Boosting Electrochemical Hydrogen Evolution of Porous Metal Phosphides Nanosheets by Coating Defective TiO2 Overlayers

Precisely Control Relationship between Sulfur Vacancy and H Absorption for Boosting Hydrogen Evolution Reaction

Effective and robust catalyst is the core of water splitting to produce hydrogen. Here, we report an anionic etching method to tailor the sulfur vacancy (VS) of NiS2 to further enhance the electrocatalytic performance for hydrogen evolution reaction (HER). With the VS concentration change from 2.4% to 8.5%, the H* adsorption strength on S sites changed and NiS2-VS 5.9% shows the most optimized H* adsorption for HER with an ultralow onset potential (68 mV) and has long-term stability for 100 h in 1 M KOH media. In situ attenuated-total-reflection Fourier transform infrared spectroscopy (ATR-FTIRS) measurements are usually used to monitor the adsorption of intermediates. The S- H* peak of the NiS2-VS 5.9% appears at a very low voltage, which is favorable for the HER in alkaline media. Density functional theory calculations also demonstrate the NiS2-VS 5.9% has the optimal |ΔGH*| of 0.17 eV. This work offers a simple and promising pathway to enhance catalytic activity via precise vacancies strategy.

Charge Self-Regulation of Metallic Heterostructure Ni2 P@Co9 S8 for Alkaline Water Electrolysis with Ultralow Overpotential at Large Current Density

Designing cost-effective alkaline water-splitting electrocatalysts is essential for large-scale hydrogen production. However, nonprecious catalysts face challenges in achieving high activity and durability at a large current density. An effective strategy for designing high-performance electrocatalysts is regulating the active electronic states near the Fermi-level, which can improve the intrinsic activity and increase the number of active sites. As a proof-of-concept, it proposes a one-step self-assembly approach to fabricate a novel metallic heterostructure based on nickel phosphide and cobalt sulfide (Ni2 P@Co9 S8 ) composite. The charge transfer between active Ni sites of Ni2 P and Co─Co bonds of Co9 S8 efficiently enhances the active electronic states of Ni sites, and consequently, Ni2 P@Co9 S8 exhibits remarkably low overpotentials of 188 and 253 mV to reach the current density of 100 mA cm-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. This leads to the Ni2 P@Co9 S8 incorporated water electrolyzer possessing an ultralow cell voltage of 1.66 V@100 mA cm-2 with ≈100% retention over 100 h, surpassing the commercial Pt/C║RuO2 catalyst (1.9 V@100 mA cm-2 ). This work provides a promising methodology to boost the activity of overall water splitting with ultralow overpotentials at large current density by shedding light on the charge self-regulation of metallic heterostructure.

Heterostructure Interface Engineering in CoP/FeP/CeOx with a Tailored d-Band Center for Promising Overall Water Splitting Electrocatalysis

Accomplishing a green hydrogen economy in reality through water spitting ultimately relies upon earth-abundant efficient electrocatalysts that can simultaneously accelerate the oxygen and hydrogen evolution reactions (OER and HER). The perspective of electronic structure modulation via interface engineering is of great significance to optimize electrocatalytic output but remains a tremendous challenge. Herein, an efficient tactic has been explored to prepare nanosheet-assembly tumbleweed-like CoFeCe-containing precursors with time-/energy-saving and easy-operating features. Subsequently, the final metal phosphide materials containing multiple interfaces, denoted CoP/FeP/CeO_x, have been synthesized via the phosphorization process. Through the optimization of the Co/Fe ratio and the content of the rare-earth Ce element, the electrocatalytic activity has been regulated. As a result, bifunctional Co3Fe/Ce0.025 reaches the top of the volcano for both OER and HER simultaneously, with the smallest overpotentials of 285 mV (OER) and 178 mV (HER) at 10 mA cm^-2 current density in an alkaline environment. Multicomponent heterostructure interface engineering would lead to more exposed active sites, feasible charge transport, and strong interfacial electronic interaction. More importantly, the appropriate Co/Fe ratio and Ce content can synergistically tailor the d-band center with a downshift to enhance the per-site intrinsic activity. This work would provide valuable insights to regulate the electronic structure of superior electrocatalysts toward water splitting by constructing rare-earth compounds containing multiple heterointerfaces.

