2D transitional-metal nickel compounds monolayer: Highly efficient multifunctional electrocatalysts for the HER,OER and ORR

The atomically dispersed Ni metal on two-dimensional (2D) monolayer species have exhibited impressive catalytic properties towards oxygen evolution and oxygen reduction reactions (OER/ORR). In this work, nickel boride and nickel carbide monolayers, which is inspired by the more different active sites of monolayer surface, such as Ni atom, that efficiently catalyze the reduction of OOH to O₂ to some extent, while B/C atom can work in the reduction of H1 to H₂, are theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of Ni atom to two free-metal atoms such as B and C to form catalytically active double sites lead to a remarkable reduction of the first or last hydrogenation free energy barrier step. The resulting Ni₂B₅ and NiC₃ exhibited ultra-low Gibbs free energy (ΔG_H1) of only 0.096 V and 0.018 V for HER and lower onset potentials of only 0.39 V (0.62 V) and 0.60 V (0.31 V) for OER (ORR), respectively, while the NiB₆ exhibited appropriate OER and ORR electrocatalytic activity. The superior bifunctional even multifunctional catalytic performance in the overall water splitting is mainly attributed to both the electron donation from the Ni metal atoms to the key intermediates, which significantly polarizes and weakens the O–H bond, and to the synergistic effect of the B/C atoms that moderates the binding strength of terminal top of H atoms. This work constitutes the DFT study of the water electroreduction processes on diverse nickel compounds monolayer catalytic sites and, consequently, paves the way towards the rational design of highly efficient bifunctional even multifunctional electrocatalysts.     

An efficient and durable bifunctional electrocatalyst based on PdO and Co2FeO4 for HER and OER

The adoption of effective, minimal, and versatile electrocatalysts for water splitting to generate hydrogen fuels is of critical importance. The bulk of newly described materials have considerable onset potential, but their electrocatalytic activity is limited by weak electrical conductivity and a limited range of catalytic sites. The combination of a few precious metals added with transition metal-based compounds is a novel and captivating approach. Herein, cobalt ferrite oxide (Co₂FeO₄) @ palladium oxide (PdO) nanostructures have been prepared through the combined use of hydrothermal and ultraviolet (UV) irradiation techniques. For hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics, the modified composition offers a high concentration of active sites, improved electrical conductivity, and stability. The Co, Fe, and Pd ions at the composite system's interface may affect the adsorption energy of reaction intermediates synergistically, enabling the process to continue with less potential. The electrocatalyst Co₂FeO₄@PdO demonstrates an excellent bifunctional approach to electrochemical water splitting (EWS) for HER and OER in alkaline medium. As-prepared electrocatalyst shows an overpotential value of 269 and 259 mV for HER and OER at 10 and 20 mA/cm² current densities respectively. The low charge transfer resistance values such as 72.2 and 62.4 Ω and durability for 48 h has been observed toward HER and OER, support this material as an efficient and durable electrocatalyst for energy conversion systems.     

Facile synthesis of the encapsulation of Co-based multimetallic alloys/oxide nanoparticles nirtogen-doped carbon nanotubes as electrocatalysts for the HER/OER

The pivotal challenge of electrocatalysis remains the development of highly effective electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this work, a universal strategy of preparing the encapsulation of Co-based multimetallic alloys/oxide nanoparticles in nitrogen-doped carbon nanotubes (named CoM@CNTs, M = Ni/Mn/Fe) was induced by annealing mixtures of the as-synthesized precursor, ethanol and different metallic acetates, including binary CoNi@CNTs, ternary CoNi/MnO@CNTs and quaternary CoNiFe/MnO@CNTs. By virtue of its unique structure with a high electrical conductive network based on CNT substrates, abundant catalytic active sites supplied by multimetallic nanoparticles and protection against nanoparticle corrosion by N-doped carbon layers, the as-synthesized CoNiFe/MnO@CNTs electrocatalyst has remarkable HER properties with a low overpotential of 122 mV and OER activity with a low overpotential of 275 mV at 10 mA cm^?2 and excellent stability and durability under long-term testing in alkaline solutions. Therefore, this strategy will provide a new route for fabricating multimetallic-based CNTs as HER/OER electrocatalysts with excellent stability and high catalytic activity.     

