Surface and interface engineering of noble-metal-free electrocatalysts for efficient overall water splitting

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

Zinc oxide nanoflakes supported copper oxide nanosheets as a bifunctional electrocatalyst for OER and HER in an alkaline medium

Bifunctional electrocatalysts are the attractive research in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in the overall water-splitting reactions. The design and development of the cost-effective OER/HER bifunctional electrocatalysts with superior catalytic activity are still remaining as the big challenges. Herein, we have developed the CuO-ZnO nanocomposite as a bifunctional OER/HER electrocatalyst via simple chemical precipitation method. The nanocomposite was investigated for its crystalline structure, surface morphology and the functions of elements using XRD, FT-IR, SEM, TEM and XPS characterization techniques, respectively. The nanocomposite exhibited the excellent activity for the overall water-splitting in an alkaline medium. The CuO-ZnO nanocomposite showed the less onset potential of 1.4 and 0.15 V versus RHE in 1M KOH (Tafel slopes value of 0.180 and 0.400 V dec-1) for OER and HER, respectively. Hence, the as-prepared bifunctional electrocatalyst displayed the high stability for 10 h in the water electrolysis processes.

Wood-Structured Nanomaterials as Highly Efficient, Self-Standing Electrocatalysts for Water Splitting

Electrocatalytic water splitting (EWS) driven by renewable energy is widely considered an environmentally friendly and sustainable approach for generating hydrogen (H2), an ideal energy carrier for the future. However, the efficiency and economic viability of large-scale water electrolysis depend on electrocatalysts that can efficiently accelerate the electrochemical reactions taking place at the two electrodes. Wood-derived nanomaterials are well-suited for serving as EWS catalysts because of their hierarchically porous structure with high surface area and low tortuosity, compositional tunability, cost-effectiveness, and self-standing integral electrode configuration. Here, recent advancements in the design and synthesis of wood-structured nanomaterials serving as advanced electrocatalysts for water splitting are summarized. First, the design principles and corresponding strategies toward highly effective wood-structured electrocatalysts (WSECs) are emphasized. Then, a comprehensive overview of current findings on WSECs, encompassing diverse structural designs and functionalities such as supported-metal nanoparticles (NPs), single-atom catalysts (SACs), metal compounds, and heterostructured electrocatalysts based on engineered wood hosts are presented. Subsequently, the application of these WSECs in various aspects of water splitting, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall water splitting (OWS), and hybrid water electrolysis (HWE) are explored. Finally, the prospects, challenges, and opportunities associated with the broad application of WSECs are briefly discussed. This review aims to provide a comprehensive understanding of the ongoing developments in water-splitting catalysts, along with outlining design principles for the future development of WSECs.

N-CoFeP/NF electrocatalyst for coupling hydrogen production and oxidation reaction of various alcohols

Replacing the oxygen evolution reaction with the alcohols oxidation reaction (AOR) in electrolytic water is not only expected to reduce the overall energy consumption, but also realize the green synthesis of high value-added chemicals. However, designing high-activity electrocatalysts toward AOR yet faces a daunting challenge due to the indefinite conversion mechanism of different alcohols. Herein, a self-supported N-CoFeP/NF electrocatalyst on a nickel foam is synthesized via hydrothermal method, followed by low temperature nitriding and phosphating. The N-CoFeP/NF exhibits a fine nanorod nanostructure and high crystallinity. The AOR using N-CoFeP/NF catalysts requires a significantly lower potential (1.38-1.42 V vs. RHE) at 100 mA cm-2, reducing the energy input and the improvement of the overall efficiency. Moreover, alcohols with secondary hydroxyl groups located in the middle of the carbon chain underwent CC bond breakage during oxidation, yielding primarily formic acid (FE = 74 %) and acetic acid (FE = 50 %), which exhibits more attractive performance than alcohols with primary hydroxyl groups located at the end group did not undergo chemical bond breakage at a high current density of 400 mA cm-2. This study provides a novel and effective method to design TMPs and the selection of alcohols for anodic reaction, which can be used as a versatile strategy to improve the performance of anodic AOR coupled hydrogen evolution.

Development of aptamer surface-enhanced Raman spectroscopy sensor based on Fe3O4@Pt and Au@Ag nanoparticles for the determination of acetamiprid

As a common chlorinated nicotinic pesticide with high insecticidal activity, acetamiprid has been widely used for pest control. However, the irrational use of acetamiprid will pollute the environment and thus affect human health. Therefore, it is crucial to develop a simple, highly sensitive, and rapid method for acetamiprid residue detection. In this study, the capture probe (Fe3O4@Pt-Aptamer) was connected with the signal probe (Au@DTNB@Ag CS-cDNA) to form an assembly with multiple SERS-enhanced effects. Combined with magnetic separation technology, a SERS sensor with high sensitivity and stability was constructed to detect acetamiprid residue. Based on the optimal conditions, the SERS intensity measured at 1333 cm-1 is in relation to the concentration of acetamiprid in the range 2.25 × 10-9-2.25 × 10-5 M, and the calculated limit of detection (LOD) was 2.87 × 10-10 M. There was no cross-reactivity with thiacloprid, clothianidin, nitenpyram, imidacloprid, and chlorpyrifos, indicating that this method has good sensitivity and specificity. Finally, the method was applied to the detection of acetamiprid in cucumber samples, and the average recoveries were 94.19-103.58%, with RSD < 2.32%. The sensor can be used to analyse real samples with fast detection speed, high sensitivity, and high selectivity.

