Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions

Efficient Energy Conversion and Storage Based on Robust Fluoride-Free Self-Assembled 1D Niobium Carbide in 3D Nanowire Network

Owing to their high robustness and conductivity, 2D transition metal carbides and nitrides known as MXenes are considered as a promising material class for electrochemical catalysis, energy conversion, and storage applications. Nevertheless, conventional hazardous fluoride-based synthesis routes and the intense intralayer bonding restrict the development of MXenes. Herein, a fluoride-free, facile, and rapid method for synthesizing self-assembled 1D architecture from an MXene-based compound is reported. The MXene nanowire (NW) not only provides a robust connection to the flexible substrate but also effectively increases the electrochemically active surface area. The kinetics-favorable structure yields a boosted performance for the hydrogen/oxygen evolution reaction and the intake of the zinc ion. The 1D NW based on MXene compound maintains high stability in a quite low overpotential of 236 mV for 24 h without detachment from the substrate and manifests an exceptional high-power density of 420 W kg-1 over 150 cycles as a flexible aqueous zinc ion battery. This work paves a novel and non-toxic synthesis method for the 1D nanofiber structure from MXene composition and demonstrates its multifunctional applications for energy conversion and storage.

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.

Highly efficient electrocatalytic hydrogen evolution reaction on carbonized porous conducting polymers

A rational design of an efficient and inexpensive electrocatalyst for water splitting still remains a challenge. Porous conducting polymers are attractive materials which not only provide a high surface area for electrocatalysis but also absorb light which can be harnessed in photoelectrocatalysis. Here, a novel and inexpensive electrochemical approach is developed to obtain nanoporous conducting copolymers with tunable light absorbance and porosity. By fine-tuning the copolymer composition and upon heat treatment, an excellent electrocatalytic hydrogen evolution reaction (HER) was achieved in alkaline solution with an overpotential of just 77 mV to obtain a current density of 10 mA cm−2. Such an overpotential is remarkably low compared with other reported values for polymers in an alkaline medium. The nanoporous copolymer developed here shows a great promise of using metal-free electrocatalysts and brings about new avenues for exploitation of these porous conducting polymers.

Highly-dispersed ruthenium precursors via a self-assembly-assisted synthesis of uniform ruthenium nanoparticles for superior hydrogen evolution reaction

For the first time, highly-dispersed ruthenium precursors via a hydrogen-bond-driven melamine–cyanuric acid supramolecular complex (denoted CAM) self-assembly-assisted synthesis of uniform ruthenium nanoparticles with superior HER performance under both acidic and alkaline conditions are reported. Electrochemical tests reveal that when the current density is −10 mA cm⁻², the optimal Ru/CNO electrocatalyst could express low overpotentials of −18 mV and −46 mV, low Tafel slopes of 46 mV dec⁻¹ and 100 mV dec⁻¹, in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. The remarkable HER performance could be attributed to uniform ruthenium with the aid of highly dispersed ruthenium precursors (Ru–CAM) and subsequent annealing results in uniform ruthenium nanoparticles.

Highly dispersed ruthenium precursors via a supramolecular self-assembly assisted synthesis of uniform ruthenium nanoparticles with excellent HER performance.

Mixed-dimensional heterostructures of hydrophobic/hydrophilic graphene foam for tunable hydrogen evolution reaction

The synergetic effect of hydrophilic and hydrophobic carbon can be used to obtain tunable hydrogen evolution reaction (HER) at the interface. Herein, graphene oxide (GO-Hummers method) was coated on graphene foam (GF) synthesized via chemical vapor deposition to develop mixed-dimensional heterostructure for the observation of HER. The porosity of GF not only provides an optimized diffusion coefficient for better mass transport but also modified surface chemistry (GF/GO-hydrophobic/hydrophilic interface), which results in an onset potential 50 mV and overpotential of 450 mV to achieve the current density 10 mA/cm². The surface analysis shows that inherent functional groups at the surface played a key role in tuning the activity of hybrid, providing a pathway to introduce non-corrosive electrodes for water splitting.

Amorphous Ni-Fe-Mo Suboxides Coupled with Ni Network as Porous Nanoplate Array on Nickel Foam: A Highly Efficient and Durable Bifunctional Electrode for Overall Water Splitting

It is a great challenge to fabricate electrode with simultaneous high activity for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, a high-performance bifunctional electrode formed by vertically depositing a porous nanoplate array on the surface of nickel foam is provided, where the nanoplate is made up by the interconnection of trinary Ni-Fe-Mo suboxides and Ni nanoparticles. The amorphous Ni-Fe-Mo suboxide and its in situ transformed amorphous Ni-Fe-Mo (oxy)hydroxide acts as the main active species for HER and OER, respectively. The conductive network built by Ni nanoparticles provides rapid electron transfer to active sites. Moreover, the hydrophilic and aerophobic electrode surface together with the hierarchical pore structure facilitate mass transfer. The corresponding water electrolyzer demonstrates low cell voltage (1.50 V @ 10 mA cm-2 and 1.63 V @ 100 mA cm-2) with high durability at 500 mA cm-2 for at least 100 h in 1 m KOH.

