Bifunctional Electrocatalysts For Water Splitting Research Articles

Self-supported and defect-rich CoP nanowire arrays with abundant catalytic sites as a highly efficient bifunctional electrocatalyst for water splitting

In 1.0 M KOH, p-CoP/NF shows outstanding HER and OER activity. Furthermore when p-CoP/NF is assembled into a two-electrode cell, a voltage of only 1.55 V is needed to achieve 10 mA cm−2, and it can maintain long-term stability.

Construction of Fe-Co-Ni-Sx/NF nanomaterial as bifunctional electrocatalysts for water splitting

Development of efficient and earth-abundant electrocatalyst for water splitting is necessary and significance. Herein, a Fe-Co-Ni-Sx/NF electrode was synthesized through hydrothermal reaction and calcination method. The Fe-Co-Ni-Sx/NF evenly distribute on the NF substrate under the action of ethylenediamine tetraacetic acid (EDTA). Fe-Co-Ni-Sx/NF electrode has excellent catalytic activity. The overpotential is 209 mV for HER, and 280 mV for OER at current density of 10 mA cm−2. The test result of two-electrode system shows that the cell potential of 1.70 V is required to reach the current density of 10 mA cm−2. The preparation strategy of Fe-Co-Ni-Sx/NF can improve the catalytic activity of transition metal sulfides, which has profound significance to further improve the catalytic activity of the catalyst.

Direct-grown nebulizer-sprayed nickel-copper mixed metal oxide nanocomposite films as bifunctional electrocatalyst for water splitting

Direct-grown nebulizer-sprayed nickel-copper mixed metal oxide nanocomposite films as bifunctional electrocatalyst for water splitting

N-doped CoSe2 nanomeshes as highly-efficient bifunctional electrocatalysts for water splitting

CoSe2 has been as one of promising electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to its earth-abundant resource, and tunable electronic structure. However, enhancing electrocatalytic activity of CoSe2 to meet the large-scale practical applications is desirable. Herein, we report a facile plasma nitridation strategy to generate N-doped CoSe2 nanomesh arrays on commercial carbon paper (N-CoSe2@CP). The obtained N-CoSe2@CP electrodes combined advantages of abundant electrochemical active sites and beneficial electronic structures, thus exhibiting a low overpotential of 106 mV and 237 mV to drive 10 mA cm−2 towards HER and OER in alkaline medium, respectively. Moreover, it can be employed as bifunctional electrocatalyst for overall water splitting, where a small potential of 1.57 V was required to reach 10 mA cm−2, and long-term stability was also realized. This work develops a promising strategy to explore highly-efficient nanocatalysts towards new energy conversion.

Bimetallic Cu-Co-Se Nanotube Arrays Assembled on 3D Framework: an Efficient Bifunctional Electrocatalyst for Overall Water Splitting.

Highly active bifunctional electrocatalysts for water splitting are of particular importance for the widespread usage of renewable energy, which require synergistic effect of ingenious architecture and intrinsic catalytic activity. Herein, a novel Cu-Co-Se nanotube array supported on 3D copper skeleton was synthesized as high-efficiency bifunctional electrocatalyst for overall water splitting via a facile two-step hydrothermal method. The rationally designed Cu-Co-Se nanotube electrocatalyst exhibited good electrocatalytic performance, with overpotential of only 152 mV to generate 10 mA cm-2 for the hydrogen evolution reaction and a small overpotential of 332 mV to drive a current density of 50 mA cm-2 for the oxygen evolution reaction. The good electrocatalytic performance was mainly due to the large electrochemical surface area and electronic coupling synergies triggered by the self-supported bimetallic nanotube architecture. The water splitting system assembled using Cu-Co-Se nanotube as cathode and anode only needed a cell voltage of 1.65 V to drive a current density of 10 mA cm-2 with long durability of 50 h for overall water splitting. Furthermore, density functional theory calculations proved that the existence of electron exchange between the neighboring bimetals as well as the coupling between Cu, Co, and Se contributed to the improvement of the water splitting performance. This work provides a general strategy to develop cost-efficient and geometrically superior bimetallic electrocatalysts toward water splitting for large-scale hydrogen production.

