Catalysts For Overall Water Splitting Research Articles

Macroporous high‐entropy spinel oxide monoliths as efficient oxygen evolution electrocatalyst

AbstractIn this paper, macroporous high‐entropy spinel oxide (HESO) (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 monoliths were successfully fabricated via a sol–gel method followed by calcination. Appropriate polyacrylic acid and propylene oxide contents allow the formation of three‐dimensional co‐continuous xerogel monoliths, and the water/glycerol ratio controls the macropore size of monoliths. Subsequent calcination achieves the precipitation of HESO (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 with a singular phase and exceptional structural stability. The macroporous HESO (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 demonstrates remarkable performance in the oxygen evolution reaction (OER) with an overpotential of 333 mV at 100 mA cm−2 and a Tafel slope of 43.2 mV dec–1, surpassing that of RuO2 (391 mV) under identical conditions. Furthermore, the catalytic stability of the HESO catalyst remains superior even after 24 h of testing. This process offers a promising avenue for the development of macroporous high‐entropy oxide OER catalysts for overall water splitting.

MOFs-derived 3D hierarchical CoFe2O4/RuO2 hollow nanosheet array for efficient overall water splitting

Accelerating hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics process is crucial to the development of transition metal oxide based materials as bifunctional catalysts for overall water splitting. Herein, a heterostructured CoFe2O4/RuO2 hollow hierarchical nanosheet array (FeRu-CoOx) has been designed by using MOF as template followed by simultaneous etching-doping process combined with calcination for enhanced HER performance without compromise of OER activity. The MOF-derived FeRu-CoOx exhibits an enhancement on HER performance with 58 mV overpotential to achieve 10 mA cm-2 and a small Tafel slop of 73.1 mV dec-1 in the alkaline solution, compared to CoFe2O4 counterpart (Fe-CoOx) using monometallic Fe etching and doping. In addition, the FeRu-CoOx electrode also shows good OER activity (10 mA cm-2 at an overpotential of 229 mV), better than those of Fe-CoOx (239 mV) and commercial RuO2 electrodes (270 mV). The introduction of Ru helps the construction of heterostructure, which is in favor of exposure of active sites, increasing the electron interaction between different composites, and accelerating charge transfer. The alkali electrolyzer using FeRu-CoOx as cathode and anode shows a small cell voltage with only 1.48 V at 10 mA cm-2 and good tolerance toward long-term stability. The MOF template combined with etching-doping strategy to construct hierarchical and heterostructured materials gives a way to develop advanced bifunctional electrocatalysts for water splitting.

Interface and Electronic Structure Regulation of RuO2 through Ru/RuO2 Heterointerface Construction and Zn Atom‐Doping for High‐Performance Overall Water Splitting in Both Alkaline and Acidic Media

AbstractRational design and fabrication of efficient catalysts for overall water splitting in an integrated electrolyzer are essential for energy storage and conversion. Herein, an ultra‐small substitutional Zn‐doped Ru/RuO2 heterostructure (5.6 nm) is synthesized through a facile pyrolyzing strategy as an efficient bi‐functional electrocatalyst for water splitting in both acidic and alkaline media. Experiments demonstrate that introducing appropriate alien atoms into RuO2 with an amorphous state can effectively optimize the electron structure and expose abundant defects, and thus increasing the number of active sites and improving the intrinsic activity of RuO2 for the oxygen evolution reaction (OER). In addition, the high hydrogen adsorption capability of metallic Ru makes Ru‐RuO2 heterointerface suitable for water splitting. Notably, the Ru/ZnRuO2 displays low overpotentials at 10 mA cm−2 with only 184 mV for OER in 0.5 m H2SO4 and 35 mV for HER in 1.0 m KOH. Besides, the as‐synthesized Ru/ZnRuO2 displays cell voltages of 1.540 and 1.567 V (at 10 mA cm−2) for water splitting with remarkable durability for more than 220 and 100 h, respectively, in acidic and alkaline media. This work provides a facile strategy for designing pH‐universal electrocatalysts toward overall water splitting.