An earth-abundant bimetallic catalyst coated metallic nanowire grown electrode with platinum-like pH-universal hydrogen evolution activity at high current density

A self-supported and flexible current collector solely made of earth-abundant elements, NiCo layered double hydroxide (LDH) wrapped around Cu nanowires (Cu-Ws) grown on top of commercially available Cu mesh (Cu-m), outperforms the benchmark 40 wt% Pt/C in catalyzing the electrochemical hydrogen evolution reaction (HER). The Cu-m/Cu-W/NiCo-LDH cathode operates both in acidic and alkaline media exhibiting high turnover frequencies (TOF) at 30 mV (0.3 H2 s-1 in 1 M KOH and 0.32 H2 s-1 in 0.5 M H2SO4, respectively) and minimal overpotentials of 15 ± 6 mV in 1 M KOH and 27 ± 2 mV in 0.5 M H2SO4 at -10 mA cm-2. Cu-m/Cu-W/NiCo-LDH outperforms the activity of 40 wt% Pt/C that needs overpotentials of 22 and 18 mV in 1 M KOH and 0.5 M H2SO4, respectively. With a tremendous advantage over Pt/C in triggering proton reduction with fast kinetics, similar mass activity and pH-universality, the current collector demonstrates outstanding operational durability even at above -1 A cm-2. The high density of electronic states near the Fermi energy level of Cu-Ws is found to be a pivotal factor for efficient electron transfer to the NiCo-LDH catalyst. This class of self-supported electrodes is expected to trigger rapid progress in developing high performance energy conversion and storage devices.

Enhancing Hydrogen Evolution Activity of Au(111) in Alkaline Media through Molecular Engineering of a 2D Polymer

The electrochemical splitting of water holds promise for the storage of energy produced intermittently by renewable energy sources. The evolution of hydrogen currently relies on the use of platinum as a catalyst-which is scarce and expensive-and ongoing research is focused towards finding cheaper alternatives. In this context, 2D polymers grown as single layers on surfaces have emerged as porous materials with tunable chemical and electronic structures that can be used for improving the catalytic activity of metal surfaces. Here, we use designed organic molecules to fabricate covalent 2D architectures by an Ullmann-type coupling reaction on Au(111). The polymer-patterned gold electrode exhibits a hydrogen evolution reaction activity up to three times higher than that of bare gold. Through rational design of the polymer on the molecular level we engineered hydrogen evolution activity by an approach that can be easily extended to other electrocatalytic reactions.

Intermetallic Cu5Zr Clusters Anchored on Hierarchical Nanoporous Copper as Efficient Catalysts for Hydrogen Evolution Reaction

Designing highly active and robust platinum-free electrocatalysts for hydrogen evolution reaction is vital for large-scale and efficient production of hydrogen through electrochemical water splitting. Here, we report nonprecious intermetallic Cu₅Zr clusters that are in situ anchored on hierarchical nanoporous copper (NP Cu/Cu₅Zr) for efficient hydrogen evolution in alkaline medium. By virtue of hydroxygenated zirconium atoms activating their nearby Cu-Cu bridge sites with appropriate hydrogen-binding energy, the Cu₅Zr clusters have a high electrocatalytic activity toward the hydrogen evolution reaction. Associated with unique architecture featured with steady and bicontinuous nanoporous copper skeleton that facilitates electron transfer and electrolyte accessibility, the self-supported monolithic NP Cu/Cu₅Zr electrodes boost violent hydrogen gas release, realizing ultrahigh current density of 500 mA cm⁻² at a low potential of -280 mV versus reversible hydrogen electrode, with exceptional stability in 1 M KOH solution. The electrochemical properties outperform those of state-of-the-art nonprecious metal electrocatalysts and make them promising candidates as electrodes in water splitting devices.