Increasing the Efficiency of Photocatalytic Water Splitting via Introducing Intermediate Bands

Photocatalytic water splitting is a potential way to utilize solar energy. To be practically useful, it is important to have a high solar-to-hydrogen (STH) efficiency. In this study, we propose a conceptually new photocatalytic water splitting model based on intermediate bands (IBs). In this new model, introducing IBs within the band gap can significantly increase the STH efficiency limit (from 30.7% to 48.1% without an overpotential and from 13.4% to 36.2% with overpotentials) compared to that in conventional single-band gap photocatalytic water splitting. First-principles calculations indicate that N-doped TiO₂, Bi-doped TiO₂, and P-doped ZnO have suitable IBs that can be used to construct IB photocatalytic water splitting systems. The STH efficiency limits of these three doped systems are 10.0%, 12.0%, and 19.0%, respectively, while those of pristine TiO₂ and ZnO without IB are only 0.9% and 1.6%, respectively. The IB photocatalytic water splitting model proposed in this study opens a new avenue for photocatalytic water splitting design.

In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting

The in situ growth of metal organic framework (MOF) derivatives on the surface of nickel foam is a novel type of promising self-supporting electrode catalyst. In this paper, this work reports for the first time the strategy of in situ growth of Ni-MOF, where the metal source is purely provided by a nickel foam (NF) substrate without any external metal ions. MOF-derived Ni2P/NPC structure is achieved by the subsequent phosphidation to yield Ni2P on porous N, P-doped carbon (NPC) backbone. Such strategy provides the as-synthesized Ni2P/NPC/NF electrocatalyst an extremely low interfacial steric resistance. Moreover, a unique three-dimensional hierarchical structure is achieved in Ni2P/NPC/NF, providing massive active sites, short ion diffusion path, and high electrical conductivity. Directly applied as the electrode, Ni2P/NPC/NF demonstrates excellent electrocatalytic performance towards both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with low overpotentials of only 58 mV and 208 mV to drive 10 mA cm−2, respectively, in 1 M KOH. Furthermore, Ni2P/NPC/NF acting as the overall water splitting electrodes can generate a current density of 10 mA cm−2 at an ultralow cell voltage of 1.53 V. This simple strategy paves the way for the construction of self-supporting transition metal-based electrocatalysts.

Enhanced electrocatalytic hydrogen evolution by molybdenum disulfide nanodots anchored on MXene under alkaline conditions

Efficient hydrogen production through electrocatalysis represents a promising path for the future clean energy. Molybdenum disulfide (MoS₂) is a good substitute for platinum-based catalysts, due to its low cost and high activity. However, the limited active sites and low electrical conductivity of MoS₂ hinder its large-scale industrial application under alkaline conditions. Herein, we constructed MoS₂ nanodots anchored on an MXene/nickel foam (MoS₂ NDs/MXene/NF) heterostructure by a cascade polymerization synthesis and in situ vulcanization. The prepared heterostructure displays an ultralow overpotential of 94 mV at a current density of 10 mA cm^-2 with a Tafel slope of only 59 mV dec^-1 in alkaline (1 M KOH) hydrogen evolution reaction (HER), and is better than conventional MoS₂ electrocatalysts reported so far. Fine structural analysis indicates that MoS₂ NDs are dispersed uniformly on the surface of the heterostructure with consistent orientation, leading to the improvement of MoS₂ conductivity with more paths for electron transfer. Moreover, the orientation of the synthesized MoS₂ NDs was verified to expose the more (002) crystal plane, which exhibits higher activity than other planes. Our results demonstrate that MoS₂ NDs with heterostructure design and preferential growth can serve as high-efficiency noble-metal free electrocatalysts for the HER in alkaline solution.

