Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Polyoxometalate precursors for precisely controlled synthesis of bimetallic sulfide heterostructure through nucleation-doping competition

Molybdenum disulfide (MoS2)-based bimetallic sulfides have drawn increasing research attention because of their unique structures and properties. Herein, a one-pot hydrothermal synthesis method is proposed to grow a series of bimetallic sulfides on carbon cloth (M-Mo-S/CC, M = Co, Ni, Fe) using Anderson-type polyoxometalates (POMs) as bimetallic sources for the first time. An ideal model of M-Mo-S/CC was used to study the growth process through the nucleation-doping competition mechanism. It is proved for the first time that M-Mo-S/CC possess certain compositions of bimetallic sulfides rather than metal doped MoS2 structures because the nucleation reaction is predominant in the nucleation-doping competition. Moreover, the nucleation rates of different metals can be compared to study the different morphologies of M-Mo-S/CC because Anderson-type POMs have fixed bimetal proportions and precise structures. Co-Mo-S and Ni-Mo-S show spherical heterostructures with CoS2 or NiS mainly inside and interconnected MoS2 nanosheets outside, while Fe-Mo-S exhibits uniform nanosheet morphology without stacking. As electrodes for alkaline water electrolysis, M-Mo-S/CC with different compositions and morphologies exhibit a variety of activities. Particularly, among the M-Mo-S/CC samples, Co-Mo-S/CC achieves the best performance for hydrogen evolution reaction, oxygen evolution reaction and overall water splitting. This study presents a facile strategy of using POMs as bimetallic precursors for studying the growth mechanism as well as the water electrolysis performances of MoS2-based bimetallic sulfides.

Oxygen Doping to Optimize Atomic Hydrogen Binding Energy on NiCoP for Highly Efficient Hydrogen Evolution

An outstanding hydrogen evolution electrocatalyst should have a free energy of adsorbed atomic hydrogen of approximately zero, which enables not only a fast proton/electron-transfer step but also rapid hydrogen release. An economic and industrially viable alternative approach for the optimization of atomic hydrogen binding energy is urgently needed. Herein, guided by density functional theory (DFT) calculations, it is theoretically demonstrated that oxygen doping in NiCoP can indeed optimize the atomic hydrogen binding energy (e.g., |ΔG_H* | = 0.08, 0.12 eV on Co, P sites). To confirm this, NiCoP electrodes with controllable oxygen doping are designed and fabricated via alteration of the reducing atmosphere. Accordingly, an optimal oxygen-doped NiCoP (≈0.98% oxygen) nanowire array is found to exhibit the remarkably low hydrogen evolution reaction (HER) overpotential of 44 mV to drive 10 mA cm^-2 and a small Tafel slope of 38.6 mV dec^-1 , and long-term stability of 30 h in an alkaline medium. In neutral solution, only a 51 mV overpotential (@10 mA cm^-2 ) is required, and the Tafel slope is 79.2 mV dec^-1 . Meanwhile, in situ Raman spectra confirm the low formation overpotential (-30 mV) of NiCo-phosphate at the surface of ≈0.98% oxygen-doped NiCoP, which enables the material to show better HER performance.

Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy

Renewable energy technology has been considered as a "MUST" option to lower the use of fossil fuels for industry and daily life. Designing critical and sophisticated materials is of great importance in order to realize high-performance energy technology. Typically, efficient synthesis and soft surface modification of nanomaterials are important for energy technology. Therefore, there are increasing demands on the rational design of efficient electrocatalysts or electrode materials, which are the key for scalable and practical electrochemical energy devices. Nevertheless, the development of versatile and cheap strategies is one of the main challenges to achieve the aforementioned goals. Accordingly, plasma technology has recently appeared as an extremely promising alternative for the synthesis and surface modification of nanomaterials for electrochemical devices. Here, the recent progress on the development of nonthermal plasma technology is highlighted for the synthesis and surface modification of advanced electrode materials for renewable energy technology including electrocatalysts for fuel cells, water splitting, metal-air batteries, and electrode materials for batteries and supercapacitors, etc.

Oxidized Laser-Induced Graphene for Efficient Oxygen Electrocatalysis

An efficient metal-free catalyst is presented for oxygen evolution and reduction based on oxidized laser-induced graphene (LIG-O). The oxidation of LIG by O₂ plasma to form LIG-O boosts its performance in the oxygen evolution reaction (OER), exhibiting a low onset potential of 260 mV with a low Tafel slope of 49 mV dec^-1 , as well as an increased activity for the oxygen reduction reaction. Additionally, LIG-O shows unexpectedly high activity in catalyzing Li₂ O₂ decomposition in Li-O₂ batteries. The overpotential upon charging is decreased from 1.01 V in LIG to 0.63 V in LIG-O. The oxygen-containing groups make essential contributions, not only by providing the active sites, but also by facilitating the adsorption of OER intermediates and lowering the activation energy.