NiFeP nanosheet arrays supported on nickel nitrogen codoped carbon nanofiber as an efficient bifunctional catalyst for overall water splitting

Transition metal phosphides have emerged as effective electrocatalysts for water splitting. In this study, novel NiFeP nanosheet arrays supported on nickel nitrogen codoped carbon nanofiber (NiFeP@Ni-NCNF) was synthesized through a combination of electrospun, template-directed growth, and phosphorization strategy. The utilization of Ni-NCNF as a substrate for constructing the nanoarrays structure with superhydrophilic and superaerophobic properties significantly enhances the electrochemical active specific surface area of the electrocatalytic material. The synergistic effect of Ni2P and Fe2P and the excellent conductivity of the Ni-NCNF substrate promote NiFeP@Ni-NCNF to exhibit excellent HER/OER electrocatalytic properties. The NiFeP@Ni-NCNF exhibits low overpotentials of 131 mV and 204 mV for HER and OER at 10 mA cm−2, respectively. Additionally, using NiFeP@Ni-NCNF as the anode and cathode of the electrolytic water device displays a voltage of 1.61 V at 10 mA cm−2, and the Faraday efficiency of the two electrodes is near 100 %. This research presents an extensible and cost-effective strategy for synthesizing phosphide-based catalysts with unique nanostructures and desirable water splitting catalytic properties.

Molybdenum and Vanadium-Codoped Cobalt Carbonate Nanosheets Deposited on Nickel Foam as a High-Efficient Bifunctional Catalyst for Overall Alkaline Water Splitting.

To address issues of global energy sustainability, it is essential to develop highly efficient bifunctional transition metal-based electrocatalysts to accelerate the kinetics of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, the heterogeneous molybdenum and vanadium codoped cobalt carbonate nanosheets loaded on nickel foam (VMoCoCOx@NF) are fabricated by facile hydrothermal deposition. Firstly, the mole ratio of V/Mo/Co in the composite is optimized by response surface methodology (RSM). When the optimized composite serves as a bifunctional catalyst, the water-splitting current density achieves 10 mA cm-2 and 100 mA cm-2 at cell voltages of 1.54 V and 1.61 V in a 1.0 M KOH electrolyte with robust stability. Furthermore, characterization is carried out using field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations reveal that the fabricated VMoCoCOx@NF catalyst synergistically decreases the Gibbs free energy of hydrogen and oxygen-containing intermediates, thus accelerating OER/HER catalytic kinetics. Benefiting from the concerted advantages of porous NF substrates and clustered VMoCoCOx nanosheets, the fabricated catalyst exhibits superior electrocatalytic performance. This work presents a novel approach to developing transition metal catalysts for overall water splitting.

Iron, Tungsten Dual-Doped Nickel Sulfide as Efficient Bifunctional Catalyst for Overall Water Splitting.

Developing low-cost and highly efficient bifunctional catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is a challenging problem in electrochemical overall water splitting. Here, iron, tungsten dual-doped nickel sulfide catalyst (Fe/W-Ni3S2) is synthesized on the nickel foam, and it exhibits excellent OER and HER performance. As a result, the water electrolyze based on Fe/W-Ni3S2 bifunctional catalyst illustrates 10mAcm-2 at 1.69V (without iR-compensation) and highly durable overall water splitting over 100h tested under 500mAcm-2. Experimental results and DFT calculations indicate that the synergistic interaction between Fe doping and Ni vacancy induced by W leaching during the in situ oxidation process can maximize exposed OER active sites on the reconstructed NiOOH species for accelerating OER kinetics, while the Fe/W dual-doping optimizes the electronic structure of Fe/W-Ni3S2 and the binding strength of intermediates for boosting HER. This study unlocks the different promoting mechanisms of incorporating Fe and W for boosting the OER and HER activity of Ni3S2 for water splitting, which provides significant guidance for designing high-performance bifunctional catalysts for overall water splitting.

Self-assembly of NiFe-LDH@Ni3S2 sub-nanosheets catalyst for overall water splitting

The development of water splitting catalysts for generating high-purity hydrogen in large-scale is still facing challenges in terms of improving efficiency and stability. Herein, Ni3S2-coated NiFe layered double hydroxides sub-nanosheets with a hierarchical heterostructure (NiFe-LDH@Ni3S2) were prepared on nickel foam (NF) by a strategy of continuous hydrothermal synthesis. The introduction of S2− broke the hydrogen bonds between NiFe-LDH layers and separated into sub-nanosheets with a thickness of only about 1 nm, which enlarged the active areas of NiFe-LDH@Ni3S2/NF by a factor of 5.7. The coexisting charge channels Ni-O-Ni and Ni-S-Ni in the material enhanced the electrochemical activity, resulting in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) overpotentials as low as 179 mV and 116 mV at 10 mA cm−2 in 1.0 M KOH. Moreover, after a 50-hour chronopotentiometry test at 50 mA cm−2, the overpotential for OER and HER decreased only 0.013 V and 0.059 V, respectively. Furthermore, under the conditions of simulating industrial hydrogen production with an electrolyte of 6.0 M KOH and a temperature of 75 °C, after a 140-hour test for durability, the current density decreased only 0.75% at the cell voltage of 1.70 V.