Subnanometric Ru clusters with upshifted D band center improve performance for alkaline hydrogen evolution reaction

Subnanometric metal clusters usually have unique electronic structures and may display electrocatalytic performance distinctive from single atoms (SAs) and larger nanoparticles (NPs). However, the electrocatalytic performance of clusters, especially the size-activity relationship at the sub-nanoscale, is largely unexplored. Here, we synthesize a series of Ru nanocrystals from single atoms, subnanometric clusters to larger nanoparticles, aiming at investigating the size-dependent activity of hydrogen evolution in alkaline media. It is found that the d band center of Ru downshifts in a nearly linear relationship with the increase of diameter, and the subnanometric Ru clusters with d band center closer to Femi level display a stronger water dissociation ability and thus superior hydrogen evolution activity than SAs and larger nanoparticles. Benefiting from the high metal utilization and strong water dissociation ability, the Ru clusters manifest an ultrahigh turnover frequency of 43.3 s^-1 at the overpotential of 100 mV, 36.1-fold larger than the commercial Pt/C.

Oxygen Vacancy Induced Boosted Visible-Light Driven Photocatalytic CO2 Reduction and Electrochemical Water Oxidation Over CuCo-ZIF@Fe2 O3 @CC Architecture

Herein, the obtained Cu_0.5 Co_0.5 -ZIF@Fe₂ O₃ @CC-150 heterojunction (termed as Cu_{1- x} Co_x -ZFC-150) showed high hydrogen and oxygen evolution reaction (HER and OER) activities with low overpotential small Tafel slope. When employed to be the bifunctional anode and cathode, they only needed a cell voltage of 1.62 V. The composite also exhibited excellent photocatalytic performance on CO₂ evolution into CO and CH₄ . The enhanced OER kinetics and Z-scheme charge transfer model for photocatalytic property have been discussed based on the experiments and density functional theory (DFT) analysis. The optimized phase interfaces, abundant active sites, optional oxygen vacancy, and adjusted Gibbs free energy were beneficial for the fast electron/ion transport enhancing the water splitting performance.

The synergistic effect of Hf-O-Ru bonds and oxygen vacancies in Ru/HfO2 for enhanced hydrogen evolution

Ru nanoparticles have been demonstrated to be highly active electrocatalysts for the hydrogen evolution reaction (HER). At present, most of Ru nanoparticles-based HER electrocatalysts with high activity are supported by heteroatom-doped carbon substrates. Few metal oxides with large band gap (more than 5 eV) as the substrates of Ru nanoparticles are employed for the HER. By using large band gap metal oxides substrates, we can distinguish the contribution of Ru nanoparticles from the substrates. Here, a highly efficient Ru/HfO₂ composite is developed by tuning numbers of Ru-O-Hf bonds and oxygen vacancies, resulting in a 20-fold enhancement in mass activity over commercial Pt/C in an alkaline medium. Density functional theory (DFT) calculations reveal that strong metal-support interaction via Ru-O-Hf bonds and the oxygen vacancies in the supported Ru samples synergistically lower the energy barrier for water dissociation to improve catalytic activities.

Although ruthenium nanomaterials have proven to be effective catalysts for H₂ evolution, there is still room for activity improvements. Here, authors develop an efficient Ru/HfO₂ electrocatalyst with tuned Ru-O-Hf bonds and oxygen vacancies that shows high activities for alkaline H₂ evolution.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

The role of metal–organic porous frameworks in dual catalysis

Metal–organic frameworks (MOFs) are a valuable group of porous crystalline solids with inorganic and organic parts that can be used in dual catalysis.

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.

