Transition Metal-based Materials Research Articles

Ultrafast and reversible anion storage of spinel nanoarchitecture for high-performance alkaline zinc full cells

Rechargeable alkaline zinc batteries are considered to be potential energy-storage systems owing to their natural abundance, low toxicity, and high capacity. However, their performance and efficiency are limited by the sluggish kinetics and irreversibility of the anode and cathode. In particular, high-capacity binary transition metal-based spinel materials that can store OH− anions are expected to replace commercial MnO cathodes owing to their abundant active sites of two or more transition metals. Herein, we report an ultrafast and reversible anion storage mechanism of spinel NiCo2O4 nanoarchitectures decorated onto N-doped reduced graphene oxide (NCO@N-rGO) for high-performance rechargeable alkaline zinc full cells. The NCO@N-rGO electrode exhibits high specific and rate capacities of 191 mA h g−1 at 1000 mA g−1 and 151 mA h g−1 even at 20 000 mA g−1, respectively, much higher than those of NCO@rGO and NCO. The as-designed cells achieve a record-high volumetric power density (7.20 W cm−3) among alkaline zinc full cells, along with a high energy density (14.93 mW h cm−3) and a capacity retention of 77% over 3000 cycles at 6000 mA g−1. These results are attributed to the facile charge-storage kinetics of the spinel framework, multiple Ni3+/Ni2+ and Co3+/Co2+ redox couples with OH−, and structural integrity of N-rGO as verified by electrochemical, ex situ XRD and XPS, and postmortem analyses. This work proposes a rational design of nanoarchitectured electrode materials for high volumetric performances and long-cycle life of rechargeable alkaline zinc batteries.

Optimal cobalt-based catalyst containing high-ratio of oxygen vacancy synthesized from metal-organic-framework (MOF) for oxygen evolution reaction (OER) enhancement

Oxygen evolution reaction (OER) is a significant reaction in metal-air battery and water splitting. Commonly, noble metals, which are expensive and less stable, have been used as catalysts for OER due to their high catalytic activity. To replace the noble metal catalyst, 3d transition metal-based materials, which are relatively inexpensive and have adequate catalytic activity, have been widely studied as promising candidates for OER catalysts. However, their moderate OER performance presents an obstacle for practical application. The introduction of oxygen vacancies is an effective way to tune the various properties of catalysts, which, in turn, can enhance the catalytic activity. Here, we manufacture cobalt-based catalysts with high ratios of oxygen vacancy from metal-organic framework (MOF) materials. Depending on the amount of treated-hexamethylenetetramine (HMT), the cobalt-based catalysts exhibit various morphologies, compositions, and ratios of oxygen vacancy. The optimal cobalt-based catalyst (300-h/Co/CC) shows significantly enhanced OER performance with a low overpotential of 280 mV at current density of 10 mA cm−2. This work presents a new approach to synthesize catalysts with high ratios of oxygen vacancy from MOF.

Dual-ligand modulation approach for improving supercapacitive performance of hierarchical zinc–nickel–iron phosphide nanosheet-based electrode

Mixed nanostructured transition metal-based complex materials with hierarchical and porous architectures, built from interconnected nano-building blocks, are considered as high-performance positive electrode materials in supercapacitors (SCs). Herein, zinc–nickel–iron phosphide (ZnNiFe–P) and Zn–Ni–Fe–hydroxide precursors (ZnNiFe–OH) were combined in a 3D hierarchical and porous structure (ZnNiFe–(P/OH)) to improve their durability and electrochemical activity by incorporating a dual-ligand synergistic modulation strategy. The 3D ZnNiFe–(P/OH) architectures, comprising perfectly aligned nanosheet arrays (NSA), were successfully grown on Ni foam using a facile hydrothermal process followed by partial phosphorization. The dual-ligand ZnNiFe–(P/OH) electrode exhibited excellent specific capacitance/areal capacitance (1708Fg−1/5.64Fcm−2 for 1Ag−1), high rate performance (62% upto 15Ag−1) and good cycle life. Moreover, the ZnNiFe–(P/OH) NSA positive electrode was coupled with an activated carbon negative electrode to design an asymmetric supercapacitor device. The device delivered an excellent capacitance of ∼187Fg−1 at 0.8Ag−1, a superior energy density of ∼58.4Whkg−1 at 600Wkg−1, and an excellent power density of 11250Wkg−1 at 34.4Whkg−1 while maintaining good cycling performance (88% after 5000 cycles).