Synergistic Effects in LaNiO3 Perovskites between Nickel and Iron Heterostructures for Improving Durability in Oxygen Evolution Reaction for AEMWE

Perovskite materials that aren't stable during the oxygen evolution reaction (OER) are unsuitable for anion-exchange membrane water electrolyzers (AEMWE). But through manipulating their electronic structures, their performance can further increase. Among the first-row transition metals, nickel and iron are widely recognized as prominent electrocatalysts; thus, the researchers are looking into how combining them can improve the OER. Recent research has actively explored the design and study of heterostructures in this field, showcasing the dynamic exploration of innovative catalyst configurations. In this study, a heterostructure is used to manipulate the electronic structure of LaNiO3 (LNO) to improve both OER properties and durability. Through adsorbing iron onto the LNO (LNO@Fe) as γ iron oxyhydroxide (γ-FeOOH), the binding energy of nickel in the LNO exhibited negative shifts, inferring nickel movement toward the metallic state. Consequently, the electrochemical properties of LNO@Fe are further improved. LNO@Fe showed excellent performance (1.98 A cm-2, 1 m KOH, 50 °C at 1.85 V) with 84.1% cell efficiency in AEMWE single cells, demonstrating great improvement relative to LNO. The degradation for the 850 h durability analysis of LNO@Fe is ≈68 mV kh-1, which is ≈58 times less than that of LNO.

Novel mixed heterovalent (Mo/Co)Ox-zerovalent Cu system as bi-functional electrocatalyst for overall water splitting

A novel hybrid ternary metallic electrocatalyst of amorphous Mo/Co oxides and crystallized Cu metal was deposited over Ni foam using a one-pot, simple, and scalable solvothermal technique. The chemical structure of the prepared ternary electrocatalyst was systematically characterized and confirmed via XRD, FTIR, EDS, and XPS analysis techniques. FESEM images of (Mo/Co)Ox-Cu@NF display the formation of 3D hierarchical structure with a particle size range of 3-5 µm. The developed (Mo/Co)Ox-Cu@NF ternary electrocatalyst exhibits the maximum activity with 188 mV and 410 mV overpotentials at 50 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Electrochemical impedance spectroscopy (EIS) results for the (Mo/Co)Ox-Cu@NF sample demonstrate the minimum charge transfer resistance (Rct) and maximum constant phase element (CPE) values. A two-electrode cell based on the ternary electrocatalyst just needs a voltage of about 1.86 V at 50 mA cm-2 for overall water splitting (OWS). The electrocatalyst shows satisfactory durability during the OWS for 24 h at 10 mA cm-2 with an increase of only 33 mV in the cell potential.

Interfacial Interaction in NiFe LDH/NiS2/VS2 for Enhanced Electrocatalytic Water Splitting

A bifunctional electrocatalyst with high efficiency and low costs for overall water splitting is critical to achieving a green hydrogen economy and coping with the energy crisis. However, developing robust electrocatalysts still faces huge challenges, owing to unsatisfactory electron transfer and inherent activity. Herein, NiFe LDH/NiS2/VS2 heterojunctions have been designed as freestanding bifunctional electrocatalysts to split water, exhibiting enhanced electron transfer and abundant catalytic sites. The optimum NiFe LDH/NiS2/VS2 electrocatalyst exhibits a small overpotential of 380 mV at 10 mA cm-2 for overall water splitting and superior electrocatalytic performance in both hydrogen and oxygen evolution reactions (HER/OER). Specifically, the electrocatalyst requires overpotentials of 76 and 286 mV at 10 mA cm-2 for HER and OER, respectively, in alkaline electrolytes, which originate from the synergistic interaction among the facilitated electron transfer and increasingly exposed active sites due to the modulation of interfaces and construction of heterojunctions.

Interface engineering of heterogeneous NiMn layered double hydroxide/vertically aligned NiCo2S4 nanosheet as highly efficient hybrid electrocatalyst for overall seawater splitting

We report the fabrication of a heterogeneous catalyst through vertically aligned NiCo2S4/Ni3S2 nanosheet with encapsulation of ultrathin NiMn layered double hydroxide over self-standing nickel foam (NM/NCS/NS/NF) via two-step hydrothermal processes. Benefiting from more adequate catalytic active centres and copious interfacial charge transfer channels, NM/NCS/NS/NF electrode demonstrates superior bifunctional activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) processes under alkaline fresh/simulated seawater electrolyte conditions. As a result, NM/NCS/NS/NF electrode requires the smallest overpotentials of 282 & 312 mV (OER) and 171 & 204 mV (HER) to attain current densities of 30 & 50 mA cm-2 respectively under alkaline simulated seawater electrolyte conditions. Besides, the presence of amorphous NiMn LDH layers over crystalline NiCo2S4/Ni3S2 catalyst stimulates surface adsorption of oxygen intermediate species, water dissociate ability on catalytic active centres, and mass transport with electron transfer at the interface. Further, the two-electrode configuration assisted electrolyser system delivers an efficient overall water splitting activity with minimum cell voltages of 1.54 V (in 1 M KOH) and 1.56 V (in 1 M KOH+0.5 M NaCl) at a current density of 10 mA cm-2. Besides, a fabricated electrolyser cell provides a more sustained water electrolysis process and robust durability for 20 h which displays NM/NCS/NS/NF electrode is a vibrant and potential candidate for realistic seawater electrolysis. Therefore, our proposed heterogeneous electrocatalyst could open up a new platform for developing efficient large-scale efficient seawater electrolysis.