Slower Removing Ligands of Metal Organic Frameworks Enables Higher Electrocatalytic Performance of Derived Nanomaterials

The widely used route of high-temperature pyrolysis for transformation of Prussian blue analogs (PBAs) to functional nanomaterials leads to the fast removal of CN^- ligands, and thus the formation of large metal aggregates and the loss of porous structures inside PBAs. Here, a controllable pyrolysis route at low temperature is reported for retaining the confined effect of CN^- ligands to metal cations during the whole pyrolysis process, thereby preparing high-surface-area cubes comprising disordered bimetallic oxides (i.e., Co₃ O₄ and Fe₂ O₃ ) nanoparticles. The disordered structure of Co₃ O₄ enables the exposure of abundant oxygen vacancies. Notably, for the first time, it is found that the in situ generated CoOOH during the oxygen evolution reaction (OER) can inherit the oxygen vacancies of pristine Co₃ O₄ (i.e., before OER), and such CoOOH with abundant oxygen vacancies adsorbs two ^- OH in the following Co³⁺ to Co⁴⁺ for markedly promoting OER. However, during the similar step, the ordered Co₃ O₄ with less oxygen vacancies only involves one ^- OH, resulting in the additional overpotentials for adsorbing ^- OH. Consequently, with high surface area and disordered Co₃ O₄ , the as-synthesized electrocatalysts have a low potential of 237 mV at 10 mA cm^-2 , surpassing most of reported electrocatalysts.

Self-Supported FeP-CoMoP Hierarchical Nanostructures for Efficient Hydrogen Evolution

Fabricating highly efficient electrocatalysts for electrochemical hydrogen generation is a top priority to relief the global energy crisis and environmental contamination. Herein, a rational synthetic strategy is developed for constructing well-defined FeP-CoMoP hierarchical nanostructures (HNSs). In general terms, the self-supported Co nanorods (NRs) are grown on conductive carbon cloth and directly serve as a self-sacrificing template. After solvothermal treatment, Co NRs are converted into well-ordered Co-Mo nanotubes (NTs). Subsequently, the small-sized Fe oxyhydroxide nanorods arrays are hydrothermally grown on the surface of Co-Mo NTs to form Fe-Co-Mo HNSs, which are then converted into FeP-CoMoP HNSs through a facile phosphorization treatment. FeP-CoMoP HNSs display high activity for hydrogen evolution reaction (HER) with an ultralow cathodic overpotential of 33 mV at 10 mA cm^-2 and a Tafel slope of 51 mV dec^-1 . Moreover, FeP-CoMoP HNSs also possess an excellent electrochemical durability in alkaline media. First-principles density functional theory (DFT) calculations demonstrate that the remarkable HER activitiy of FeP-CoMoP HNSs originates from the synergistic effect between FeP and CoMoP.

Preparation of Hierarchical Cube-on-plate Metal Phosphides as Bifunctional Electrocatalysts for Overall Water Splitting

To rationally design efficient and cost-effective electrocatalysts, a simple but efficient strategy has been developed to directly anchor prussian blue analogue (PBA) nanocubes on cobalt hydroxide nanoplates (PBA@Co(OH)₂ ) via the in-situ interfacial precipitation process. Subsequently, the thermal treatment in the presence of sodium hydrogen phosphite enabled the successful transition into metal phosphides with the hierarchical cube-on-plate structure. When used as electrocatalytsts, the obtained bimetal phosphides exhibited good bifunctional electrocatalytic activities for hydrogen and oxygen evolution reactions with good long-term stability. Thus, an enhanced performance for overall water splitting can be achieved, which could be ascribed to the hierarchical structure and favorable composition of as-prepared bimetal phosphide for rapid electron and mass transfer. The present study demonstrates a favorable approach to modulate the composition and structure of metal phosphide for enhancing the electrocatalytic ability toward water splitting.

Defect engineering in two-dimensional electrocatalysts for hydrogen evolution

The electrocatalytic hydrogen evolution reaction (HER) is an efficient and economic pathway to generate clean hydrogen energy in a sustainable manner. To improve the HER activity of Earth-abundant catalysts, reducing the dimension of materials is an effective strategy, and in this context two-dimensional (2D) materials have received substantial research attention owing to their large surface area and 2D charge transport channels. However, the thermodynamically stable basal surface of 2D catalysts is usually inactive in catalysis, which significantly impedes further optimization of the 2D HER catalysts. In this Minireview, we highlight in detail that defect engineering in 2D catalysts could bring multiple benefits in improving the HER activity. From the point of view of kinetics, defect sites could serve as active sites for catalyzing the HER process directly, and the introduction of defect structures may result in the optimization of electronic structures of the catalysts, thereby facilitating the HER process. Besides, for catalytically inert substrate materials, the defect sites could act as anchoring sites for catalyst loading, thus realizing efficient HER performance with the aid of enhanced electric conductivity. We anticipated that this Minireview could provide useful guidance for designing advanced HER catalysts in the future.

