Recent development of transition metal doped carbon materials derived from biomass for hydrogen evolution reaction

Achieving high catalytic performance with the lowest cost is critical for hydrogen evolution reduction. In recent years, biomass-derived carbon catalysts have triggered huge interest in catalytic reactions owing to the low cost, high energy conversion efficiency and environmental friendliness. A rapid growth of novel electrocatalysts is witnessed especially those based on non-precious metals, some of which approach the activity of precious metals. Synergistic interactions between metals and heteroatoms can significantly improve the electrocatalytic activity, thus transition metal-decorated biomass-based carbon materials were commonly adopted to improve HER performance. The resulting electrocatalytic activities are introduced and compared to conventional Pt/C-based electrocatalysts in present research. Moreover, the remaining challenges in the development process and future prospects of hydrogen evolution reduction catalysts are discussed.     

Nickel sulfide-based electrocatalysts for overall water splitting

Hydrogen is a green energy with sustainability and high energy density. Electrochemical water splitting (EWS) is a promising green strategy for hydrogen production. Noble metal electrocatalysts exhibit excellent electrocatalytic activity in EWS. However, the applications of noble metals in EWS are limited because of their scarcity and high price. Therefore, the research on non-noble metal electrocatalysts has attracted much attention. Among them, nickel sulfide electrocatalysts, with a unique 3D structure, pretty conductivity, and adjustable electronic structure, show significant electrocatalytic activity in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, the mechanism of the electrocatalytic reaction, electrochemical parameters, and preparation methods of nickel sulfide are introduced first. Then, the five methods including atomic doping (including cations, anions and diatoms), morphological control, hybridization, integration with nanocarbon, and high-index facets exposure to regulate the electronic structure and active sites of nickel sulfide were illustrated, so as to improve the electrocatalytic activity of nickel sulfide. The electrocatalytic properties of these nickel sulfides were reviewed. However, there are some problems in the research of electrocatalysis, such as how to further improve the conductivity of the electrocatalyst, and the calculation method of current density is not unified. Therefore, our future development direction is to prepare a stable nickel sulfide electrocatalyst, study relevant strategies to simultaneously increase active sites and improve conductivity, and effectively make nickel sulfide into an EWS catalyst with higher performance.     

Activity engineering to transition metal phosphides as bifunctional electrocatalysts for efficient water-splitting

At present, water splitting has been regarded as one of the most promising ways for hydrogen production. Therefore, exploitation of cost-effective electrocatalysts is essential to realize the industrialization of electrocatalytic techniques. In recent years, transition metal phosphides (TMPs) as non-noble metal electrocatalysts have gained a great deal of attention owing to their multifunctional active sites, tunable structure and composition, as well as unique physicochemical properties. However, the poor electrical conductivity of TMPs and high adsorption energy of hydrogen intermediates during the hydrogen evolution severely restrict its large-scale application. Therefore, it is of great importance to develop effective activity engineering to TMPs. Herein, the reaction mechanisms of water splitting including hydrogen evolution and oxygen evolution reactions and the key performance parameters are briefly clarified. Then, the strategies to improve the performance of TMPs are summarized in four aspects, including modulation of electronic structure, tailoring microstructures, selection of working electrode, and replacing OER with an energy-saving reaction. Finally, a summary and perspective for further opportunities and challenges are highlighted for the TMPs from the point of characterization methodologies, theoretical calculation and practical application.     

Graphene-based electrocatalysts: Hydrogen evolution reactions and overall water splitting

Recently, carbon-based materials (e.g., graphene, carbon nanotubes, carbon quantum dots) have been used as electrocatalysts to catalyze the reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Among them, graphene has attracted attention as an electrocatalyst, and its electrocatalytic performances have been improved by doping with metals and non-metals, surface and defect engineering, and hybrid development. In this perspective, the present paper reviewed the recent advances (2018 onwards) on the progress of graphene-based electrocatalysts for HER and overall water splitting (OWS). It is emphasizing strategies for optimizing electrocatalytic properties followed by challenges and future outlook. This review will provide the essential ideas and strategies that can help design graphene-based electrocatalysts of high performance that can be implemented for sustainable energy application.     

