Flexible Bifunctional Electrode for Alkaline Water Splitting with Long-Term Stability

Progress in electrochemical water-splitting devices as future renewable and clean energy systems requires the development of electrodes composed of efficient and earth-abundant bifunctional electrocatalysts. This study reveals a novel flexible and bifunctional electrode (NiO@CNTR) by hybridizing macroscopically assembled carbon nanotube ribbons (CNTRs) and atmospheric plasma-synthesized NiO quantum dots (QDs) with varied loadings to demonstrate bifunctional electrocatalytic activity for stable and efficient overall water-splitting (OWS) applications. Comparative studies on the effect of different electrolytes, e.g., acid and alkaline, reveal a strong preference for alkaline electrolytes for the developed NiO@CNTR electrode, suggesting its bifunctionality for both HER and OER activities. Our proposed NiO@CNTR electrode demonstrates significantly enhanced overall catalytic performance in a two-electrode alkaline electrolyzer cell configuration by assembling the same electrode materials as both the anode and the cathode, with a remarkable long-standing stability retaining ∼100% of the initial current after a 100 h long OWS run, which is attributed to the "synergistic coupling" between NiO QD catalysts and the CNTR matrix. Interestingly, the developed electrode exhibits a cell potential (E10) of only 1.81 V with significantly low NiO QD loading (83 μg/cm2) compared to other catalyst loading values reported in the literature. This study demonstrates a potential class of carbon-based electrodes with single-metal-based bifunctional catalysts that opens up a cost-effective and large-scale pathway for further development of catalysts and their loading engineering suitable for alkaline-based OWS applications and green hydrogen generation.

Enhancing Hydrogen Evolution Reaction Activity of Palladium Catalyst by Immobilization on MXene Nanosheets

Efficient catalysts with minimal content of catalytically active noble metals are essential for the transition to the clean hydrogen economy. Catalyst supports that can immobilize and stabilize catalytic nanoparticles and facilitate the supply of electrons and reactants to the catalysts are needed. Being hydrophilic and more conductive compared with carbons, MXenes have shown promise as catalyst supports. However, the controlled assembly of their 2D sheets creates a challenge. This study established a lattice engineering approach to regulate the assembly of exfoliated Ti3C2Tx MXene nanosheets with guest cations of various sizes. The enlargement of guest cations led to a decreased interlayer interaction of MXene lamellae and increased surface accessibility, allowing intercalation of Pd nanoparticles. Stabilization of Pd nanoparticles between interlayer-expanded MXene nanosheets improved their electrocatalytic activity. The Pd-immobilized K+-intercalated MXene nanosheets (PdKMX) demonstrated exceptional electrocatalytic performance for the hydrogen evolution reaction with the lowest overpotential of 72 mV (@10 mA cm-2) and the highest turnover frequency of 1.122 s-1 (@ an overpotential of 100 mV), which were superior to those of the state-of-the-art Pd nanoparticle-based electrocatalysts. Weakening of the interlayer interaction during self-assembly with K+ ions led to fewer layers in lamellae and expansion of the MXene in the c direction during Pd anchoring, providing numerous surface-active sites and promoting mass transport. In situ spectroscopic analysis suggests that the effective interfacial electron injection from the Pd nanoparticles strongly immobilized on interlayer-expanded PdKMX may be responsible for the improved electrocatalytic performance.

Noble Metal Phosphides Supported on CoNi Metaphosphate for Efficient Overall Water Splitting

Transition metal metaphosphates and noble metal phosphides prepared under similar conditions are potential hybrid catalysts for electrocatalytic water splitting, which is of great significance for H2 production. Herein, the structure and electrocatalytic activity of different noble metal species (i.e., Rh, Pd, Ir) on CoNiP4O12 nanoarrays have been systematically studied. Due to the different formation energies of noble metal phosphides, the phosphides of Rh (RhPx) and Pd (PdPx) as well as the noble metal Ir are obtained under the same phosphorylation conditions perspectively. RhPx/CoNiP4O12 and PdPx/CoNiP4O12 exhibit much better HER activity than Ir/CoNiP4O12 due to the advantages of phosphides. Density functional theory (DFT) calculations reveal that the extraordinary activity of RhPx/CoNiP4O12 originated from the strong affinity to H2O and optimal adsorption for H*. The best RhPx/CoNiP4O12 only requires a low overpotential of 30 and 234 mV to deliver 10 mA cm-2 for HER and OER, respectively, and therefore is effective for overall water splitting (requiring 1.57 V to achieve a current density of 10 mA cm-2). This work not only develops a novel RhPx/CoNiP4O12 electrocatalyst for overall water splitting but also provides deep insight into the formation mechanism of noble metal phosphides.

Electrografting of Phenyl Phosphate Layers onto Glassy Carbon for Tuning Catalytic Activity toward the Hydrogen Evolution Reaction

In this study, we explored the surface modification of a glassy carbon electrode through the electrografting of 4-Aminophenyl phosphate, which features heteroatoms and ionic properties. The electrochemical grafting process involves reducing in situ-generated diazonium derivatives. The primary objective of this research was to immobilize organic layers and assess their electrochemical and surface properties. Subsequently, the generated surface serves as a template for the electrochemical growth of Pd and Co nanoparticles on functionalized electrodes. The electrocatalytic performances of these hybrid electrodes in driving the hydrogen evolution reaction were investigated. The obtained results indicate an enhancement in the electrocatalytic activity of the modified electrodes, where lower overpotential and higher stability were observed when the catalyst was electrodeposited onto the attached ionic layer. These findings highlight the synergistic effect between the attached phenyl phosphate moieties and electrocatalysts.

Electronic Structure Regulated Nickel-Cobalt Bimetal Phosphide Nanoneedles for Efficient Overall Water Splitting

Transition metal phosphides (TMPs) have been widely studied for water decomposition for their monocatalytic property for anodic or cathodic reactions. However, their bifunctional catalytic activity still remains a major challenge. Herein, hexagonal nickel-cobalt bimetallic phosphide nanoneedles with 1-3 μm length and 15-30 nm diameter supported on NF (NixCo2-xP NDs/NF) with adjusted electron structure have been successfully prepared. The overall alkaline water electrolyzer composed of the optimal anode (Ni0.67Co1.33P NDs/NF) and cathode (Ni1.01Co0.99P NDs/NF) provide 100 mA cm-2 at 1.62 V. Gibbs Free Energy for reaction paths proves that the active site in the hydrogen evolution reaction (HER) is Ni and the oxygen evolution reaction (OER) is Co in NixCo2-xP, respectively. In the HER process, Co-doping can result in an apparent accumulation of charge around Ni active sites in favor of promoting HER activity of Ni sites, and ΔGH* of 0.19 eV is achieved. In the OER process, the abundant electron transfer around Co-active sites results in the excellent ability to adsorb and desorb *O and *OOH intermediates and an effectively reduced ∆GRDS of 0.37 eV. This research explains the regulation of electronic structure change on the active sites of bimetallic materials and provides an effective way to design a stable and effective electrocatalytic decomposition of alkaline water.

Noble Metal Phosphides: Robust Electrocatalysts toward Hydrogen Evolution Reaction

Facing with serious carbon emission issues, the production of green H2 from electrocatalytic hydrogen evolution reaction (HER) has received extensive research interest. Almost all kinds of noble metal phosphides (NMPs) consisting of Pt-group elements (i.e., Ru, Rh, Pd, Os, Ir and Pt) are all highly active and pH-universal electrocatalysts toward HER. In this review, the recent progress of NMP-based HER electrocatalysts is summarized. It is further take typical examples for discussing important impact factors on the HER performance of NMPs, including crystalline phase, morphology, noble metal element and doping. Moreover, the synthesis and HER application of hybrid catalysts consisting of NMPs and other materials such as transition metal phosphides, oxides, sulfides and phosphates, carbon materials and noble metals is also reviewed. Reducing the use of noble metal is the key idea for NMP-based hybrid electrocatalysts, while the expanded functionality and structure-performance relationship are also noticed in this part. At last, the potential opportunities and challenges for this kind of highly active catalyst is discussed.

