Ultrafine metal phosphide nanoparticles in situ encapsulated in porous N,P-codoped nanofibrous carbon coated on carbon paper for effective water splitting

NiFe-layered double hydroxide/CoP2@MnP heterostructures of clustered flower nanowires on MXene-modified nickel foam for overall water-splitting

Exploiting efficient and economical electrocatalysts is indispensable to promoting the sluggish kinetics of overall water-splitting. Herein, we designed a phosphate reaction and two-step hydrothermal method to construct a 3D porous clustered flower-like heterogeneous structure of NiFe-layered double hydroxide (NiFe) and CoP2@MnP (CMP) grown in-situ on MXene-modified nickel foam (NF) substrate (denoted as NiFe/CMP/MX), with favorable kinetics. Density functional theory calculations (DFT) demonstrate that the self-driven transfer of heterojunction charges causes electron redistribution of the catalyst, and optimizes the electron transfer rate of the active site and the d-band center near the Fermi level, thereby reducing the adsorption energy of H and O reaction intermediates (H*, OH*, OOH*). As expected, the combination of CMP and NiFe with naturally conductive MXene forms a strong chemical and electron synergistic effect, which enables the synthesized NiFe/CMP/MX heterogeneous structure exhibits good activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with a low overpotential of 200 mV and 126 mV at 10 mA cm-2, respectively. Furthermore, the overpotential of 1.58 V is enough to drive a current density of 10 mA cm-2 in a two-electrode configuration, which is better than noble metals (RuO2(+)//Pt/C(-)) (1.68 V).

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Unveiling the advantages of an ultrathin N-doped carbon shell on self-supported tungsten phosphide nanowire arrays for the hydrogen evolution reaction experimentally and theoretically

Packaging electrocatalysts with carbon shells offers an opportunity to develop stable and effective hydrogen evolution reaction (HER) materials. Here, an ultrathin N-doped carbon-coated self-supported WP nanowire array (WP@NC NA) hybrid has been synthesized. Owing to the encapsulation of the ultrathin N-doped carbon shell on the WP surface, the as-prepared WP@NC NA hybrid exhibits enhanced physicochemical stability, more active sites, and superior conductivity compared with WP NA without carbon coating. Besides, density functional theory calculations demonstrate that the carbon shell can optimize the hydrogen adsorption step in the acidic HER, and simultaneously facilitate water physical adsorption, water dissociation, and hydroxyl group desorption steps during the alkaline HER. These findings demonstrate the intrinsic mechanism of how a carbon shell promotes the acidic and alkaline HER kinetics, and provide scientific guidance for the packaging design of promising carbon-encapsulating self-supported electrocatalysts.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Transition Metal Phosphide-Based Materials for Efficient Electrochemical Hydrogen Evolution: A Critical Review

As hydrogen has been increasingly considered as promising sustainable energy supply, electrochemical overall water splitting driven by highly efficient non-noble metal electrocatalysts has aroused extensive attention. Transition metal phosphides (TMPs) have demonstrated remarkable electrocatalytic performance, including high activity and robust durability towards hydrogen evolution reaction (HER) in acidic and alkaline as well as neutral electrolytes. In this Review, up-to-date progress of TMP-based HER electrocatalysts is summarized. Various synthesis strategies of TMPs based on selected phosphorus sources are presented, and the reaction mechanisms of HER as well as the contribution of phosphorus in the TMPs to HER activity are briefly discussed. The multiscale approaches for promoting the activity and stability of TMP-based catalysts are discussed with respect to intrinsic electronic structure, hybrids, microstructure, and working electrode interface. Some crucial issues and future perspectives of TMPs are pointed out. These modulated approaches and challenges are also instructive for constructing other high-activity energy-related electrocatalysts.

Puzzle-inspired carbon dots coupled with cobalt phosphide for constructing a highly-effective overall water splitting interface

The unique bamboo-like CDs/CoP nanoarray is beneficial to produce more H radicals and accelerate water decomposition.

Preparation of Yolk-Shell-Structured Cox Fe1-x P with Enhanced OER Performance

The design and development of low-cost, highly efficient, and stable electrocatalysts to take the place of noble-metal catalysts for the oxygen evolution reaction (OER) remain a significant challenge. Herein, the synthesis of yolk-shell-structured binary transition metal phosphide Co_x Fe_1-x P with different Co/Fe ratios by phosphidation of a cobalt ferrite precursor is reported. The as-synthesized Co_x Fe_1-x P catalysts were used for the OER. All yolk-shell Co_x Fe_1-x P catalysts with different Co/Fe ratios showed much better performance than the corresponding solid catalyst. The formation of Co oxides on the catalyst surface during OER and the optimal Co/Fe ratio were found to be critical to their activity. Among the as-prepared Co_x Fe_1-x P catalysts, that with a Co/Fe ratio of 0.47/0.53 (Co_0.47 Fe_0.53 P) exhibited the best performance. Co_0.47 Fe_0.53 P has an overpotential of 277 mV at a current density of 10 mA cm^-2 , a Tafel slope of 37 mV dec^-1 , and superior stability in alkaline medium. The outstanding performance is partly ascribed to the transfer of valence electrons from Co to P and Fe. The Co_0.47 Fe_0.53 P matrix with excellent conductivity and Fe phosphate that is stable on the surface of the catalyst are also helpful for the OER performance. In addition, the yolk-shell structure of Co_0.47 Fe_0.53 P increases the contact area between electrolyte and catalyst. These characteristics of Co_0.47 Fe_0.53 P greatly improve its OER performance. This optimized binary transition metal phosphide provides a new approach for the design of nonprecious-metal electrocatalysts.