2D MXenes and their heterostructures for HER,OER and overall water splitting: A review

MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides that have surface termination groups such as –OH, –O, and –F. The presence of transition metal imparts conductivity, surface termination groups induce hydrophilicity and layered structure offers large surface area which makes MXenes a potential candidate to be utilized as an electro-catalyst with enhanced efficiency. The Water Electrolysis (WE) efficiency of an electro-catalysts is dependent on the performance of half-cell reactions i.e. Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). The OER kinetics of most of the bi-functional electrocatalysts are considered sluggish due to which they are tested in alkaline media. However, due to the metallic nature and surface properties of MXenes, they as substrate not only improve HER performance of grown electro-catalyst but also facilitate OER kinetics which is considered sluggish for most bi-functional electrocatalysts. This review presents the significance of MXenes as HER, OER, and bi-functional electrocatalysts by discussing the electrocatalytic properties of a wide range of MXenes and how their hetero-structures affect HER, OER, and bi-functional electrocatalytic performance. In the end, the current challenges, and future perspectives of MXenes and their nanocomposites for water electrolysis have been discussed.     

Co3Mo3N nanosheets arrays on nickel foam as highly efficient bifunctional electrocatalysts for overall urea electrolysis

Replacing dynamics-restricted oxygen evolution reaction (OER) with smart urea oxidation reaction (UOR) is very important for reducing the power consumption for hydrogen production. Here, the Co₃Mo₃N-400/NF is prepared using a facial way, which exhibits remarkable catalytic performances for UOR, hydrogen evolution reaction (HER) and overall urea electrolysis (OUE) because of the more exposed active sites and high electrical conductivity. At 100 mA/cm², the Co₃Mo₃N-400/NF shows a small potential of 1.356 V vs. RHE (reversible hydrogen electrode) for UOR, which is much lower than that for OER. Furthermore, for HER, to reach to 100 mA/cm², a low overpotential of 299 mV is required, and the urea has negligible influence on the HER process. For OUE, the Co₃Mo₃N-400/NF||Co₃Mo₃N-400/NF shows a small cell potential of 1.481 V at 100 mA/cm² along with a good durability. Our work provides more choice for future OUE to generate hydrogen.     

Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER

Tetra-carboxylic acid based 3D porous MOFs 1 and 2, named {[Co_2.5(L)]·5H₂O}_n and 0.5{[Cu(L)]·2H₂O}_n (L = 4,4′-di(ethoxy)biphenyl-3,3′,5,5′-tetra-(phenyl-4-carboxylic acid)), bearing metal clusters have been facilely constructed by hydrothermal synthesis. Structural studies indicate that 1 presents a 3-nodal (4, 4, 8)-connected topology with the point notation of {4⁴.6²}2{4⁸.6⁷.8¹³}, while 2 shows a uninodal (4, 4)-connected network with point symbol of {4⁴.6²}. The pristine MOFs are directly utilized for electrocatalysis and poor HER activities are obtained in alkaline solution, which promote the further design and fabrication of a mixed-metal Co/Cu-MOF (3). As expected, 3 shows significantly improved performances for HER with overpotential of 391 mV (10 mA cm⁻² current density), low Tafel slope of 94 mV dec⁻¹ and long-term operation stability (14 h). More importantly, the direct utilization of 3 for accelerating OER also presents a fascinating performance in overpotential at 10 mA cm⁻² current and durability. The above electrocatalytic performance of pristine 3 can be ascribed to the result of hybridizing strategy for constructing MOFs under hydrothermal procedure, which may favorably produces synergistic effect and more open metal sites. This work provides in-depth understanding of hybrid pristine MOFs for electrocatalysis.     