Ultra-Stable Ti Vacancies-Pt Atomic Clusters Structure on Titanium Oxycarbide Supports for High Current Density Hydrogen Evolution Reaction

Electrocatalysts with low Pt loading mass to achieve high current density (≥1 A cm-2 ) for hydrogen evolution reaction (HER) are still extremely challenging due to the limited intrinsic activity and weak stability of catalytic sites. The modulation of the electronic microenvironment of the support-Pt structure is crucial to enhance the intrinsic activity and stability of catalytic sites. Herein, an innovative titanium oxycarbide (TiV CO) solid solution with Ti vacancies (TiV ) is proposed as support to anchor sub-nanoscale Pt atomic clusters (Pt ACs) and a stable "TiV -Pt ACs" structure is carefully designed. The electronic microenvironment of "TiV -Pt ACs" is indirectly optimized by an unsaturated C/O site near TiV . Thanks to this, novel "TiV -Pt ACs" structure (Pt@TiV CO) with low Pt loading mass (2.44 wt.%) exhibits excellent HER activity in acidic solution and the mass activity is more than ten times that of commercial 20% Pt/C at the overpotentials of 50 and 100 mV. Particularly, Pt@TiV CO shows amazing stability at high and fluctuating current density of 1-2 A cm-2 for 120 h. This work provides a novel and promising method to develop stable and low-loading Pt-based catalysts adapting to high current density.

Mo2 C-Based Ceramic Electrode with High Stability and Catalytic Activity for Hydrogen Evolution Reaction at High Current Density

Developing robust electrodes with high catalytic performance is a key step for expanding practical HER (hydrogen evolution reaction) applications. This paper reports on novel porous Mo2 C-based ceramics with oriented finger-like holes directly used as self-supported HER electrodes. Due to the suitable MoO3 sintering additive, high-strength (55 ± 6 MPa) ceramic substrates and a highly active catalytic layer are produced in one step. The in situ reaction between MoO3 and Mo2 C enabled the introduction of O in the Mo2 C crystal lattice and the formation of Mo2 C(O)/MoO2 heterostructures. The optimal Mo2 C-based electrode displayed an overpotential of 333 and 212 mV at 70 °C under a high current intensity of 1500 mA cm-2 in 0.5 m H2 SO4 and 1.0 m KOH, respectively, which are markedly better than the performance of Pt wire electrode; furthermore, its price is three orders of magnitude lower than Pt. The chronopotentiometric curves recorded in the 50 - 1500 mA cm-2 range, confirmed its excellent long-term stability in acidic and alkaline media for more than 260 h. Density functional theory (DFT) calculations showed that the Mo2 C(O)/MoO2 heterostructures has an optimum electronic structure with appropriate *H adsorption-free energy in an acidic medium and minimum water dissociation energy barrier in an alkaline medium.

Synthesis of Silver Nanoparticles from Silver Closo-Dodecaborate Film for Enhanced Hydrogen Evolution

The icosahedral closo dodecaborate cluster [B12 H12 ]2- is gaining increasing interest due to its unique properties including the ease of functionalization, 3D aromaticity, and formation of metal salts with high ion conductivity. In this work, simple and effective preparation of silver closo dodecaborte (Ag2 B12 H12 ) films is reported by an electrochemical route. The size of the Ag2 B12 H12 particles in the films can be tuned from nanometers to micrometers by varying the electrochemical parameters. Ag nanoclusters with controllable sizes are successfully generated via electrochemical reduction reactions or thermal anneal of the Ag2 B12 H12 films. When tested for hydrogen evolution reaction (HER) in an acidic solution, the as-prepared Ag nanoparticles deliver a current density of 10 mA cm-2 at 376 mV overpotential. This research sheds light on a new synthesis of [B12 H12 ]2- based thin films, the generation of metal nano-powders, and their application in HER or other applications.

Multiple-Strategy Design of MOF-Derived N, P Co-Doped MoS2 Electrocatalysts Toward Efficient Alkaline Hydrogen Evolution and Overall Water Splitting

The multiple strategy design is crucial for enhancing the efficiency of nonprecious electrocatalysts in hydrogen evolution reaction (HER). In this work, we successfully synthesized N, P-codoped MoS2 nanosheets as highly efficient catalysts by integrating doping effects and phase engineering using a porous metal-organic framework (MOF) template. The electrocatalysts exhibit excellent bifunctional activity and stability in alkaline media. The N, P codoping induces electron redistribution to enhance conductivity and promote the intrinsic activity of the electrocatalysts. It optimizes the H* adsorption free energy and the dissociative adsorption energy, resulting in significant enhancement of HER activity. Moreover, the porous MOF structure exposes a large number of electrochemically active sites and facilitates the diffusion of ions and gases, which improve charge transfer efficiency and structural stability. Specifically, at a current density of 10 mA cm-2, the overpotential of the HER is only 32 mV, with a Tafel slope of 47 mV dec-1 and Faradaic efficiency as high as 98.51% (at 100 mA cm-2). Only a 338 mV overpotential is required to achieve a current density of 50 mA cm-2 for oxygen evolution reaction (OER), and a potential of 1.49 V (at 10 mA cm-2) is sufficient to drive overall water splitting. Further experimental measurements and first-principles calculations evidence that the exceptional performance is primarily attributed to the dual functionality of N and P dopants, which not only activate additional S sites but also initialize the phase transition of 2H to 1T-MoS2 to facilitate the rapid charge transfer. Through in-depth exploration of the combined design of multiple strategies for efficient catalysts, our work paves a new way for the development of future efficient nonprecious metal catalysts.

Construction of Thiadiazole-Linked Covalent Organic Frameworks via Facile Linkage Conversion with Superior Photocatalytic Properties

The establishment of facile synthetic routes to engineer covalent organic frameworks (COFs) with fully conjugated structure and excellent stability is highly desired for practical applications in optoelectronics and photocatalysis. Herein, a novel linkage conversion strategy is reported to prepare crystalline thiadiazole-linked COFs via thionation, cyclization, and oxidation of N-acylhydrazole bonds with Lawesson's reagent (LR). The as-prepared thiadiazole-linked COFs not only remain porosity and crystallinity, but enhance its chemical stability. Furthermore, thiadiazole-linked COFs are more favorable to lower exciton binding energy and promote π-electron delocalization over the whole reticular framework than N-acylhydrazone-linked COFs. Notably, the extended π-conjugation structure and decent crystallinity of the resulting TDA-COF are reflected by its higher photocatalytic H2 evolution rate (61.3 mmol g-1 in 5 h) in comparison with that (7.5 mmol g-1 ) of NAH-COF.

Bimetallic Nanoalloy Catalysts for Green Energy Production: Advances in Synthesis Routes and Characterization Techniques

Bimetallic Nanoalloy catalysts have diverse uses in clean energy, sensing, catalysis, biomedicine, and energy storage, with some supported and unsupported catalysts. Conventional synthetic methods for producing bimetallic alloy nanoparticles often produce unalloyed and bulky particles that do not exhibit desired characteristics. Alloys, when prepared with advanced nanoscale methods, give higher surface area, activity, and selectivity than individual metals due to changes in their electronic properties and reduced size. This review demonstrates the synthesis methods and principles to produce and characterize highly dispersed, well-alloyed bimetallic nanoalloy particles in relatively simple, effective, and generalized approaches and the overall existence of conventional synthetic methods with modifications to prepare bimetallic alloy catalysts. The basic concepts and mechanistic understanding are represented with purposely selected examples. Herein, the enthralling properties with widespread applications of nanoalloy catalysts in heterogeneous catalysis are also presented, especially for Hydrogen Evolution Reaction (HER), Oxidation Reduction Reaction (ORR), Oxygen Evolution Reaction (OER), and alcohol oxidation with a particular focus on Pt and Pd-based bimetallic nanoalloys and their numerous fields of applications. The high entropy alloy is described as a complicated subject with an emphasis on laser-based green synthesis of nanoparticles and, in conclusion, the forecasts and contemporary challenges for the controlled synthesis of nanoalloys are addressed.