Design and multilevel regulation of transition metal phosphides for efficient and industrial water electrolysis

Renewable energy electrolysis of water to produce hydrogen is an effective measure to break the energy dilemma. However, achieving activity and stability at a high current density is still a key problem in water electrolyzers. Transition metal phosphides (TMPs), with high activity and relative inexpensiveness, have become excellent candidates for the production of highly pure green hydrogen for industrial applications. In this mini-review, multilevel regulation strategies including nanoscale control, surface composition and interface structure design of high-performance TMPs for hydrogen evolution are systematically summarized. On this basis, in order to achieve large-scale hydrogen production in industry, the hydrogen evolution performance and stability of TMPs at a high current density are also discussed. Peculiarly, the practical application and requirements in proton exchange membrane (PEM) or anion exchange membrane (AEM) electrolyzers can guide the advanced design of regulatory strategies of TMPs for green hydrogen production from renewable energy. Finally, the challenges and prospects in the future development trend of TMPs for efficient and industrial water electrolysis are given.

Hierarchical Superhydrophilic/Superaerophobic Ni(OH)2@NiFe-PBA Nanoarray Supported on Nickel Foam for Boosting the Oxygen Evolution Reaction

The design of hierarchical electrocatalysts with plentiful active sites and high mass transfer efficiency is critical to efficiently and sustainably carrying out the oxygen evolution reaction (OER), which presents a challenging and pressing need. In this study, a hierarchical Ni(OH)2@NiFe-Prussian blue analogue nanoarray grown on nickel foam (NF) [labeled as Ni(OH)2@NiFe-PBA/NF] was synthesized by combining a mild electrodeposition method with an ion-exchange strategy. The resultant Ni(OH)2@NiFe-PBA/NF displays superhydrophilic/superaerophobic properties that optimize the contact with the electrolyte, improve mass transfer efficiency, and expedite detachment of O2 bubbles during the electrocatalytic OER. Specifically, Ni(OH)2@NiFe-PBA/NF exhibits exceptional capability in the OER with low overpotentials of 224 and 240 mV at the current densities of 50 and 100 mA cm-2, respectively, accompanied by a low Tafel slope of 37.1 mV dec-1 and outstanding stability over 100 h at a fixed potential of 1.78 V vs reversible hydrogen electrode (RHE). Furthermore, Ni(OH)2@NiFe-PBA/NF demonstrates remarkable OER performance even in alkaline simulated seawater. During the OER process, active metal-OOH intermediates were formed by the partial self-reconstruction of NiFe-PBA in the heterostructure, as revealed by in situ Raman spectroscopy.

Heteroatom Doping Promoting CoP for Driving Water Splitting

CoP nanomaterials have been extensively regarded as one of the most promising electrocatalysts for overall water splitting due to their unique bifunctionality. Although the great promise for future applications, some important issues should also be addressed. Heteroatom doping has been widely acknowledged as a potential strategy for improving the electrocatalytic performance of CoP and narrowing the gap between experimental study and industrial applications. Recent years have witnessed the rapid development of heteroatom-doped CoP electrocatalysts for water splitting. Aiming to provide guidance for the future development of more effective CoP-based electrocatalysts, we herein organize a comprehensive review of this interesting field, with the special focus on the effects of heteroatom doping on the catalytic performance of CoP. Additionally, many heteroatom-doped CoP electrocatalysts for water splitting are also discussed, and the structure-activity relationship is also manifested. Finally, a systematic conclusion and outlook is well organized to provide direction for the future development of this interesting field.

An efficient cerium dioxide incorporated nickel cobalt phosphide complex as electrocatalyst for All-pH hydrogen evolution reaction and overall water splitting

Transition metal phosphides (TMPs) have been considered as potential electrocatalysts with adjustable valence states, metal characteristics, and phase diversity. However, it is necessary but remains a major challenge to obtain efficient and durable TMPs catalysts, which can realize efficiently for not only all-pH hydrogen evolution reaction (HER), but also oxygen evolution reaction (OER). Hence, cerium dioxide incorporated nickel cobalt phosphide growth on nickel foam (CeO2/NiCoP) is fabricated by hydrothermal and phosphating reaction. CeO2/NiCoP shows excellent activity for all-pH HER (overpotentials of 48, 58 and 72 mV in alkaline, neutral and acidic solution at the current density of 10 mA cm-2), and has a small OER overpotential (231 mV @ 10 mA cm-2). Moreover, the voltage of overall water splitting in alkaline solution and simulated seawater electrolyte is only 1.46 and 1.41 V (10 mA cm-2), respectively, coupled with outstanding operational stability and corrosion resistance. Further mechanism research shows that CeO2/NiCoP possesses rich heterointerfaces, which serves more exposed active sites and possesses a promising superhydrophilic and superaerophobic surface. Density functional theory calculations manifest that CeO2/NiCoP has appropriate energy for intermediates of reactions. This work provides a deep insight into the CeO2/NiCoP catalyst for high-performance water/seawater electrolysis.