Organic template-based ZnO embedded Mn3O4 nanoparticles: synthesis and evaluation of their electrochemical properties towards clean energy generation

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts. In this work, a natural organic agent has been used to synthesize nano-structured ZnO/Mn₃O₄ with high surface area and enhanced electrocatalytic performance. Upon pre-annealing treatment, mixed metal oxide precipitates are formed due to the complex formation between a metal oxide and organic extract. The thermally annealed mixed oxide ZnO/Mn₃O₄ was characterized by XRD diffractometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Gas chromatography-mass spectrometry (GC-MS) identified methyldecylamine as a major stabilizing agent of the synthesized nanomaterial. Using a Tauc plot, the calculated band energy for the synthesized ZnO/Mn₃O₄ mixed metal oxide was 1.65 eV. Moreover, we have demonstrated the effects of incorporated organic compounds on the surface chemistry, morphology and electrochemical behavior of ZnO/Mn₃O₄. The phyto-functionalized ZnO/Mn₃O₄ was deposited on Ni-foam for electrocatalytic studies. The fabricated electrode revealed good performance with low over-potential and Tafel slope, suggesting it to be suitable as a potential catalyst for water splitting application, in particular for the oxygen evolution reaction (OER). The overall findings of the current study provide a cost-effective and efficient organic template for functionalization and sustainable fabrication of ZnO/Mn₃O₄ nanomaterial for application as an electrocatalyst.

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts.

Hydrogen Evolution Reaction-From Single Crystal to Single Atom Catalysts

Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.

Co/Cu-modified NiO film grown on nickel foam as a highly active and stable electrocatalyst for overall water splitting

The development and utilization of low-cost and efficient electrocatalysts for overall water splitting is of great significance for future energy supplies. Herein, a Co-doped NiCu mixed oxide film on Ni foam as a bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is synthesized by a facile solvothermal method using methanol as a reactant followed by annealing in air and it exhibits remarkably enhanced HER and OER activities. The well-constructed surface and porous skeleton structure with a large volume provide a large number of catalytically active sites during the electrochemical reaction. Notably, CuO plays an important role in improving the catalytic activity of the electrode; meanwhile, Co doping is beneficial for increasing the conductivity and activating the Ni sites at lower overpotentials via the charge transfer effect. Accordingly, the optimized CuO-NiO/Ni foam electrode exhibits a comparatively low overpotential of 38 mV and 172 mV at 10 mA cm^-2 for the HER and OER in 1.0 M KOH, respectively. Moreover, the electrode shows excellent long-term stability for 1000 cyclic voltammetric cycles in both the HER and OER. A self-assembled overall water splitting device using this electrode as both the anode and cathode achieves a current density of 10 mA cm^-2 at a low cell voltage of 1.51 V. This study is promising and provides a simple method for depositing a multimetal mixed oxide on a metal substrate resulting in an efficient bifunctional electrocatalyst, holding great significance for future energy applications.

N, Ru Codoped Pellet Drum Bundle-Like Sb2S3: An Efficient Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction Electrocatalyst in Alkaline Medium

Though investigations have been made on several metal chalcogenides in hydrogen evolution reactions (HERs) and hydrogen oxidation reactions (HORs), antimony sulfide (Sb₂S₃) has not generated much attention. In this direction, the present work reports on the synthesis of N, Ru codoped pellet drum bundle-like antimony sulfide (Sb₂S₃) via a simple reflux method. Subsequent HER and HOR electrocatalytic investigations in 1 M KOH revealed their suitability as an efficient and inexpensive alternative to platinum, as is evident from the overpotential (72 mV at a current density of 10 mA cm^-2), Tafel slope (193 mV/decade), exchange current density (1.42 mA/cm²), and breakdown potential at ∼0.6 V vs RHE, respectively. Such remarkable HER and HOR performance of N, Ru codoped Sb₂S₃ could be ascribed to the presence of relatively larger active sites compared to Sb₂S₃ and N-doped Sb₂S₃ individually due to synergistic effects arising from N and Ru dopants. Further, N, Ru codoped Sb₂S₃ demonstrated high intrinsic catalytic activity as indicated by its turnover frequency (2.03 s^-1) and current loss, corresponding to 35% after 10 h of continuous amperometric i-t operation. Alternatively, such excellent catalytic performance of N, Ru codoped Sb₂S₃ arises due to geometric lattice defects with surface oxygen vacancy, and the availability of abundant edges and its pellet drum-like morphology also cannot be overruled.

Covalent organic framework-supported platinum nanoparticles as efficient electrocatalysts for water reduction

The hydrogen evolution reaction (HER) is one of the most effective and sustainable ways to produce hydrogen gas as an alternative clean fuel. The rate of this electrocatalytic reaction is highly dependent on the properties (dispersity and stability) of electrocatalysts. Herein, we developed well-dispersed and highly stable platinum nanoparticles (PtNPs) supported on a covalent organic framework (COF-bpyTPP), which exhibit excellent catalytic activities toward HER as well as the hydride reduction reaction. The nanoparticles have an average size of 2.95 nm and show superior catalytic performance compared to the commercially available Pt/C under the same alkaline conditions, producing 13 times more hydrogen with a far more positive onset potential (-0.13 V vs.-0.63 V) and ca. 100% faradaic efficiency. The reaction rate of the hydride reduction of 4-nitrophenol was also 10 times faster in the case of PtNPs@COF compared to the commercial Pt/C under the same loading and conditions. More importantly, the PtNPs@COF are highly stable under the aqueous reactions conditions and can be reused without showing noticeable aggregation and activity degradation.