Active Site Engineering and Theoretical Aspects of "Superhydrophilic" Nanostructure Array Enabling Efficient Overall Water Electrolysis

The rational design of noble metal-free electrocatalysts holds great promise for cost-effective green hydrogen generation through water electrolysis. In this context, here, the development of a superhydrophilic bifunctional electrocatalyst that facilitates both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline conditions is demonstrated. This is achieved through the in situ growth of hierarchical NiMoO4 @CoMoO4 ·xH2 O nanostructure on nickel foam (NF) via a two-step hydrothermal synthesis method. NiMoO4 @CoMoO4 ·xH2 O/NF facilitates OER and HER at the overpotentials of 180 and 220 mV, respectively, at the current density of 10 mA cm-2 . The NiMoO4 @CoMoO4 ·xH2 O/NF ǁ NiMoO4 @CoMoO4 ·xH2 O/NF cell can be operated at a potential of 1.60 V compared to 1.63 V displayed by the system based on the Pt/C@NFǁRuO2 @NF standard electrode pair configuration at 10 mA cm-2 for overall water splitting. The density functional theory calculations for the OER process elucidate that the lowest ΔG of NiMoO4 @CoMoO4 compared to both Ni and NiMoO4 is due to the presence of Co in the OER catalytic site and its synergistic interaction with NiMoO4 . The preparative strategy and mechanistic understanding make the windows open for the large-scale production of the robust and less expensive electrode material for the overall water electrolysis.

Self-Enhancement of Water Electrolysis by Electrolyte-Poled Ferroelectric Catalyst

Efficient and durable electrocatalysts with superior activity are needed for the production of green hydrogen with a high yield and low energy consumption. Electrocatalysts based on transition metal oxides hold dominance due to their abundant natural resources, regulable physical properties, and good adaptation to a solution. In numerous oxide catalyst materials, ferroelectrics, possessing semiconducting characteristics and switchable spontaneous polarization, have been considered promising photoelectrodes for solar water splitting. However, few investigations noted their potential as electrocatalysts. In this study, we report an efficient electrocatalytic electrode made of a BiFeO3/nickel foam heterostructure, which displays a smaller overpotential and higher current density than the blank nickel foam electrode. Moreover, when in contact with an alkaline solution, the bond between hydroxyls and the BiFeO3 surface induces a large area of upward self-polarization, lowering the adsorption energy of subsequent adsorbates and facilitating oxygen and hydrogen evolution reaction. Our work demonstrates an infrequent pathway of using functional semiconducting materials for exploiting highly efficient electrocatalytic electrodes.

Preparation of NH4Cl-Modified Carbon Materials via High-Temperature Calcination and Their Application in the Negative Electrode of Lead-Carbon Batteries

The performance of lead-acid batteries could be significantly increased by incorporating carbon materials into the negative electrodes. In this study, a modified carbon material developed via a simple high-temperature calcination method was employed as a negative electrode additive, and we have named it as follows: N-doped chitosan-derived carbon (NCC). The performance of this material was compared with a control battery containing activated carbon (AC). X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy were engaged in analyzing the crystal structure and morphology of the material. Afterwards, the electrochemical and battery performance was examined through cyclic voltammetry (CV), linear voltammetry (LSV) and constant current charge-discharge testing. Markedly, the electrode plate containing 1 wt.% NCC indicates the highest specific capacity (106.48 F g^-1) as compared to the control battery, which is 1.56 times higher than the AC electrode plate and 4.75 times higher than the blank electrode plate. The linear voltammetry shows that the hydrogen precipitation current density of the 1 wt.% NCC electrode plate is only -0.028 A cm^-2, a much higher value than that of the AC electrode plate. In addition, the simulated battery containing 1 wt.% NCC has a cycle life of 4324 cycles, which is 2.36 times longer than that of the same amount of additive AC battery (1834 cycles) and 5.34 times longer than that of the blank battery (809 cycles). In summary, NCC carbon has the advantage of extending the life of lead-acid batteries, rendering it a promising candidate for lead-acid battery additives.

Nature-Inspired Design of Nano-Architecture-Aligned Ni5P4-Ni2P/NiS Arrays for Enhanced Electrocatalytic Activity of Hydrogen Evolution Reaction (HER)

The projection of developing sustainable and cost-efficient electrocatalysts for hydrogen production is booming. However, the full potential of electrocatalysts fabricated from earth-abundant metals has yet to be exploited to replace Pt-group metals due to inadequate efficiency and insufficient design strategies to meet the ever-increasing demands for renewable energies. To improve the electrocatalytic performance, the primary challenge is to optimize the structure and electronic properties by enhancing the intrinsic catalytic activity and expanding the active catalytic surface area. Herein, we report synthesizing a 3D nanoarchitecture of aligned Ni₅P₄-Ni₂P/NiS (plate/nanosheets) using a phospho-sulfidation process. The durability and unique design of prickly pear cactus in desert environments by adsorbing moisture through its extensive surface and ability to bear fruits at the edges of leaves inspire this study to adopt a similar 3D architecture and utilize it to design an efficient heterostructure catalyst for HER activity. The catalyst comprises two compartments of the vertically aligned Ni₅P₄-Ni₂P plates and the NiS nanosheets, resembling the role of leaves and fruits in the prickly pear cactus. The Ni₅P₄-Ni₂P plates deliver charges to the interface areas, and the NiS nanosheets significantly influence H_ad and transfer electrons for the HER activity. Indeed, the synergistic presence of heterointerfaces and the epitaxial NiS nanosheets can substantially improve the catalytic activity compared to nickel phosphide catalysts. Notably, the onset overpotential of the best-modified ternary catalysts exhibits (35 mV) half the potential required for nickel phosphide catalysts. This promising catalyst demonstrates 70 and 115 mV overpotentials to attain current densities of 10 and 100 mA cm^-2, respectively. The obtained Tafel slope is 50 mV dec^-1, and the measured double-layer capacitance from cyclic voltammetry (CV) for the best ternary electrocatalyst is 13.12 mF cm^-2, 3 times more than the nickel phosphide electrocatalyst. Further, electrochemical impedance spectroscopy (EIS) at the cathodic potentials reveals that the lowest charge transfer resistance is linked to the best ternary electrocatalyst, ranging from 430 to 1.75 Ω cm^-2. This improvement can be attributed to the acceleration of the electron exchangeability at the interfaces. Our findings demonstrate that the epitaxial NiS nanosheets expand the active catalytic surface area and simultaneously elevate the intrinsic catalytic activity by introducing heterointerfaces, which leads to accommodating more H_ad at the interfaces.