Cobalt-based MOF-derived carbon electrocatalysts with tunable architecture for enhanced oxygen evolution reaction

Development of the hydrogen economy requires the design of catalysts that increase the rate of the accompanying sluggish kinetic oxygen evolution reaction (OER). This is a key process in electrochemical energy conversion and storage, such as water splitting and metal-air batteries. The OER needs high overpotential and typically expensive precious metal-based catalysts. Therefore, designing low-cost and efficient electrocatalysts for OER is of paramount importance. In addition to focusing on the number of active sites or high specific surface area, the correlation between catalyst particle shape and performance should be considered. This work presents an electrocatalytic activity comparison of cobalt-containing carbons with different morphologies in the OER process. Employing metal-organic frameworks as carbon and metal precursors, the materials in the shape of polyhedrons, needles, unique spherical hedgehogs, and sea urchins were obtained. The effect of MOF template infiltration with additional carbon source on the physicochemical properties of electrocatalysts was also examined. The furfuryl alcohol-impregnated needle-shaped particles were characterized by a high content of cobalt active sites, surrounded by nitrogen-containing graphite layers. Electrochemical tests confirmed their best activity (overpotential 317 mV@10 mA/cm2), long stability (up to 20 h), as well as low reagents diffusion limitations (Tafel slope 57 mV/dec up to 24 mA/cm2). The vertically aligned structure of the catalyst contributed to improved detachment of the oxygen bubbles produced.

Interfacial and Electronic Modulation of W Bridging Heterostructure Between WS2 and Cobalt-Based Compounds for Efficient Overall Water Splitting

The development of high performance electrocatalysts for effective hydrogen production is urgently needed. Herein, three hybrid catalysts formed by WS2 and Co-based metal-organic frameworks (MOFs) derivatives are constructed, in which the small amount of W in the MOFs derivatives acts as a bridge to provide the charge transfer channel and enhance the stability. In addition, the effects of the surface charge distribution on the catalytic performance are fully investigated. Due to the optimal interfacial electron coupling and rearrangement as well as its unique porous morphology, WS2 @W-CoPx exhibits superior bifunctional performance in alkaline media with low overpotentials in hydrogen evolution reaction (HER) (62 mV at 10 mA cm-2 ) and oxygen evolution reaction (OER) (278 mV at 100 mA cm-2 ). For overall water splitting (OWS), WS2 @W-CoPx only requires a cell voltage of 1.78 V at 50 mA cm-2 and maintains good stability within 72 h. Density functional theory calculations verify that the combination of W-CoPx with WS2 can effectively enhance the activity of OER and HER with weakened OH (or O) adsorption and enhanced H atom adsorption. This work provides a feasible idea for the design and practical application of WS2 or phosphide-based catalysts in OWS.

Surface Engineering over Metal-Organic Framework Nanoarray to Realize Boosted and Sustained Urea Oxidation

Facilitating C─N bond cleavage and promoting *COO desorption are essential yet challenging in urea oxidation reactions (UORs). Herein a novel interfacial coordination assembly protocol is established to modify the Co-phytate coordination complex on the Ni-based metal-organic framework (MOF) nanosheet array (CC/Ni-BDC@Co-PA) toward boosted and sustained UOR electrocatalysis. Comprehensive experimental and theoretical investigations unveil that surface Co-PA modification over Ni-BDC can manipulate the electronic state of Ni sites, and in situ evolved charge-redistributed surface can promote urea adsorption and the subsequent C─N bond cleavage. Impressively, Co-PA functionalization can impart a negatively charged catalyst surface with improved aerophobicity, not only weakening *COO adsorption and promoting CO2 departure, but also repelling CO3 2- approaching to deactivate Ni species, eventually alleviating CO2 poisoning and enhancing operational durability. Beyond that, improved hydrophilic and aerophobic characteristics would also contribute to better mass transfer kinetics. Consequently, CC/Ni-BDC@Co-PA exhibits prominent UOR performance with an ultralow potential of 1.300 V versus RHE to attain 10 mA cm-2 , a small Tafel slope of 45 mV dec-1 , and strong durability, comparable to the best Ni-based electrocatalysts documented thus far. This work affords a novel paradigm to construct MOF-based materials for promoted and sustained UOR catalysis through elegant surface engineering based on a metal-PA complex.

Constructing interface engineering and tailoring a nanoflower-like FeP/CoP heterostructure for enhanced oxygen evolution reaction

The inexpensive and highly efficient electrocatalysts toward oxygen evolution reaction (OER) in water splitting electrolysis have displayed promising practical applications to relieve energy crisis. Herein, we prepared a high-yield and structurally regulated bimetallic cobalt-iron phosphide electrocatalyst by a facile one-pot hydrothermal reaction and subsequent low-temperature phosphating treatment. The tailoring of nanoscale morphology was achieved by varying the input ratio and phosphating temperature. Thus, an optimized FeP/CoP-1-350 sample with the ultra-thin nanosheets assembled into a nanoflower-like structure was obtained. FeP/CoP-1-350 heterostructure displayed remarkable activity toward the OER with a low overpotential of 276 mV at a current density of 10 mA cm^-2, and a low Tafel slope of only 37.71 mV dec^-1. Long-lasting durability and stability were maintained with the current with almost no obvious fluctuation. The enhanced OER activity was attributed to the presence of copious active sites from the ultra-thin nanosheets, the interface between CoP and FeP components, and the synergistic effect of Fe-Co elements in the FeP/CoP heterostructure. This study provides a feasible strategy to fabricate highly efficient and cost-effective bimetallic phosphide electrocatalysts.

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.

