic Green Hydrogen Cycling

With the ultimate atomic utilization, well-defined configuration of active sites and unique electronic properties, catalysts with single-atom sites (SASs) exhibit appealing performance for electrocatalytic green hydrogen generation from water splitting and further utilization via hydrogen-oxygen fuel cells, such that a vast majority of synthetic strategies toward SAS-based catalysts (SASCs) are exploited. In particular, room-temperature electrosynthesis under atmospheric pressure offers a novel, safe, and effective route to access SASs. Herein, the recent progress in ambient electrosynthesis toward SASs for electrocatalytic sustainable hydrogen generation and utilization, and future opportunities are discussed. A systematic summary is started on three kinds of ambient electrochemically synthetic routes for SASs, including electrochemical etching (ECE), direct electrodeposition (DED), and electrochemical leaching-redeposition (ELR), associated with advanced characterization techniques. Next, their electrocatalytic applications for hydrogen energy conversion including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, and oxygen reduction reaction are reviewed. Finally, a brief conclusion and remarks on future challenges regarding further development of ambient electrosynthesis of high-performance and cost-effective SASCs for many other electrocatalytic applications are presented.

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)Fe₂ O₄ 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.

Hybrid Heterostructure Ni3 N|NiFeP/FF Self-Supporting Electrode for High-Current-Density Alkaline Water Electrolysis

Exploring earth-abundant and efficient electrocatalysts for oxygen evolution reaction (OER) is an urgent need and significant to water electrolysis. Although great achievements have been made, it is still challenging to achieve industrial current density and stability. Herein, a hybrid heterostructure electrode based on Ni₃ N and NiFeP over Fe foam substrate (Ni₃ N|NiFeP/FF) is reported, along with 3D-interconnected hierarchical porous architecture, achieving the low overpotentials of 287, 178, and 290 mV at 500 mA cm^-2 in 1 m KOH, 30 wt% KOH, and alkaline simulated seawater, respectively, with excellent durability at 800 mA cm^-2 over 120 h, which can satisfy the requirements of industrial water electrolysis. Here, the hybrid heterostructure can ensure the low energy barrier of the catalytic active sites, the 3D-interconnected hierarchical porous architecture can facilitate the fast mass/ions/electrons transformation, which contributes together to boost the superb water splitting performance. Furthermore, the COMSOL simulations confirm the multiple merits of the designed electrode during the water electrocatalysis. The present work provides a new strategy in the design and engineering of high-performance electrodes for industrial water electrolysis.

Lamella-heterostructured nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride electrodes as stable catalysts for oxygen evolution

Developing robust nonprecious-metal electrocatalysts with high activity towards sluggish oxygen-evolution reaction is paramount for large-scale hydrogen production via electrochemical water splitting. Here we report that self-supported laminate composite electrodes composed of alternating nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride (FeCo/CeO_2-xN_x) heterolamellas hold great promise as highly efficient electrocatalysts for alkaline oxygen-evolution reaction. By virtue of three-dimensional nanoporous architecture to offer abundant and accessible electroactive CoFeOOH/CeO_2-xN_x heterostructure interfaces through facilitating electron transfer and mass transport, nanoporous FeCo/CeO_2-xN_x composite electrodes exhibit superior oxygen-evolution electrocatalysis in 1 M KOH, with ultralow Tafel slope of ~33 mV dec^-1. At overpotential of as low as 360 mV, they reach >3900 mA cm^-2 and retain exceptional stability at ~1900 mA cm^-2 for >1000 h, outperforming commercial RuO₂ and some representative oxygen-evolution-reaction catalysts recently reported. These electrochemical properties make them attractive candidates as oxygen-evolution-reaction electrocatalysts in electrolysis of water for large-scale hydrogen generation.

Engineering cation vacancies in high-entropy layered double hydroxides for boosting the oxygen evolution reaction

High-entropy layered double hydroxides (HE-LDHs) are emerging as promising electrocatalysts towards the OER due to their high-entropy effect and the cocktail effect. However, the catalytic activity and stability of HE-LDHs is, as yet, unsatisfactory. Herein, we designed FeCoNiCuZn LDHs with rich cation vacancies, which need only low overpotentials of 227, 275 and 293 mV to reach 10, 100 and 200 mA cm^-2, respectively, and show almost no decay up to 200 h at 200 mA cm^-2. DFT calculations validate that the cation vacancies can boost the intrinsic activity of HE-LDHs through optimizing the adsorption energy of OER intermediates.