NiO as a Bifunctional Promoter for RuO2 toward Superior Overall Water Splitting

Conventional development of nanomaterials for efficient electrocatalysis is largely based on performance-oriented trial-and-error/iterative approaches, while a rational design approach at the atomic/molecular level is yet to be found. Here, inspired by a fundamental understanding of the mechanism for both oxygen and hydrogen evolution half reactions (OER/HER), a unique strategy is presented to engineer RuO₂ for superior alkaline water electrolysis through coupling with NiO as an efficient bifunctional promoter. Benefitting from desired potential-induced interfacial synergies, NiO-derived NiOOH improves the oxygen binding energy of RuO₂ for enhanced OER, and NiO also promotes water dissociation for enhanced HER on RuO₂ -derived Ru. The resulting hybrid material exhibits remarkable bifunctional activities, affording 2.6 times higher OER activity than that of RuO₂ and an HER activity comparable to Pt/C. As a result, the simple system requires only 1.5 V to deliver 10 mA cm^-2 for overall alkaline water splitting, outperforming the benchmark PtC/NF||IrO₂ /NF couple with high mass loading. Comprehensive electrochemical investigation reveals the unique and critical role of NiO on the optimized RuO₂ /NiO interface for synergistically enhanced activities, which may be extended to broader (electro)catalytic systems.

Noble Metal-Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in-depth study of the structural-activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy-rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy-rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy-rich noble metal-free electrocatalysts with high performance are discussed.

Room temperature-formed iron-doped nickel hydroxide on nickel foam as a 3D electrode for low polarized and high-current-density oxygen evolution

Unique room temperature-formed iron-doped nickel hydroxide on Ni foam as a 3D electrode in alkaline electrolyte offers fast gas dissipation, a high number of active sites and a high oxidation state of Ni for an improved oxidation ability for the oxygen evolution reaction (OER), which delivers an OER current density of 100 mA cm-2 at an overpotential of 0.312 V and 96.3% retention after 100 h at an overpotential of 0.370 V in 1 M KOH, and 1000 mA cm-2 at an overpotential of 0.265 V in 30 wt% KOH.

High-Index Faceted Porous Co3O4 Nanosheets with Oxygen Vacancies for Highly Efficient Water Oxidation

Because of sluggish kinetics of the oxygen evolution reaction (OER), designing low-cost, highly active, and stable electrocatalysts for OER is important for the development of sustainable electrochemical water splitting. Here, {112} high-index facet exposed porous Co₃O₄ nanosheets with oxygen vacancies on the surface have been successfully synthesized via a simple hydrothermal method followed by NaBH₄ reduction. As compared with the pristine and other faceted porous Co₃O₄ nanosheets (e.g., {110} and {111}), the as-prepared {112} faceted porous nanosheets exhibit a much lower overpotential of 318 mV at a current density of 10 mA cm^-2. Importantly, these nanosheets also give excellent electrochemical stability, displaying an insignificant change in the required overpotential at a current density of 10 mA cm^-2 even after a 14 h long-term chronoamperometric test. All these superior OER activity and stability could be attributed to their unique hierarchical structures assembled by ultrathin porous nanosheets, {112} high-index exposed facets with higher ratio of Co²⁺/Co³⁺ and oxygen vacancies on the surface, which can substantially enhance the charge transfer rate and increase the number of active sites. All these findings not only demonstrate the potency of our Co₃O₄ nanosheets for efficient water oxidation but also provide further insights into developing cost-effective and high-performance catalysts for electrochemical applications.

Encapsulation of Ni/Fe3O4 heterostructures inside onion-like N-doped carbon nanorods enables synergistic electrocatalysis for water oxidation

The rational modulation of composition and structure is critical for the development of robust and efficient oxygen evolution reaction (OER) catalysts for water splitting. In this study, an onion-like N-doped carbon nanorods hybrid (denoted as ONC) with encapsulated Ni/Fe₃O₄ heterostructures has been fabricated by the pyrolysis of an NiFe-based coordination polymer under a N₂ atmosphere. The nanorod-like morphology is transferred from the polymer to the hybrids and generates ONC nanolayers encapsulated with core-shell Ni/Fe₃O₄ nanostructures. The synergistic effects between the ONC layers and the encapsulated Ni/Fe₃O₄ heterostructures result in high electronic conductivity due to the nitrogen-doped carbon with an appropriate level of defects and enlarged electrochemical surface area due to the well-defined mesoporous morphology. Compared with Ni@ONC, Fe₃O₄@ONC, NiFe₂O₄ and commercial RuO₂ electrocatalysts, the as-prepared Ni/Fe₃O₄@ONC exhibits extraordinary electrocatalytic activity for water oxidation with an overpotential of merely 296 mV at 10 mA cm^-2 and a small Tafel slope of 61 mV dec^-1. This Ni/Fe₃O₄@ONC OER catalyst highlights the great potential of integrating hetero-composite nanocatalysts with hetero-atom doped nanocarbon supports for the development of high-performance electrocatalysts for renewable energy applications.