Iron anchored microporous cobalt phosphonate novel nanostructure for efficient oxygen evolution reaction

Very recently, microporous transition metal-based organic-inorganic hybrid phosphonate nanostructured materials have attracted attention for their good potential in electrocatalytic application involving oxygen evolution reaction (OER). Newly designed Iron anchored microporous cobalt-based phosphonate materials i.e. FeCoNTMP has been synthesized utilizing nitrilotri(methylphosphonic acid) under the hydrothermal reaction condition in absence of any structure directing agent (SDA). Moreover, FeCoNTMP material exhibits excellent catalytic performance towards electrochemical oxygen evolution reaction with the low overpotential value of 289 mV at the current density of 10 mA cm−2 in 1.0 M basic solution whereas Tafel slope was 59.3 mV dec-1. In addition, FeCoNTMP catalyst displays the outstanding durability up to 25 h, suggesting the potential heterogeneous electrocatalyst.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide.

Hydrogen peroxide (H2 O2 ) is a highly value-added and environmentally friendly chemical with various applications. The production of H2 O2 by electrocatalytic 2e- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H2 O2 synthesis by 2e- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H2 O2 , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e- /4e- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H2 O2 are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H2 O2 electrogeneration through 2e- ORR, from the standpoints of mechanistic studies and practical application.

Strongly coupled Te-SnS2/MXene superstructure with self-autoadjustable function for fast and stable potassium ion storage

Potassium‐ion batteries (PIBs) are a promising candidate for next‐generation electric energy storage applications because of the abundance and low cost of potassium. However, the development of PIBs is limited by sluggish kinetics and huge volume expansion of anodes, leading to poor rate capability and cycling stability. Herein, an advanced superstructure anode, including Te-doped SnS2 nanosheets uniformly anchored on MXene surface (Te-SnS2/MXene), is rationally designed for the first time to boost K+ storage performance. Featuring with strong interface interaction and self-autoadjustable interlayer spacings, the Te-SnS2/MXene can efficiently accelerate electron/ion transfer, accommodate volume expansion, inhibit crack formation, and improve pseudocapacitive contribution during cycling. Thus, the novel Te-SnS2/MXene anode delivers a high reversible capacity (343.2 mAh g−1 after 50 cycles at 0.2 A g−1), outstanding rate capability (186.4 mAh g−1 at 20 A g−1), long cycle stability (165.8 mAh g−1 after 5000 cycles at 10 A g−1 with a low electrode swelling rate of only 15.4%), and reliable operation in flexible full battery. The present Te-SnS2/MXene becomes among the best transition metal-based anode materials for PIBs reported to date.

The Electrochemical Tuning of Transition Metal-Based Materials for Electrocatalysis

The Electrochemical Tuning of Transition Metal-Based Materials for Electrocatalysis

High-rate transition metal-based cathode materials for battery-supercapacitor hybrid devices.

With the rapid development of portable electronic devices, electric vehicles and large-scale grid energy storage devices, there is a need to enhance the specific energy density and specific power density of related electrochemical devices to meet the fast-growing requirements of energy storage. Battery-supercapacitor hybrid devices (BSHDs), combining the high-energy-density feature of batteries and the high-power-density properties of supercapacitors, have attracted mass attention in terms of energy storage. However, the electrochemical performances of cathode materials for BSHDs are severely limited by poor electrical conductivity and ion transport kinetics. As the rich redox reactions induced by transition metal compounds are able to offer high specific capacity, they are an ideal choice of cathode materials. Therefore, this paper reviews the currently advanced progress of transition metal compound-based cathodes with high-rate performance in BSHDs. We discuss some efficient strategies of enhancing the rate performance of transition metal compounds, including developing intrinsic electrode materials with high conductivity and fast ion transport; modifying materials, such as inserting defects and doping; building composite structures and 3D nano-array structures; interfacial engineering and catalytic effects. Finally, some suggestions are proposed for the potential development of cathodes for BSHDs, which may provide a reference for significant progress in the future.