Sulfur doping enhanced desorption of intermediates on NiCoP for efficient alkaline hydrogen evolution

Electrocatalysts with high catalytic activity, high stability and low cost are critical to the hydrogen evolution reaction (HER). In this paper, sulfur(S)-doped NiCoP nanowire arrays on a carbon fiber paper skeleton (S-NiCoP NW/CFP) are prepared, and it is demonstrated that the electrocatalytic properties of NiCoP in alkaline solution could be well improved by sulfur doping. In 1.0 M KOH, only an overpotential of 172 mV (vs. RHE) at 100 mA cm-2 is required for S doped NiCoP nanowires on CFP, and the turnover frequency (TOF) is 1.71 times that of NiCoP at an overpotential of 100 mV, indicating its superior intrinsic activity. Density functional theory (DFT) calculations show that S doping could lower the center of the d-band, and thus weaken the interaction between NiCoP and the intermediates. This leads to an optimized hydrogen adsorption Gibbs free energy (ΔGH*) and faster desorption of OH*. This study offers a promising way to design and optimize electrocatalysts for the HER in alkaline solution.

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

Innovative renewable routes are potentially able to sustain the transition to a decarbonized energy economy. Green synthetic fuels, including hydrogen and natural gas, are considered viable alternatives to fossil fuels. Indeed, they play a fundamental role in those sectors that are difficult to electrify (e.g., road mobility or high-heat industrial processes), are capable of mitigating problems related to flexibility and instantaneous balance of the electric grid, are suitable for large-size and long-term storage and can be transported through the gas network. This article is an overview of the overall supply chain, including production, transport, storage and end uses. Available fuel conversion technologies use renewable energy for the catalytic conversion of non-fossil feedstocks into hydrogen and syngas. We will show how relevant technologies involve thermochemical, electrochemical and photochemical processes. The syngas quality can be improved by catalytic CO and CO2 methanation reactions for the generation of synthetic natural gas. Finally, the produced gaseous fuels could follow several pathways for transport and lead to different final uses. Therefore, storage alternatives and gas interchangeability requirements for the safe injection of green fuels in the natural gas network and fuel cells are outlined. Nevertheless, the effects of gas quality on combustion emissions and safety are considered.

Amorphous-MoO3−x/MoS2 heterostructure: in situ oxidizing amorphization of S-vacancy MoS2 for enhanced alkaline hydrogen evolution

Amorphous-MoO_3−x/MoS₂ heterostructure prepared by in situ oxidizing amorphization of S-vacancy MoS₂ enables the acceleration of water dissociation and alkaline hydrogen evolution.

Polymorphs of a copper coordination compound: interlinking active sites enhance the electrocatalytic activity of the coordination polymer compared to the coordination complex

A copper coordination polymer exhibits highly enhanced HER activity in neutral medium compared to a coordination complex with a similar coordination environment.

Enhancement of HER kinetics with RhNiFe for high-rate water electrolysis

The NiFeCHH is best for OER and poor for HER which by introducing Rh, showed total water splitting in KOH. It requires an ultra-low overpotential of just 36 mV at 50 mA cm⁻² for HERs and 286 mV at 50 mA cm⁻² for TWS.

Nature-inspired electrocatalysts and devices for energy conversion

A NICE approach for the design of nature-inspired electrocatalysts and electrochemical devices for energy conversion.

Adsorption energy scaling relation on bimetallic magnetic surfaces: role of surface magnetic moments

The scaling relationships between the adsorption energies of different reaction intermediates have a tremendous effect in the field of surface science, particularly in predicting new catalytic materials. In the last few decades, these scaling laws have been extensively studied and interpreted by a number of research groups which makes them almost universally accepted. In this work, we report the breakdown of the standard scaling law in magnetic bimetallic transition metal (TM) surfaces for hydrogenated species of oxygen (O), carbon (C), and nitrogen (N), where the adsorption energies are estimated using density functional theory (DFT). We propose that the scaling relationships do not necessarily rely solely on the adsorbates, they can also be strongly dependent on the surface properties. For magnetic bimetallic TM surfaces, the magnetic moment plays a vital role in the estimation of adsorption energy, and therefore towards the linear scaling relation.