One-pot hydrothermal synthesis: Enhanced MOR and OER performance using low-cost Mn3O4 electrocatalyst

Mn₃O₄ NPs were synthesized using a simple, rapid, and cost-effective hydrothermal method. High-resolution transmission electron microscopy showed that most of the synthesized NPs were cubic, with some NPs being spherical and hexagonal. The electrochemical results showed that the Mn₃O₄ cubes were exhibited good oxygen evolution activity (OER) and methanol oxidation activity (MOR) and stability in alkaline media, making them a promising and efficient material in energy applications.     

Hydrothermally/electrochemically decorated FeSe on Ni-foam electrode: An efficient bifunctional electrocatalysts for overall water splitting in an alkaline medium

A new hybrid catalyst based on Ni foam (NF) and FeSe was prepared by a facial hydrothermal method, in which Se-decorated NF was subsequently electrochemically doped by Fe. Binder-free catalyst containing electrodes were directly tested for the hydrogen and oxygen evolution reaction (HER/OER). The FeSe/NF electrode displayed an OER current density of 100 mA cm⁻² at potential of 1.42 V, and a relatively small Tafel slope of 109 mV dec⁻¹ in a 1 M KOH solution. Also, FeSe/NF electrode exhibited reasonable HER overpotential of 200 mV at 10 mAcm⁻² current density with Tafel slope of 145 mV dec⁻¹. The XRD and TEM studies revealed that the formation of heterogeneous interfaces of NiSe₂ and FeSe₂,generated more active sites that can promote better ions and electron transport in the electrode/electrolyte interfaces. Furthermore, HRTEM analysis indicates that FeSe₂ rich in Se vacancy defects can be created with suitable M − O and M − H bond for better OER and HER performance, respectively. In a-two electrode alkaline water electrolyzer, current densities of 10 mA cm⁻² and 50 mA cm⁻² were obtained at cell voltages of 1.52 V and 1.85 V, respectively, using pure FeSe–NF as both the cathode and anode.     

Controllable synthesis of one-dimensional NiS2 nanotube and nanorod arrays on nickel foams for efficient electrocatalytic water splitting

One-dimensional NiS₂ nanotube arrays and nanorod arrays are controllably grown on Ni foam surface. The electrocatalytic test shows that the NiS₂ nanotube arrays require competitive overpotentials of 209 mV for HER and 367 mV for OER (to achieve a current density of 50 mA/cm²), respectively, which are much lower than the NiS₂ nanorod arrays and other NiS₂ nanostructures reported before. Specifically, the NiS₂ nanotube arrays can be employed as an efficient bi-functional catalyst for overall water splitting, with a low cell voltage (1.58 V) to deliver a current density of 10 mA/cm². The outstanding performance can be attributed to the special structural characteristics of nanotubes, which have high specific surface areas along with abundant active sites. The present study not only enriches the morphology of NiS₂ nanostructures for highly efficient electrocatalytic reaction, but also provides an interesting self-assembly path for the synthesis of one-dimensional NiS₂ nanostructures.     

Catalytic activity and underlying atomic rearrangement in monolayer CoOOH towards HER and OER

For efficient hydrogen and oxygen production, design and synthesis of cost-effective, stable and active materials are inevitable. In this work, the catalytic activity of 2D CoOOH towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been investigated using first principles calculations based on density functional theory. The adatom induced structural rearrangement have been investigated from structural parameters as well as charge redistribution in 2D CoOOH. The preferred site for hydrogen and oxygen adsorption were found to be the top site of oxygen atom of 2D CoOOH. The catalytic activity of HER and OER towards 2D CoOOH was studied by calculating the Gibbs free energy. Our study revealed that the 2D CoOOH serve better as a catalyst for HER than OER with adsorption energy of −0.45 and −3.68 eV respectively suggesting its efficient use for hydrogen production. We further investigated the changes in electronic properties of 2D CoOOH on adsorption of hydrogen and oxygen atom.     