Understanding the Role of (W, Mo, Sb) Dopants in the Catalyst Evolution and Activity Enhancement of Co3 O4 during Water Electrolysis via In Situ Spectroelectrochemical Techniques

Unlocking the potential of the hydrogen economy is dependent on achieving green hydrogen (H₂ ) production at competitive costs. Engineering highly active and durable catalysts for both oxygen and hydrogen evolution reactions (OER and HER) from earth-abundant elements is key to decreasing costs of electrolysis, a carbon-free route for H₂ production. Here, a scalable strategy to prepare doped cobalt oxide (Co₃ O₄ ) electrocatalysts with ultralow loading, disclosing the role of tungsten (W), molybdenum (Mo), and antimony (Sb) dopants in enhancing OER/HER activity in alkaline conditions, is reported. In situ Raman and X-ray absorption spectroscopies, and electrochemical measurements demonstrate that the dopants do not alter the reaction mechanisms but increase the bulk conductivity and density of redox active sites. As a result, the W-doped Co₃ O₄ electrode requires ≈390 and ≈560 mV overpotentials to reach ±10 and ±100 mA cm^-2 for OER and HER, respectively, over long-term electrolysis. Furthermore, optimal Mo-doping leads to the highest OER and HER activities of 8524 and 634 A g^-1 at overpotentials of 0.67 and 0.45 V, respectively. These novel insights provide directions for the effective engineering of Co₃ O₄ as a low-cost material for green hydrogen electrocatalysis at large scales.

F and rare V4+ doped cobalt hydroxide hybrid nanostructures: excellent OER activity with ultralow overpotential

Highly efficient and stable Earth abundant transition metal electrocatalysts are in great demand for the oxygen evolution reaction (OER), a bottleneck process involved in the water splitting reaction and metal-air batteries. Herein, we have demonstrated a single step direct fabrication of cobalt hydroxide (Co(OH)₂) nanowires doped with vanadium(V) in a less stable +4 oxidation state and fluoride (F) ions (V-Co(OH)₂) on a carbon cloth electrode that showed highly enhanced OER activity under alkaline conditions. V-Co(OH)₂ nanowires synthesized under the optimized conditions produced excellent OER activity with an ultralow overpotential of 136 mV at 10 mA cm^-2 (scan rate 1 mV s^-1), a small Tafel slope (51.6 mV dec^-1) and good stability over 72 h. To the best of our knowledge, this is the lowest overpotential reported for cobalt-based electrocatalysts to achieve a geometric current density of 10 mA cm^-2. The controlled synthesis and HR-TEM studies revealed the formation of hybrid nanostructures (nanowires along with spherical assembly of nanoparticles) and codoping of V and F ions played an important role in enhancing the OER activity. The detailed chemical composition and oxidation state analysis by X-ray photoelectron spectroscopy (XPS) confirmed the doping of V⁴⁺ and ionic F in V-Co(OH)₂ with mixed valence states of Co²⁺/Co³⁺ and a higher Co²⁺ ratio. The outstanding OER activity of V-Co(OH)₂ is attributed to the formation of a spherical assembly of nanoparticles with nanowires, which provided a high number of catalytically active sites with enhanced charge transport, and doping of higher valence V⁴⁺ and strongly electronegative F in V-Co(OH)₂ with a higher ratio of Co²⁺/Co³⁺ promoted OOH* intermediate generation and significantly boosted the OER activity. Overall, the present work highlights the possibility of achieving highly active Earth abundant OER electrocatalysts by controlling the mixed oxidation state of Co with a judicious choice of dopants along with maintaining optimal nanostructure morphologies.

Facile synthesis of Co-Ni layered double hydroxides nanosheets wrapped on a prism-like metal-organic framework for efficient oxygen evolution reaction

The construction of low-cost oxygen evolution reaction (OER) electrocatalysts with high activity and good durability is a considerable challenge for facilitating the efficient utilization of green energy. Herein, the prism-like materials of institute lavoisier frameworks-88 (MIL-88) was first synthesized by a hydrothermal method. Then, Co-Ni layered double hydroxides (CoNi-LDHs) nanosheets were directly wrapped on the MIL-88 surface by electrodeposition to form core-shell MIL-88@CoNi-LDHs composites. Due to the distinct structure and synergistic effect between the MIL-88 core and CoNi-LDHs shell, it was found that MIL-88@CoNi-LDHs had outstanding OER activity with a small Tafel slope (45.55 mV dec^-1), low overpotential (314 mV) at 10 mA cm^-2, and superior durability. This study provides a prospective pathway to exploit highly efficient low-cost electrocatalysts for OER.

Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

Flexible synthetic routes to crystalline metal-rich to phosphorus-rich nickel phosphides are highly desired for comparable electrocatalytic HER studies. This report details solvent-free, direct, and tin-flux-assisted synthesis of five different nickel phosphides from NiCl₂ and phosphorus at moderate temperatures (500 °C). Direct reactions are thermodynamically driven via PCl₃ formation and tuned through reaction stoichiometry to produce crystalline Ni-P materials from metal-rich (Ni₂P, Ni₅P₄) to phosphorus-rich (cubic NiP₂) compositions. A tin flux in NiCl₂/P reactions allows access to monoclinic NiP₂ and NiP₃. Intermediates in tin flux reactions were isolated to help identify phosphorus-rich Ni-P formation mechanisms. These crystalline micrometer-sized nickel phosphide powders were affixed to carbon-wax electrodes and investigated as HER electrocatalysts in acidic electrolyte. All nickel phosphides show moderate HER activity in a potential range of -160 to -260 mV to achieve current densities of 10 mA/cm² ordered as c-NiP₂ ≥ Ni₅P₄ > NiP₃ > m-NiP₂ > Ni₂P, with NiP₃ activity showing some particle size influence. Phosphorus-rich c/m-NiP₂ appears most stable under acidic conditions during extended reactions. The HER activity of these different nickel phosphides appears influenced by a combination of factors such as particle size, phosphorus content, polyphosphide anions, and surface charge.

Tunable Ordered Nanostructured Phases by Co-assembly of Amphiphilic Polyoxometalates and Pluronic Block Copolymers

The assembly of polyoxometalate (POM) metal-oxygen clusters into ordered nanostructures is attracting a growing interest for catalytic and sensing applications. However, assembly of ordered nanostructured POMs from solution can be impaired by aggregation, and the structural diversity is poorly understood. Here, we present a time-resolved small-angle X-ray scattering (SAXS) study of the co-assembly in aqueous solutions of amphiphilic organo-functionalized Wells-Dawson-type POMs with a Pluronic block copolymer over a wide concentration range in levitating droplets. SAXS analysis revealed the formation and subsequent transformation with increasing concentration of large vesicles, a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually a hexagonal phase formed at concentrations above 110 mM. The structural versatility of co-assembled amphiphilic POMs and Pluronic block copolymers was supported by dissipative particle dynamics simulations and cryo-TEM.

Photoexcitation of Fe3 O Nodes in MOF Drives Water Oxidation at pH=1 When Ru Catalyst Is Present

Artificial photosynthesis strives to convert the energy of sunlight into sustainable, eco-friendly solar fuels. However, systems with light-driven water oxidation reaction (WOR) at pH=1 are rare. Broadly used [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bipyridine) photosensitizer has a fixed +1.23 V potential which is insufficient to drive most water oxidation catalysts (WOCs) in acid, while Fe₂ O₃ , featuring the highly oxidizing holes, is not stable at low pH. Here, the key examples of Fe-based metal-organic framework (MOF) water oxidation photoelectrocatalysts active at pH=1 are presented. Fe-MIL-126 and Fe MOF-dcbpy structures were formed with 4,4'-biphenyl dicarboxylate (bpdc), 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) linkers and their mixtures. Presence of dcbpy linkers allows integration of metal-based catalysts via coordination to 2,2'-bipyridine fragments. Fe-based MOFs were doped with Ru-based precursors to achieve highly active MOFs bearing [Ru(bpy)(dcbpy)(H₂ O)₂ ]²⁺ WOC. Materials were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infra-red (FTIR) spectroscopy, resonance Raman, X-ray absorption spectroscopy, fs optical pump-probe, electron paramagnetic resonance (EPR), diffuse reflectance and electric conductivity measurements and were modeled by band structure calculations. It is shown that under reaction conditions, Fe^III and Ru^III oxidation states are present, indicating rate-limiting electron transfer in MOF. Fe₃ O nodes emerge as photosensitizers able to drive prolonged O₂ evolution in acid. Further developments are possible via MOF's linker modification for enhanced light absorption, electrical conductivity, reduced MOF solubility in acid, Ru-WOC modification for faster WOC catalysis, or Ru-WOC substitution to 3d metal-based systems. The findings give further insight for development of light-driven water splitting systems based on Earth-abundant metals.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO2 reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Designing In Situ Grown Ternary Oxide/2D Ni-BDC MOF Nanocomposites on Nickel Foam as Efficient Electrocatalysts for Electrochemical Water Splitting

