Amorphization activated ruthenium-tellurium nanorods for efficient water splitting

Engineering Ruthenium-Based Electrocatalysts for Effective Hydrogen Evolution Reaction

The investigation of highly effective, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is a prerequisite for the upcoming hydrogen energy society. To establish a new hydrogen energy system and gradually replace the traditional fossil-based energy, electrochemical water-splitting is considered the most promising, environmentally friendly, and efficient way to produce pure hydrogen. Compared with the commonly used platinum (Pt)-based catalysts, ruthenium (Ru) is expected to be a good alternative because of its similar hydrogen bonding energy, lower water decomposition barrier, and considerably lower price. Analyzing and revealing the HER mechanisms, as well as identifying a rational design of Ru-based HER catalysts with desirable activity and stability is indispensable. In this review, the research progress on HER electrocatalysts and the relevant describing parameters for HER performance are briefly introduced. Moreover, four major strategies to improve the performance of Ru-based electrocatalysts, including electronic effect modulation, support engineering, structure design, and maximum utilization (single atom) are discussed. Finally, the challenges, solutions and prospects are highlighted to prompt the practical applications of Ru-based electrocatalysts for HER.

Nonmetal-doping of noble metal-based catalysts for electrocatalysis

In response to the shortage of fossil fuels, efficient electrochemical energy conversion devices are attracting increasing attention, while the limited electrochemical performance and high cost of noble metal-based electrode materials remain a daunting challenge. The electrocatalytic performance of electrode materials is closely bound with their intrinsic electronic/ionic states and crystal structures. Apart from the nanoscale design and conductive composite strategies, heteroatom doping, particularly for nonmetal doping (e.g., hydrogen, boron, sulfur, selenium, phosphorus, and tellurium), is also another effective strategy to greatly promote the intrinsic activity of the electrode materials by tuning their atomic structures. From the perspective of electrocatalytic reactions, the effective atomic structure regulation could induce additional active sites, create rich defects, and optimize the adsorption capability, thereby contributing to the promotion of the electrocatalytic performance of noble metal-based electrocatalysts. Encouraged by the great progress achieved in this field, we have reviewed recent advancements in nonmetal doping for electrocatalytic energy conversion. Specifically, the doping effect on the atomic structure and intrinsic electronic/ionic state is also systematically illustrated and the relationship with the electrocatalytic performance is also investigated. It is believed that this review will provide guidance for the development of more efficient electrocatalysts.

Chlorine-assisted synthesis of CuCo2S4@(Cu,Co)2Cl(OH)3 heterostructures with an efficient nanointerface for electrocatalytic oxygen evolution

The demand for sustainable energy sources urges the development of efficient and earth-abundant electrocatalysts. Herein, chlorine assisted ion-exchange and in-situ sulfurization processes were combined to construct CuCo₂S₄@(Cu,Co)₂Cl(OH)₃ heterostructures from Cu(OH)₂ nanoarrays. Chlorine element in the cobalt source stimulated the formation of (Cu,Co)₂Cl(OH)₃ precursor, and further facilitated partial transformation of the precursor to CuCo₂S₄ on the surface to achieve composite structure. The mixed valences of Co element (Co³⁺ in CuCo₂S₄ and Co²⁺ in (Cu,Co)₂Cl(OH)₃) and OS interpenetrated nanointerface in the composite catalysts provided low electron transfer resistance for good alkaline oxygen evolution reaction (OER) activities. In 1 mol L^-1 KOH electrolyte, the overpotentials of the optimal composite catalyst reached 253 and 290 mV respectively at the current density of 20 and 50 mA cm^-2, which is comparable to the activity of commercial Ir/C (281 mV@20 mA cm^-2). These findings could provide opportunities for designing effective and inexpensive composite electrocatalysts through nanointerface engineering strategy.

Advances in hydrogen production from electrocatalytic seawater splitting

As one of the most abundant resources on the Earth, seawater is not only a promising electrolyte for industrial hydrogen production through electrolysis, but also of great significance for the refining of edible salt. Despite the great potential for large-scale hydrogen production, the implementation of water electrolysis requires efficient and stable electrocatalysts that can maintain high activity for water splitting without chloride corrosion. Recent years have witnessed great achievements in the development of highly efficient electrocatalysts toward seawater splitting. Starting from the historical background to the most recent achievements, this review will provide insights into the current state, challenges, and future perspectives of hydrogen production through seawater electrolysis. In particular, the mechanisms of overall water splitting, key features of seawater electrolysis, noble-metal-free electrocatalysts for seawater electrolysis and the underlying mechanisms are also highlighted to provide guidance for fabricating more efficient electrocatalysts toward seawater splitting.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