Ultrathin porous nanosheet-assembled hollow cobalt nickel oxide microspheres with optimized compositions for efficient oxygen evolution reaction

Hollow nanosheet-assembled cobalt nickel oxide microspheres were prepared via a facile “self-template” route, serving as an efficient and stable catalyst for the OER.

Electrocatalytic valorisation of biomass derived chemicals

Recent progress in electro-valorization of biomass-derived intermediates is reviewed, while a perspective on future R&D in this field is provided.

CoC2O4·2H2O derived Co3O4 nanorods array: a high-efficiency 1D electrocatalyst for alkaline oxygen evolution reaction

Self-standing Co₃O₄ nanorods array on Co foil as a 1D OER catalyst electrode, only needs overpotential of 308 mV to drive 15 mA cm⁻² in 1.0 M KOH, with good long-term electrochemical durability and a high turnover frequency.

In situ growth of well-ordered NiFe-MOF-74 on Ni foam by Fe2+ induction as an efficient and stable electrocatalyst for water oxidation

Uniform and well-ordered NiFe-MOF-74 was in situ grown on Ni foam by Fe²⁺ induction, exhibiting enhanced activity toward the OER with a very low overpotential of 223 mV at a current density of 10 mA cm⁻².

Efficient Co@CoPx core–shell nanochains catalyst for the oxygen evolution reaction

Co@CoP_x core–shell nanochains were prepared via a direct-current arc-discharge method and subsequent phosphorization at 350 °C.

Polyoxometalate-encapsulated twenty-nuclear silver-tetrazole nanocage frameworks as highly active electrocatalysts for the hydrogen evolution reaction

Two unprecedented polyoxometalate (POM)-encapsulated twenty-nuclear silver-tetrazole nanocage frameworks have been successfully synthesized, which exhibit high activity in the hydrogen evolution reaction.

An Fe(TCNQ)2 nanowire array on Fe foil: an efficient non-noble-metal catalyst for the oxygen evolution reaction in alkaline media

An Fe-(tetracyanoquinodimethane)₂ nanowire array in situ developed on Fe foil (Fe(TCNQ)₂/Fe) acts as an efficient and durable electrocatalyst for water oxidation, needing an overpotential of 340 mV to attain a current density of 10 mA cm⁻² in 1.0 M KOH.

Recent advances in three-dimensional graphene based materials for catalysis applications

This review presents recent theoretical and experimental progress in the construction, properties, and catalytic applications of 3D graphene-based materials.

Nickel oxide–polypyrrole nanocomposite electrode materials for electrocatalytic water oxidation

Electrochemically prepared nickel oxide nanoparticles entrapped into a polymer matrix as efficient material for O₂ evolution.

Cathodic electrochemical activation of Co3O4 nanoarrays: a smart strategy to significantly boost the hydrogen evolution activity

The room-temperature cathodic polarization of Co₃O₄ leads to an ultrathin amorphous Co–P shell as an active layer. Such a Co–P@Co₃O₄ hybrid nanoarray needs an overpotential of 73 mV to drive a geometrical catalytic current density of 10 mA cm⁻² in 1.0 M KOH.

An Fe-MOF nanosheet array with superior activity towards the alkaline oxygen evolution reaction

Fe-MOF/NF shows superior electrocatalytic performance towards the OER in 1.0 M KOH with a low overpotential of 240 mV at 50 mA cm⁻².

In situ development of amorphous Mn–Co–P shell on MnCo2O4 nanowire array for superior oxygen evolution electrocatalysis in alkaline media

Core–shell Mn–Co–P@MnCo₂O₄ nanoarray on Ti mesh (Mn–Co–P@MnCo₂O₄/Ti) acts as a durable water oxidation electrocatalyst with an overpotential of 269 mV to drive 10 mA cm⁻² in 1.0 M KOH.

Core–shell structured hierarchically porous NiO microspheres with enhanced electrocatalytic activity for oxygen evolution reaction

We present a one-pot synthetic strategy to synthesize hierarchically porous NiO microspheres with enhanced activity for OER.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.