Oxygen-Deficient FeNbO4-x In-Situ Growth in Honey-Derived N-Doping Porous Carbon for Overall Water Splitting.

It is still a great challenge to reasonably design green, low cost, high activity and good stability catalysts for overall water splitting (OWS). Here, we introduce a novel catalyst with ferric niobate (FeNbO4) in-situ growing in honey-derived porous carbon of high specific surface area, and its catalytic activity is further enhanced by micro-regulation (oxygen vacancy and N-doping). From the experimental results and density functional theory (DFT) calculations, the oxygen vacancy in catalyst FeNbO4-x@NC regulates the local charge density of active site, thus increasing conductivity and optimizing hydrogen/oxygen species adsorption energy. FeNbO4 in-situ grows within N-doping honey-derived porous carbon, which can enhance active specific surface area exposure, strengthen gaseous substances escape rate, and accelerate electrons/ions transfer and electrolytes diffusion. Moreover, in-situ Raman also confirms O-species generation in oxygen evolution reaction (OER). As a result, the catalyst FeNbO4-x@NC shows good electrochemical performance in OER, HER and OWS.

In situ growth of highly wrinkled 3D volcano-like Fe3S4/Ni3S2/WO3/NiFe LDH/NF electrocatalyst for overall water splitting

The rational design of efficient, cost-effective, and durable electrocatalysts is crucial for the realization of hydrogen production through water splitting. The present study describes the synthesis of a novel three-dimensional (3D) volcano-like Fe3S4/Ni3S2/WO3/NiFe LDH/NF (Layered Double Hydroxides/Nickel Foam) electrocatalyst with a highly wrinkled structure through a two-step hydrothermal method. The as-synthesized electrocatalyst exhibits an exceptionally low overpotential of 77 mV for the hydrogen evolution reaction (HER) and 131 mV for the oxygen evolution reaction (OER) at a current density of 10 mA cm−2, owing to the unique morphology and strong electronic interactions between Fe3S4, Ni3S2 and WO3 heterostructures. The Fe3S4/Ni3S2/WO3/NiFe LDH/NF electrocatalyst demonstrates exceptional performance in overall water splitting when employed as both anode and cathode, achieving a current density of 10 mA cm−2 at an impressively low cell voltage of 1.5 V as well as exhibiting remarkable durability with little degradation over a period of 12 hours. This study presents a straightforward approach for the synthesis of highly efficient and durable non-noble metal catalysts for overall water splitting under hydrothermal conditions.

Coupling NiFe-MOF with antiperovskite nickel-based nitrides as efficient electrocatalysts for overall water splitting

The exploration of high-performance, durable and cost-effective catalysts for overall water splitting, including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), has always been a vibrant topic in electrochemical water splitting techniques. Here we report a facile strategy for the synthesis of antiperovskite bimetallic nitrides (FeNNi3 and CuNNi3) and their NiFe-MOF morphology on nickel foams (NF) for HER and OER respectively in alkaline media. The NiFe-MOF and FeNNi3 nanoparticle exhibit strong coupling and synergistic effects, resulting in a solid integrated structure and fast electron transfer. The FeNNi3 towards HER shows an overpotential of 57 mV at 10 mA cm−2 in alkaline media, and its MOF morphology shows a low overpotential of 219 mV at 10 mA cm−2 towards OER. The alkaline electrolyzer of FeNNi3/NF || NiFe-MOF@FeNNi3/NF for overall water splitting demonstrates a low cell voltage of 1.49 V at 10 mA cm−2 with high cycling durability. This work investigates antiperovskite nitrides' capabilities for overall water splitting and provides guidance on designing high-performance catalytic materials.

Reversible surface reconstruction of bifunctional NiCu/FeNi2S4 heterostructure catalyst for overall water splitting

The synergistic effects of bifunctional catalysts in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) must be better understood. Herein, the 3D hierarchical heterostructure consisting of NiCu surface layer assembled on the FeNi2S4 (or Ni-Fe layered-double-hydroxide, NiFe-LDH) is constructed on the nickel foam (NF) and the bifunctional catalytic effects of HER/OER are investigated in an alkaline solution. Although the high OER activity of NiFe-LDH and HER activity of NiCu/NF, NiCu/FeNi2S4 heterostructures is demonstrated by the small overpotentials of HER/OER (306 mV at a current density of 10 mA cm-2), in comparison with NiCu/NF (315 mV) and NiCu/NiFe-LDH (357 mV). An ultra-low cell voltage of 1.54 V is accomplished in overall water splitting. The high-performance bifunctional catalytic effects can be attributed to the synergistic effects of reversible surface reconstruction in the presence of S element, enabling the formation of the NiO/Ni interface at the low potential for HER and high oxidation states (Ni2+δ) at the high potential for OER. The results provide insights into the design of bifunctional electrocatalysts with high activity.