Theoretical Investigation of HER and OER Electrocatalysts Based on the 2D R-graphyne Completely Composed of Anti-Aromatic Carbon Rings

Based on the DFT calculations, two-dimensional (2D) R-graphyne has been demonstrated to have high stability and good conductivity, which can be conducive to the relevant electrocatalytic activity of the material. Different from the poor graphene, R-graphyne, which is completely composed of anti-aromatic structural units, can exhibit certain HER catalytic activity. In addition, doping the TM atoms in Group VIIIB can be considered an effective strategy to enhance the HER catalytic activity of R-graphyne. Particularly, Fe@R-graphyne, Os@R-graphyne, Rh@R-graphyne and Ir@R-graphyne can exhibit higher HER catalytic activities due to the formation of more active sites. Usually, the shorter the distance between the TM and C atoms is, the better the HER activity of the C-site is. Furthermore, doping Ni and Rh atoms of Group VIIIB can significantly improve the OER catalytic performance of R-graphyne. It can be found that ΔG_O* can be used as a good descriptor for the OER activities of TM@R-graphyne systems. Both Rh@R-graphyne and Ni@R-graphyne systems can exhibit bifunctional electrocatalytic activities for HER/OER. In addition, all the relevant catalytic mechanisms are analyzed in detail. This work not only provides nonprecious and highly efficient HER/OER electrocatalysts, but also provides new ideas for the design of carbon-based electrocatalysts.

Design of XS2 (X = W or Mo)-Decorated VS2 Hybrid Nano-Architectures with Abundant Active Edge Sites for High-Rate Asymmetric Supercapacitors and Hydrogen Evolution Reactions

Two-dimensional layered transition metal dichalcogenides have emerged as promising materials for supercapacitors and hydrogen evolution reaction (HER) applications. Herein, the molybdenum sulfide (MoS₂ )@vanadium sulfide (VS₂ ) and tungsten sulfide (WS₂ )@VS₂  hybrid nano-architectures prepared via a facile one-step hydrothermal approach is reported. Hierarchical hybrids lead to rich exposed active edge sites, tuned porous nanopetals-decorated morphologies, and high intrinsic activity owing to the strong interfacial interaction between the two materials. Fabricated supercapacitors using MoS₂ @VS₂  and WS₂ @VS₂  electrodes exhibit high specific capacitances of 513 and 615 F g^{- 1} , respectively, at an applied current of 2.5 A g^{- 1}  by the three-electrode configuration. The asymmetric device fabricated using WS₂ @VS₂  electrode exhibits a high specific capacitance of 222 F g^{- 1}  at an applied current of 2.5 A g^{- 1}  with the specific energy of 52 Wh kg^{- 1}  at a specific power of 1 kW kg^{- 1} . For HER, the WS₂ @VS₂  catalyst shows noble characteristics with an overpotential of 56 mV to yield 10 mA cm^{- 2} , a Tafel slope of 39 mV dec^-1 , and an exchange current density of 1.73 mA cm^{- 2} . In addition, density functional theory calculations are used to evaluate the durable heterostructure formation and adsorption of hydrogen atom on the various accessible sites of MoS₂ @VS₂  and WS₂ @VS₂  heterostructures.

Recent Advances in Transition Metal Tellurides (TMTs) and Phosphides (TMPs) for Hydrogen Evolution Electrocatalysis

The hydrogen evolution reaction (HER) is a developing and promising technology to deliver clean energy using renewable sources. Presently, electrocatalytic water (H₂O) splitting is one of the low-cost, affordable, and reliable industrial-scale effective hydrogen (H₂) production methods. Nevertheless, the most active platinum (Pt) metal-based catalysts for the HER are subject to high cost and substandard stability. Therefore, a highly efficient, low-cost, and stable HER electrocatalyst is urgently desired to substitute Pt-based catalysts. Due to their low cost, outstanding stability, low overpotential, strong electronic interactions, excellent conductivity, more active sites, and abundance, transition metal tellurides (TMTs) and transition metal phosphides (TMPs) have emerged as promising electrocatalysts. This brief review focuses on the progress made over the past decade in the use of TMTs and TMPs for efficient green hydrogen production. Combining experimental and theoretical results, a detailed summary of their development is described. This review article aspires to provide the state-of-the-art guidelines and strategies for the design and development of new highly performing electrocatalysts for the upcoming energy conversion and storage electrochemical technologies.

Exploring the Magnetic and Electrocatalytic Properties of Amorphous MnB Nanoflakes

Two-dimensional (2D) metal borides are a class of ceramic materials with diverse structural and topological properties. These diverse material properties of metal borides are what forms the basis of their interdisciplinarity and their applicability in various research fields. In this study, we highlight which fundamental and practical parameters need to be taken into consideration when designing nanomaterials for specific applications. A simple one-pot chemical reduction method was applied for the synthesis of manganese mono-boride nanoflakes at room temperature. How the specific surface area and boron-content of the as-synthesized manganese mono-boride nanoflakes influence their magnetic and electrocatalytic properties is reported. The sample with the highest specific surface area and boron content demonstrated the best magnetic and electrocatalytic properties in the HER. Whereas the sample with the lowest specific surface area and boron content exhibited the best electric conductivity and electrocatalytic properties in the OER.

Observation of H2 Evolution and Electrolyte Diffusion on MoS2 Monolayer by In Situ Liquid-Phase Transmission Electron Microscopy

Unit-cell-thick MoS₂ is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its tunable catalytic activity, which is determined based on the energetics and molecular interactions of different types of HER active sites. Kinetic responses of MoS₂ active sites, including the reaction onset, diffusion of the electrolyte and H₂ bubbles, and continuation of these processes, are important factors affecting the catalytic activity of MoS₂ . Investigating these factors requires a direct real-time analysis of the HER occurring on spatially independent active sites. Herein, the H₂ evolution and electrolyte diffusion on the surface of MoS₂ are observed in real time by in situ electrochemical liquid-phase transmission electron microscopy (LPTEM). Time-dependent LPTEM observations reveal that different types of active sites are sequentially activated under the same conditions. Furthermore, the electrolyte flow to these sites is influenced by the reduction potential and site geometry, which affects the bubble detachment and overall HER activity of MoS₂ .

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

A Ni(II) Metal-Organic Framework with Mixed Carboxylate and Bipyridine Ligands for Ultrafast and Selective Sensing of Explosives and Photoelectrochemical Hydrogen Evolution

We report a Ni-MOF (nickel metal-organic framework), Ni-SIP-BPY, synthesized by using two linkers 5-sulfoisophthalic acid (SIP) and 4,4'-bipyridine (BPY) simultaneously. It displays an orthorhombic crystal system with the Ama2 space group: a = 31.425 Å, b = 19.524 Å, c = 11.2074 Å, α = 90°, β = 90°, γ = 90°, and two different types of nickel(II) centers. Interestingly, Ni-SIP-BPY exhibits excellent sensitivity (limit of detection, 87 ppb) and selectivity toward the 2,4,6-trinitrophenol (TNP)-like mutagenic environmental toxin in the pool of its other congeners via "turn-off" fluorescence response by the synergism of resonance energy transfer, photoinduced electron transfer, intermolecular charge transfer, π-π interactions, and competitive absorption processes. Experimental studies along with corroborated theoretical experimentation, vide density functional theory studies, shed light on determining the plausible mechanistic pathway in selective TNP detection, which is highly beneficial in the context of homeland security perspective. Along with the sensing of nitroaromatic explosives, the moderately low band gap and the p-type semiconducting behavior of Ni-SIP-BPY make it suitable as a photoanode material for visible-light-driven water splitting. Highly active surface functionalities and sufficient conduction band minima effectively reduce the water and result in a seven times higher photocurrent density under visible-light illumination.