Recent progress in development of efficient electrocatalyst for methanol oxidation reaction in direct methanol fuel cell

The human craving for energy is continually mounting and becoming progressively difficult to Gratify. At present, the world's massive energy demands are chiefly encountered by nonrenewable and benign fossil fuels. However, the development of dynamic energy cradles for a gradually thriving world to lessen fossil fuel reserve depletion and environmental concerns are persistent issues for society at present. The discovery of copious nonconventional resources to fill the gap between energy petition and supply is the extreme obligation of the modern era. A new emergent, clean, and robust alternate of fossil fuels are the fuel cells. Among the different types of fuel cells, direct methanol fuel cells are an outstanding option for light-duty vehicles and portable devices. A critical tactic for striving and sustainable energy sources is the production of highly proficient, economical, and green catalysts for energy storage and conversion devices. Up till now a broad range of research work is available to use Pt and modify Pt-based electrocatalysts to augment the CH3OH oxidation process. Platinum-based nanocubes, nanorods, nanoflower, hybrids of Pt with metal-oxides such as Fe2O3, TiO2, SnO2, MnO, Cu2O, ZnO, and with conducting polymers are extensively utilized in both acidic and basic media. Moreover, Pd-based materials, transition metal-based materials as well as MOF and MOF-derived materials are also the point of interest for researchers nowadays. However, the problem associated with their practical applications is that they can be effectively employed in basic media due to comparative less stability in acidic medium. This review article will deliver a broad vision of the current progress of the MOR process concerning noble metals, transition metals, and MOF-based materials.

(Invited) Si-Based Water Splitting Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts

The development of a carbon-free hydrogen production method by photoelectrochemical (PEC) water splitting is a promising pathway to deal with the increased energy demands and deleterious environmental issues derived from the usage of fossil fuels. Silicon, which is the second most earth-abundant element and a small band-gap material, is an applicable candidate for an efficient solar water splitting photoelectrode. However, the stability of Si-based photoelectrodes hampers efficient water splitting because of its thermodynamic instability and etching of silicon surface. Until now, much research has been conducted to deal with the challenges of using Si to fabricate efficient and stable photoelectrodes. Over the decades, earth-abundant transition metal-based electrocatalysts (metals, sulfides, and hydroxides, phosphies) have been investigated intensively. We briefly introduce the photoelectrochemistry and important parameters for evaluating the performance of the Si photoelectrodes. We present our strategies to overcome the challenges of silicon by combining the advantages of transition metal-based electrocatalysts, which are cost-effective, stable, and highly active for both oxygen evolution and hydrogen evolution reactions. To realize spontaneous water splitting, we introduce Si-based PEC-photovoltaic cells using transition metal-based materials.

Insight into Diffusion Mechanism in Cathode Materials NaVPO5 and NaVFPO4 for Sodium Ion Batteries: DFT Investigation