Gold doping induced strong enhancement of carbon quantum dots fluorescence and oxygen evolution reaction catalytic activity of amorphous cobalt hydroxide

Au doping leads to tunable and strong enhancement of SCQDs fluorescence and OER activity of amorphous Co(OH)₂.

Preparation of Co–N carbon nanosheet oxygen electrode catalyst by controlled crystallization of cobalt salt precursors for all-solid-state Al–air battery

Production of bifunctional catalysts for catalyzing both ORR and OER is highly advisable but challenging with respect to the applications of these catalysts in renewable energy conversion and storage technologies.

MOF-derived Mn doped porous CoP nanosheets as efficient and stable bifunctional electrocatalysts for water splitting

Mn-CoP nanosheets were synthesized from ZIF-67 via an etching-carbonization–phosphidation strategy and showed efficient electrocatalytic activity in both HER and OER under acidic and alkaline conditions.

Fabrication of Pt nanoparticles on nitrogen-doped carbon/Ni nanofibers for improved hydrogen evolution activity

Among various methods for acquiring hydrogen fuel, electrocatalytic water splitting is considered to be as one of the most efficient and promising approaches. Pt is regarded as the best electrocatalyst for the hydrogen evolution reaction (HER) during water splitting process, however, the scarcity and costliness of Pt restrict the large-scale practical application. On the other hand, transition metal Ni is abundant in earth and exhibits favorable HER catalytic activity theoretically. In this work, we have demonstrated a facile electrospinning combined with calcination and chemical reduction processes to fabricate Pt nanoparticles loaded on nitrogen-doped carbon/Ni nanofibers (Ni-NCNFs-Pt) as efficient HER electrocatalysts. The as-prepared Ni-NCNFs-Pt not only reduced the usage of noble metal Pt but also revealed an excellent electrochemical activity at all values of pH, including the smaller overpotentials of 47, 22 and 84 mV (at j = 10 mA cm^-2) in 0.5 M H₂SO₄, 1 M KOH and 0.1 M phosphate buffer solution, respectively. In addition, Ni-NCNFs-Pt nanocatalyst displayed an extraordinary low Tafel slope and long durability over a wide range of pH. The remarkable HER performance could be ascribed to the increased electrochemical active surface area through the introduction of Ni nanoparticles, synergistic interactions between Ni and Pt nanoparticles and the introduction of the conductive nitrogen-doped carbon nanofibers (NCNFs) substrate which facilitated the fast electron transport. This investigation provides a potential route to construct efficient and low cost HER electrocatalysts with promising practical applications in renewable energy conversion and storage devices.

Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting

Rational design of highly efficient bifunctional electrocatalysts based on 3D transition-metal-based materials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for sustainable energy conversion processes. Herein, a novel strategy involving outer and inner structural engineering is developed for superior water splitting via in situ vertical growth of 2D amorphous FePO₄ nanosheets on Ni foam (Am FePO₄ /NF). Careful experiments and density functional theory calculations show that the inner and outer structural engineering contributing to the synergistic effects of 2D morphology, amorphous structure, conductive substrate, and Ni-Fe mixed phosphate lead to superior electrocatalytic activity toward OER and HER. Furthermore, a two-electrode electrolyzer assembled using Am FePO₄ /NF as an electrocatalyst at both electrodes gives current densities of 10 and 100 mA cm^-2 at potentials of 1.54 and 1.72 V, respectively, which is comparable to the best bifunctional electrocatalyst reported in the literature. The strategies, introduced in the present work, may open new opportunities for the rational design of other 3D transition-metal-based electrocatalyst through an outer and inner structural control to strengthen the electrocatalytic performance.

Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions

Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries. Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance. Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial. The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected. More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups. To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed. An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided. The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects. The defect-activity relationship is also explored by theoretical methods.

In Situ Coupling Strategy for the Preparation of FeCo Alloys and Co4 N Hybrid for Highly Efficient Oxygen Evolution

An in situ coupling approach is developed to create a new highly efficient and durable cobalt-based electrocatalyst for the oxygen evolution reaction (OER). Using a novel cyclotetramerization, a task-specific bimetallic phthalocyanine-based nanoporous organic framework is successfully built as a precursor for the carbonization synthesis of a nonprecious OER electrocatalyst. The resultant material exhibits an excellent OER activity with a low overpotential of 280 mV at a current density of 10 mA cm^-2 and high durability in an alkaline medium. This impressive result ranks among the best from known Co-based OER catalysts under the same conditions. The simultaneous installation of multiple diverse cobalt-based active sites, including FeCo alloys and Co₄ N nanoparticles, plays a critical role in achieving this promising OER performance. This innovative approach not only enables high-performance OER activity to be achieved but simultaneously provides a means to control the surface features, thereby tuning the catalytic property of the material.