Ultrafast Carbothermal Shock Constructing Ni3Fe1-xCrx Intermetallic Integrated Electrodes for Efficient and Durable Overall Water Splitting

The development of the electrocatalyst-integrated electrodes with HER/OER bifunctional activity is desirable to reduce the cost and simplify the system of the practical water electrolyzers. Herein, we construct a new type of Ni₃Fe_1-xCr_x (0 ≤ x < 0.3) intermetallic integrated electrodes for overall water splitting via an ultrafast carbothermal shock method. The obtained Ni₃Fe_0.9Cr_0.1/CACC electrode exhibits the optimum performance among all developed electrocatalyst electrodes in this work, and the overpotential is merely 239 mV for OER and 128 mV for HER at 10 mA cm^-2. In addition, the Ni₃Fe_0.9Cr_0.1/CACC electrode shows excellent durability during both OER and HER stability tests at a high current density of 100 mA cm^-2. An electrolyzer, which was assembled with Ni₃Fe_0.9Cr_0.1/CACC electrodes as both the anode and cathode, operates with a low cell voltage of 1.59 V at 10 mA cm^-2. It has been found that the impressive OER activity of Ni₃Fe_0.9Cr_0.1 nanoparticles (NPs) can be ascribed to the stimulative formation of the OER-active Ni³⁺/Fe³⁺ species by the substituted Cr, while the enhanced HER activity is caused by the Cr substitution, which decreases the water dissociation energy barrier. This work provides an ultrafast and facile strategy to develop electrocatalyst-integrated electrodes with low cost and impressive HER/OER bifunctional performance for overall water splitting.

Ir Single Atom Catalyst Loaded on Amorphous Carbon Materials with High HER Activity

The research of high efficiency water splitting catalyst is important for the development of renewable energy economy. Here, the progress in the preparation of high efficiency hydrogen evolution reaction (HER) catalyst is reported. The support material is based on a polyhexaphenylbenzene material with intrinsic holes, which heals into carbon materials upon heating. The healing process is found to be useful for anchoring various transition metal atoms, among which the supported Ir Single-atom catalyst (SAC) catalyst shows much higher electrocatalytic activity and stability than the commercial Pt/C and Ir/C in HER. There is only 17 mV overpotential at 10 mA cm-2 , which is significantly lower than that of commercial Pt/C and Ir/C catalysts respectively by 26 and 3 mV, and the catalyst has an ultra-high mass activity (MA) of 51.6 A mg Ir - 1 ${\text{ A mg}}_{{\rm{Ir}}}^{ - 1}$ at 70 mV potential and turn over frequencies (TOF) of 171.61 s-1 at the potential of 100 mV. The density functional theory (DFT) calculation reveals the significant role of carbon coordination around the Ir center. A series of monatomic PBN-300-M are synthesized by using of designed carbon materials. The findings provide an enabling and versatile platform for facile accessing SACs toward many industrial important reactions.

Unusual Activity of Rationally Designed Cobalt Phosphide/Oxide Heterostructure Composite for Hydrogen Production in Alkaline Medium

Design and development of an efficient, nonprecious catalyst with structural features and functionality necessary for driving the hydrogen evolution reaction (HER) in an alkaline medium remain a formidable challenge. At the root of the functional limitation is the inability to tune the active catalytic sites while overcoming the poor reaction kinetics observed under basic conditions. Herein, we report a facile approach to enable the selective design of an electrochemically efficient cobalt phosphide oxide composite catalyst on carbon cloth (CoP-Co_xO_y/CC), with good activity and durability toward HER in alkaline medium (η₁₀ = -43 mV). Theoretical studies revealed that the redistribution of electrons at laterally dispersed Co phosphide/oxide interfaces gives rise to a synergistic effect in the heterostructured composite, by which various Co oxide phases initiate the dissociation of the alkaline water molecule. Meanwhile, the highly active CoP further facilitates the adsorption-desorption process of water electrolysis, leading to extremely high HER activity.

Using Exciton/Trion Dynamics to Spatially Monitor the Catalytic Activities of MoS2 during the Hydrogen Evolution Reaction

The adsorption and desorption of electrolyte ions strongly modulates the carrier density or carrier type on the surface of monolayer-MoS₂ catalyst during the hydrogen evolution reaction (HER). The buildup of electrolyte ions onto the surface of monolayer MoS₂ during the HER may also result in the formation of excitons and trions, similar to those observed in gate-controlled field-effect transistor devices. Using the distinct carrier relaxation dynamics of excitons and trions of monolayer MoS₂ as sensitive descriptors, an in situ microcell-based scanning time-resolved liquid cell microscope is set up to simultaneously measure the bias-dependent exciton/trion dynamics and spatially map the catalytic activity of monolayer MoS₂ during the HER. This operando probing technique used to monitor the interplay between exciton/trion dynamics and electrocatalytic activity for two-dimensional transition metal dichalcogenides provides an excellent platform to investigate the local carrier behaviors at the atomic layer/liquid electrolyte interfaces during electrocatalytic reaction.

Real-Time Modulation of Hydrogen Evolution Activity of Graphene Electrodes Using Mechanical Strain

This paper reports the effect of mechanically applied elastic strain on the hydrogen evolution reaction (HER) activity of graphene under acidic conditions. An applied tensile strain of 0.2% on a graphene electrode is shown to lead to a 1-3% increase in the HER current. The tensile strain increases HER activity, whereas compressive strain decreases it. Density functional theory (DFT) calculations using a periodic graphene slab model predict an increase in the adsorption energy of the H atom with growing tensile strain, consistent with an enhancement of the current density in HER, similar to that observed experimentally.

Insights into Antiperovskite Ni3 In1-x Cux N Multi-Crystalline Nanoplates and Bulk Cubic Particles as Efficient Electrocatalysts on Hydrogen Evolution Reaction

Intrinsic hydrogen evolution reaction (HER) activity and the mechanism of antiperovskite Ni₃ In_1-x Cu_x N bulk cubic particles and multi-crystalline nanoplates are thoroughly investigated. Stoichiometric Ni₃ In_0.6 Cu_0.4 N reaches the best HER performance, with an overpotential of 102 mV in its multi-crystalline nanoplates obtained from the LDH-derived method, and 143 mV in its bulk cubic particles from the citric method. DFT calculation reveals that Ni-In or Ni-Cu paired on the (100) plane serve as primary active sites. The Ni-Cu pair exhibits stronger OH* and H* affinity that correspondingly reduce OH* and H* adsorption free energy. Introducing specific amounts of the Ni-Cu pair, that is In:Cu = 0.6:0.4 in Ni₃ In_0.6 Cu_0.4 N, can optimize OH* and H* adsorption free energy to facilitate water dissociation in the HER process, while avoiding OH* adsorption getting too strong to block active sites. Besides, Ni₃ In_0.6 Cu_0.4 N turns the water adsorption step spontaneous, which may be attributed to the shifted d-band center and polarizing effect from surface In-Cu charge distribution. This work expands the scope for material design in an antiperovskite system by tailoring the chemical components and morphology for optimal reaction free energy and performance.

Ternary Mo2 NiB2 as a Superior Bifunctional Electrocatalyst for Overall Water Splitting

Transition metal borides are considered as promising electrocatalysts for water splitting due to their metallic conductivity and good durability. However, the currently reported monometallic and noncrystalline multimetallic borides only show generic and monofunctional catalytic activity. In this work, the authors design and successfully synthesize highly crystalline ternary borides, Mo₂ NiB₂ , via a facile solid-state reaction from pure elemental powders. The as-synthesized Mo₂ NiB₂ exhibits very low overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), that is, 280 and 160 mV to reach a current density of 10 mA cm^-2 , in alkaline media. These values are much lower from the ones observed over monometallic borides, that is, Ni₂ B and MoB, and the lowest among all nonprecious metal borides. By loading Mo₂ NiB₂ onto Ni foams as both cathode and anode electrode for overall water splitting applications, a low cell voltage of 1.57 V is required to achieve a current density of 10 mA cm^-2 , comparable with the value required from the Pt/C||IrO₂ /C couple (1.56 V). The proposed synthesis strategy can be used for the preparation of cost-effective, multi-metallic crystalline borides, as multifunctional electrocatalysts.

