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

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.

P-Doped Iron-Nickel Sulfide Nanosheet Arrays for Highly Efficient Overall Water Splitting

Iron-nickel sulfide ((Ni,Fe)₃S₂) is one of the most promising bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media because of their metallic conductivity and low cost. However, the reported HER activity of (Ni,Fe)₃S₂ is still unsatisfactory. Herein, three-dimensional self-supported phosphorus-doped (Ni,Fe)₃S₂ nanosheet arrays on Ni foam (P-(Ni,Fe)₃S₂/NF) are synthesized by a simple one-step simultaneous phosphorization and sulfuration route, which exhibits dramatically enhanced HER activity as well as drives remarkable OER activity. The incorporation of P significantly optimized the hydrogen/water absorption free energy (ΔG_H*/ΔG_H2O*), enhanced electrical conductivity, and increased electrochemical surface area. Accordingly, the optimal P-(Ni,Fe)₃S₂/NF exhibits relatively low overpotentials of 98 and 196 mV at 10 mA cm^-2 for HER and OER in 1 M KOH, respectively. Furthermore, an alkaline electrolyzer comprising the P-(Ni,Fe)₃S₂/NF electrodes needs a very low cell voltage of 1.54 V at 10 mA cm^-2 and exhibits long-term stability and outperforms most other state-of-the-art electrocatalysts. The reported electrocatalyst activation approach by anion doping can be adapted for other transition-metal chalcogenides for water electrolysis, offering great promise for future applications.

A Roadmap to Low-Cost Hydrogen with Hydroxide Exchange Membrane Electrolyzers

Hydrogen is an ideal alternative energy carrier to generate power for all of society's energy demands including grid, industrial, and transportation sectors. Among the hydrogen production methods, water electrolysis is a promising method because of its zero greenhouse gas emission and its compatibility with all types of electricity sources. Alkaline electrolyzers (AELs) and proton exchange membrane electrolyzers (PEMELs) are currently used to produce hydrogen. AELs are commercially mature and are used in a variety of industrial applications, while PEMELs are still being developed and find limited application. In comparison with AELs, PEMELs have more compact structure and can achieve higher current densities. Recently, however, an alternative technology to PEMELs, hydroxide exchange membrane electrolyzers (HEMELs), has gained considerable attention due to the possibility to use platinum group metal (PGM)-free electrocatalysts and cheaper membranes, ionomers, and construction materials and its potential to achieve performance parity with PEMELs. Here, the state-of-the-art AELs and PEMELs along with the current status of HEMELs are discussed in terms of their positive and negative aspects. Additionally discussed are electrocatalyst, membrane, and ionomer development needs for HEMELs and benchmark electrocatalysts in terms of the cost-performance tradeoff.

The mechanistic role of a support-catalyst interface in electrocatalytic water reduction by Co3O4 supported nanocarbon florets

Comprehending the mechanistic involvement of a support-catalyst interface is critical for effective design of industrially relevant electrocatalytic processes such as the alkaline hydrogen evolution reaction (alHER). The understanding of the kinetically sluggish alHER exhibited by both Pt and Pt-group-metal-free catalysts is primarily derived from indirect electrochemical parameters such as the Tafel slope. To address these issues, we establish the critical role of a nanocarbon floret (NCF) based electrochemical support in generating a key cobalt-oxohydroxo (OH-Co[double bond, length as m-dash]O) intermediate during the alHER through operando Raman spectro-electrochemistry. Specifically, interfacial nano-engineering of a newly designed carbon support (NCF) with a spinel Co₃O₄ nanocube catalyst is demonstrated to achieve a facile alHER (-0.46 V@10 mA cm^-2). Such an efficient alHER is mainly attributed to the unique lamellar morphology with a high mesoporous surface area (936 m² g^-1) of the NCF which catalyses the rate-determining water dissociation step and facilitates rapid ion diffusion. The dissociated water drives the formation of the OH-Co[double bond, length as m-dash]O intermediate, spectroscopically captured for the first time through the emergence of a νOH-Co[double bond, length as m-dash]O Raman peak (1074 cm^-1). The subsequent alHER proceeds through the Volmer-Heyrovsky route (119 mV dec^-1) via the T_d Co²⁺↔ Co³⁺↔ Co⁴⁺ oxidative pathway. Concomitant graphitization of the NCF through the disappearance of νsp³C-H (2946 cm^-1) supports the co-operative dynamics at the Co₃O₄-NCF interface. Thus, the NCF positively contributes towards the lowering of the overpotential with a low charge-transfer resistance (R_ct = 35.8 Ω) and high double layer capacitance (C_dl = 410 mF cm^-2).