Multifunctional Electrocatalysis on a Porous N-Doped NiCo2O4@C Nanonetwork

Developing a multifunctional electrocatalyst with eminent activity, strong durability, and cheapness for the hydrogen/oxygen evolution reaction (HER/OER) and oxygen reduction reaction (ORR) is critical to overall water splitting and regenerative fuel cells. Herein, a nitrogen-doped nanonetwork assembled by porous and defective NiCo₂O₄@C nanowires grown on nickel foam (N-NiCo₂O₄@C@NF) is crafted via biomimetic mineralization and following carbonization of phase-transited lysozyme (PTL)-coupled NiCo₂O₄. The as-obtained N-NiCo₂O₄@C@NF electrocatalysts exhibit an exceptional catalytic activity with ultralow overpotentials for the HER (42 mV) and OER (242 mV) to afford 10 mA cm^-2 while maintaining good stability in alkaline media. Meanwhile, the N-NiCo2O4@C electrocatalysts presents a superior catalytic activity for ORR and a favorable four-electron pathway. The unprecedented catalytic performance arises from a highly porous structure and abundant defects and synergistic effects of components. This work may offer a new possibility in the exploration of multifunctional electrocatalysts for various energy-related electrocatalytic reactions.

Constructing Bifunctional 3D Holey and Ultrathin CoP Nanosheets for Efficient Overall Water Splitting

Pursuing cost-effective water-splitting catalysts is still a significant scientific challenge to produce renewable fuels and chemicals from various renewable feedstocks. The construction of controllable binder-free nanostructures with self-standing holey and ultrathin nanosheets is one of the promising approaches. Herein, by employing a combination of the potentiodynamic mode of electrodeposition and low-temperature phosphidation, three-dimensional (3D) holey CoP ultrathin nanosheets are fabricated on a carbon cloth (PD-CoP UNSs/CC) as bifunctional catalysts. Electrochemical tests show that the PD-CoP UNSs/CC exhibits outstanding hydrogen evolution reaction performance at all pH values with overpotentials of 47, 90, and 51 mV to approach 10 mA cm^-2 in acidic, neutral, and alkaline media, respectively. Meanwhile, only a low overpotential of 268 mV is required to drive 20 mA cm^-2 for the oxygen evolution reaction in alkaline media. Cyclic voltammetry and impedance studies suggest the enhanced performance is mainly attributed to the unique 3D holey ultrathin nanosheets, which could increase the electrochemically active area, facilitate the release of gas bubbles from electrode surfaces, and improve effective electrolyte diffusion. This work suggests an efficient path to design and fabricate non-noble bifunctional electrocatalysts for water splitting at a large scale.

Nonmetal Doping as a Robust Route for Boosting the Hydrogen Evolution of Metal-Based Electrocatalysts

Recently, nonmetal doping has exhibited its great potential for boosting the hydrogen evolution reaction (HER) of transition-metal (TM)-based electrocatalysts. To this end, this work overviews the recent achievements made on the design and development of the nonmetal-doped TM-based electrocatalysts and their performance for the HER. It is also shown that by rationally doping nonmetal elements, the electronic structures of TM-based electrocatalysts can be effectively tuned and in turn the Gibbs free energy of the TM for adsorption of H* intermediates (ΔG_H* ) optimized, consequently enhancing the intrinsic activity of TM-based electrocatalysts. Notably, we highlight that concurrently doping two nonmetal elements can continuously and precisely regulate the electronic structures of the TM, thereby maximizing the activity for HER. Moreover, nonmetal doping also accounts for enhancing the physical properties of the TM (i.e. surface area). Therefore, nonmetal doping is a robust strategy for simultaneous regulation of the chemical and physical features of the TM.