Iron-Salt Thermally Emitted Strategy to Prepare Graphene-like Carbon Nanosheets with Trapped Fe Species for an Efficient Electrocatalytic Oxygen Reduction Reaction in the All-pH Range

Earth-abundant, highly active, and durable electrocatalysts toward oxygen reduction reaction (ORR) in the all-pH range are highly required for practical application of electrochemical energy conversion technologies. Here, non-noble-metal graphene-like carbon nanosheets with trapped Fe species (Fe-N/GPC) are developed by an iron-salt thermally emitted strategy, which integrates the modulation of the electronic structure for boosted intrinsic activity with the engineering of hierarchical porosity for enriched active sites. The ORR electrocatalytic performance of Fe-N/GPC-800 achieves the half-wave potentials of 0.86 and 0.77 V with limiting current densities of 6.1 and 4.7 mA cm^-2 in 0.1 M KOH and 0.1 M PBS solutions, respectively, as well as respectable stability. Furthermore, Fe-N/GPC-800 also shows considerable ORR catalytic activity in acid media accompanied by stability superior to those of Pt/C catalysts. The as-prepared Fe-N/GPC-800, as a cathodic catalyst, is assessed in a Zn-air battery test and delivers an open-circuit voltage of 1.44 V with a power density of 134 mW cm^-2 as well as the outstanding durability after 350 cycles at 10 mA cm^-2, demonstrating appreciable promise in application of metal-air batteries. This work provides an enabling and versatile strategy for facile and scale-up preparation of high-performance non-noble-metal electrocatalysts.

Self-supported Al-doped cobalt phosphide nanosheets grown on three-dimensional Ni foam for highly efficient water reduction and oxidation

Al-doped CoP nanosheets self-supported on Ni foam are shown to be an efficient bifunctional electrocatalyst for long-time overall water splitting.

Well-Defined Mo2C Nanoparticles Embedded in Porous N-Doped Carbon Matrix for Highly Efficient Electrocatalytic Hydrogen Evolution

On the design of efficient and affordable electrocatalysts for water reduction half reaction, this paper fabricates molybdenum carbide nanoparticles uniformly loaded in highly porous N-doped carbon matrix derived from polyaniline-molybdate monolith with the use of graphitic carbon nitride (g-C₃N₄) as template. The obtained molybdenum carbide-carbon hybrid catalysts (MoC@NCS) exhibit extraordinarily electrochemical hydrogen evolution activity with a small overpotential of 89 and 81 mV to deliver a current density of 10 mA cm^-2 in alkaline (1.0 M KOH) and acidic (0.5 M H₂SO₄) medium, respectively, even comparable to noble-metal Pt/C benchmark. Specially, MoC@NCS also shows excellent long-term durability in alkaline or acidic electrolyte. Furthermore, the obtained carbon matrix (NCS) featuring high content of N dopants and hierarchically porous architecture exhibits high catalytic efficiency for oxygen evolution reaction in alkaline electrolyte. For a further step, the obtained NCS coupled with the MoC@NCS, working as anodic and cathodic electrodes, in a two-electrode alkaline electrolyzer for overall water splitting, which can obtain a current density of 10 mA cm^-2 at 1.69 V, along with robust operation durability. The synergistic effect of the porous carbon matrix of high nitrogen content and the molybdenum carbide nanoparticles of uniform distribution, together with hierarchically porous structure, should be responsible for the outstanding electrocatalytic HER performance. This work presents an easy and cost-effective strategy to prepare molybdenum-based materials with controlled size for electrocatalytic hydrogen evolution.

Uniquely integrated Fe-doped Ni(OH)2 nanosheets for highly efficient oxygen and hydrogen evolution reactions

Developing high-efficiency electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is vital for the production of hydrogen on a large scale by electrocatalytic splitting of water. Herein, Fe-doped Ni(OH)₂ nanosheets directly grown on commercial Ni foam (FeNiOH/NF) were fabricated through a facile hydrothermal method in (NH₄)₂S₂O₈ aqueous solution containing iron salts. The integrated architecture with hierarchical pores is beneficial for exposing sufficient catalytically active sites and providing evaluated structural and electrical properties. In particular, the Fe-induced partial-charge-transfer greatly modifies the electronic structure of Ni(OH)₂, which evidently promotes the electrocatalytic activity of the as-fabricated FeNiOH/NF for OER and HER. Thus, as an electrocatalyst for OER, FeNiOH/NF exhibits excellent activity with overpotentials of 271 and 318 mV to deliver current densities of 20 and 100 mA cm^-2, respectively, with a small Tafel slope of 72 mV dec^-1 in 1.0 M KOH, demonstrating the very high level of novelty and sufficient improvement over the current state-of-the-art IrO₂ electrocatalyst. Most importantly, there is an increase in overpotential by only 23 mV during continuous reaction for over 20 h at an applied potential of 1.62 V to deliver current density of 500 mA cm^-2. The as-fabricated electrocatalyst also enables high HER activity with robust stability. Finally, an overall water splitting current density of 10 mA cm^-2 can be obtained at a cell voltage of 1.67 V in a two-electrode alkaline electrolyzer using FeNiOH/NF as both anode and cathode, along with impressive operation stability. This development with significant over the state-of-the-art IrO₂ electrocatalyst can be widely extended to large-scale fabrication of versatile electrocatalysts for efficient water splitting technology.