Metal organic frameworks as electrocatalysts: Hydrogen evolution reactions and overall water splitting

The development of clean energy technologies to protect the environment is an important demand of the times. Electrocatalysis is emerging as a promising method for evolution of hydrogen and overall water splitting. Nowadays, metal organic frameworks (MOFs) have emerged as electrocatalysts having uniformly distributed active sites and high electrical conductivity. This review summarizes the latest advances in heterogeneous catalysis by MOFs and their composite/derivatives for efficient hydrogen evolution reaction (HER) and water splitting. Pristine MOFs with their recent development are summarized first followed by composites of MOFs with their enhanced electrocatalytic performances. Overall water splitting by using bifunctional electrocatalysts derived from MOFs with different synthetic approaches is provided and this review gives the metal-based categorisation of precursor MOFs. Different strategies to improve chemical stability, conductivity, and overall electrocatalytic properties have been discussed. In the last, perspectives on the synthesis of efficient MOF-based electrocatalyst materials are provided.     

Recent advances in amorphous metal phosphide electrocatalysts for hydrogen evolution reaction

Hydrogen is a green energy with the long-term sustainability and high energy density. Hydrogen evolution reaction via electrocatalysis is a prospective strategy for green hydrogen. Pt-based electrocatalysts have exhibited excellent electrocatalytic activities on hydrogen evolution reaction. But the scarce and costly Pt limits the application of hydrogen evolution reaction. Therefore, non-Pt/low-Pt electrocatalysts have attracted much research attention. Amorphous metal phosphide electrocatalysts have shown significant electrocatalytic activities on hydrogen evolution reaction for their special long-range disordered but short-/medium-range ordered structures with abundant active sites and adjustable electronic structures. Mechanisms and electrochemical parameters of hydrogen evolution reaction as well as characterization technologies and atomic configurations of amorphous metal phosphides are firstly illustrated in the review. Amorphous monometallic, bimetallic, trimetallic and other multimetallic phosphides were investigated for modulation of electronic structures and active sites by heteroatom incorporation, nanoporous structure and heterostructure construction. The electrocatalytic performances of these amorphous metal phosphides are summarized in the review. Whereas some questions have emerged in recent researches, like atom leaching, uncertain self-constructions and lack of atomic configurations. Therefore, the future perspectives for the development of amorphous metal phosphide electrocatalysts on hydrogen evolution reaction are construction of stable amorphous metal phosphide electrocatalysts, exploration of self-construction mechanism and convenient construction of atomic configurations.     

Recent advances in CoSe2 electrocatalysts for hydrogen evolution reaction

Hydrogen as a sustainable alternative fuel is recognized as a primary choice for future energy supply due to its high gravimetric energy density and zero carbon emission upon combustion. Electrochemical water splitting is a promising strategy for effective and sustainable hydrogen production. Nowadays, research is focused on developing non-precious, stable, and highly efficient electrocatalysts for hydrogen evolution reaction (HER). Among them, CoSe₂ has attracted tremendous attention as HER electrocatalyst due to its unique electronic configuration that ensures fast charge transport, excellent catalytic activity, and good chemical stability. So far, a lot of reviews on electrocatalytic water splitting based on transition metal dichalcogenides and cobalt-based materials are reported. However, the review on CoSe₂ electrocatalyst for hydrogen evolution reaction is limited up-to-date. Hence in the present review, a comprehensive literature survey on CoSe₂ electrocatalyst for hydrogen evolution reaction is done and reported. In this review, the crystal structures of CoSe₂, their phase transformation strategy, their hydrogen evolution reaction mechanism in acidic and alkaline electrolytes are highlighted. The various synthesis procedures adopted to produce CoSe₂ based materials, the relation between its structure and composition with their electrocatalytic activities are discussed. Moreover, the effective ways to enhance the electrocatalytic performance of CoSe₂ based materials such as its morphological modification, constructing heterostructures, and heteroatom doping are reviewed.     