Comparative studies on the electrocatalytic hydrogen evolution property of Cu2SnS3 and Cu4SnS4 ternary alloys prepared by solvothermal method

The development of non-precious metal based electrocatalysts is essential for Hydrogen Evolution Reaction (HER) from the splitting of water. Recently copper tin sulphide (CTS) has gained attention due to its favourable properties in photovoltaic devices and photocatalysis. In this work, CTS has been explored as an electrocatalyst in HER. Cu₂SnS₃ (CTS (A)) and Cu₄SnS₄ (CTS (B)) are prepared by a simple solvothermal method. From the XRD analysis, tetragonal and orthorhombic phase is confirmed for CTS (A) and CTS (B), respectively. Morphology and composition of the prepared samples has been confirmed by SEM and EDAX analysis. Optical study reveals that the band gap energy is around 1.55 and 1.20 eV for CTS (A) and CTS (B), respectively. From photoluminescence studies, CTS (B) has been observed to have a greater recombination of charge carriers than CTS (A). The prepared materials were tested and compared for its performance as electrocatalysts for HER, where the current density of CTS (A) is higher than that of CTS (B). Furthermore, the calculated Tafel slope was 98 mV/dec and 110 mV/dec for CTS (A) and CTS (B), respectively. On the other hand, CTS (A) and CTS (B) have showed good stability even after 500 cycles in acidic medium.     

Development of RuO2/CeO2 heterostructure as an efficient OER electrocatalyst for alkaline water splitting

Here, the synthesis of RuO₂ loaded CeO₂ with varying amount of Ru loading with enhanced amount of Ce³⁺ and surface area, through synthesis of CeO₂ using cerium ammonium carbonate complex as procure followed by Ru loading by impregnation and calcination at 300 °C, is presented. Corresponding characterizations by XRD, SEM, TEM, XPS of all the samples reveal the formation of highly crystalline mesoporous CeO₂ nanoparticles with uniformly dispersed RuO₂ particles on the CeO₂ surface having approximately 45% Ce³⁺. All the samples were utilized as oxygen evolution reaction (OER) catalyst for electrocatalytic H₂ generation through water electrolysis. Electrocatalytic experiments reveal that synthesized 1 wt% RuO₂ loaded CeO₂ (1-RuO₂/CeO₂) showed superior OER activity. A quite low over-potential of 350 mV is required to attain a current density of 10 mA/cm² (ɳ₁₀), with a Tafel slope of 74 mVdec⁻¹ for OER in 1 M KOH solution. The synthesized 1-RuO₂/CeO₂ electrocatalyst also exhibited superior long term stability in basic medium and redox atmosphere.     

Synthesis of Co2FeAl alloys as highly efficient electrocatalysts for alkaline hydrogen evolution reaction

The development of cost-effective non-precious metal electrocatalysts is a major challenge for water splitting applications, but it is important for the realization of renewable energy systems. Alloying has proved an effective way to design metal-based electrocatalysts, and by controlling the annealing temperature, the surface morphology and crystallinity of the alloy can be tuned to control the hydrogen evolution reaction (HER) performance. In this work, with a simple coprecipitation method, we have prepared Co₂FeAl alloys at different annealing temperatures (550 °C–670 °C), which exhibit excellent crystallinity and electrocatalytic performance for HER in alkaline solution. Among all conditions, the Co₂FeAl alloys prepared at 620 °C shows the better crystallinity and the higher purity, and it could achieve a low overpotential of 149 mV at 10 mA cm^?2 in alkaline solution. The overpotential demonstrates persistent stability with only 3 mV change after over 1000 cycles. Both density functional theory (DFT) calculations and experimental results revealed that alloying optimizes the electronic structure near the Fermi surface of the system, improving the electron transport efficiency and enhancing the catalytic activity. These Co₂FeAl alloys are appealing candidates for high-performance alkaline HER electrocatalytic electrodes in water electrolysis due to their outstanding electrocatalytic properties.     