Interface and defect engineering of hybrid nanostructures toward an efficient HER catalyst

The hydrogen evolution reaction (HER) plays a key role in hydrogen production for clean energy harvesting. Designing novel efficient and robust electrocatalysts with sufficient active sites and excellent conductivity is one of the key parameters for hydrogen production using water splitting devices. Recently, low-dimensional carbon materials have gained attention as metal-free catalysts for hydrogen production. Such nanostructures need to be engineered to improve their catalytic activity. Here, we designed and synthesized a B and N doped carbon nanostructure (CNS)-hBN heterostructure as an improved HER catalyst. The hBN layers on CNS could provide exposed defects and edges that act as active sites for proton adsorption and reduction. The composition, structure and chemical properties of the B and N doped CNS-hBN heterostructure were tuned to obtain excellent HER activity. Detailed morphological, structural and electrochemical characterization demonstrated that the synergistic effect rising from the interaction between B and N doped CNS and hBN structures contributes to enhance the electrocatalytic performances. To get more insight into the role of defects and doping, we performed density functional theory (DFT) calculations on the CNS-hBN heterostructure.

Low-Cost Porous Ruthenium Layer Deposited on Nickel Foam as a Highly Active Universal-pH Electrocatalyst for the Hydrogen Evolution Reaction

Low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) are a key constituent of a low-carbon industrial economy based on intermittent energy production in the near future. A facile wet-chemistry strategy has been developed for the synthesis of a porous Ru layer deposited onto Ni foam (NF) as a competitive candidate for HER over the whole pH range, especially under economical alkaline conditions. The catalyst shows outstanding HER performance, which stems from the porosity of the Ru layer, the electronic structure of the electrode, and the charge transfer between the NF and the Ru layer, which gives rise to the strong activity of the Ru layer in the HER process. Moreover, the Ru loading was as low as approximately 1.1 wt %, representing significant potential for application in cost-effective HER.

Design and Mechanistic Study of Highly Durable Carbon-Coated Cobalt Diphosphide Core-Shell Nanostructure Electrocatalysts for the Efficient and Stable Oxygen Evolution Reaction

The facile synthesis of hierarchically functional, catalytically active, and electrochemically stable nanostructures holds a tremendous promise for catalyzing the efficient and durable oxygen evolution reaction (OER) and yet remains a formidable challenge. Herein, we report the scalable production of core-shell nanostructures composed of carbon-coated cobalt diphosphide nanosheets, C@CoP₂, via three simple steps: (i) electrochemical deposition of Co species, (ii) gas-phase phosphidation, and (iii) carbonization of CoP₂ for catalytic durability enhancement. Electrochemical characterizations showed that C@CoP₂ delivers an overpotential of 234 mV, retains its initial activity for over 80 h of continuous operation, and exhibits a fast OER rate of 63.8 mV dec^-1 in base.

Synergy of Nb Doping and Surface Alloy Enhanced on Water-Alkali Electrocatalytic Hydrogen Generation Performance in Ti-Based MXene

Presented are the theoretical calculation and experimental studies of a Ti3C2T x MXene-based nanohybrid with simultaneous Nb doping and surface transition metal alloy modification. Guided by the density functional theory calculation, the Nb doping can move up the Fermi energy level to the conduction band, thus enhancing the electronic conductivity. Meanwhile, the surface modification by Ni/Co alloy can moderate the surface M-H affinity, which will further enhance the hydrogen evolution reaction (HER) activity. A series of Ni/Co alloy attached on Nb-doped Ti3C2T x MXene nanohybrids (denoted as NiCo@NTM) are successfully prepared. As expected, the Ni0.9Co0.1@ NTM nanohybrids present an extraordinary HER activity in alkaline solution, which only needs an overpotential (η) of 43.4 mV to reach the current density of 10 mA cm-2 in 1 m KOH solution and shows good stability. The performance of the Ni0.9Co0.1@ NTM nanohybrids is comparable to the commercial 10% Pt/C electrode (34.4 mV@10 mA cm-2) and is better than most state-of-the-art Pt-free HER catalysts. Inspired by the facile synthesis process and chemical versatility of both MXene and transition metal alloys, the nanohybrids reported here are promising non-noble metal electrocatalysts for water-alkali electrolysis.