General Synthesis of Amorphous PdM (M = Cu, Fe, Co, Ni) Alloy Nanowires for Boosting HCOOH Dehydrogenation

Noble metal-based nanomaterials with amorphous structures are promising candidates for developing efficient electrocatalysts. However, their synthesis remains a significant challenge, especially under mild conditions. In this paper, we report a general strategy for preparing amorphous PdM nanowires (a-PdM NWs, M = Fe, Co, Ni, and Cu) at low temperatures by exploiting glassy non-noble metal (M) nuclei generated by special ligand adsorption as the amorphization dictator. When evaluated as electrocatalysts toward formic acid oxidation, a-PdCu NWs can deliver the mass and specific activities as high as 2.93 A/mg_Pd and 5.33 mA/cm², respectively; these are the highest values for PdCu-based catalysts reported thus far, far surpassing the crystalline-dominant counterparts and commercial Pd/C. Theoretical calculations suggest that the outstanding catalytic performance of a-PdCu NWs arises from the amorphization-induced high surface reactivity, which can efficiently activate the chemically stable C-H bond and thereby significantly facilitate the dissociation of HCOOH.

Deposition of Atomically Thin Pt Shells on Amorphous Palladium Phosphide Cores for Enhancing the Electrocatalytic Durability

As an excellent electrocatalyst, platinum (Pt) is often deposited as a thin layer on a nanoscale substrate to achieve high utilization efficiency. However, the practical application of the as-designed catalysts has been substantially restricted by the poor durability arising from the leaching of cores. Herein, by employing amorphous palladium phosphide (a-Pd-P) as substrates, we develop a class of leaching-free, ultrastable core-shell Pt catalysts with well-controlled shell thicknesses and surface structures for fuel cell electrocatalysis. When a submonolayer of Pt is deposited on the 6 nm nanocubes, the resulting Pd@a-Pd-P@Pt_SML core-shell catalyst can deliver a mass activity as high as 4.08 A/mg_Pt and 1.37 A/mg_Pd+Pt toward the oxygen reduction reaction at 0.9 V vs the reversible hydrogen electrode and undergoes 50 000 potential cycles with only ∼9% activity loss and negligible structural deformation. As elucidated by the DFT calculations, the superior durability of the catalysts originates from the high corrosion resistance of the disordered a-Pd-P substrates and the strong interfacial Pt-P interactions between the Pt shell and amorphous Pd-P layer.

Hollow Pd/Te nanorods for the effective electrooxidation of methanol

Methanol electrooxidation is significant in realizing effective C1 liquid fuel applications. Herein, hollow Pd/Te nanorods were fabricated and evaluated for methanol oxidation, and they were found to exhibit high catalytic efficiency for methanol oxidation in alkaline electrolyte compared to Pd or Pd/C catalysts. The hybrid structure of hexagonal crystal Te and face-centered cubic Pd was formed by microwave assisted Pd nanoparticle deposition over the surface of Te nanorods. Strong electronic effects and facile oxophilic properties were indicated in the Pd/Te system by spectroscopic analysis, which mainly accounts for the high catalytic performance for methanol oxidation. Specifically, they showed a peak current density of 90.1 mA cm^-2 for methanol oxidation, around 3.5 times higher than that of commercial Pd/C (26.3 mA cm^-2). High catalytic stability was also observed for Pd/Te, with a current retention of 64.3% after 3600 s of chronoamperometric testing, much higher than for Pd catalysts (20.1%). High anti-CO poisoning ability of the Pd/Te catalyst was demonstrated in the CO-stripping voltammetry results, and faster catalytic kinetics were also observed for this catalyst system. The electron-rich state of Pd and high active site exposure are responsible for the high performance of the Pd/Te catalyst in methanol oxidation.

Benchmarking Phases of Ruthenium Dichalcogenides for Electrocatalysis of Hydrogen Evolution: Theoretical and Experimental Insights

The hydrogen evolution reaction (HER) is a significant cathode step in electrochemical devices, especially in water splitting, but developing efficient HER catalysts remains a great challenge. Herein, comprehensive density functional theory calculations are presented to explore the intrinsic HER behaviors of a series of ruthenium dichalcogenide crystals (RuX₂ , X = S, Se, Te). In addition, a simple and easily scaled production strategy is proposed to synthesize RuX₂ nanoparticles uniformly deposited on carbon nanotubes. Consistent with theoretical predictions, the RuX₂ catalysts exhibit impressive HER catalytic behavior. In particular, marcasite-type RuTe₂ (RuTe₂ -M) achieves Pt-like activity (35.7 mV at 10 mA cm^-2 ) in an acidic electrolyte, and pyrite-type RuSe₂ presents outstanding HER performance in an alkaline media (29.5 mV at 10 mA cm^-2 ), even superior to that of commercial Pt/C. More importantly, a RuTe₂ -M-based proton exchange membrane (PEM) electrolyzer and a RuSe₂ -based anion exchange membrane (AEM) electrolyzer are also carefully assembled, and their outstanding single-cell performance points to them being efficient cathode candidates for use in hydrogen production. This work makes a significant contribution to the exploration of a new class of transition metal dichalcogenides with remarkable activity toward water electrolysis.