Solvothermal synthesis of g-C3N4 and ZnO nanoparticles on TiO2 nanotube as photoanode in DSSC

In this research, the dye-sensitized solar cells (DSSCs) were fabricated by using g-C₃N₄ and ZnO modified TiO₂nanotube (TNT) arrays as photoanodes. The TNT arrays were synthesized by the anodizing method. G-C₃N₄ and ZnO modified TNTs were synthesized via a solvothermal method in which ethylene glycol served as a solvent. The short circuit current (I_SC) and open-circuit voltage (V_OC) of DSSCs based on ZnO + g-C₃N₄modified TNTs photoanode considerably increased from 9.25 mA/cm⁻²to 14.68 mA/cm⁻², and from 0.707 mV to 0.695 mV, respectively, resulting in a135% increase in the efficiency compared with the pure TNT arrays photoanode. However, the DSSC fabricated with g-C₃N₄ TNT did not show much improvement in the conversion efficiency compared with the pure TNT arrays photoanode, implying more effective processes of the carrier production and transport between ZnO, g-C₃N₄ and TNTs. Also, the electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) analyses showed that ZnO + g-C₃N₄ can promote the electron collecting rate, suppress he electron recombination and extend the electron lifetime.     

Phytic acid-derived Co2-xNixP2O7-C/RGO and its superior OER electrocatalytic performance

A phytic acid-derived Co_2-xNi_xP₂O₇-C/RGO composite was designed and facilely synthesized, in which phytic acid acted as both a phosphoric source and carbon source. Both carbon derived from phytic acid and reduced graphene oxide (RGO) in composite, enhanced the conductivity and thus improve its electrocatalytical capability. As-synthesized Co_1.22Ni_0.78P₂O₇-C/RGO composite exhibited excellent oxygen evolution reaction (OER) catalytic performances: At the current density of 10 mA cm⁻², only a low overpotential of 283 mV and a small Tafel slope of 51 mV dec⁻¹ were observed. Good OER catalytic performance was retained even after 10 h continuously running at a constant voltage, which is even comparable to those of first-rate noble metal catalyst RuO₂. In addition, the performances of Co_2-xNi_xP₂O₇-C/RGO catalysts were also strongly dependent on Ni content.     

Self-supported Ni3S2@MoS2 core/shell nanorod arrays via decoration with CoS as a highly active and efficient electrocatalyst for hydrogen evolution and oxygen evolution reactions

A hierarchically porous MoS₂ on Ni₃S₂ nanorod array on Ni foam (MoS₂/Ni₃S₂/NF) was firstly fabricated through a simple microwave-assisted hydrothermal method, and then followed by electrochemical deposition approach in which MoS₂/Ni₃S₂/Ni foam is decorated with CoS (CoSMoS₂/Ni₃S₂/NF). In contrast to conventional hydrothermal approach, microwave irradiation accelerates the synthesis of MoS₂/Ni₃S₂/Ni foam from time of >20 h–2 h. The characterization of CoSMoS₂/Ni₃S₂/NF by scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) indicate that a whole scale of 1D the Ni₃S₂ nanorods were hierarchically integrated with MoS₂ and CoS nanosheets. The as-synthesized CoSMoS₂/Ni₃S₂/NF hybrid not only endows the ease transport of electrons along Ni₃S₂ nanorods to Ni foam, but also accommodates maximal exposure of active edge sites to the reactants through hierarchically porous CoS doped MoS₂ nanosheets, accomplishing the promoted kinetics and activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). By electrochemical measurements such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscope (EIS), we find that the CoSMoS₂/Ni₃S₂/NF hybrid shows markedly enhanced electrochemical performance for both HER and OER. Specifically, the optimal CoSMoS₂/Ni₃S₂/NF8C possesses the low overpotentials (η₁₀) of 85 and 225 mV at current density (|j|) of 10 mA cm^?2 in 1.0 M KOH and the small 62.3 and 46.1 mV dec^?1 Tafel slope for HER and OER, respectively, outperforming those of most of the current noble metal-free electrocatalysts. These results highlight the fact that CoSMoS₂/Ni₃S₂/NF is a high-performance, noble-metal-free electro-catalyst, and provide a potential avenue toward achieving an enhanced electrocatalytic activity towards both in HER and OER. Yet the duration of the as prepared catalyst in OER still need to be improved.     