Benchmarking Three Ruthenium Phosphide Phases for Electrocatalysis of the Hydrogen Evolution Reaction: Experimental and Theoretical Insights

The outstanding electrocatalytic activity of ruthenium (Ru) phosphides toward the hydrogen evolution reaction (HER) has received wide attention. However, the effect of the Ru phosphide phase on the HER performance remains unclear. Herein, a two-step method was developed to synthesize nanoparticles of three types of Ru phosphides, namely, Ru₂ P, RuP, and RuP₂ , with similar morphology, dimensions, loading density, and electrochemical surface area on graphene nanosheets by simply controlling the dosage of phytic acid as P source. Electrochemical tests revealed that Ru₂ P/graphene shows the highest intrinsic HER activity, followed by RuP/graphene and RuP₂ /graphene. Ru₂ P/graphene affords a current density of 10 mA cm^-2 at an overpotential of 18 mV in acid media. Theoretical calculations further showed that P-deficient Ru₂ P has a lower free energy of hydrogen adsorption on the surface than other two, P-rich Ru phosphides (RuP, RuP₂ ), which confirms the excellent intrinsic HER activity of Ru₂ P and is consistent with experiment results. The work reveals for the first time a clear trend of HER activity among three Ru phosphide phases.

Engineering 2D Metal-Organic Framework/MoS2 Interface for Enhanced Alkaline Hydrogen Evolution

2D metal-organic frameworks (MOFs) have been widely investigated for electrocatalysis because of their unique characteristics such as large specific surface area, tunable structures, and enhanced conductivity. However, most of the works are focused on oxygen evolution reaction. There are very limited numbers of reports on MOFs for hydrogen evolution reaction (HER), and generally these reported MOFs suffer from unsatisfactory HER activities. In this contribution, novel 2D Co-BDC/MoS₂ (BDC stands for 1,4-benzenedicarboxylate, C₈ H₄ O₄ ) hybrid nanosheets are synthesized via a facile sonication-assisted solution strategy. The introduction of Co-BDC induces a partial phase transfer from semiconducting 2H-MoS₂ to metallic 1T-MoS₂ . Compared with 2H-MoS₂ , 1T-MoS₂ can activate the inert basal plane to provide more catalytic active sites, which contributes significantly to improving HER activity. The well-designed Co-BDC/MoS₂ interface is vital for alkaline HER, as Co-BDC makes it possible to speed up the sluggish water dissociation (rate-limiting step for alkaline HER), and modified MoS₂ is favorable for the subsequent hydrogen generation step. As expected, the resultant 2D Co-BDC/MoS₂ hybrid nanosheets demonstrate remarkable catalytic activity and good stability toward alkaline HER, outperforming those of bare Co-BDC, MoS₂ , and almost all the previously reported MOF-based electrocatalysts.

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

Integrating Catalysis of Methane Decomposition and Electrocatalytic Hydrogen Evolution with Ni/CeO2 for Improved Hydrogen Production Efficiency

Ni/CeO₂ enables either methane decomposition or water electrolysis for pure hydrogen production. Ni/CeO₂ , prepared by a sol-gel method with only one heat treatment step, was used to catalyze methane decomposition for the generation of H₂ . The solid byproduct, Ni/CeO₂ /carbon nanotube (CNT), was further employed as an electrocatalyst in the hydrogen evolution reaction (HER) for H₂ production. The Ni/CeO₂ catalyst exhibits excellent activity for methane decomposition because CeO₂ prevents carbon encapsulation of Ni nanoparticles during the preparation process and forms a special metal-support interface with Ni. The derived CNTs act as antenna to improve conductivity and promote the dispersion of agglomerated Ni/CeO₂ . In addition, they provide H₂ diffusion paths and prevent Ni/CeO₂ from peeling off the HER electrode. Although long-term methane decomposition reduces the HER activity of Ni/CeO₂ /CNTs (owing to degradation of the delicate Ni/CeO₂ interface), the tunable nature of the synthesis makes this an attractive sustainable approach to synthesize future high-performance materials.