Extreme Environmental Thermal Shock Induced Dislocation-Rich Pt Nanoparticles Boosting Hydrogen Evolution Reaction

Crystal structure engineering of nanomaterials is crucial for the design of electrocatalysts. Inducing dislocations is an efficient approach to generate strain effects in nanomaterials to optimize the crystal and electronic structures and improve the catalytic properties. However, it is almost impossible to produce and retain dislocations in commercial mainstream catalysts, such as single metal platinum (Pt) catalysts. In this work, a non-equilibrium high-temperature (>1400 K) thermal-shock method is reported to induce rich dislocations in Pt nanocrystals (Dr-Pt). The method is performed in an extreme environment (≈77 K) created by liquid nitrogen. The dislocations induced within milliseconds by thermal and structural stress during the crystallization process are kinetically frozen at an ultrafast cooling rate. The high-energy surface structures with dislocation-induced strain effects can prevent surface restructuring during catalysis. The findings indicate that a novel extreme environmental high-temperature thermal-shock method can successfully introduce rich dislocations in Pt nanoparticles and significantly boost its hydrogen evolution reaction performance.

Defect-Assisted Anchoring of Pt Single Atoms on MoS2 Nanosheets Produces High-Performance Catalyst for Industrial Hydrogen Evolution Reaction

Pt-based catalysts are currently the most efficient electrocatalysts for the hydrogen evolution reaction (HER), but the scarcity and high cost of Pt limit industrial applications. Downsizing Pt nanoparticles (NPs) to single atoms (SAs) can expose more active sites and increase atomic utilization, thus decreasing the cost. Here, a solar-irradiation strategy is used to prepare hybrid SA-Pt/MoS₂ nanosheets (NSs) that demonstrate excellent HER activity (the overpotential at a current density of 10 mA cm^-2 (η₁₀ ) of 44 mV, and Tafel slope of 34.83 mV dec^-1 in acidic media; η₁₀ of 123 mV, and Tafel slope of 76.71 mV dec^-1 in alkaline media). Defects and deformations introduced by thermal pretreatment of the hydrothermal MoS₂ NSs promote anchoring and stability of Pt SAs. The fabrication of Pt SAs and NPs is easily controlled using different Pt-precursor concentrations. Moreover, SA-Pt/MoS₂ produced under natural sunlight exhibits high HER performance (η₁₀ of 55 mV, and Tafel slope of 43.54 mV dec^-1 ), which indicates its viability for mass production. Theoretical simulations show that Pt improves the absorption of H atoms and the charge-transfer kinetics of MoS₂ , which significantly enhance HER activity. A simple, inexpensive strategy for preparing SA-Pt/MoS₂ hybrid catalysts for industrial HER is provided.

Engineering interfacial coupling between 3D net-like Ni3(VO4)2 ultrathin nanosheets and MoS2 on carbon fiber cloth for boostinghydrogen evolution reaction

The use of cheap and efficient electrocatalyst for the production of hydrogen is the key to solving the current energy crisis. Herein, we used a two-step hydrothermal process to fabricate noble-metal-free 3D net-like Ni₃(VO₄)₂ ultrathin nanosheets coupled with MoS₂@CFC interface. Unlike the traditional two-dimensional composite materials, Ni₃(VO₄)₂ ultrathin nanosheets intersect with MoS₂ nanosheets grown on CFC in a 3D net-like structure (Ni₃(VO₄)₂/MoS₂@CFC). Due to the mutual combination of structures and the interfacial coupling cooperation effect between Ni₃(VO₄)₂ nanosheet and MoS₂@CFC, the catalytically active area was expanded, and the intrinsic activity toward HER was significantly improved. Ni₃(VO₄)₂/MoS₂@CFC showed high activity at the industrial temperature (75 °C), with an overpotential of 77 mV (10 mA/cm²) and a 65 mV/dec Tafel slope. This material showed good stability at 0.5 M H₂SO₄. This work provides a heterostructure scheme for the construction of a novel noble metal-free electrocatalyst to promote hydrogen evolution reaction.

Tungsten Oxide/Reduced Graphene Oxide Aerogel with Low-Content Platinum as High-Performance Electrocatalyst for Hydrogen Evolution Reaction

Designing cost-effective, highly active, and durable platinum (Pt)-based electrocatalysts is a crucial endeavor in electrochemical hydrogen evolution reaction (HER). Herein, the low-content Pt (0.8 wt%)/tungsten oxide/reduced graphene oxide aerogel (LPWGA) electrocatalyst with excellent HER activity and durability is developed by employing a tungsten oxide/reduced graphene oxide aerogel (WGA) obtained from a facile solvothermal process as a support, followed by electrochemical deposition of Pt nanoparticles. The WGA support with abundant oxygen vacancies and hierarchical pores plays the roles of anchoring the Pt nanoparticles, supplying continuous mass transport and electron transfer channels, and modulating the surface electronic state of Pt, which endow the LPWGA with both high HER activity and durability. Even under a low loading of 0.81 μg_Pt cm^-2 , the LPWGA exhibits a high HER activity with an overpotential of 42 mV at 10 mA cm^-2 , an excellent stability under 10000-cycle cyclic voltammetry and 40 h chronopotentiometry at 10 mA cm^-2 , a low Tafel slope (30 mV dec^-1 ), and a high turnover frequency of 29.05 s^-1 at η = 50 mV, which is much superior to the commercial Pt/C and the low-content Pt/reduced graphene oxide aerogel. This work provides a new strategy to design high-performance Pt-based electrocatalysts with greatly reduced use of Pt.

Atomically Dispersed Platinum Modulated by Sulfide as an Efficient Electrocatalyst for Hydrogen Evolution Reaction

Catalytically active metals atomically dispersed on supports presents the ultimate atom utilization efficiency and cost-effective pathway for electrocatalyst design. Optimizing the coordination nature of metal atoms represents the advanced strategy for enhancing the catalytic activity and the selectivity of single-atom catalysts (SACs). Here, we designed a transition-metal based sulfide-Ni3S2 with abundant exposed Ni vacancies created by the interaction between chloride ions and the functional groups on the surface of Ni3S2 for the anchoring of atomically dispersed Pt (PtSA-Ni3S2). The theoretical calculation reveals that unique Pt-Ni3S2 support interaction increases the d orbital electron occupation at the Fermi level and leads to a shift-down of the d -band center, which energetically enhances H2O adsorption and provides the optimum H binding sites. Introducing Pt into Ni position in Ni3S2 system can efficiently enhance electronic field distribution and construct a metallic-state feature on the Pt sites by the orbital hybridization between S-3p and Pt-5d for improved reaction kinetics. Finally, the fabricated PtSA-Ni3S2 SAC is supported by Ag nanowires network to construct a seamless conductive three-dimensional (3D) nanostructure (PtSA-Ni3S2@Ag NWs), and the developed catalyst shows an extremely great mass activity of 7.6 A mg-1 with 27-time higher than the commercial Pt/C HER catalyst.

Enhanced photocatalytic hydrogen production activity of Janus Cu1.94S-ZnS spherical nanoheterostructures

Photocatalytic hydrogen evolution is one of the most promising approaches for efficient solar energy conversion. The light-harvesting ability and interfacial structure of heterostructured catalysts regulate the processes of photon injection and transfer, which further determines their photocatalytic performances. Here, we report a Janus Cu_1.94S-ZnS nano-heterostructured photocatalyst synthesized using a facile stoichiometrically limited cation exchange reaction. Djurleite Cu_1.94S and wurtzite ZnS share the anion skeleton, and the lattice mismatch between immiscible domains is ∼1.7%. Attributing to the high-quality interfacial structure, Janus Cu_1.94S-ZnS nanoheterostructures (NHs) show an enhanced photocatalytic hydrogen evolution rate of up to 0.918 mmol h^-1 g^-1 under full-spectrum irradiation, which is ∼38-fold and 17-fold more than those of sole Cu_1.94S and ZnS nanocrystals (NCs), respectively. The results indicate that cation exchange reaction is an efficient approach to construct well-ordered interfaces in hybrid photocatalysts, and it also demonstrates that reducing lattice mismatch and interfacial defects in hybrid photocatalysts is essential for enhancing their solar energy conversion performance.