Manageable Bubble Release Through 3D Printed Microcapillary for Highly Efficient Overall Water Splitting

Porous metal foams (e.g., Ni/Cu/Ti) are applied as catalyst supports extensively for water splitting due to their large specific area and excellent conductivity, however, intrinsic bubble congestion is unavoidable because of the irregular three-dimensional (3D) networks, resulting in high polarization and degraded electrocatalytic performances. To boost the H₂ O decomposition kinetics, the immediate bubble removal and water supply sequential in the gas-liquid-solid interface is essential. Inspired by the high efficiency of water/nutrient transport in the capillaries plants, this work designs a graphene-based capillary array with side holes as catalyst support to manage the bubble release and water supply via a Z-axis controllable digital light processing (DLP) 3D printing technology. Like planting rice, a low-cost, high-active CoNi carbonate hydroxide (CoNiCH) is planted on support. A homemade cell can reach 10 mA cm^-2 in 1.51 V, and be kept at 30 mA cm^-2 for 60 h without noticeable degradation, surpassing most of the known cells. This research provides a promising avenue to design and prepare advanced catalysts in various fields, including energy applications, pollutant treatment, and chemical synthesis.

Interfacial built-in electric-field for boosting energy conversion electrocatalysis

The formation of a built-in electric field (BIEF) can induce electron-rich and electron-poor counterparts to synergistically modify electronic configurations and optimize the binding strengths with intermediates, thereby leading to outstanding electrocatalytic performance. Herein, a critical review regarding the concept, modulation strategies, and applications of BIEFs is comprehensively summarized, which begins with the fundamental concepts, together with the advantages of BIEF for boosting electrocatalytic reactions. Then, a systematic summary of the advanced strategies for the modulation of BIEF along with the in-detail mechanisms in its formation are also added. Finally, the applications of BIEF in driving electrocatalytic reactions and some cascade systems for illustrating the conclusive role from the induced BIEF are also systematically discussed, followed by perspectives on the future deployment and opportunity of the BIEF design.

Rational Synthesis of Core-Shell-Structured Nickel Sulfide-Based Nanostructures for Efficient Seawater Electrolysis

Versatile electrocatalysis at higher current densities for natural seawater splitting to produce hydrogen demands active and robust catalysts to overcome the severe chloride corrosion, competing chlorine evolution, and catalyst poisoning. Hereto, the core-shell-structured heterostructures composed of amorphous NiFe hydroxide layer capped Ni₃ S₂ nanopyramids which are directly grown on nickel foam skeleton (NiS@LDH/NF) are rationally prepared to regulate cooperatively electronic structure and mass transport for boosting oxygen evolution reaction (OER) performance at larger current densities. The prepared NiS@LDH/NF delivers the anodic current density of 1000 mA cm^-2 at the overpotential of 341 mV in 1.0 m KOH seawater. The feasible surface reconstruction of Ni₃ S₂ -FeNi LDH interfaces improves the chemical stability and corrosion resistance, ensuring the robust electrocatalytic activity in seawater electrolytes for continuous and stable oxygen evolution without any hypochlorite production. Meanwhile, the designed Ni₃ S₂ nanopyramids coated with FeNi₂ P layer (NiS@FeNiP/NF) still exhibit the improved hydrogen evolution reaction (HER) activity in 1.0 m KOH seawater. Furthermore, the NiS@FeNiP/NF||NiS@LDH/NF pair requires cell voltage of 1.636 V to attain 100 mA cm^-2 with a 100% Faradaic efficiency, exhibiting tremendous potential for hydrogen production from seawater.

Research on engineered electrocatalysts for efficient water splitting: a comprehensive review

Water electrolysis plays an interesting role toward hydrogen generation for overcoming global environmental crisis and solving the energy storage problem. However, there is still a deficiency of efficient electrocatalysts to overcome sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Great efforts have been employed to produce potential catalysts with low overpotential, rapid kinetics, and excellent stability for HER and OER. At present, hydrogen economy is driven by electrocatalysts with excellent characteristics; thus, systematic design strategy has become the driving force to exploit earth-abundant transition metal-based electrocatalysts toward H₂ economy. In this review, the recent progress on newer materials including metals, alloys, and transition metal oxides (manganese oxides, cobalt oxides, nickel oxides, PBA-derived metal oxides, and metal complexes) as photocatalysts/electrocatalysts has been overviewed together with some methodologies for efficient water splitting. Metal-organic framework (MOF)-based electrocatalysts have been highly exploited owing to their interesting functionalities. The photovoltaic-electrocatalytic (PV-EC) process focused on harvesting high solar-to-hydrogen efficiency (STH) among various solar energy conversion as well as storage systems. Electrocatalysts/photocatalysts with high efficiency have become an urgent need for overall water splitting. Also, cutting-edge achievements in the fabrication of electrocatalysts along with theoretical consideration have been discussed.

Engineering sensitive gas sensor based on MOF-derived hollow metal-oxide semiconductor heterostructures

Metal-organic frameworks (MOFs) derived hollow heterostructured metal oxide semiconductors (MOSs) are a class of functional porous materials exhibiting distinctive physiochemical properties. Owing to the unique advantages, including large specific surface, high intrinsic catalytic performance, abundant channels for facilitating electron transfer and mass transport, and strong synergistic effect between different components, MOF-derived hollow MOSs heterostructures can work as promising candidates for gas sensing, which have thus attracted increasing attention. Aiming to provide a deep understanding on the design strategy and MOSs heterostructure, this review presents a comprehensive overview on the advantages and applications of MOF-derived hollow MOSs heterostructures when they used n for the detection of toxic gases. In addition, a deep discussion about the perspective and challenge of this interesting field is also well organized, hoping to provide guidance for the future design and development of more accurate gas sensors.