In transition metal-based cathode materials, the oxidation and reduction of transition metal ions cause local distortion in the crystal structure during intercalation and de-intercalation, resulting in the formation of a quasiparticle, named small polaron. Since a strong binding energy of 500 meV between an alkali ion and a small polaron was reported[1], the polaron would migrate simultaneously with the diffusion of the alkali ions[2]. Using the density functional method, we investigate the crystal, electronic structure, and electrochemical properties of orthorhombic NaxVPO5 and NaxVPO4F, focusing on the diffusion mechanism of a Na ion accompanied with polaron. The diffusion of Na ions can be described as a process of the complex of the Na vacancy and accompanying positive polaron at high Na concentrations and as that of the Na ion and negative polaron at low Na concentrations. When a Na vacancy is introduced to fully occupied structure (x=1), the positive small polaron prefers locating at one of the two first nearest neighbour (1NN) vanadium sites V1NN to the Na vacancy. Similarly, after a Na ion is inserted to fully unoccupied structures (x=0), the negative polaron is formed at a V1NN site of Na ion. Three elementary diffusion processes (EPDs), including the single, crossing,and parallel diffusion processes are explored. The single process occurs when the polaron that strongly binding with the Na vacancy stays at the same position during the Na vacancy diffusion. On the other hand, the parallel (crossing) diffusion process takes place when polaron hops from a V1NN site to the adjacent V1NN site along the direction paralleled (crossed) to Na diffusion path. It is found that the [010] direction is preferable for Na ions diffusing during charging or discharging processes. At a high Na concentration regime, the activation energy E a gains approximately 400 meV on all three EPDs in NaVPO5, whereas it lowers to 190 meV, 320 meV and 345 meV for single, crossing and parallel EDPs, respectively, in NaVPO4F. These values of E a indicate that the effect of polaron migration is negligibly weak in NaVPO5 at high Na concentrations. At low Na concentrations, such a effect of negative polaron migration on Na ion diffusion is considerably substantial in both materials. While the parallel diffusion process is less preferred (Ea = 830 meV and 570 meV for VPO5 and VPO4F, respectively), the single and crossing processes can occur with same activation energy of about 630 meV and 530 meV for VPO5 and VPO4F, respectively. In general, energy barriers required for diffusion in NaVPO4F are notably lower than those in NaVPO5. Compared with some common cathode materials such as the olivines, it is expected that NaVOPO4 and NaVPO5 perform as good as LiFePO4 for LIBs.Figure 1: Diffusion pathway of Na vacancy–positive polaron complex along the [010] direction in de-intercalation. The brown and green octahedra indicate the 1NN and 2NN VO6 groups relative to the Na vacancy, respectively. The red, green and blue balls illustrate the trace of the crossing, single and parallel diffusion processes, respectively. Curved arrows illustrate the migration directions of the polaron in each EDP. [1]T. Maxisch, F. Zhou and G. Ceder, Phys. Rev. B: Condens. Matter Mater. Phys., 73, 104301 (2006). [2]V. A. Dinh, J. Nara, and T. Ohno, Appl. Phys. Express, 5,045801(2012). Figure 1

Modification strategies on transition metal-based electrocatalysts for efficient water splitting

Electrocatalytic water splitting driven by electrocatalysts is recognized as a promising strategy to generate clean hydrogen fuel. Searching and constructing high-efficient and low-cost electrocatalysts is vital in the practical applications of electrocatalytic water splitting. Although transition metal-based materials have been considered as promising electrocatalysts, the satisfactory activities are usually not built on the bulk materials, but strongly relying on elaborately designing these electrocatalysts. Herein, the recent theoretical and experimental progress on modification strategies to improve the intrinsic activities is summarized, especially including element doping, phase engineering, structure cooperation, interface engineering, vacancy engineering, strain engineering and self-functionalization. Finally, the future opportunities and challenges on these modification strategies are also proposed. Overall, it is anticipated that these modification strategies offer some new understandings on rationally constructing non-noble electrocatalysts for efficient electrocatalytic water splitting.

Ni-BTC Derived Porous Ni/C/Graphene Composite for Highly Sensitive Non-enzymatic Electrochemical Glucose Detection