The effects of power ultrasound (24 kHz) on the electrochemical reduction of CO2 on polycrystalline copper electrodes

The electrochemical CO₂ reduction reaction (CO2RR) on polycrystalline copper (Cu) electrode was performed in a CO₂-saturated 0.10 M Na₂CO₃ aqueous solution at 278 K in the absence and presence of low-frequency high-power ultrasound (f = 24 kHz, P_T ~ 1.23 kW/dm³) in a specially and well-characterized sonoelectrochemical reactor. It was found that in the presence of ultrasound, the cathodic current (I_c) for CO₂ reduction increased significantly when compared to that in the absence of ultrasound (silent conditions). It was observed that ultrasound increased the faradaic efficiency of carbon monoxide (CO), methane (CH₄) and ethylene (C₂H₄) formation and decreased the faradaic efficiency of molecular hydrogen (H₂). Under ultrasonication, a ca. 40% increase in faradaic efficiency was obtained for methane formation through the CO2RR. In addition, and interestingly, water-soluble CO₂ reduction products such as formic acid and ethanol were found under ultrasonic conditions whereas under silent conditions, these expected electrochemical CO2RR products were absent. It was also found that power ultrasound increases the formation of smaller hydrocarbons through the CO2RR and may initiate new chemical reaction pathways through the sonolytic di-hydrogen splitting yielding other products, and simultaneously reducing the overall molecular hydrogen gas formation.

Super-Hydrophilic Hierarchical Ni-Foam-Graphene-Carbon Nanotubes-Ni2P-CuP2 Nano-Architecture as Efficient Electrocatalyst for Overall Water Splitting

Water splitting via an electrochemical process to generate hydrogen is an economic and green approach to resolve the looming energy and environmental crisis. The rational design of multicomponent materials with seamless interfaces having robust stability, facile scalability, and low-cost electrocatalysts is a grand challenge to produce hydrogen by water electrolysis. Herein, we report a superhydrophilic homogeneous bimetallic phosphide of Ni₂P-CuP₂ on Ni-foam-graphene-carbon nanotubes (CNTs) heterostructure using facile electrochemical metallization followed by phosphorization without any intervention of metal-oxides/hydroxides. This bimetallic phosphide shows ultralow overpotentials of 12 (HER, hydrogen evolution reaction) and 140 mV (OER, oxygen evolution reaction) at current densities of 10 and 20 mA/cm² in acidic and alkaline mediums, respectively. The excellent stability lasts for at least for 10 days at a high current density of 500 mA/cm² without much deviation, inferring the practical utilization of the catalyst toward green fuel production. Undoubtedly, the catalyst is capable enough for overall water splitting at a very low cell voltage of 1.45 V @10 mA/cm² with an impressive stability of at least 40 h, showing a minimum loss of potential. Theoretical study has been performed to understand the reaction kinetics and d-band shifting among metal atoms in the heterostructure (Ni₂P-CuP₂) that favor the HER and OER activities, respectively. In addition, the catalyst demonstrates an alternate transformation of solar energy to green H₂ production using a standard silicon solar cell. This work unveils a smart design and synthesizes a highly stable electrocatalyst against an attractive paradigm of commercial water electrolysis for renewable electrochemical energy conversion.

Electronic Structure Tuning of 2D Metal (Hydr)oxides Nanosheets for Electrocatalysis

2D metal (hydr)oxide nanosheets have captured increasing interest in electrocatalytic applications aroused by their high specific surface areas, enriched chemically active sites, tunable physiochemical properties, etc. In particular, the electrocatalytic reactivities of materials greatly rely on their surface electronic structures. Generally speaking, the electronic structures of catalysts can be well adjusted via controlling their morphologies, defects, and heterostructures. In this Review, the latest advances in 2D metal (hydr)oxide nanosheets are first reviewed, including the applications in electrocatalysis for the hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction. Then, the electronic structure-property relationships of 2D metal (hydr)oxide nanosheets are discussed to draw a picture of enhancing the electrocatalysis performances through a series of electronic structure tuning strategies. Finally, perspectives on the current challenges and the trends for the future design of 2D metal (hydr)oxide electrocatalysts with prominent catalytic activity are outlined. It is expected that this Review can shed some light on the design of next generation electrocatalysts.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW-1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV-Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Fabrication of Magnetic Superstructure NiFe2O4@MOF-74 and Its Derivative for Electrocatalytic Hydrogen Evolution with AC Magnetic Field

As an ideal hydrogen production route, electrolyzed water still faces the challenges of high cost of noble-metal electrocatalysts and low performance of non-noble-metal catalysts in scalable applications. Recently, introduction of external fields (such as magnetic fields, light fields, etc.) to improve the electrocatalytic water splitting performance of non-noble-metal catalysts has attracted great attention due to their simplicity. Here, a simple method for preparing magnetic superstructure (NiFe₂O₄@MOF-74) is described, and the hydrogen evolution reaction (HER) behavior of its carbonized derivative, a ferromagnetic superstructure, is revealed in a wide range of applied voltage under an AC magnetic field. The overpotential (@10 mA cm^-2) required for the HER of the obtained ferromagnetic superstructure in 1 M KOH was reduced by 31 mV (7.7%) when a much small AC magnetic field (only 2.3 mT) is applied. Surprisingly, the promotion effect of the AC magnetic field is not monotonically increasing with the increase of the applied voltage or the strength of AC magnetic field, but increasing first, then weakening. This unusual behavior is believed to be mainly caused by the enhanced induced electromotive force and the additional energy by the applied AC magnetic field. This discovery provides a new idea for adjusting the performance of electrocatalytic reactions.

Interfacial Structure of Water as a New Descriptor of the Hydrogen Evolution Reaction

Driven by the persisting poor understanding of the sluggish kinetics of the hydrogen evolution reaction (HER) on Pt in alkaline media, a direct correlation of the interfacial water structure and activity is still yet to be established. Herein, using Pt and Pt-Ni nanoparticles we first demonstrate a strong dependence of the proton donor structure on the HER activity and pH. The structure of the first layer changes from the proton acceptors to the donors with increasing pH. In the base, the reactivity of the interfacial water varied its structure, and the activation energies of water dissociation increased in the sequence: the dangling O-H bonds < the trihedrally coordinated water < the tetrahedrally coordinated water. Moreover, optimizing the adsorption of H and OH intermediates can re-orientate the interfacial water molecules with their H atoms pointing towards the electrode surface, thereby enhancing the kinetics of HER. Our results clarified the dynamic role of the water structure at the electrode-electrolyte interface during HER and the design of highly efficient HER catalysts.