Strategies for Promoting Catalytic Performance of Ru-based Electrocatalysts towards Oxygen/Hydrogen Evolution Reaction

Ru-based materials hold great promise for substituting Pt as potential electrocatalysts toward water electrolysis. Significant progress is made in the fabrication of advanced Ru-based electrocatalysts, but an in-depth understanding of the engineering methods and induced effects is still in their early stage. Herein, we organize a review that focusing on the engineering strategies toward the substantial improvement in electrocatalytic OER and HER performance of Ru-based catalysts, including geometric structure, interface, phase, electronic structure, size, and multicomponent engineering. Subsequently, the induced enhancement in catalytic performance by these engineering strategies are also elucidated. Furthermore, some representative Ru-based electrocatalysts for the electrocatalytic HER and OER applications are also well presented. Finally, the challenges and prospects are also elaborated for the future synthesis of more effective Ru-based catalysts and boost their future application.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

An inclusive review and perspective on Cu-based materials for electrochemical water splitting

In recent years, there has been a resurgence of interest in developing green and renewable alternate energy sources as a solution to the energy and environmental problems produced by conventional fossil fuel use. As a very effective energy transporter, hydrogen (H₂) is a possible candidate for the future energy supply. Hydrogen production by water splitting is a promising new energy option. Strong, efficient, and abundant catalysts are required for increasing the efficiency of the water splitting process. Cu-based materials as an electrocatalyst have shown promising results for application in the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting. In this review, our aim is to cover the latest developments in the synthesis, characterisation, and electrochemical behaviour of Cu-based materials as a HER, and OER electrocatalyst, highlighting the impact that these advances have had on the field. It is intended that this review article will serve as a roadmap for developing novel, cost-effective electrocatalysts for electrochemical water splitting based on nanostructured materials with particular emphasis on Cu-based materials for electrocatalytic water splitting.

Hollow Heterostructured Nanocatalysts for Boosting Electrocatalytic Water Splitting

The implementation of electrochemical water splitting demands the development and application of electrocatalysts to overcome sluggish reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Hollow nanostructures, particularly for hollow heterostructured nanomaterials can provide multiple solutions to accelerate the HER/OER kinetics owing to their advantageous merit. Herein, the recent advances of hollow heterostructured nanocatalysts and their excellent performance for water splitting are systematically summarized. Starting by illustrating the intrinsically advantageous features of hollow heterostructures, achievements in engineering hollow heterostructured electrocatalysts are also highlighted with the focus on structural design, interfacial engineering, composition regulation, and catalytic evaluation. Finally, some perspective insights and future challenges of hollow heterostructured nanocatalysts for electrocatalytic water splitting are also discussed.

Pd-Based Metallenes for Fuel Cell Reactions

Pd-based metallenes, atomically thin layers composed primarily of under-coordinated Pd atoms, have emerged as the newest members in the family of two-dimensional (2D) nanomaterials. Moreover, the unique physiochemical properties, high intrinsic activity associated with metallenes coupled with the ease of applying chemical modifications result in great potential in catalyst engineering for fuel cell reactions. Especially in recent years, interest in Pd-based metallenes is growing, as evidenced by surge in available literatures. Herein, we have reviewed the recent findings achieved in Pd-based metallenes in fuel cells by highlighting the technologies available for deriving metallenes and manifesting the modification strategies for designing them to better suit the application demand. Moreover, we also discuss the perspective insights of Pd-based metallenes for fuel cells regarding the surfactant-free synthesis method, strain engineering, constructing high-entropy alloy, and so on.

Synergistic Effect in a Metal-Organic Framework Boosting the Electrochemical CO2 Overall Splitting

It is a very important but still challenging task to develop bifunctional electrocatalysts for highly efficient CO₂ overall splitting. Herein, we report a stable metal-organic framework (denoted as PcNi-Co-O), composed of (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)nickel(II) (PcNi-(O^-)₈) ligands and the planar CoO₄ nodes, for CO₂ overall splitting. When working as both cathode and anode catalysts (i.e., PcNi-Co-O||PcNi-Co-O), PcNi-Co-O achieved a commercial-scale current density of 123 mA cm^-2 (much higher than the reported values (0.2-12 mA cm^-2)) with a Faradic efficiency (CO) of 98% at a low cell voltage of 4.4 V. Mechanism studies suggested the synergistic effects between two active sites, namely, (i) electron transfer from CoO₄ to PcNi sites under electric fields, resulting in the raised oxidizability/reducibility of CoO₄/PcNi sites, respectively; (ii) the energy-level matching of cathode and anode catalysts can reduce the energy barrier of electron transfer between them and improve the performance of CO₂ overall splitting.

Coupling Transition Metal Compound with Single-Atom Site for Water Splitting Electrocatalysis

Single-atom site catalysts (SACs) provide an ideal platform to identify the active centers, explore the catalytic mechanism, and establish the structure-property relationships, and thus have attracted increasing interests for electrocatalytic energy conversion. Substantial endeavors have been devoted to the construction of carbon-supported SACs, and their progress have been comprehensively reviewed. Compared with carbon-supported SACs, transition metal compounds (TMCs)-supported SACs are still in their infancy in the field of electrocatalysis. However, they have also aroused ever-increasing attention for driving electrocatalytic water splitting, and emerged as an indispensable class of SACs in recent years, predominately owing to their inherently structural features, such as rich anchoring sites, surface defects, and lattice vacancy. Herein, in this review, we have systematically summarized the recent advances of a variety of TMC supported SACs toward electrocatalytic water splitting. The advanced characterization techniques and theoretical analyses for identifying and monitoring the atomic structure of SACs are firstly manifested. Subsequently, the anchoring and stabilization mechanisms for TMC supported SACs are also highlighted. Thereafter, the advances of TMC supported SACs for driving water electrolysis are systematically unraveled.