Spinel NiCo2O4 3-D nanoflowers supported on graphene nanosheets as efficient electrocatalyst for oxygen evolution reaction

Efficient oxygen evolution reaction (OER) electrocatalysts with non-noble metals are very critical for the large-scale exploitation of electrocatalytic hydrogen production systems. To improve the catalytic activity of OER electrocatalysts, several design strategies, such as construction of nanostructures, porous structures and composite materials have been proposed. Herein, spinel NiCo₂O₄ 3-D nanoflowers supported on graphene nanosheets (GNs) are prepared by a simple solvothermal synthesis method as non-noble metal electrocatalysts for OER. The present NiCo₂O₄/GNs composite integrates multiple advantages of nanostructures, porous structures and composite materials, including high surface area, abundant catalytic sites and high stability. Benefiting from the favorable features, the NiCo₂O₄/GNs composite exhibits a better OER performance than NiCo₂O₄ and RuO₂ in alkaline medium, which has a low onset potential (1.50 V), a small Tafel slope (137 mV dec⁻¹). The present work opens a new window for the construction of the carbon-supported 3-D nanostructure of transition metal catalysts with optimizable electrocatalytic performances for electrocatalytic hydrogen production.     

Electrochemical fabrication of Ni–Mo nanostars with Pt-like catalytic activity for both electrochemical hydrogen and oxygen evolution reactions

Synthesis of electrocatalysts with excellent performance for hydrogen and oxygen evolution are the main challenges for production of hydrogen by electrochemical water splitting method. Here, Ni–Mo nanostars were created by electrochemical deposition process at different morphologies and their electrocatalytic behavior was studied for hydrogen and oxygen evolution reactions in 1.0 M KOH solution. Increased electrochemically active surface area due to the nanostars formation, improved intrinsic electrocatalytic activity, increased surface wettability, as well as being binder-free during electrode production, resulted in excellent electrocatalytic behavior. For optimized condition, 60 mV and 225 mV overpotential are needed for generating the current density of 10 mA.cm-² in HER and OER process respectively in the alkaline medium. The lower slope of the electrode compared to the other electrodes also indicated that the kinetics of HER on the surface of the electrode was better. Also, there was very little change in the potential during the stability test, indicating the excellent electrocatalytic stability of the synthesized electrode. The present study introduces a rational, cost-effective and binder-free method for the synthesis of high performance electrocatalysts.     

Electrocatalysis for hydrogen electrode reactions in the light of fermi dynamics and structural bonding FACTORS-I. individual electrocatalytic properties of transition metals

The Brewer valence-bond theory for bonding in metals and intermetallic phases has been employed, together with Fermi dynamics, to correlate with the electrocatalytic properties of both individual and composite transition metal catalysts for the hydrogen electrode reactions (HELR). It has been inferred that the electrocatalytic activity of both individual transition metals and their intermetallic phases and alloys for both hydrogen evolution (HER) and its oxidation (HOR), primarily correlates with the electronic density of states and obeys typical laws of catalysis reflected in the first place in the existence of volcano plots along the Periodic Table. Since the bonding effectiveness of both individual and intermetallic hypo-hyper-d-electronic transition metal composite electrocatalysts correlates in a straightforward manner with their electrocatalytic activity, such evidence strongly suggests Fermi energy, as a typical elementary binding energy, which otherwise stays in the linear relation with cohesive energy, this forms the basis in investigation and correlation of electrocatalytic activity. Due to the fact that the Fermi wave-vector represents the individual and collective (alloys and intermetallic phases) bulk property of the available electronic number density (or its concentration, n, i.e., k_F = (3π²n) ), and in a straightforward manner correlates with the electronic density of states at the Fermi level, and thereby defines all metallic properties of a metal (and intermetallics) as “a solid with a Fermi surface”, including electrocatalytic features, it has been taken as the main parameter to correlate with the exchange current density in the hydrogen electrode reactions.     