Beyond electrolysis: old challenges and new concepts of electricity-driven chemical reactors

With renewable electricity becoming the most widely available, versatile energy form on Earth, the electricity-driven chemical reactors will play crucial role in the transition to green, environmentally-neutral manufacturing of fuels and chemicals.

Urea-assisted enhanced electrocatalytic activity of MoS2–Ni3S2 for overall water splitting

Urea-assisted enhanced electrocatalytic activity of MoS₂–Ni₃S₂ as a bifunctional electrocatalyst for overall water splitting.

Metallic nanostructures with low dimensionality for electrochemical water splitting

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure-property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.

Highly enhanced bifunctional electrocatalytic activity of mixed copper–copper oxides on nickel foam via composition control

Fabricating Cu₂O–CuO and CuO directly on the conducting nickel foam resulted in highly enhanced OER and HER electrocatalytic activity in an alkaline medium, respectively.

Metal-organic Framework-driven Porous Cobalt Disulfide Nanoparticles Fabricated by Gaseous Sulfurization as Bifunctional Electrocatalysts for Overall Water Splitting

Both high activity and mass production potential are important for bifunctional electrocatalysts for overall water splitting. Catalytic activity enhancement was demonstrated through the formation of CoS2 nanoparticles with mono-phase and extremely porous structures. To fabricate porous structures at the nanometer scale, Co-based metal-organic frameworks (MOFs), namely a cobalt Prussian blue analogue (Co-PBA, Co3[Co(CN)6]2), was used as a porous template for the CoS2. Then, controlled sulfurization annealing converted the Co-PBA to mono-phase CoS2 nanoparticles with ~ 4 nm pores, resulting in a large surface area of 915.6 m2 g-1. The electrocatalysts had high activity for overall water splitting, and the overpotentials of the oxygen evolution reaction and hydrogen evolution reaction under the operating conditions were 298 mV and -196 mV, respectively, at 10 mA cm-2.

Glycine-Induced Electrodeposition of Nanostructured Cobalt Hydroxide: A Bifunctional Catalyst for Overall Water Splitting

Herein, an interconnected α-Co(OH)₂ structure with a network-like architecture was used as a bifunctional electrocatalyst for the overall water splitting reaction in alkaline medium. The complexing ability of glycine with a transition metal was exploited to form [Co(gly)₃ ]^- dispersion at pH 10, which was used for the electrodeposition. High-resolution TEM, UV/Vis-diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy were used to confirm that the as-synthesized materials had an α-Co(OH)₂ phase. The electrocatalytic oxygen and hydrogen evolution activity of the glycine-coordinated α-Co(OH)₂ was found to be approximately 320 and 145 mV, respectively, at 10 mA cm^-2 . The material required approximately 1.60 V (vs. reversible hydrogen electrode; RHE) to achieve the benchmark of 10 mA cm^-2 for overall water splitting with a mass activity of approximately 63.7 A g^-1 at 1.60 V (vs. RHE). The chronoamperometric response was measured to evidence the stability of the material for overall water splitting for up to 24 h. Characterization of the catalyst after the oxygen and hydrogen evolution reactions was performed by XPS and showed the presence of a Co^II /Co^III oxidation state.

Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH

Polyoxometalates (POMs) are promising catalysts for the electrochemical hydrogen production from water owing to their high intrinsic catalytic activity and chemical tunability. However, poor electrical conductivity and easy detachment of the POMs from the electrode cause significant challenges under operating condition. Herein, a simple one-step hydrothermal method is reported to synthesize a series of Dexter-Silverton POM/Ni foam composites (denoted as NiM-POM/Ni; M=Co, Zn, Mn), in which the stable linkage between the POM catalysts and the Ni foam electrodes lead to high activity for the hydrogen evolution reaction (HER). Among them, the highest HER performance can be observed in the NiCo-POM/Ni, featuring an overpotential of 64 mV (at 10 mA cm-2 , vs. reversible hydrogen electrode), and a Tafel slope of 75 mV dec-1 in 1.0 m aqueous KOH. Moreover, the NiCo-POM/Ni catalyst showed a high faradaic efficiency ≈97 % for HER. Post-catalytic of NiCo-POM/Ni analyses showed virtually no mechanical or chemical degradation. The findings propose a facile and inexpensive method to design stable and effective POM-based catalysts for HER in alkaline water electrolysis.