A critical review of research progress for metal alloy materials in hydrogen evolution and oxygen evolution reaction

Hydrogen produced by electrolyzing water has attracted extensive attention as an effective way to generate and store new energy by using renewable energy. Electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were the core reactions in the process of hydrogen production by water electrolysis, however, due to the low efficiency of the electrolytic device caused by its slow kinetic reaction and the dependence on noble metal catalysts (platinum and iridium/ruthenium (oxide)), which limited its wide application. The preparation of high-efficiency catalysts with high catalytic activity, stability, low cost and scalability played a vital role in promoting the development of hydrogen production technology from electrolytic water and has become a current research hotspot. Metal alloy catalysts have been widely studied as high-efficiency electrocatalysts. This study introduced and analyzed the mechanism and application of metal alloy catalyst in hydrogen and oxygen evolution reaction, summarized and discussed the progress in the design, preparation and application of metal alloy electrocatalysts. Finally, the strategy and prospect of new high-efficiency electrocatalysts were proposed.

Mo doping and Se vacancy engineering for boosting electrocatalytic water oxidation by regulating the electronic structure of self-supported Co9Se8@NiSe

Oxygen evolution reactions (OERs) are regarded as the rate-determining step of electrocatalytic overall water splitting, which endow OER electrocatalysts with the advantages of high activity, low cost, good conductivity, and excellent stability. Herein, a facile H₂O₂-assisted etching method is proposed for the fabrication of Mo-doped ultrathin Co₉Se₈@NiSe/NF-X heterojunctions with rich Se vacancies to boost electrocatalytic water oxidation. After step-by-step electronic structure modulation by Mo doping and Se vacancy engineering, the self-standing Mo-Co₉Se₈@NiSe/NF-60 heterojunctions deliver a current density of 50 mA cm^-2 with an overpotential of 343 mV and a cell voltage of only 1.87 V at 50 mA cm^-2 for overall water splitting in 1.0 M KOH. Our study opens up the possibility of realizing step-by-step electronic structure modulation of nonprecious OER electrocatalysts via heteroatom doping and vacancy engineering.

Coupling Transition Metal Catalysts with Ir for Enhanced Electrochemical Water Splitting Activity

Developing advanced electrocatalysts is of great significance for boosting electrochemical water splitting to produce hydrogen. The electrocatalytic activity of a catalyst is associated with the surface/interface, geometric structure, and electronic properties. Coupling Ir with transition metal compounds is an effective strategy to improve their electrocatalytic performance. In this review, we summarize the recent progress of Ir coupled transition metal compound catalysts for the application in driving electrochemical water splitting. The significant role of Ir played in the promotion of electrocatalytic performance is firstly illustrated. Then, the applications of Ir-based catalysts in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are comprehensively discussed, with an emphasis on correlating the structure-function relationships. Lastly, the challenges and future directions for the fabrication of more advanced Ir coupled electrocatalysts are also presented.

Cation-Anion Dual Doping Modifying Electronic Structure of Hollow CoP Nanoboxes for Enhanced Water Oxidation Electrocatalysis

Tuning the electronic state of a nanocatalyst is of vital importance for elevating its catalytic performance toward oxygen evolution reaction (OER). Herein, a cation-anion dual doping strategy has been proposed for modifying the electronic structure of CoP via doping Fe and S atoms. Impressively, Fe doping has been demonstrated to be favorable for improving the carrier density of CoP to produce more hydroxyl radicals (^•OH), while S doping can further modify the electronic structure of CoP to improve the charge-transfer characteristics, thereby synergistically decreasing the energy barrier for the transformation of O* to OOH* and promoting the electrocatalytic OER performance. More importantly, the highly open nanobox structure is also beneficial for the exposure of more accessible catalytically active sites, which can substantially facilitate the electron and mass transport, leading to the superb catalytic OER performance. The successful modulation of OER performance via dual-doping strategy will pose a new strategy for designing more advanced nanocatalysts for energy-related catalysis process.

Nanoscale hetero-interfaces for electrocatalytic and photocatalytic water splitting

As green and sustainable methods to produce hydrogen energy, photocatalytic and electrochemical water splitting have been widely studied. In order to find efficient photocatalysts and electrocatalysts, materials with various composition, size, and surface/interface are investigated. In recent years, constructing suitable nanoscale hetero-interfaces can not only overcome the disadvantages of the single-phase material, but also possibly provide new functionalities. In this review, we systematically introduce the fundamental understanding and experimental progress in nanoscale hetero-interface engineering to design and fabricate photocatalytic and electrocatalytic materials for water splitting. The basic principles of photo-/electro-catalytic water splitting and the fundamentals of nanoscale hetero-interfaces are briefly introduced. The intrinsic behaviors of nanoscale hetero-interfaces on electrocatalysts and photocatalysts are summarized, which are the electronic structure modulation, space charge separation, charge/electron/mass transfer, support effect, defect effect, and synergistic effect. By highlighting the main characteristics of hetero-interfaces, the main roles of hetero-interfaces for electrocatalytic and photocatalytic water splitting are discussed, including excellent electronic structure, efficient charge separation, lower reaction energy barriers, faster charge/electron/mass transfer, more active sites, higher conductivity, and higher stability on hetero-interfaces. Following above analysis, the developments of electrocatalysts and photocatalysts with hetero-structures are systematically reviewed.