Electrochemical detection of glucose, including enzymatic and non-enzymatic detections, has gained widespread attention due to its rapid response, unparalleled sensitivity, and common applications in both biomedical and healthcare fields. Compared with enzymatic glucose detection, non-enzymatic detection has lower cost, better stability and simpler immobilization operation. However, due to relatively inert nature of glucose and prone-to-contaminant characteristic of solid electrodes, non-enzymatic glucose detection suffers from weak sensitivity and reproducibility. To address these problems, various transition metal-based materials and graphene-based materials have been explored to improve electrochemical responses to glucose.Herein, a highly sensitive non-enzymatic glucose detection electrocatalyst was synthesized by in-situ growing porous NiO nanoparticles derived from Ni-BTC (1,3,5-Benzenetricarboxylic acid) on graphene (NiO/G). By virtue of highly porous Ni-BTC as a precursor, the composite exhibited homogeneous metal distribution, massive exposure of active sites, and spatially ordered structure. The NiO/G demonstrated superior steadiness, electron transfer rate, excellent electrocatalytic activity towards glucose oxidation, and high sensitivity (1030 μA mM−1 cm−2) in alkaline media. Thus, the NiO/G can be utilized as electrocatalysts for applications in non-enzymatic glucose detection, and the preparation strategy can be used to synthesize other nano-composites with low cost, such as CuO/G and Co/G, for non-enzyme glucose sensors.

Heterogeneous CoSe2–CoO nanoparticles immobilized into N-doped carbon fibers for efficient overall water splitting

The rational construction of heterogeneous interface is considered as a promising approach to improve the electrocatalytic performance of non-noble metal materials. Herein, an electrospinning–pyrolysis–partial selenization strategy is proposed to construct the heterostructure nanoparticles of CoSe2CoO, which are encapsulated into N-doped carbon fibers (CoSe2CoO/NCF). The assembled CoSe2CoO/NCF hybrid enables to integrate and optimize the multicomponent features from different components through electronic coupling effects. The heterojunction between CoO and CoSe2 owns abundant nanointerface, which could induce the electron rearrangement to enhance the charge transfer ability and optimize the adsorption towards reaction intermediates. As a result, CoSe2CoO/NCF presents superior cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) activities with low overpotentials, small Tafel slopes and robust durability. Impressively, the overpotentials to afford 10 mA cm−2 current density for acidic HER, alkaline HER and OER upon CoSe2CoO/NCF are 72, 117 and 279 mV, respectively, much lower than those counterparts. Further, the full water splitting device with CoSe2CoO/NCF as both cathode and anode requires a low voltage of 1.604 V to attain 10 mA cm−2. The current work proposes a new insight for interface engineering to improve the electrocatalytic performance of transition metal-based materials.

FeOOH/Ni heterojunction nanoarrays on carbon cloth as a robust catalyst for efficient oxygen evolution reaction

Developing catalysts based on transition metal-based materials for oxygen evolution reaction (OER), which are cheap and efficient, is one of the keys to increase the rate of electrolysis of water to produce hydrogen. Herein, we successfully synthesize iron hydr(oxy)oxide nano-arrays on carbon cloth (FeOOH@CC), and then metallic nickel is electrodeposited on its surface to fabricate FeOOH/Ni heterojunction nanoarrays. Notably, the optimal FeOOH/Ni heterojunction nanoarrays catalyst shows high electrocatalytic performance toward OER with a small overpotential of 257.8 mV at 50 mA cm−2, a Tafel slope of 30.8 mV dec−1 and outstanding long-term stability in alkaline media. The superior OER performance could be ascribed to the introducing of metallic nickel. The nickel in-situ grows on the surface of FeOOH, which not only can improve the conductivity of FeOOH, but also cooperate with FeOOH to form the FeOOH/Ni heterogeneous interfaces for further enhancing OER electrocatalytic activities. This work provides a simple and efficient strategy of interface engineering to fabricate oxyhydroxide/metal heterojunction nanoarrays as high-efficiency OER catalysts.