Effect of Ni Doping on the MoS2 Structure and Its Hydrogen Evolution Activity in Acid and Alkaline Electrolytes

We have investigated three-dimensional (3D) MoS2 nanoarchitectures doped with different amount of Ni to boost the hydrogen evolution reaction (HER) in alkaline environment, where this reaction is normally hindered. As a comparison, the activity in acidic media was also investigated to determine and compare the role of the Ni sites in both media. The doping of MoS2, especially at high loadings, can modify its structural and/or electronic properties, which can also affect the HER activity. The structural and electronic properties of the Ni doped 3D-MoS2 nanoarchitecture were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning and transmission electronic microscopy (SEM; TEM), and X-ray photoemission Spectroscopy (XPS). XPS also allowed us to determine the Ni-based species formed as a function of the dopant loading. The HER activity of the materials was investigated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH. By combining the physicochemical and electrochemical results, we concluded that the Ni sites have a different role in the HER mechanism and kinetics in acidic and in alkaline media. Thus, NiSx species are essential to promote HER in alkaline medium, whereas the Ni-Mo-S ones enhance the HER in acid medium.

Intermetallic compounds with high hydrogen evolution reaction performance: a case study of a MCo2 (M = Ti, Zr, Hf and Sc) series

Noble metals (e.g., Ru, Ir and Pt) or their derivatives exhibit very appealing activity toward the hydrogen evolution reaction (HER), but their high price and low reserves impede their wide use. Herein, we propose a strategy in which, through the manipulation of crystal and electronic structure, one can convert a common metal to have a Pt-like performance for HER. To achieve this goal, a series of MCo2 (M = Ti, Zr, Hf and Sc) has been synthesized by using a rapid arc-melting method. TiCo2 exhibits comparable HER activity to that of Pt/C, for which it requires only -70 mV (V vs. RHE) to reach 10 mA cm-2 with a Tafel slope of 33 mV decade-1 in 1.0 M KOH. Moreover, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) indicate that the lower adsorption energy (ΔGH*) of H on the Co atom in TiCo2, due to the change in Co electronic state, is another key factor to account for its high HER activity. This case study offers a good illustration of how to transform a non-noble metal so it behaves like a noble metal toward HER and can potentially be applied under other conditions.

Hydrogen evolution reaction from bare and surface-functionalized few-layered MoS2 nanosheets in acidic and alkaline electrolytes

Hydrogen is considered as an ideal and sustainable energy carrier because of its high energy density and carbon-free combustion. Electrochemical water splitting is the only solution for uninterrupted, scalable, and sustainable production of hydrogen without carbon emission. However, a large-scale hydrogen production through electrochemical water splitting depends on the availability of earth-abundant electrocatalysts and a suitable electrolyte medium. In this article, we demonstrate that hydrogen evolution reaction (HER) performance of electrocatalytic materials can be controlled by their surface functionalization and selection of a suitable electrolyte solution. Here, we report syntheses of few-layered MoS₂ nanosheets, NiO nanoparticles (NPs), and multiwalled carbon nanotubes (MWCNTs) using scalable production methods from earth-abundant materials. Magnetic measurements of as-produced electrocatalyst materials demonstrate that MoS₂ nanoflakes are diamagnetic, whereas surface-functionalized MoS₂ and its composite with carbon nanotubes have strong ferromagnetism. The HER performance of the few-layered pristine MoS₂ nanoflakes, MoS₂/NiO NPs, and MoS₂/NiO NPs/MWCNT nanocomposite electrocatalysts are studied in acidic and alkaline media. For bare MoS₂, the values of overpotential (η₁₀) in alkaline and acidic media are 0.45 and 0.54 V, respectively. Similarly, the values of current density at 0.5 V overpotential are 27 and 6.2 mA/cm² in alkaline and acidic media, respectively. The surface functionalization acts adversely in the both alkaline and acidic media. MoS₂ nanosheets functionalized with NiO NPs also demonstrated excellent performance for oxygen evolution reaction with anodic current of ~60 mA/cm² and Tafel slope of 78 mVdec^-1 in alkaline medium.

Spatially confined electrochemical conversion of metal-organic frameworks into metal-sulfides and their in situ electrocatalytic investigation via scanning electrochemical microscopy

There is an on-going search for new earth-abundant electrocatalytic materials, suitable for replacing noble-metals as efficient accelerators of energy-conversion reactions. In this regard, over the last few years, metal-organic framework (MOF)-converted materials have demonstrated promising electrocatalytic properties. Nevertheless, the discovery of new catalytic materials requires development of methods combining high-throughput synthesis and electrochemical-activity screening. To do so, here we couple the synthetical and the analytical virtues of scanning electrochemical microscopy (SECM). Namely, we first utilized an SECM tip electrode to induce spatially confined (μm-scale) electrochemical conversion of cobalt-based ZIF-67 MOFs into patterns of cobalt sulfide with a tuned chemical composition. In turn, the same SECM setup was used to map the H₂ evolution activity of the as-formed cobalt sulfide. Hence, the presented method should have great implications for future screening of new electrocatalytic materials for a variety of energy-related applications.