The security of future energy, hydrogen, is subject to designing high-performance, stable, and low-cost electrocatalysts for hydrogen and oxygen evolution reactions (HERs and OERs), for the realization of efficient overall water splitting. Two-dimensional (2D) metal-organic frameworks (MOFs) introduce a large family of materials with versatile chemical and structural features for a variety of applications, such as supercapacitors, gas storage, and water splitting. Herein, a series of nanocomposites based on NCM/Ni-BDC@NF (N=Ni, C=Co, M:F=Fe, C=Cu, and Z=Zn, BDC: benzene dicarboxylic acid, NF: nickel foam) were directly developed on NF using a facile yet scalable solvothermal method. After coupling, the electronic structure of metallic atoms was well-modulated. Based on the XPS results, for the NCF/Ni-BDC, cationic atoms shifted to higher oxidation states, favorable for the OER. Conversely, for the NCZ/Ni-BDC and NCC/Ni-BDC nanocomposites, cationic atoms shifted to lower oxidation states, advantageous for the HER. The as-prepared NCF/Ni-BDC demonstrated prominent OER performance, requiring only 1.35 and 1.68 V versus a reversible hydrogen electrode to afford 10 and 50 mA cm-2 current densities, respectively. On the cathodic side, NCZ/Ni-BDC exhibited the best HER activity with an overpotential of 170 and 350 mV to generate 10 and 50 mA cm-2, respectively, under 1.0 M KOH medium. In a two-electrode alkaline electrolyzer, the assembled NCZ/Ni-BDC (cathode) ∥ NCF/Ni-BDC (anode) couple demanded a cell voltage of only 1.58 V to produce 10 mA cm-2. The stability of NCF/Ni-BDC toward OER was also exemplary, experiencing a continuous operation at 10, 20, and 50 mA cm-2 for nearly 45 h. Surprisingly, the overpotential after OER stability at 50 mA cm-2 dropped drastically from 450 to 200 mV. Finally, the faradaic efficiencies for the overall water splitting revealed the respective values of 100 and 85% for the H2 and O2 production at a constant current density of 20 mA cm-2.

Polypyrrole modification on BiVO4 for photothermal-assisted photoelectrochemical water oxidation

The bismuth vanadate (BiVO₄) photoanode receives extensive attention in photoelectrochemical (PEC) water splitting. However, the high charge recombination rate, low electronic conductivity, and sluggish electrode kinetics have inhibited the PEC performance. Increasing the reaction temperature for water oxidation is an effective way to enhance the carrier kinetics of BiVO₄. Herein, a polypyrrole (PPy) layer was coated on the BiVO₄ film. The PPy layer could harvest the near-infrared light to elevate the temperature of the BiVO₄ photoelectrode and further improve charge separation and injection efficiencies. In addition, the conductive polymer PPy layer acted as an effective charge transfer channel to facilitate photogenerated holes moving from BiVO₄ to the electrode/electrolyte interface. Therefore, PPy modification led to a significantly improved water oxidation property. After loading the cobalt-phosphate co-catalyst, the photocurrent density reached 3.64 mA cm^-2 at 1.23 V vs the reversible hydrogen electrode, corresponding to an incident photon-to-current conversion efficiency of 63% at 430 nm. This work provided an effective strategy for designing a photothermal material assisted photoelectrode for efficient water splitting.

Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am-)x/m, A = CO32-, NO3-, F-, Cl-] consisting of OH- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32-, NO3-, F-, Cl-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Interface and electronic structure regulation of Mo-doped NiSe2-CoSe2 heterostructure aerogel for efficient overall water splitting