Structural engineering of Fe2.8Sn0.2O4@C micro/nano composite as anode material for high-performance lithium ion batteries

The design and synthesis of high-performance anode materials using environmentally friendly routes are demanded to meet the high-energy and high-power density requirements of next-generation lithium-ion batteries (LIBs). Herein, a facile and green gallic acid-assisted in situ synthetic strategy is demonstrated to fabricate novel Fe2.8Sn0.2O4@C composites as the anode materials of high-performance LIBs. The Fe2.8Sn0.2O4@C composites reveal a sphere-like morphology (2–3 μm in diameter), which is composed of highly dispersed nano iron-tin oxide particles (~25 nm) embedded in an interconnected carbon skeleton. This unique structure and composition allow Fe2.8Sn0.2O4@C to inhibit electrolyte adverse reactions, improve electron conductivity, and buffer mechanical stress for the entire electrode, resulting in its outstanding electrochemical performances. A high specific capacity of 2047 mA h g−1 at 0.2 A g−1 after 250 cycles, long cycling stability of 1061 mAh g−1 at 1 A g−1 after 1000 cycle, and rate capability of 572 mAh g−1 at 10 A g−1 are obtained. It is also indicated that the lithium storage in the Fe2.8Sn0.2O4@C anode involves both diffusion and surface capacitance mechanisms. The strategy reports here may open up new prospects for the design and manufacture of high-performance transition metal-based anode materials.

Three-dimensional self-supporting NiFe-X (X = OH, O, P) nanosheet arrays for high-efficiency overall water splitting

Three dimensional (3D) self-supporting materials are a class of promising catalysts for electrochemical water splitting, because 3D materials can not only effectively prevent the catalyst from falling off by the generated vigorous H2 or O2 bubbles, but also provide large catalytically active surface areas by designing porous structures. However, the relationship between anion species and their electrocatalytic performances of such materials in alkaline media has not been clearly known. Herein, a simple strategy is used to grow NiFe-based hydroxides/oxides/phosphides (NiFe-X, X = OH, O, P) nanosheet arrays directly on nickel foam (NF) as electrode materials for overall water splitting. We found that the electrocatalytic activity is strongly dependent on the composition of the prepared 3D composites. The electrochemical results reveal that the NiFeP-NF shows the highest electrocatalytic activity for oxygen evolution reaction (OER) with ultra-low overpotentials of 194 and 220 mV to reach the current densities of 20 and 50 mA cm−2, while the NiFeO-NF has the highest catalytic activity for hydrogen evolution reaction (HER) with an overpotential of 162 mV to reach the current density of 10 mA cm−2. In addition, both materials exhibit high long-term durability. Based on the excellent catalytic properties for OER and HER, the two catalysts are then assembled as cathode and anode, respectively, and only 1.58 V is needed to achieve the current density of 10 mA cm−2 for overall water splitting. This work presents a strategy to fabricate excellent OER and HER catalysts for electrochemical water splitting and demonstrates the effects of various anion species on the catalytic activities of transition metal-based nanosheet materials.

Highly Porous Nickel-Phosphorous Electrode Synthesized By a Facile Electrodeposition Method for Efficient Alkaline Electrolysis