Subnano Amorphous Fe-Based Clusters with High Mass Activity for Efficient Electrocatalytic Oxygen Reduction Reaction

The development of cost-effective and efficient oxygen-relative electrocatalysts with high mass activity is extremely critical for modern sustainable fuel cells. Here, we present a new type of subnano amorphous transition-metal clusters supported on a hierarchical carbon framework as a promising oxygen reduction reaction (ORR) electrocatalyst, synthesized by a novel "amino-induced spatial confinement" strategy. This developed Fe subnano-cluster/3D-C could deliver outstanding ORR performance with a large mass activity of ∼8600 A g_Fe^-1 at a half-wave potential of 0.92 V, ∼10 times that of the benchmarking Pt/C electrocatalyst. The atomic characterizations and theoretical calculations jointly reveal the robust surface-covalent Fe-N bonds, and the synergistic effect of hetero Fe^2+/0 species is essentially beneficial for the adsorption of *O₂ and the formation of key *O intermediate during the ORR process, contributing to high oxygen-relative electrocatalytic activity for subnano amorphous Fe clusters.

CeOx-Decorated Hierarchical NiCo2S4 Hollow Nanotubes Arrays for Enhanced Oxygen Evolution Reaction Electrocatalysis

Developing hollow self-supported nanotube arrays with hierarchical microporous and abundant multiactive sites shows great promise for oxygen evolution reaction (OER) electrocatalysis. Herein, a facile and low-cost strategy of NiCo₂S₄ hollow nanotubes arrays decorated with CeO_x nanoparticles (NPs) assembled on a flexible support carbon cloth (CC) for enhanced OER performance is reported. The obtained hierarchical nanoarrays CeO_x/NiCo₂S₄/CC exhibited excellent activity toward OER with an overpotential (270 mV) at 10 mA cm^-2, relatively weak Tafel slope, and distinguished durability. CeO_x/NiCo₂S₄/CC nanoarrays not only provide fast electronic transmission and well-defined connection to the substrate but also defective sites and electron transfer by the introduction of CeO_x NPs. This new strategy was offered to construct low-cost and effectively hierarchical structural electrocatalysts containing rare-earth species.

Design Superior Alkaline Hydrogen Evolution Electrocatalyst by Engineering Dual Active Sites for Water Dissociation and Hydrogen Desorption

In alkaline media, the water-dissociation-related Volmer process always suppresses the hydrogen formation/desorption process, which makes it challenging to develop non-noble-metal alkaline electrocatalysts with excellent catalytic activity. Here, we proposed a two-pronged strategy to simultaneously promote the kinetic process of both water dissociation and hydrogen desorption with the Co-doped WO₂/amorphous Co_xW hybrid electrocatalyst. Impressively, the optimized hybrid exhibits an outstanding hydrogen evolution reaction (HER) activity with the quite small Tafel slope of 19.77 mV dec^-1 and ultralow overpotential of just 25 mV to reach a current density of 10 mA cm^-2 in alkaline media. Both experiments and density functional theory calculations reveal that the top-level HER performance can be attributed to the cooperation of two different active components, in which the water molecule can easily be activated on the amorphous Co_xW with low energy barrier (ΔG_w = 0.46 eV), while hydrogen atoms can rapidly desorb from the Co-doped WO₂ with an optimal Gibbs free energy of hydrogen adsorption (ΔG_H* = -0.06 eV). Also, the density functional theory calculation further confirms that the H* tends to combine with another H* via Tafel step rather than Heyrovsky step. The findings provide unique insights for the development of the state-of-the-art non-noble-metal HER electrocatalyst with a Pt-like kinetic behavior in alkaline media.

Engineering the In-Plane Structure of Metallic Phase Molybdenum Disulfide via Co and O Dopants toward Efficient Alkaline Hydrogen Evolution