A Chronocoulometric Method to Measure the Corrosion Rate on Zinc Metal Electrodes

Research studies on zinc metal-based batteries have attracted considerable attention as a candidate for post-lithium-ion batteries. Zinc is one of the few metal anodes that is compatible with aqueous and non-aqueous electrolytes, providing a large theoretical capacity of 820 mAh g^-1. However, in aqueous electrolytes, the zinc metal anode suffers from hydrogen evolution reaction (HER), by which zinc is irreversibly consumed or corroded continually. Exact estimation of the corrosion rate has been a challenge in the development of Zn-based batteries. Measurement of the corrosion rate by conventional Tafel analysis meets serious problems because the cathodic current reflects deposition of Zn metal as well as HER, inhibiting exact measurement of the corrosion rate. Herein, we developed a chronocoulometric "deposition-rest-dissolution" method to quantify the corrosion rate without such interference from the deposition of Zn. The method was successfully applied to the quantification of the rate of chemical corrosion of Zn in aqueous electrolytes with various pH and concentration values. The "deposition-rest-dissolution" method and electrochemical impedance spectroscopy confirmed that saturated ZnSO₄ (ca. 3.2 M) + 0.075 M Li₂SO₄ delivers the lowest corrosion rate compared to the other electrolytes, probably because the activity of water in such a concentrated electrolyte is low enough to suppress the kinetics of HER. Moreover, this method can be generally applied to determine the rate of chemical corrosion on various metal electrodes.

Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2

Forming pits on molybdenum disulfide (MoS₂ ) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS₂ , and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS₂ monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS₂ monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS₂ , rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS₂ .

Enhancing the Hydrogen Evolution Reaction Activity of Platinum Electrodes in Alkaline Media Using Nickel-Iron Clusters

Herein, we demonstrate an easy way to improve the hydrogen evolution reaction (HER) activity of Pt electrodes in alkaline media by introducing Ni-Fe clusters. As a result, the overpotential needed to achieve a current density of 10 mA cm^-2 in H₂ -saturated 0.1 m KOH is reduced for the model single-crystal electrodes down to about 70 mV. To our knowledge, these modified electrodes outperform any other reported electrocatalysts tested under similar conditions. Moreover, the influence of 1) Ni to Fe ratio, 2) cluster coverage, and 3) the nature of the alkali-metal cations present in the electrolyte on the HER activity has been investigated. The observed catalytic performance likely originates from both the improved water dissociation at the Ni-Fe clusters and the subsequent optimal hydrogen adsorption and recombination at Pt atoms present at the Ni-Fe/Pt boundary.

Flexible Honeycombed Nanoporous/Glassy Hybrid for Efficient Electrocatalytic Hydrogen Generation

Hydrogen evolution reaction (HER) in alkaline media urgently requires electrocatalysts concurrently possessing excellent activity, flexible free-standing capability, and low cost. A honeycombed nanoporous/glassy sandwich structure fabricated through dealloying metallic glass (MG) is reported. This free-standing hybrid shows outstanding HER performance with a very small overpotential of 37 mV at 10 mA cm^-2 and a low Tafel slope of 30 mV dec^-1 in alkaline media, outperforming commercial Pt/C. By alloying 3 at% Pt into the MG precursor, a honeycombed Pt₇₅ Ni₂₅ solid solution nanoporous structure, with fertile active sites and large contact areas for efficient HER, is created on the dealloyed MG surface. Meanwhile, the surface compressive lattice-strain effect is also introduced by substituting the Pt lattice sites with the smaller Ni atoms, which can effectively reduce the hydrogen adsorption energy and thus improve the hydrogen evolution. Moreover, the outstanding stability and flexibility stemming from the ductile MG matrix also make the hybrid suitable for practical electrode application. This work not only offers a reliable strategy to develop cost-effective and flexible multicomponent catalysts with low Pt usage for efficient HER, but also sheds light on understanding the alloying effects of the catalytic process.

Catalytically Active Carbon From Cattail Fibers for Electrochemical Reduction Reaction

Catalytically active carbons derived from plant biomass are conducive to the construction of renewable energy source system and utilization of sustainable resources. In this article, natural cattail fibers are used to fabricate porous nitrogen-doped carbon via direct chemical activation and heteroatom modification treatments. The graphene-like sheets from biomass pyrolysis are assembled into three-dimensional carbon frameworks. The chemical activation of KHCO₃ generated unique porous structure and N-containing molecules pyrolysis modification provided nitrogen doping atoms. High surface area up to 2,345 m²·g⁻¹ with simultaneous hierarchical pores (from micro to meso and macro) with abundant edge defects are achieved for these carbon materials. These materials have a very large external surface area up to 1,773 m²·g⁻¹. The above strategy exhibits a significant synergistic effect on the improvement of catalytic properties toward hydrogen evolution reaction and oxygen reduction reaction. The small over potentials and Tafel slopes of these catalytically active carbons demonstrate excellent potential applications in renewable energy conversion and storage systems. This research established a new link among environmental improvement, biomass conversion and renewable energy utilization.

Simultaneously Engineering Electron Conductivity, Site Density and Intrinsic Activity of MoS2 via the Cation and Anion Codoping Strategy

The catalytic activity of 2H-MoS₂ is retarded by the deficiency in active sites, inferior intrinsic activity, and slow electron transfer kinetics. However, the strategies to concurrently resolve these issues have been challenging and rarely reported. Herein, we successfully endow MoS₂ with exceptional acidic HER performance by concurrently doping nitrogen and metal atoms into the basal plane of MoS₂. The experimental results reveal that the N dopant that induces the intervalence charge transfer between two ions (Mo⁴⁺/Mo³⁺) and the atoms rearrangement can enable the successful synthesis of 1T MoS₂ on reduced graphene oxides, which can concurrently increase the active-site density and facilitate the charge transfer from the substrate to the catalyst active sites. The spontaneous doping of metal cation atoms further improves the intrinsic activity of MoS₂ by creating more sulfur vacancy sites and tailoring the energy level matching. The optimized electrocatalyst exhibited unprecedented activity and stability for HER with a low overpotential of 143 mV at 150 mA cm^-2 and a high exchange current density of 1 mA cm^-2. Therefore, our work opens up possibility to manipulate the MoS₂ catalytic performance to rival Pt, which is of significant importance to both fundamental study and industry applications.

Synthesis of MoWS2 on Flexible Carbon-Based Electrodes for High-Performance Hydrogen Evolution Reaction

We directly synthesize MoWS₂ nanosheets on conductive carbon nanotube yarn (MoWS₂/CNTY) and carbon fiber cloth (MoWS₂/CC) through a fast and low-temperature thermolysis method to obtain flexible catalyst electrodes that show high performance in the hydrogen evolution reaction. Small Tafel slopes of 41.8 and 46.7 mV dec^-1 are achieved for MoWS₂/CC and MoWS₂/CNTY, respectively, by optimizing the density of the exposed active edge sites of vertically aligned MoWS₂ on CNTY and CC. Furthermore, the catalyst electrodes demonstrate good electrocatalytic stability over 36 h. The proposed technique for fabricating high-performance, binder-free, and flexible catalyst electrodes is more accessible and faster than conventional methods.

Stimulated Electrocatalytic Hydrogen Evolution Activity of MOF-Derived MoS2 Basal Domains via Charge Injection through Surface Functionalization and Heteroatom Doping

The design of MoS2-based electrocatalysts with exceptional reactivity and robustness remains a challenge due to thermodynamic instability of active phases and catalytic passiveness of basal planes. This study details a viable in situ reconstruction of zinc-nitrogen coordinated cobalt-molybdenum disulfide from structure directing metal-organic framework (MOF) to constitute specific heteroatomic coordination and surface ligand functionalization. Comprehensive experimental spectroscopic studies and first-principle calculations reveal that the rationally designed electron-rich centers warrant efficient charge injection to the inert MoS2 basal planes and augment the electronic structure of the inactive sites. The zinc-nitrogen coordinated cobalt-molybdenum disulfide shows exceptional catalytic activity and stability toward the hydrogen evolution reaction with a low overpotential of 72.6 mV at -10 mA cm-2 and a small Tafel slope of 37.6 mV dec-1. The present study opens up a new opportunity to stimulate catalytic activity of the in-plane MoS2 basal domains for enhanced electrochemistry and redox reactivity through a "molecular reassembly-to-heteroatomic coordination and surface ligand functionalization" based on highly adaptable MOF template.