Special atmosphere annealed Co3O4 bifunctional catalysts with oxygen defects for high-efficient water splitting

Electrochemical water splitting is widely recognized as an economical and sustainable method for producing high-purity hydrogen using renewable energy sources. Herein, we prepared 3D urchin-like sphere structures of Co3O4 to serve as an excellent bifunctional catalyst for overall water splitting. Different Co3O4 samples were directly synthesized on Ni foam through hydrothermal and annealing processes under various atmospheric conditions, leading to the formation of oxygen vacancies with varying concentrations. Specifically, the Co3O4/NF-Ar/O2 demonstrates outstanding catalytic performance for both OER (η20 = 269 mV) and HER (η10 = 135 mV), while maintaining excellent durability in 1 M KOH. Electrolysis cells employing Co3O4/NF-Ar/O2 as both anode and cathode demonstrate a low cell voltage of 1.56 V to achieve 10 mV cm−2 in alkaline conditions. The excellent catalytic performance can be mainly attributed to the urchin-like nanostructure and the appropriate concentration of oxygen vacancies. This research provides further insights into the application of the method to induce oxygen vacancies in low-cost and high-efficiency transition metal oxides (TMOs)-based catalysts for electrochemical reaction.

An ultrathin flocculent Co9S8/Ni3S2 hybrid as high-performance bifunctional catalyst for overall water splitting

Modern industrial hydrogen production is a crucial solution to address the energy crisis. Therefore, the development of a cost-effective and highly efficient hydrolytic hydrogen production catalyst holds significant practical importance. In this study, we propose to synthesize an efficient bifunctional electrocatalyst of Co9S8/Ni3S2 grown on nickel foam (NF) by a one-step hydrothermal method, which has a large specific surface area due to its flocculated structure. Through electrochemical testing, the Co9S8/Ni3S2/NF electrode exhibits remarkable performance in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Specifically, low overpotentials of 53.6, 90.2, and 209.2 mV are required to achieve current densities of 10, 20, and 100 mA/cm2 during HER, respectively. Similarly, for OER, the overpotentials are only 107.6, 150.6, and 350.4 mV. Additionally, the Co9S8/Ni3S2/NF electrode demonstrates a small Tafel slope of 68.7 mV/dec for HER and 91.8 mV/dec for OER, indicating efficient reaction kinetics. Notably, the electrode exhibits exceptional long-term stability. Moreover, in a self-assembled monolithic water splitting device with Co9S8/Ni3S2/NF electrodes as cathode and anode, a cell potential of 1.55 V is required to drive the overall water splitting process at a current density of 10 mA/cm2, and the reaction shows excellent stability with a loss of voltage of only 16.5 mV over a 10-hour period. This paper presents a simple and innovative approach for constructing bimetallic sulfides electrocatalysts.

Interfacial Electronic Modulation of Mo5N6/Ni3S2 Heterojunction Array Boosts Electrocatalytic Alkaline Overall Water Splitting.

Bifunctional electrocatalysts with excellent activity and durability are highly desirable for alkaline overall water splitting, yet remain a significant challenge. In this contribution, palm-like Mo5N6/Ni3S2 heterojunction arrays anchored in conductive Ni foam (denoted as Mo5N6-Ni3S2 HNPs/NF) are developed. Benefiting from the optimized electronic structure configuration, hierarchical branched structure and abundant heterogeneous interfaces, the as-synthesized Mo5N6-Ni3S2 HNPs/NF electrode exhibits remarkably stable bifunctional electrocatalytic activity in 1m KOH solution. It only requires ultralow overpotentials of 59 and 190mV to deliver a current density of 10mAcm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1m KOH solution, respectively. Importantly, the overall water splitting electrolyzer assembled by Mo5N6-Ni3S2 HNPs/NF exhibits an exceptionally low cell voltage (1.48V@10mAcm-2) and outstanding durability, surpassing most of the reported Ni-based bifunctional materials. Density functional theory (DFT) further confirms the heterostructure can optimize the Gibbs free energies of H and O-containing intermediates (OH, O, OOH) during HER and OER processes, thereby accelerating the catalytic kinetics of electrochemical water splitting. The findings provide a new design strategy toward low-cost and excellent catalysts for overall water splitting.