3D Heteroatom-Doped Carbon Nanomaterials as Multifunctional Metal-Free Catalysts for Integrated Energy Devices

Sustainable and cost-effective energy generation has become crucial for fulfilling present energy requirements. For this purpose, the development of cheap, scalable, efficient, and reliable catalysts is essential. Carbon-based heteroatom-doped, 3D, and mesoporous electrodes are very promising as catalysts for electrochemical energy conversion and storage. Various carbon allotropes doped with a variety of heteroatoms can be utilized for cost-effective mass production of electrode materials. 3D porous carbon electrodes provide multiple advantages, such as large surface area, maximized exposure to active sites, 3D conductive pathways for efficient electron transport, and porous channels to facilitate electrolyte diffusion. However, it is challenging to synthesize and functionalize isotropic 3D carbon structures. Here, various synthesis processes of 3D porous carbon materials are summarized to understand how their physical and chemical properties together with heteroatom doping dictate the electrochemical catalytic performance. Prospects of attractive 3D carbon structural materials for energy conversion and efficient integrated energy systems are also discussed.

Highly Efficient Hybrid Ni/Nitrogenated Graphene Electrocatalysts for Hydrogen Evolution Reaction

Two nickel/nitrogenated graphene hybrid electrodes (Ni-NrGO NH3 and Ni-NrGO APTES ) were synthesized, and their catalytic activity with respect to the hydrogen evolution reaction (HER) in alkaline media was analyzed. Incorporation of nitrogen to the carbon structure in graphene oxide (GO) or reduced GO (rGO) flakes in aqueous solutions was carried out based on two different configurations. NrGO NH 3 particles were obtained by a hydrothermal method using ammonium hydroxide as the precursor, and NGO APTES particles were obtained by silanization (APTES functionalization) of GO sheets. Aqueous dispersions containing NrGO NH 3 and NGO APTES particles were added to the traditional nickel Watts plating bath in order to prepare the Ni-NrGO NH 3 and Ni-NrGO APTES catalysts, respectively. Nickel substrates were coated with the hybrid nickel electrodeposits and used as electrodes for hydrogen production. The Ni-NrGO catalysts show a higher activity than the conventional nickel electrodeposited electrodes, particularly the ones containing APTES molecules because they allow obtaining a hydrogen current density 130% higher than conventional Ni-plated electrodes with a Watts bath in the absence of additives. In addition, both catalysts show a low deactivation rate during the ageing treatment, which is a sign of a longer midlife for the catalyst. Cyclic voltammetry and electrochemical impedance spectroscopy measurements were used for examination of the catalytic efficiency of hybrid Ni-NrGO electrodes for HER in KOH solution. High values of exchange current densities, 8.53 × 10-4 and 2.53 × 10-5 mA cm-2 for HER in alkaline solutions on Ni-NrGO NH 3 and Ni-NrGO APTES electrodes, respectively, were obtained.

Over 56.55% Faradaic efficiency of ambient ammonia synthesis enabled by positively shifting the reaction potential

Ambient electrochemical N₂ reduction is emerging as a highly promising alternative to the Haber–Bosch process but is typically hampered by a high reaction barrier and competing hydrogen evolution, leading to an extremely low Faradaic efficiency. Here, we demonstrate that under ambient conditions, a single-atom catalyst, iron on nitrogen-doped carbon, could positively shift the ammonia synthesis process to an onset potential of 0.193 V, enabling a dramatically enhanced Faradaic efficiency of 56.55%. The only doublet coupling representing ¹⁵NH₄⁺ in an isotopic labeling experiment confirms reliable NH₃ production data. Molecular dynamics simulations suggest efficient N₂ access to the single-atom iron with only a small energy barrier, which benefits preferential N₂ adsorption instead of H adsorption via a strong exothermic process, as further confirmed by first-principle calculations. The released energy helps promote the following process and the reaction bottleneck, which is widely considered to be the first hydrogenation step, is successfully overcome.

While direct N₂ reduction using electrochemistry offers an appealing method to obtain usable nitrogen, materials typically show poor activities and efficiencies. Here, authors demonstrate a single-atom catalyst, iron on N-doped carbon, to have dramatically enhanced N₂ reduction efficiencies.

Copper coordination polymer electrocatalyst for strong hydrogen evolution reaction activity in neutral medium: influence of coordination environment and network structure

Water-coordinated copper coordination polymer exhibited strong enhancement of HER activity in neutral medium with good stability compared to non-water-coordinated coordination polymer.

The Co2+/Ni2+ ion-mediated formation of a topochemically converted copper coordination polymer: structure-dependent electrocatalytic activity

The presence of Co²⁺/Ni²⁺ ions strongly influenced the formation of copper coordination polymers that showed a structure-dependent hydrogen evolution reaction catalytic activity.