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

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

A Glass-Ceramic with Accelerated Surface Reconstruction toward the Efficient Oxygen Evolution Reaction

The effective non-precious metal catalysts toward the oxygen evolution reaction (OER) are highly desirable for electrochemical water splitting. Herein, we prepare a novel glass-ceramic (Ni_1.5 Sn@triMPO₄ ) by embedding crystalline Ni_1.5 Sn nanoparticles into amorphous trimetallic phosphate (triMPO₄ ) matrix. This unique crystalline-amorphous nanostructure synergistically accelerates the surface reconstruction to active Ni(Fe)OOH, due to the low vacancy formation energy of Sn in glass-ceramic and high adsorption energy of PO₄ ^3- at the V_O sites. Compared to the control samples, this dual-phase glass-ceramic exhibits a remarkably lowered overpotential and boosted OER kinetics after surface reconstruction, rivaling most of state-of-the-art electrocatalysts. The residual PO₄ ^3- and intrinsic V_O sites induce redistribution of electron states, thus optimizing the adsorption of OH* and OOH* intermediates on metal oxyhydroxides and promoting the OER activity.

Oxidation Etching-Induced Post-Crystallization of Palladium Nanosheets for Efficient Catalytic Hydrogenation

The performances of catalysts are highly dependent on their crystallinities. It is a significant challenge to successively manipulate the crystallinities of noble metal nanocatalysts due to the strong metallic bonds, especially under ambient conditions. Herein, a post-crystallization approach is developed for successive control of the crystallinity of Pd nanosheets via selective oxidation etching of the amorphous domains. This strategy can be extended to crystallize other Pd and Ru nanomaterials. By carefully modulating the crystallinity of Pd nanosheets, the time for the complete conversion of 4-nitrostyrene via hydrogenation is reduced by 20 times. Also, crystallization can turn the selectivity of the products and improve the stability of Pd nanosheets. These findings may advance the crystal engineering of metal nanomaterials for wide applications.

Double exchange interaction promoted high-valence metal sites for neutral oxygen evolution reaction

A unique double-exchange strategy is adopted to access active high-valent transition metal sites during neutral oxygen evolution reaction (OER). This double-exchange is realized through electronic interaction between transition metal ions and foreign dopants in a transition metal oxide. Based on systematical evaluation on dopants with varied d-electron numbers, we demonstrate that the d electron-poor dopant exhibits more significant double-exchange interaction with the transition metal ions, and therefore obtains more active high-valence metal sites, and thus achieves better neutral OER performance.

One-dimensional nitrogen-doped carbon frameworks embedded with zinc-cobalt nanoparticles for efficient overall water splitting

Metal-organic frameworks (MOFs)-derived catalysts exhibit highly-efficient hydrogen or oxygen evolution performance on water splitting. However, it is an urgent problem to construct bifunctional electrocatalysts for both hydrogen and oxygen evolution performance. Herein, we adopted Ag nanowires as templates to prepare one-dimensional Ag nanowire@ZIF-8@ZIF-67 precursors (1D AgNW@ZIF-8@ZIF-67). Through pyrolysis, AgNW@ZIF-8@ZIF-67 precursors transformed into nitrogen-doped carbon frameworks (NCF) embedded with zinc-cobalt (ZnCo) nanoparticles on the surface of Ag NWs (denoted as Ag@ZnCo/NCF nanohybrids). The nanohybrids were consisted of Ag NWs with good conductivity and ZnCo/NCF nanohybrids with rich accessible active sites. Benefiting from their large specific surface area, accessible active sites and synergistic effect among components, Ag@ZnCo/NCF nanohybrids exhibit lower overpotentials of 139 mV and 279 mV at the current density of 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution, severally. Compared with other catalysts, Ag@ZnCo/NCF nanohybrids possess smaller Tafel slope, indicating their higher catalytic activity. This work provides a new perspective for designing low-cost and highly efficient bifunctional electrocatalysts 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.