Transition metal selenides (TMSes) with cubic pyrite-type crystal structure have been widely explored as electrocatalysts for oxygen evolution reaction (OER), but the insufficient hydrogen evolution reaction (HER) performance hinders the application of overall water splitting. Herein, we designed and prepared a Mo doped NiSe₂-CoSe₂ heterostructure aerogel as bifunctional electrocatalyst via facile spontaneous gelation and selenium vapor deposition. The active sites on the heterointerface possessed desirable Gibbs free energy of hydrogen adsorption, leading to better HER performance than single NiSe₂ or CoSe₂. Moreover, systematically experimental research and density functional theory (DFT) calculations revealed that fine regulated Mo doping improved the electropositivity of heterostructure, promoting the nucleophilic adsorption of water molecule. Benefit from those improvements, the optimal Mo doped NiSe₂-CoSe₂ aerogel exhibited an extremely low overpotential of 57 mV at the current density of 10 mA·cm^-2 for HER with a small Tafel slope value of 38 mV·dec^-1. Meanwhile, Mo doping provided higher electron transfer efficiency and better adsorptive property toward reaction intermediate in anodic reaction, resulting in low overpotential of 270 mV at the current density of 100 mA·cm^-2 for OER with good electrocatalytic stability. This work provides an anticipated perspective of rational combination of metal doping and heterostructure for advanced electrocatalysts.

Reconstructured Electrocatalysts during Oxygen Evolution Reaction under Alkaline Electrolytes

The development of electrocatalysts with high-efficiency and clear structure-activity relationship towards the sluggish oxygen evolution reaction (OER) is essential for the wide application of water electrolyzers. Recently, the dynamic reconstruction phenomenon of the catalysts' surface structures during the OER process has been discovered. With the help of various advanced ex situ and in situ characterization, it is demonstrated that such surface reconstruction could yield actual active species to catalyze the water oxidation process. However, the attention and studies of potential interaction between reconstructed species and substrate are lacking. This review summarizes the recent development of typical reconstructed electrocatalysts and the substrate effect. First, the advanced characterization for electrocatalytic reconstruction is briefly discussed. Then, typical reconstructed electrocatalysts are comprehensively summarized and the key role of substrate effects during the OER process is emphasized. Finally, the future challenges and perspectives of surface reconstructed catalysts for water electrolysis are discussed.

Metal-organic frameworks-derived layered double hydroxides: From controllable synthesis to various electrochemical energy storage/conversion applications

Over the past years, metal-organic frameworks (MOF) have been directly used as electrodes or as a precursor for MOF-derived materials in energy storage and conversion systems. In the wide range of existing MOF derivatives, MOF-derived layered double hydroxides (LDHs) are determined to be promising materials due to their unique structure and features. However, MOF-derived LDHs (MDL) materials can suffer from insufficient intrinsic conductivity and agglomeration during formation. Various techniques and approaches were designed and applied to tackle these problems, such as using ternary LDHs, ion-doping, sulphurization, phosphorylation, selenization, direct growth, and conductive substrates. All the mentioned enhancement techniques aim to create the ideal electrode materials with maximum performance. In this review, we gathered and discussed the most recent progressive advances, different synthesis methodologies, unsolved challenges, applications, and electrochemical and electrocatalytic performance of MDL materials. We hope this work will be a reliable source for future progress and synthesis of these materials.

Revealing the enhanced photoelectrochemical water oxidation activity of Fe-based metal-organic polymer-modified BiVO4 photoanode

Metal-organic polymers (MOPs) can enhance the photoelectrochemical (PEC) water oxidation performance of BiVO₄ photoanodes, but their PEC mechanisms have yet to be comprehended. In this work, we constructed an active and stable composite photoelectrode by overlaying a uniform MOP on the BiVO₄ surface using Fe²⁺ as the metal ions and 2,5-dihydroxyterephthalic acid (DHTA) as ligand. Such modification on the BiVO₄ surface yielded a core-shell structure that could effectively enhance the PEC water oxidation activity of the BiVO₄ photoanode. Our intensity-modulated photocurrent spectroscopy analysis revealed that the MOP overlayer could concurrently reduce the surface charge recombination rate constant (k_sr) and enhance the charge transfer rate constant (k_tr), thus accelerating water oxidation activity. These phenomena can be ascribed to the passivation of the surface that inhibits the recombination of the charge carrier and the MOP catalytic layer that improves the hole transfer. Our rate law analysis also demonstrated that the MOP coverage shifted the reaction order of the BiVO₄ photoanode from the third-order to the first-order, resulting in a more favorable rate-determining step where only one hole accumulation is required to overcome water oxidation. This work provides new insights into the reaction mechanism of MOP-modified semiconductor photoanodes.