A robust electrocatalytic activity toward the hydrogen evolution reaction from W/W2C heterostructured nanoparticles coated with a N,P dual-doped carbon layer

W/W₂C heterostructured nanoparticles encapsulated by N,P dual-doped carbon require low overpotentials of 55 mV and 82 mV vs. RHE to achieve cathodic current density of 10 mA cm⁻² in acidic and alkaline electrolytes, respectively.

Dual tuning of nickel sulfide nanoflake array electrocatalyst through nitrogen doping and carbon coating for efficient and stable water splitting

Simultaneous carbon coating and nitrogen incorporation of a Ni₃S₂ nanoflake array electrocatalyst with enhanced activity and stability for water splitting.

Iron phosphide encapsulated in P-doped graphitic carbon as efficient and stable electrocatalyst for hydrogen and oxygen evolution reactions

The development of durable and efficient non-noble electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is highly desirable but challenging for the commercialization of renewable energy systems. Herein, a facile strategy is developed for the synthesis of iron phosphide (FeP) nanoparticles protected with an overcoat of "multifunctional" P-doped graphitic carbon as a cost-effective electrocatalyst. The key point is the utilization of MOF-derived iron nanoparticles embedded in graphitic carbon (Fe@GC), which are synthesized via the pyrolysis of the Fe-MIL-88 template and subsequent phosphorization of Fe and simultaneous doping of P in carbon. Compared to the direct phosphorization of Fe-MIL-88, resulting in Fe₂P on amorphous carbon (Fe₂P@APC), this strategy gives easier access to phosphorization and P doping through pyrolysis temperature regulation. High-temperature pyrolysis can also yield the graphitic carbon encapsulated nanoparticle structure (FeP@GPC), which increases conductivity and prevents agglomeration as well as dissolution under harsh operating conditions, and thus contributes to enhanced activity and long-time stability. The optimized FeP@GPC exhibits superior activity compared to Fe₂P/FeP@GPC and Fe₂P@APC, which is attributed to the modified electronic structure of FeP due to its greater P proportion than Fe₂P together with the strong synergy between the nanoparticles and graphitic carbon. In detail, FeP@GPC exhibits an ultralow overpotential of 72 mV and 278 mV to achieve the current density of 10 mA cm^-2 for the HER in acid and OER in alkaline media, respectively, together with negligible degradation after 20 h, which ranks among the best Fe-based electrocatalysts.

Fe2 N/S/N Codecorated Hierarchical Porous Carbon Nanosheets for Trifunctional Electrocatalysis

Construction of multifunctional highly active earth-abundant electrocatalysts on a large scale is a great challenge due to poor control over nanostructural features and limited active sites. Here, a simple methodology to tailor metal-organic frameworks (MOFs) to extract highly active multifunctional electrocatalysts on a large scale for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution reaction (HER) is presented. The N, S codoped Fe₂ N decorated highly porous and defect-rich carbon nanosheets are grown using MOF xerogels, melamine, and polyvinylpyrollidone. The resulting catalyst exhibits excellent activity for ORR with an onset (0.92 V) and half-wave (0.81 V) potential similar to state-of-the-art Pt/C catalysts. The catalyst also shows outstanding OER and HER activities with a small overpotential of 360 mV in 1 m KOH and -123 mV in 0.5 m H₂ SO₄ at a current density of 10 mA cm^-2 , respectively. Excellent catalytic properties are further supported by theoretical calculations where relevant models are built and various possible activation sites are identified by first-principles calculations. The results suggest that the carbon atoms adjacent to heteroatoms as well as Fe₂ -N sites present the active sites for improved catalytic response, which is in agreement with the experimental results.

Hollow Porous Heterometallic Phosphide Nanocubes for Enhanced Electrochemical Water Splitting

Highly efficient earth-abundant electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of great importance for renewable energy conversion systems. Herein, hollow porous heterometallic phosphide nanocubes are developed as a highly active and robust catalyst for electrochemical water splitting via one-step phosphidation of a NiCoFe Prussian blue analogue. Through modulation of the composition of metals in the precursors, the optimal NiCoFeP exhibiting increased electrical conductivity and abundant electrochemically active sites, leading to high electrocatalytic activities and outstanding kinetics for both HER and OER, is successfully obtained. NiCoFeP shows low overpotentials of 273 mV for OER and 131 mV for HER at a current density of 10 mA cm^-2 and quite low Tafel slopes of 35 mV dec^-1 for OER and 56 mV dec^-1 for HER.