Stainless Steel-Supported Amorphous Nickel Phosphide/Nickel as an Electrocatalyst for Hydrogen Evolution Reaction

Recently, nickel phosphides (Ni-P) in an amorphous state have emerged as potential catalysts with high intrinsic activity and excellent electrochemical stability for hydrogen evolution reactions (HER). However, it still lacks a good strategy to prepare amorphous Ni-P with rich surface defects or structural boundaries, and it is also hard to construct a porous Ni-P layer with favorable electron transport and gas-liquid transport. Herein, an integrated porous electrode consisting of amorphous Ni-P and a Ni interlayer was successfully constructed on a 316L stainless steel felt (denoted as Ni-P/Ni-316L). The results demonstrated that the pH of the plating solution significantly affected the grain size, pore size and distribution, and roughness of the cell-like particle surface of the amorphous Ni-P layer. The Ni-P/Ni-316L prepared at pH = 3 presented the richest surface defects or structural boundaries as well as porous structure. As expected, the as-developed Ni-P/Ni-316L demonstrated the best kinetics, with η10 of 73 mV and a Tafel slope of ca. 52 mV dec-1 for the HER among all the electrocatalysts prepared at various pH values. Furthermore, the Ni-P/Ni-316L exhibited comparable electrocatalytic HER performance and better durability than the commercial Pt (20 wt%)/C in a real water electrolysis cell, indicating that the non-precious metal-based Ni-P/Ni-316L is promising for large-scale processing and practical use.

Defect and Interface Engineering of Three-Dimensional Open Nanonetcage Electrocatalysts for Advanced Electrocatalytic Oxygen Evolution Reaction

Defect engineering and interface engineering are two efficient approaches to promote the electrocatalytic performance of transition metal oxides (TMOs) by modulating the local electronic structure and inducing a synergistic effect but usually require costly and complicated processes. Herein, a facile electrochemical etching method is proposed for the controllable tailoring of the defects in a three-dimensional (3D) open nanonetcage CoZnRuO_x heterostructure via the in situ electrochemical etching to remove partial ZnO. The highly open 3D nanostructures, numerous defects, and multicomponent heterointerfaces endow the CoZnRuO_x nanonetcages with more accessible active sites, moderated local electronic structure, and strong synergistic effect, thereby enabling them to not only deliver an ultralow overpotential (244 mV @ 10 mA cm^-2) for oxygen evolution reaction (OER) but also high-performance overall water electrolysis by coupling with commercial Pt/C, with a potential of 1.52 V at 10 mA cm^-2. Moreover, experiments and characterizations also reveal that the remaining Zn²⁺ can facilitate OH^- adsorption and charge transfer, which also further improves the electrocatalytic OER performance. This work proposes a promising strategy for creating surface defects in heterostructured TMOs and provides insights to understand the defect- and interface-induced enhancement of OER electrocatalysis.

Synthesis, Physical Properties and Electrocatalytic Performance of Nickel Phosphides for Hydrogen Evolution Reaction of Water Electrolysis

Nickel phosphides have been investigated as an alternative to noble metals and have emerged as potential catalysts that can efficiently catalyze the hydrogen evolution reaction (HER). However, the impacts of facet morphology and crystal structure of the nickel phosphides on their catalytic reactivity have not been systematically investigated. Herein, nickel phosphides with different crystalline states were prepared through a facile calcination treatment. It was found that the calcination treatment had important effects on the phase compositions, morphologies, and crystallinity of nickel phosphides, which are closely related to their HER activity. Generally, the crystallized Ni-P catalysts exhibited faster kinetics than the amorphous Ni-P. In particular, the Ni-P 300 showed remarkable HER performance with η10 of ca. 65 mV, along with a very low Tafel slope of ca. 44 mV dec^-1 due to the increased catalytically active sites. Furthermore, the Ni-P 300 exhibited negligible decay during the 140 h galvanostatic electrolysis, showing better catalytic stability than the commercial Pt/C catalyst. Compared with the amorphous Ni-P, the boosted HER activity of the Ni-P 300 could benefit from the mixed nanocrystalline Ni₂P and Ni₃P, which could contribute to the H_ads adsorption/desorption abilities and helped provide more activity sites, promoting the HER performance.

Hierarchical Hollow CoWO4-Co(OH)2 Heterostructured Nanoboxes Enabling Efficient Water Oxidation Electrocatalysis

Rational design and construction of well-defined hollow heterostructured nanomaterials assembled by ultrathin nanosheets overtakes crucial role in developing high-efficiency oxygen evolution reaction (OER) electrocatalysts. Herein, a reliable metal-organic framework-mediated and cation-exchange strategy to tune the geometric structure and multicomponent heterostructures has been proposed for the fabrication of hollow CoWO₄-Co(OH)₂ hierarchical nanoboxes assembled by rich ultrathin nanosheets. Benefiting from the hierarchical hollow nanostructure, the CoWO₄-Co(OH)₂ nanoboxes offer plenty of metal active centers available for reaction intermediates. Moreover, the well-defined nanointerfaces between CoWO₄ and Co(OH)₂ can function as the bridge for boosting the efficient electron transfer from CoWO₄ to Co(OH)₂. As a consequence, the optimized CoWO₄-Co(OH)₂ nanoboxes can exhibit outstanding electrocatalytic performance toward OER by delivering 10 mA cm^-2 with a low overpotential of 280 mV and a small Tafel slope of 70.6 mV dec^-1 as well as outstanding electrochemical stability. More importantly, this CoWO₄-Co(OH)₂ heterostructured nanocatalyst can couple with Pt/C to drive overall water splitting to achieve 10 mA cm^-2 with a voltage of 1.57 V.