Revealing the Formation Mechanism and Optimizing the Synthesis Conditions of Layered Double Hydroxides for the Oxygen Evolution Reaction

Layered double hydroxides (LDHs), whose formation is strongly related to OH^- concentration, have attracted significant interest in various fields. However, the effect of the real-time change of OH^- concentration on LDHs' formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH₃ gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi-LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.

Non-covalent ligand-oxide interaction promotes oxygen evolution

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak "non-bonding" interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO₂ interaction that substantially elevates the population of Co⁴⁺ sites for improved water oxidation. We find that phenanthroline only coordinates with Co²⁺ forming soluble Co(phenanthroline)₂(OH)₂ complex in alkaline electrolytes, which can be deposited as amorphous CoO_xH_y film containing non-bonding phenanthroline upon oxidation of Co²⁺ to Co^3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm^-2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO₂ through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

Self-Supporting Co/CeO2 Heterostructures for Ampere-Level Current Density Alkaline Water Electrolysis

Remodeling the active surface through fabricating heterostructures can substantially enhance alkaline water electrolysis driven by renewable electrical energy. However, there are still great challenges in the synthesis of highly reactive and robust heterostructures to achieve both ampere-level current density hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report a new Co/CeO₂ heterojunction self-supported electrode for sustainable overall water splitting. The self-supporting Co/CeO₂ heterostructures required only low overpotentials of 31.9 ± 2.2, 253.3 ± 2.7, and 316.7 ± 3 mV for HER and 214.1 ± 1.4, 362.3 ± 1.9, and 400.3 ± 3.7 mV for OER at 0.01, 0.5, and 1.0 A·cm^-2, respectively, being one of the best Co-based bifunctional electrodes. Electrolyzer constructed from this electrode acting as an anode and cathode merely required cell voltages of 1.92 ± 0.02 V at 1.0 A·cm^-2 for overall water splitting. Multiple characterization techniques combined with density functional theory calculations disclosed the different active sites on the anode and cathode, and the charge redistributions on the heterointerfaces that can optimize the adsorption of H and oxygen-containing intermediates, respectively. This study presents the tremendous prospective of self-supporting heterostructures for effective and economical overall water splitting.

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN₄ ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN₄ centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe^II (P⋅)]^- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Dynamic charge collecting mechanisms of cobalt phosphate on hematite photoanodes studied by photoinduced absorption spectroscopy

Reaction sites, surface states, and surface loaded electrocatalysts are photoinduced charge storage sites and critical to photoelectrochemical (PEC) performance, however the charge transfer mechanisms involved in the three remain poorly understood. Herein, we studied the charge transfer processes in hematite (Fe2O3) without/with loaded cobalt phosphate (CoPi) by operando photoinduced absorption (PIA) spectroscopy. The loaded CoPi receives trapped holes in surface states at low potential and directly captures holes in the valence band at high potential. Through the dynamic hole storage mechanisms, loaded CoPi on Fe2O3 facilitates spatial charge separation and serves as a charge transfer mediator, instead of serving as a catalyst to change the water oxidation mechanism (constant third-order reaction). The spatial separation of photoinduced charges between Fe2O3 and CoPi results in more long-lived holes on the Fe2O3 surface to improve PEC water oxidation kinetically. The dynamic charge collecting mechanism sheds light on the understanding and designing of electrocatalyst loaded photoanodes.

Nanowire photochemical diodes for artificial photosynthesis

Artificial photosynthesis can provide a solution to our current energy needs by converting small molecules such as water or carbon dioxide into useful fuels. This can be accomplished using photochemical diodes, which interface two complementary light absorbers with suitable electrocatalysts. Nanowire semiconductors provide unique advantages in terms of light absorption and catalytic activity, yet great control is required to integrate them for overall fuel production. In this review, we journey across the progress in nanowire photoelectrochemistry (PEC) over the past two decades, revealing design principles to build these nanowire photochemical diodes. To this end, we discuss the latest progress in terms of nanowire photoelectrodes, focusing on the interplay between performance, photovoltage, electronic band structure, and catalysis. Emphasis is placed on the overall system integration and semiconductor-catalyst interface, which applies to inorganic, organic, or biologic catalysts. Last, we highlight further directions that may improve the scope of nanowire PEC systems.