Ni(OH)2 -WP Hybrid Nanorod Arrays for Highly Efficient and Durable Hydrogen Evolution Reactions in Alkaline Media

The development of efficient non-noble-metal hydrogen evolution electrocatalysts in alkaline media is crucial for sustainable, ecofriendly production of H₂ through water electrolysis. An alkaline hydrogen evolution reaction (HER) catalyst composed of Ni(OH)₂ -decorated thungsten phosphide (WP) nanorod arrays on carbon paper was synthesized by thermal evaporation and electrodeposition. This hybrid catalyst displayed outstanding HER activity and required a low overpotential of only 77 mV to obtain a current density of 10 mA cm^-2 and a Tafel slope of 71 mV dec^-1 . The hybrid catalyst also showed long-term electrochemical stability, maintaining its activity for 18 h. This improved HER efficiency was attributed to the synergetic effect of WP and Ni(OH)₂ : Ni(OH)₂ effectively lowers the energy barrier during water dissociation and also provides active sites for hydroxyl adsorption, whereas WP adsorbs hydrogen intermediates and efficiently produces H₂ gas. This interfacial cooperation offers not only excellent HER catalytic activity but also new strategies for the fabrication of effective non-noble-metal-based electrocatalysts in alkaline media.

Mesoporous Pd@Ru Core-Shell Nanorods for Hydrogen Evolution Reaction in Alkaline Solution

The activity and stability of bimetallic nanocatalysts strongly depend on their structures, compositions, and interfaces. Here, we report the synthesis of mesoporous Pd@Ru core-shell bimetallic nanorods composed of face-centered cubic Pd and hexagonal close-packed Ru. The nanorods have two types of cavities with diameters of 3.0 ± 0.9 and 20.3 ± 8.1 nm. The mutual diffusion process between Ru and Pd is characterized by the high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy mapping, and the synchrotron radiation photoemission spectroscopy measurements. The mesoporous Pd@Ru nanorods exhibit superior catalytic performance and stability for hydrogen evolution reactions (overpotentials of 30 mV at 10 mA·cm^-2 in 1.0 M KOH solution and 37 mV at 10 mA·cm^-2 in 0.5 M H₂SO₄ solution).

Enhancing the catalytic activity of the alkaline hydrogen evolution reaction by tuning the S/Se ratio in the Mo(SxSe1-x)2 catalyst

The alkaline hydrogen evolution reaction (HER) plays a key role in photo(electro)catalytic water splitting technologies, particularly in water-alkali electrolyzers. Unfortunately, although transition metal dichalcogenide (TMD) materials, especially MoS2 and MoSe2, are considered efficient, Earth-abundant catalysts for the HER in an acidic electrolyte, they are much less effective under high pH conditions due to a sluggish water dissociation process (Volmer-step) and strong adsorption of the OH- intermediate on their surfaces. Herein we show a novel synergetic effect obtained by tailoring the S/Se ratio in Mo(SxSe1-x)2 alloys. We were able to influence the metal oxidation state and d-band to optimize the catalytic sites for HOH dissociation and OH- desorption while maintaining favourable M-H energetics. The Mo(SxSe1-x)2 (x = 0.58) catalyst exhibited high performance with an onset potential of -43 mV in 0.5 M KOH, i.e., ∼3 and ∼4-fold less than that for MoSe2 and MoS2, respectively. The results obtained in the current study have implications for the rational design of cost-effective electro-catalysts for water reduction based on TMD alloys.

Graphite carbon nitride/boron-doped graphene hybrid for efficient hydrogen generation reaction

Metal-free carbon materials, with tuned surface chemical and electronic properties, hold great potential for the hydrogen evolution reaction (HER). We designed and synthesized a CN/BG hybrid electrocatalytic system with a porous and active graphite carbon nitride (CN) layer on boron-doped graphene (BG). A porous CN layer on graphene could provide exposed defects and edges that act as active sites for proton adsorption and reduction. The composition, structure, surface electronics, and chemical properties of this CN/BG hybrid system were tuned to obtain excellent HER activity and stability. Detailed surface chemical, morphological, and structural analyses demonstrated the synergetic effect arising from the electronic interaction between CN and BG, which contributed to the enhanced electrocatalytic performances.