Water electrolysis is an electrolysis reaction of water, which can produce a large amount of hydrogen environmentally friendly. The purposes of water electrolysis are responding to current hydrogen demand and storing electric energy produced from renewable energy such as solar or wind power to solve their intermittency. Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The theoretical equilibrium potential of the water electrolysis is 1.23 V, but the electrolysis reaction of the actual water requires much higher voltage due to the slow kinetics of oxygen and hydrogen reaction. Therefore, in order to increase hydrogen production efficiency, it is necessary to develop a catalyst having high activity and low cost for OER and HER.At present, transition metal-based materials such as Fe, Co, and Ni are mainly studied as high efficiency catalysts for HER and OER for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide/hydroxide, phosphide, sulfide and other transition metal-based materials. Among various type of compounds, Nickel phosphides/phosphorous have drawn attention for use in the alkaline water electrolysis system due to their chemical stability, conductivity, and bifunctional catalytic activity towards HER and OER. Different properties between Ni and P such as electronegativity and atomic radius induce the modulation of the Ni electronic structure. The modulated electronic structure of Ni can tailor the adsorption properties of adsorbates participating in HER and OER, and accelerate the kinetics of both reactions. However, since most of Ni phosphide/phosphorous catalysts require long time for synthesis process, including high temperature heat treatment, which acts as a hurdle to apply to industry field.In this study, we have developed the Ni-P electrodes for cathode and anode for alkaline electrolysis. An electrodeposition method has been applied, because it is a facile and reproducible technique that can be carried out at room temperature. The synthesis procedure of the Ni-P electrodes only needs 10 seconds, and hydrogen bubbles vigorously generated during synthesis act as a dynamic template, resulting in a porous structure of Ni-P. The surface area of highly porous Ni-P (HP Ni-P) was 3.42 times larger than that of the flat Ni electrode. The HP Ni-P exhibits remarkable electro-catalytic activity and stability towards the HER and OER in alkaline solutions. For the HER, overpotentials at -10 and -100 mA/cm2 were 97 and 145 mV, respectively, which are superior catalytic activity than that of Pt/C. For the OER, HP Ni-P shows higher catalytic activity than IrO2at the entire current region; overpotentials at 10 and 100 mA/cm2 were 286 and 331 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO2) in the long-term chronopotentiometry test. Our experimental and theoretical studies demonstrate that the formation of Ni-P bond induces the lack of electrons of Ni in Ni-P and increases hydrogen adsorption sites. In addition, electronic structure change of Ni in Ni-P promotes water dissociation and lowers hydrogen adsorption energy. These changes enable the Volmer-Tafel reaction and accelerate the HER kinetics. Furthermore, the oxidation of P during OER could help to accelerate the OER rate.Fig 1. SEM images of (a), (b) highly porous Ni-P, and (c), (d) highly porous NiFig 2. (a) HER and (b) OER activities of HP Ni-P, HP Ni and precious metal-based catalysts in 1 M KOH. Figure 1

Overview of transition metal-based composite materials for supercapacitor electrodes.

Supercapacitors (SCs) can bridge the gap between batteries and conventional capacitors, playing a critical role as an efficient electrochemical storage device in intermittent renewable energy sources. Transition metal-based electrode materials have been investigated extensively as a class of electrode materials for SC application, but they have some limitations due to the sluggish ion/electron diffusion and inferior electronic conductivity, restricting their electrochemical performances towards energy storage. Developing advanced transition metal-based electrode materials is crucial for high energy density along with high specific power and fast charging/discharging rates towards high performance SCs. In this review, we highlight the state-of-the-art of transition metal-based electrode materials (transition metal oxides and their composites, transition metal sulfides and their composites, and transition metal phosphides and their composites), focusing on specific morphologies, components, and power characteristics. We also provide future prospects for transition metal-based electrode materials for SCs and hope this review will shed light on the achievement of higher performance and hold great promise in vast applications for future energy storage and conversion.

Si-Based Water Oxidation Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts

The development of a carbon-free hydrogen production method by photoelectrochemical water splitting is a promising pathway to deal with the increased energy demands and deleterious environmental issues derived from the usage of fossil fuels. Silicon, which is the second most earth-abundant element and a small band-gap material, is an applicable candidate for an efficient solar water splitting photoelectrode. However, the stability of Si-based photoelectrode hampers efficient water splitting, especially for the Si-based photoanodes because of its thermodynamic instability and etching of silicon surface. Until now, much research has been conducted to deal with the challenges of using Si to fabricate efficient and stable photoanodes. Over the decades, to expedite the oxygen evolution reaction, a complicated 4-electron transfer process and requires large overpotential, cheap and earth-abundant transition metal-based electrocatalysts have been investigated. In this Review, we briefly introduce the photoelectrochemistry and important parameters for evaluating the performance of the Si photoanodes. We summarize transition metal-based catalysts, focusing on Ni-, Co-, and Fe-based oxygen evolving catalysts. Then, we present various strategies to overcome the challenges of silicon by combining the advantages of transition metal-based oxygen evolving catalysts, which are cost-effective, stable, and highly active for oxygen evolution reaction. Finally, to realize spontaneous water splitting, we introduce Si-based tandem cells combined with transition metal-based materials.