Ultrafast nanomanufacturing via high-temperature shock of La0.6Sr0.4CoO3 catalysts for overall water splitting

Electrochemical water splitting, as an effective sustainable and eco-friendly energy conversion strategy, can produce high-purity hydrogen (H2) and oxygen (O2) via hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, altering the nonrenewable fossil fuels. Here, La0.6Sr0.4CoO3 perovskite oxide nanoparticles with an orthorhombic phase were synthesized within 2 min in a one-step reaction, using a rapid and efficient high-temperature shock (HTS) method. Impressively, the as-prepared La0.6Sr0.4CoO3 with orthorhombic phase (HTS-2) exhibited better OER and HER performance than the hexagonal phase counterpart prepared using the traditional muffle furnace calcination method. The electrocatalytic performance enhancement of orthorhombic La0.6Sr0.4CoO3 can be attributed to the novel orthorhombic structure, such as confined strontium segregation, a higher percentage of highly oxidative oxygen species, and more active sites on the surface. This facile and rapid synthesis technique shows great potential for the rational design and crystal phase engineering of nanocatalysts.

Tuneable and coral-like NiCoP for enhanced oxygen and hydrogen evolution reaction

Meticulous tuning of nonprecious catalysts for overall water splitting is highly challenging but it is one of the most promising routes toward the future hydrogen economy. Here, we present a highly active, robust and earth abundant NiCoP electrocatalyst with a tuning capacity to excel in both oxygen and hydrogen evolution reactions. The composition of Ni and Co has been varied in a facile two-step method to produce coral-like NiCoP. The variant Ni0.25Co0.75 P has shown remarkable OER activity with overpotential as low as 240 mV at 10 mA/cm2 current density and Tafel slope of 68 mV dec−1 in alkaline medium. On the other hand, Ni0.75Co0.25P exhibited commendable HER performance with an overpotential of 120 mV and a Tafel slope of 123 mV dec−1 in an acid medium. Long-term durability and minimal loading of the catalyst ascertain the significance of the present catalyst. Moreover, our theoretical study finds that NiCoP provides a much higher electron density of d-states near the Fermi level compared to the individual metal phosphide and the low-index surface (100) of composite phosphide has a moderate level of desorption energy of oxygen and hydrogen compared to that of NiP2 & CoP corroborating the superiority of NiCoP in OER/HER performance.

Molybdenum-doped Co3S4 nanoarrays as outstanding catalysts for overall water splitting

Developing cobalt-based electrocatalysts that can replace precious metals is significant for hydrogen production in water splitting, however, this is still a complex and demanding task.

FeS nanosheets assembled with 1T-MoS2 nanoflowers on iron foam for efficient overall water splitting

The exploration of highly efficient noble-metal-free catalysts for overall water splitting is crucial for industrial application.

MnOx-decorated MOF-derived nickel-cobalt bimetallic phosphide nanosheet arrays for overall water splitting.

Exploring non-noble metal dual-functional electrocatalysts with high activity and stability for water splitting is highly desirable. In this study, using zeolitic imidazolate framework-L (ZIF-L) nanoarrays as the precursor, manganese oxide-decorated porous nickel-cobalt phosphide nanosheet arrays have been prepared on nickel foam (denoted as MnOx/NiCoP/NF) through cation etching, phosphorization and electrodeposition, which are utilized as an efficient dual-functional electrocatalyst for overall water splitting. The hierarchical porous nanosheet arrays provide abundant active sites for the electrochemical process, while the MnOx modification induces strong interfacial interaction, benefiting charge transfer. Thus, the MnOx/NiCoP/NF exhibits excellent electrocatalytic activity toward the hydrogen evolution reaction (HER, overpotential of 93 mV at 10 mA cm-2), oxygen evolution reaction (OER, overpotential of 240 mV at 10 mA cm-2) and overall water splitting (cell voltage of 1.59 V at 10 mA cm-2). Furthermore, it shows superior stability during continuous overall water splitting for 200 h. This work provides a simple and effective approach for developing efficient non-noble metal dual-functional catalysts for overall water splitting.

Bimetallic atom-improved Ni3S2 bifunctional electrocatalysts for efficient hydrogen evolution reaction and overall water splitting performance

An efficient bifunctional Ni3S2–Fe–Ni electrocatalyst was prepared by two-step methods. DFT calculations indicate that the adsorption energy of the catalyst for overall water-splitting was modulated due to the introduction of Fe and Ni atoms.