One-step synthesis of Co2P/N-P co-doped porous carbon composites derived from soybean derivatives as acidic and alkaline HER electrocatalysts

Developing inexpensive and efficient electrocatalysts for hydrogen evolution reaction (HER) in both acidic and alkaline mediums is of great significance to the hydrogen energy industry. Hereby, we prepared a mixture of precursors with homogeneous composition by using the chelating ability of soybean protein isolate (C and N source) and phytic acid (dopant and phosphating agent) with cobalt ions, and achieved one-step synthesis and construction of Co₂P/N–P co-doped porous carbon composite by carbonization at 800 °C. The as-synthesized Co₂P/NPPC-800 electrocatalyst exhibits low HER overpotentials of 121 and 125 mV at 10 mA cm^?2 in 0.5 M H₂SO₄ and 1.0 M KOH, which are close to those of the commercial Pt/C catalyst. Additionally, the NPPC substrate surrounding the Co₂P could diminish the corrosion during the HER, and Co₂P/NPPC-800 displays good stability and durability. Furthermore, this work offers a convenient synthesis strategy for phosphide/doped porous carbon composites in other electrochemical energy technologies.     

3D graphene decorated with hexagonal micro-coin of Co(OH)2: A competent electrocatalyst for hydrogen and oxygen evolution reaction

Herein, 3D graphene is synthesized from the cation exchange resin by a cheap and efficient strategy, and then hexagonal micro-coin Co(OH)₂ particles are loaded by a simple double displacement reaction. Different analytical techniques confirm that the 3D graphene exhibit rose petal-like structure, which is decorated with Co(OH)₂ hexagonal micro coin structure. The hexagonal micro-coin Co(OH)₂ are the actual active sites for electrochemical reactions, while conductive graphene eases the transport of electrons which may further heighten their performance. The as-synthesized electrocatalysts are used to study different electrochemical measurement in an alkaline (1 M KOH) solution. The as-prepared Co(OH)₂-3 dimensional graphene-0.5 delivered the overpotential value of −0.367 and 1.599 V (vs RHE) (10 mA cm⁻²), the calculated Tafel slope values were 96 and 110 mV dec⁻¹ for hydrogen and oxygen evolution reactions correspondingly. Different concentrations of Co were used to study the effect of Co on electrochemical measurements. The data shows that Co(OH)₂-3DG-0.5 exhibited better performance than the other as-prepared Co(OH)₂ based electrocatalysts. The as-prepared electrocatalyst also shows low R_ct and reasonable stability for hydrogen and oxygen evolution reaction.     

One-pot synthesis of Fe2O3/C by urea combustion method as an efficient electrocatalyst for oxygen evolution reaction

The hematite type Fe₂O₃/C catalyst for oxygen evolution reaction (OER) was prepared using a facile urea combustion method. The Fe₂O₃/C electrode showed excellent electrocatalytic ability towards OER in alkaline medium. The phase and morphology of the product were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The sintering temperature is 600 °C, and the calcination time is 3 h, which is the best preparation process condition. The particles were irregular spherical with the size of 10–30 nm and dispersed uniformly. The current density of the Fe₂O₃/C electrode was 147 mA cm⁻² at 0.6 V (vs. HgO/Hg) in 6 mol L⁻¹ KOH electrolyte at room temperature.