Preparation and characterization of IrxPt1−xO2 anode electrocatalysts for the oxygen evolution reaction

Towards competent production of clean and electrolytic hydrogen, proton exchange membrane (PEM) water electrolysis offers several advantages, such as high purity of the produced gases, very high level of operation safety and direct storage of gases under high pressure. The work presented deals with the development of efficient PEM water electrolyzers, employing high specific surface area Ir_xPt_1−xO₂ electrocatalysts synthesized by the modified Adams fusion method. A typical three-electrode cell was used to evaluate the performance of the materials for water splitting. The performance of electrodes for oxygen evolution reaction was assessed by steady-state current–potential measurements while their electrochemical characteristics and stability were studied by cyclic voltammetry. It was found that Ir–Pt bimetallic oxide electrodes present a stable performance for oxygen evolution reaction. Their intrinsic electrocatalytic activity in combination with their large surface area and stability are quite promising for the development of economically feasible electrocatalysts for PEM water electrolyzers.     

One-step potentiostatic electrodeposition of Ni–Se–Mo film on Ni foam for alkaline hydrogen evolution reaction

Exploring efficient, abundant, low-cost and stable materials for hydrogen evolution reaction (HER) is highly desired but still a challenging task. Herein, Ni–Se–Mo electrocatalysts supported on nickel foam (NF) substrate were synthesized by a facile one-step electrodeposition method. The Ni–Se–Mo film presents high electrocatalytic activity and stability toward HER, with a low overpotential of 101 mV to afford a current density of 10 mA cm⁻² in 1.0 M KOH medium. Such excellent HER performance of Ni–Se–Mo film induced by the synergistic effects from Mo-doped Ni–Se film leads to the fast electron transfer. This work provides the validity of interface engineering strategy in preparing highly efficient transition metal chalcogenides based HER electrocatalysts.     

Molecular metal nanoclusters for ORR,HER and OER: Achievements,opportunities and challenges

With the rapid development of economy, the past decades have experienced more and more severe energy depletion and environmental pollution issues, hence it is urgent to develop more environmental-friendly energy devices, such as fuel cells, metal-air batteries, water electrolysis and so on. However, such devices have long been suffering from the sluggish reaction kinetics and high energy barriers, plus the conventional electrocatalysts used in these devices mostly are noble-metal-based materials, such as Pt/C, IrO₂, and RuO₂. These noble-metal-based electrocatalysts possess significant disadvantages such as high price, limited reserves, and undesirable stability, and these factors together hinder their large-scale industrial application and the inhomogeneity of the catalyst structure at the atomic level also impose great challenges to disclose the underlying catalytic mechanism. Noble metal nanoclusters, as a promising type of electrocatalyst with definitive composition and structure, have been attracting increasingly research efforts. This review aims to summarize the recent achievements of molecular metal nanoclusters employed in electrocatalytic processes, with particular elaboration on oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), as well as to unravel the catalytic mechanism and establish the relationship between its structure and functionality. Specifically, the size effect, the metal core configuration, charge effect, size effect, ligand effect, and metal-ligand binding motifs of the metal clusters that would impact the electrocatalytic performance are comprehensively discussed. In the end, the outlook and perspective including challenges and opportunities are proposed. We anticipate this review would be beneficial for gaining a deeper understanding of engineering nanoclusters for electrocatalysis and to expand its application in electrocatalysis.     