Atomically dispersed iridium catalysts on silicon photoanode for efficient photoelectrochemical water splitting

Stabilizing atomically dispersed single atoms (SAs) on silicon photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites. Here, we elaborately demonstrate the decoration of iridium SAs on silicon photoanodes and assess the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, is capable of embedding the iridium SAs in its lattices by locally modifying the electronic structure. The isolated iridium SAs enable the effective photogenerated charge transport by suppressing the charge recombination and lower the thermodynamic energy barrier in the potential-determining step. The Ir SAs/NiO/Ni/ZrO₂/n-Si photoanode exhibits a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode and 130 h stability. This study proposes the rational design of SAs on silicon photoelectrodes and reveals the potential of the iridium SAs to boost photogenerated charge carrier kinetics.

Recent Advances and Future Perspectives of Metal-Based Electrocatalysts for Overall Electrochemical Water Splitting

Recently, the growing demand for a renewable and sustainable fuel alternative is contingent on fuel cell technologies. Even though it is regarded as an environmentally sustainable method of generating fuel for immediate concerns, it must be enhanced to make it extraordinarily affordable, and environmentally sustainable. Hydrogen (H₂ ) synthesis by electrochemical water splitting (ECWS) is considered one of the foremost potential prospective methods for renewable energy output and H₂ society implementation. Existing massive H₂ output is mostly reliant on the steaming reformation of carbon fuels that yield CO₂ together with H₂ and is a finite resource. ECWS is a viable, efficient, and contamination-free method for H₂ evolution. Consequently, developing reliable and cost-effective technology for ECWS was a top priority for scientists around the globe. Utilizing renewable technologies to decrease total fuel utilization is crucial for H₂ evolution. Capturing and transforming the fuel from the ambient through various renewable solutions for water splitting (WS) could effectively reduce the need for additional electricity. ECWS is among the foremost potential prospective methods for renewable energy output and the achievement of a H₂ -based economy. For the overall water splitting (OWS), several transition-metal-based polyfunctional metal catalysts for both cathode and anode have been synthesized. Furthermore, the essential to the widespread adoption of such technology is the development of reduced-price, super functional electrocatalysts to substitute those, depending on metals. Many metal-premised electrocatalysts for both the anode and cathode have been designed for the WS process. The attributes of H₂ and oxygen (O₂ ) dynamics interactions on the electrodes of water electrolysis cells and the fundamental techniques for evaluating the achievement of electrocatalysts are outlined in this paper. Special emphasis is paid to their fabrication, electrocatalytic performance, durability, and measures for enhancing their efficiency. In addition, prospective ideas on metal-based WS electrocatalysts based on existing problems are presented. It is anticipated that this review will offer a straight direction toward the engineering and construction of novel polyfunctional electrocatalysts encompassing superior efficiency in a suitable WS technique.

A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts

Electrochemical splitting of water is an appealing solution for energy storage and conversion to overcome the reliance on depleting fossil fuel reserves and prevent severe deterioration of the global climate. Though there are several fuel cells, hydrogen (H2) and oxygen (O2) fuel cells have zero carbon emissions, and water is the only by-product. Countless researchers worldwide are working on the fundamentals, i.e. the parameters affecting the electrocatalysis of water splitting and electrocatalysts that could improve the performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and overall simplify the water electrolysis process. Noble metals like platinum for HER and ruthenium and iridium for OER were used earlier; however, being expensive, there are more feasible options than employing these metals for all commercialization. The review discusses the recent developments in metal and metalloid HER and OER electrocatalysts from the s, p and d block elements. The evaluation perspectives for electrocatalysts of electrochemical water splitting are also highlighted.

Lead-Free Metal Halide Perovskite Nanocrystals: From Fundamentals to Applications

Lead (Pb) halide perovskite nanocrystals, with the general formula APbX₃ , where A=CH₃ NH³⁺ , CH(NH₂ )²⁺ , or Cs⁺ and X=Cl^- , Br^- , or I^- , have emerged as a class of materials with promising properties due to their remarkable optical properties and solar cell performance. However, important issues still need to be addressed to enable practical applications of these materials, such as instability, mass production, and Pb toxicity. Recent studies have carried out the replacement of Pb by various less-toxic cations as Sn, Ge, Sb, and Bi. This variety of chemical compositions provide Pb-free perovskite and metal halide nanostructures with a wide spectral range, in addition to being considered less toxic, therefore having greater practical applicability. Highlighting the necessity to address and solve the toxicity problems related to Pb-containing perovskite, this review considers the prospects of the Pb-free perovskite, involving synthesis methods, and properties of them, including advantages, disadvantages, and applications.