Molybdenum disulfide (MoS₂) has attracted much attention as a promising alternative to Pt-based catalysts for highly efficient hydrogen generation. However, it suffers sluggish kinetics for driving the hydrogen evolution reaction (HER) process because of inert basal planes, especially in alkaline solution. Here, we show a combination of heteroatom doping and phase transformation strategies to engineer the in-plane structure of MoS₂, that trigger their catalytic activities. Systematic characterizations are performed with advanced aberration-corrected microscopy and X-ray techniques, indicating that an as-designed MoS₂ catalyst has a distorted zigzag-chain superlattice in metallic phase, while its in-plane structure was engineered via the incorporation of cobalt and oxygen species. The optimal Co, O dual-doped metallic phase molybdenum disulfide (1T-MoS₂) electrocatalyst shows a significantly enhanced HER activity with a low overpotential of 113 mV at 10 mA cm^-2 and corresponding small Tafel slope of 50 mV dec^-1, accompanied by the robust stability in alkaline media. The calculated turnover frequency is higher than 6.65 H₂ s^-1 at an overpotential of 200 mV. More in-depth insights from the first-principle calculations illustrate that the water dissociation as a rate-determining step was largely accelerated by the in-plane Co-O-Mo species and fast electron transfer of the catalyst. Benefiting from ingenious design and fine identifications, this work provides a fundamental understanding of the relationships among heteroatom doping, phase transformation, and performance for MoS₂-based catalysts.

Steering elementary steps towards efficient alkaline hydrogen evolution via size-dependent Ni/NiO nanoscale heterosurfaces

Alkaline hydrogen evolution reaction (HER), consisting of Volmer and Heyrovsky/Tafel steps, requires extra energy for water dissociation, leading to more sluggish kinetics than acidic HER. Despite the advances in electrocatalysts, how to combine active sites to synergistically promote both steps and understand the underlying mechanism remain largely unexplored. Here, Density Functional Theory (DFT) calculations predict that NiO accelerates the Volmer step while metallic Ni facilitates the Heyrovsky/Tafel step. A facile strategy is thus developed to control Ni/NiO heterosurfaces in uniform and well-dispersed Ni-based nanocrystals, targeting both reaction steps synergistically. By systematically modulating the surface composition, we find that steering the elementary steps through tuning the Ni/NiO ratio can significantly enhance alkaline HER activity, and Ni/NiO nanocrystals with a Ni/NiO ratio of 23.7% deliver the best activity, outperforming other state-of-the-art analogues. The results suggest that integrating bicomponent active sites for elementary steps is effective for promoting alkaline HER, but they have to be balanced.

Catalytic Surface Specificity of Ni(OH)2 -Decorated Pt Nanocubes for the Hydrogen Evolution Reaction in an Alkaline Electrolyte

Because the hydrogen evolution reaction (HER) in alkaline electrolyzers is initiated by water dissociation, the hydrogen evolution kinetics are sluggish even on highly active Pt catalysts. Here, we have synthesized Ni(OH)₂ -decorated Pt nanocubes as a bifunctional catalyst to enhance the HER kinetics in an alkaline medium. Electrochemical cyclic voltammetry and CO-stripping measurements confirmed the selective deposition of Ni(OH)₂ on the Pt(1 0 0) facets of nanocubes. Electrocatalytic HER activity of the Ni(OH)₂ -decorated Pt nanocubes demonstrated that the bifunctional catalytic surface promotes the Volmer step kinetics and thus the Volmer/Tafel coupling dominant. As the result, catalytic surface specificity of Ni(OH)₂ -decorated Pt nanocubes enhanced water dissociation, reduced contamination of OH_ad on Pt surface, and maintained long-term HER performance in alkaline electrolytes.

Enhanced Hydrogen Evolution Reaction Performance of NiCo2P by Filling Oxygen Vacancies by Phosphorus in Thin-Coating CeO2

A series of NiCo₂P-based electrocatalysts, which were wrapped by CeO₂ whose oxygen vacancies (V_O) are partially filled with phosphorus atoms (named as NiCo₂P^x/P^xFVo-CeO₂, where x refers to the consumption of NaH₂PO₂·H₂O), have been fabricated to improve the electrocatalytic reactivity of NiCo₂P toward hydrogen evolution in alkaline solution. In the novel catalysts, the P atoms fill the oxygen vacancies, elevate the chemical valence state of Ni²⁺ and Co³⁺, and increase the hydride acceptors, which reinforcing the promoting effect of CeO₂ in the hydrogen evolution reaction (HER). Moreover, the negatively charged P atoms capture the positively charged protons more easily, benefiting the Volmer step during HER. Furthermore, the synergistic effect between oxygen vacancies and the filled P atoms accelerates the migration rate of electrons/ions and increases the electrochemical active area. All of the above are advantageous to the hydrogen evolution of NiCo₂P^x/P^xFVo-CeO₂ in alkaline electrolyte. As a result, the overpotential as low as 33.6 mV is achieved for NiCo₂P^0.3/P^0.3FVo-CeO₂ in alkaline media to drive a current density of 10 mA cm^-2. The reactivity is superior to that of Pt/C at a large current density along with a Tafel slope of 61.24 mV dec^-1 and long-term durability, which giving a new technology for efficient transition-metal catalyst candidates toward HER in alkaline solution.