Accelerated oxygen evolution kinetics on NiFeAl-layered double hydroxide electrocatalysts with defect sites prepared by electrodeposition

NiFe layered double hydroxides (LDHs) is considered to be one of the LDHs electrocatalyst materials with the best electrocatalytic oxygen evolution properties. However, its poor conductivity and inherently poor electrocatalytic activity are considered to be the limiting factors inhibiting the electrocatalytic properties for oxygen evolution reaction (OER). The amorphous NiFeAl-LDHs electrocatalysts were prepared by electrodeposition with nickel foam as the support, and the D-NiFeAl-LDHs electrocatalyst with defect sites was then obtained by alkali etching. The mechanism of catalysts with defect sites in OER was analyzed. The ingenious defects can selectively accelerate the adsorption of OH⁻, thus enhancing the electrochemical activity. The D-NiFeAl-LDHs electrocatalyst had higher OER electrocatalytic activity than NiFe-LDHs electrocatalyst: its accelerated OER kinetics were mainly due to the introduction of iron and nickel defects in NiFeAl-LDHs nanosheets, which effectively adjusted the surface electronic structure and improved OER electrocatalytic performance. There was only a low overpotential of 262 mV with the current density of 10 mA cm⁻², and the Tafel slope was as low as 41.67 mV dec⁻¹. The OER electrocatalytic performance of D-NiFeAl-LDHs was even better than those of most of the reported NiFe-LDHs electrocatalysts.     

Impact in rational synthesis Co-ZIF templates for derived the structural Co@NC catalysis for high efficient hydrogen and oxygen evolution reactions

Developing highly active and stable non-noble metal bifunctional electrocatalysts are urgently demanded in overall water splitting. Herein, tunable precursor ratio synthesis of cobalt-based ZIFs as a template derived active cobalt embedded N-doped carbon (Co@NC) catalyst. The rational synthesis of ZIF templates significantly impacts the complex nanostructure and properties of the catalyst (Co@NC). Consequently, the different nanostructures on Co@NC exhibit significance for the electrocatalyst of hydrogen and oxygen evolution reactions. The optimized Co@NC-20 provides excellent electrocatalytic activity with the lowest overpotential of 172 and 301 mV for HER and OER, respectively, at the current density of 10 mA cm^?2. The bifunctional Co@NC-20 reveals a potential for overall water splitting as low as 1.68 V of 10 mA cm^?2. After continuously working for 24h, the exceptional stability activity maintains 75% of the catalytic performance on Co@NC-20. The beneficial character in the synergistic effects between high-active Co species with well-protection of the metal core by carbon shell promotes their excellent performance. This study provides an essential reference for the rational design of ZIF templates for electrocatalysts with more complex structures in the future.     

Recent developments in nickel based electrocatalysts for ethanol electrooxidation

A survey is done to gain a general idea in the development of various nickel based anode electrocatalysts for ethanol electrooxidation reaction. Platinum and other noble metal electrocatalysts are very well known but their cost and scarcity is a major issue hampering its use on a commercial level. Apart from cost, the poisoning of noble metal electrocatalysts due to CO is also another issue. These issues can be tackled by partially or fully replacing the noble metal electrocatalysts by non-noble metal electrocatalysts. The use of electrocatalytically active non-noble metal like nickel provides an excellent alternative. Hence, major thrust is laid upon the use of nickel in the form of either as a single or a complementary element in the electrocatalysts containing two (binary), three (ternary), four (quaternary) or more noble and/or non-noble metals to improve the electrocatalytic activity for ethanol electrooxidation reaction. The quality of an electrocatalyst is decided on a number of factors. Onset potential and current density are the two main parameters representing the activity of electrocatalysts. Complete oxidation of ethanol to give CO₂ is a major requirement to extract maximum current. Literature survey shows that support, synthesis approaches and elemental compositions greatly contributes to enhance the electrocatalytic performance of nickel based electrocatalysts towards ethanol electrooxidation reaction.     