Pt and Pt-Ni(OH)2 Electrodes for the Hydrogen Evolution Reaction in Alkaline Electrolytes and Their Nanoscaled Electrocatalysts

The design and synthesis of Pt-based electrocatalysts for the hydrogen evolution reaction (HER) are of great importance for the successful development of hydrogen-based alternative energy technologies. Although Pt is considered to be the most active catalyst for the HER, its reaction performance is limited in alkaline solutions owing to a slow rate for water dissociation. Therefore, many research groups have intensively investigated reaction mechanisms and developed system designs and efficient Pt-based catalysts to enhance the alkaline HER. Herein, we summarize the catalytic surface specificity of Pt and Pt-Ni(OH)₂ materials to control the kinetics of the alkaline HER. In particular, we increase our understanding of Ni(OH)₂ -modified Pt surfaces and the corresponding nanoscaled Pt-Ni(OH)₂ electrocatalysts to improve the sluggish water-dissociation step, and this knowledge will guide us to future sustainable energy applications of advanced nanomaterials.

Sulfide-Derived Copper for Electrochemical Conversion of CO2 to Formic Acid

The electrochemical CO₂ reduction reaction (CO2RR) has gained attention recently due to rising concern over atmospheric carbon levels, but catalyst selectivity and efficiency remain a challenge, particularly for products other than CO. Here, we report the selective formation of formate using a sulfide-derived copper (SD-Cu) catalyst for CO2RR. On the basis of in situ and postelectrolysis spectroscopy, we propose that this selectivity is due to stronger binding of the CO intermediate originating from remaining subsurface sulfur atoms.

In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution

Sluggish water dissociation kinetics on nonprecious metal electrocatalysts limits the development of economical hydrogen production from water-alkali electrolyzers. Here, using Co₃N electrocatalyst as a prototype, we find that during water splitting in alkaline electrolyte a cobalt-containing hydroxide formed on the surface of Co₃N, which greatly decreased the activation energy of water dissociation (Volmer step, a main rate-determining step for water splitting in alkaline electrolytes). Combining the cobalt ion poisoning test and theoretical calculations, the efficient hydrogen production on Co₃N electrocatalysts would benefit from favorable water dissociation on in-situ formed cobalt-containing hydroxide and low hydrogen production barrier on the nitrogen sites of Co₃N. As a result, the Co₃N catalyst exhibits a low water-splitting activation energy (26.57 kJ mol^-1) that approaches the value of platinum electrodes (11.69 kJ mol^-1). Our findings offer new insight into understanding the catalytic mechanism of nitride electrocatalysts, thus contributing to the development of economical hydrogen production in alkaline electrolytes.

N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen Production

Molybdenum (Mo) carbide-based electrocatalysts are considered promising candidates to replace Pt-based materials toward the hydrogen evolution reaction (HER). Among different crystal phases of Mo carbides, although Mo₂C exhibits the highest catalytic performance, the activity is still restricted by the strong Mo-H bonding. To weaken the strong Mo-H bonding, creating abundant Mo₂C/MoC interfaces and/or doping a proper amount of electron-rich (such as N and P) dopants into the Mo₂C crystal lattice are effective because of the electron transfer from Mo to surrounding C in carbides and/or N/P dopants. In addition, Mo carbides with well-defined nanostructures, such as one-dimensional nanostructure, are desirable to achieve abundant catalytic active sites. Herein, well-defined N,P-codoped Mo₂C/MoC nanofibers (N,P-Mo _xC NF) were prepared by pyrolysis of phosphomolybdic ([PMo₁₂O₄₀]^3-, PMo₁₂) acid-doped polyaniline nanofibers at 900 °C under an Ar atmosphere, in which the hybrid polymeric precursor was synthesized via a facile interfacial polymerization method. The experimental results indicate that the judicious choice of pyrolysis temperature is essential for creating abundant Mo₂C/MoC interfaces and regulating the N,P-doping level in both Mo carbides and carbon matrixes, which leads to optimized electronic properties for accelerating HER kinetics. As a result, N,P-Mo _xC NF exhibits excellent HER catalytic activity in both acidic and alkaline media. It requires an overpotential of only 107 and 135 mV to reach a current density of 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH, respectively, which is comparable and even superior to the best of Mo carbide-based electrocatalysts and other noble metal-free electrocatalysts.