Recent advances in cobalt disulfide for electrochemical hydrogen evolution reaction

Hydrogen production by water electrolysis is the most promising green hydrogen supply method in the future. Electrocatalytic hydrogen evolution reaction (HER), an essential step in water electrolysis, has received continuous interest for a long time. Noble metal-based electrocatalysts exhibit excellent performance for HER, while their high price, limited reserves, and insufficient durability limit their large-scale applications. Transition metal sulfides (TMSs) have been extensively studied as potential alternative catalysts, among which cobalt disulfide (CoS₂) stands out due to its unique structure, low price, and good electrical conductivity. Although remarkable progress has been made, the catalytic activity and stability of CoS₂ electrode materials themselves are still insufficient for large-scale industrial applications, so effective improvement of the HER catalytic performance of CoS₂ remains the focus of research. In this review, we briefly outline the reaction mechanism of HER, focusing on strategies to improve the catalytic performance of CoS₂, including morphology engineering, carbon materials combination, heteroatom doping, and heterostructure construction. Furthermore, the key challenges and opportunities for CoS₂ electrode materials as an electrocatalytic material for HER are discussed.     

Palladium nanoparticles grown on β-Mo2C nanotubes as dual functional electrocatalysts for both oxygen reduction reaction and hydrogen evolution reaction

Developing efficient dual functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is critical for boosting the performance of fuel cells and metal air batteries, as well as production of clean and sustainable energy source. Herein, Pd nanoparticles grown on Mo₂C nanotubes were prepared as dual functional electrocatalysts for both ORR and HER. A series of samples with different Pd loadings were fabricated, while the morphology and the structural features were well examined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Interestingly, both the ORR activity and HER activity first increased then decreased with the increasing of Pd loading, and the sample of Mo₂C-Pd-9% exhibited the best performance among the series, superior than commercial Pd/C in both ORR and HER tests. Furthermore it also exhibited markedly higher long-term stability than Pd/C for both electrocatalytic reactions. The results may shed light on rational design of novel bi-functional electrocatalysts in the renewable energy field.     

Electrocatalysts for hydrogen evolution reaction

In addition to the historical importance of water electrolysis, hydrogen evolution reaction (HER) is the heart of various energy storage and conversation systems in the future of renewable energy. The HER electrocatalysis can be well conducted by Pt with a low overpotential close to zero and a Tafel slope around 30 mV dec^?1; however, the practical developments to satisfy the growing demands require cheaper electrocatalysts. Noble metals are still the promising candidates, though further improvement is needed to enhance the HER efficiency in performance. Three categories of non-noble metal electrocatalysts are under heavy investigations: (i) alloys, (ii) transition metal compounds, and (iii) carbonaceous nanomaterials. The most practical option, based on the electrocatalytic activity and electrochemical stability, seems to be the transition metal compounds MX (where M is Mo, W, Ni, Co, etc. and X is S, Se, P, C, N, etc.). Among these compounds, some like MoS₂ and WC can display metallic properties and a Pt-like electrocatalytic activity, but they still need serious modifications for the practical performance. In general, similar strategies have been employed to improve the HER performance of all of these materials such as doping (both cation and anion), controlling the crystallinity and amorphism, and increasing the active sites by changing the morphology. Another important issue is the chemical and physical structure of the carbon-based catalyst support, as carbon is normally a vital component even for the Pt electrocatalysts.     

Facile synthesis of polyoxometalate-based composite with doped ternary NiCoFe cations as electrocatalyst for oxygen evolution reaction

The construction of efficient and low-cost electrocatalysts for oxygen evolution reactions (OER) to replace precious catalysts is a necessity to achieve economic production of hydrogen. Herein, we report an efficient tri-metallic electrocatalysts for the OER that is prepared by incorporate nickel, cobalt and iron cations on Triton X-100/phosphotungstic acid organic-inorganic composite without utilize any binders or energy consumer procedure. Considering to the synergy effect of simultaneous absorption of NiCoFe cations on composite substrate, the as-made tri-metallic catalyst exhibits excellent OER activity with a small overpotential of 210 and 330 mV at a current density of 10 and 100 mA cm^?2, respectively. Moreover, remarkable trends in electrocatalytic activity of mono-, bi- and tri-metallic electrocatalysts at low (10 mA) and high (100 mA) current density are observed. In addition, this new families of non-precious metal catalyst shows long-term durability in 1 M KOH.