One stone two birds: Vanadium doping as dual roles in self-reduced Pt clusters and accelerated water splitting

Integrating active Pt clusters into transition-metal oxides with water-dissociation ability is effective to prepare a bifunctional electrocatalyst for water splitting in alkaline. However, the additional utilization of a reductant and/or the operation at the elevating temperature causes the over-growth and agglomeration of Pt clusters, thus losing the high catalytic performance. Herein, we report that V dopant not only favors self-reducing Pt clusters on NiFe layered double hydroxide (LDH) (Pt/NiFeV) at room temperature, but also regulates interfacial charge redistribution to enhance the water-splitting performance. Experimental and theoretical studies reveal that V dopant into NiFe LDH triggers more electrons to transfer to adjacent Fe atoms, thus leading to a higher reducing ability compared to that without V-doping. When used as water-splitting electrocatalyst, V doping promotes electron loss of Pt clusters in Pt/NiFeV, optimizing the free energy of hydrogen adsorption and proton recombination kinetics at the cathode. Meanwhile, it also moves the d-band center of Ni away from the Fermi level to optimize the adsorption of *OH intermediates and facilitate the desorption of oxygen molecules at the anode. Thereby, Pt/NiFeV exhibits much higher bifunctional performance than V-free Pt/NiFe LDH toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This work can spark inspiration of designing other bifunctional electrocatalysts for energy conversion and storage.

Coumarin bearing asymmetrical zinc(II) phthalocyanine functionalized SWCNT hybrid nanomaterial: Synthesis, characterization and investigation of bifunctional electrocatalyst behavior for water splitting

A novel asymmetrical zinc(II) phthalocyanine (Coum-Pc) bearing three 7-ethynyl-3-(3,4,5-trimethoxyphenyl) coumarin and one ethyloxy azido groups was synthesized and characterized for the first time. This novel asymmetric Coum-Pc was covalently attached to single walled carbon nanotube (SWCNT) containing terminal ethynyl groups via the azide-alkyne Huisgen cycloaddition (Click) reaction for preparation of coumarin bearing zinc(II) phthalocyanine functionalized SWCNT hybrid nanomaterial (SWCNT-Coum-Pc). Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalyst for water splitting behavior of SWCNT-Coum-Pc hybrid nanomaterial were evaluated. The SWCNT-Coum-Pc hybrid reduced the onset potential required for HER by approximately 200 mV compared to SWCNT catalyst in acidic medium. The SWCNT-Coum-Pc catalyst reduced the HER overpotential of 570 mV at a current density of 10 mA cm−2 in alkaline medium. It also exhibited outstanding activity in OER, required only an overpotential of 330 mV at a current density of 10 mA cm−2. Heterogeneous rate constants of the electrodes from impedance analysis were used to calculate the value of charge transfer resistance and the conductivity of the composite film. The presence of Coum-Pc groups on the SWCNT surface increased for both the conductivity and electron transfer rate of the SWCNT. Therefore, SWCNT-Coum-Pc hybrid significantly improved the performance of OER and HER for water splitting. The present work provided a promising ground for the of SWCNT-Coum-Pc hybrid as a bifunctional electrocatalyst for water splitting.

Synergistically integrated Co9S8@NiFe-layered double hydroxide core-branch hierarchical architectures as efficient bifunctional electrocatalyst for water splitting

Efficient, low-cost, and robust electrocatalysts development for overall water splitting is highly desirable for renewable energy production yet still remains challenging. In this work, Co9S8 nanoneedles arrays are synergistically integrated with NiFe-layered double hydroxide (NiFe-LDH) nanosheets to form Co9S8@NiFe-LDH core-branch hierarchical architectures supported on nickel foam (Co9S8@NiFe-LDH HAs/NF). The Co9S8@NiFe-LDH HAs/NF exhibits high catalytic performances for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with overpotential of 190 and 145 mV at 10 mA cm−2, respectively. The density functional theory calculations predict that the synergy between Co9S8 and NiFe-LDH contributes to the high catalytic performance by lowering the energy barrier of HER. When used as both anode and cathode electrocatalyst, it can deliver 10 mA cm−2 at a low cell voltage of 1.585 V with excellent long-term durability. This work opens a new avenue toward the exploration of highly efficient and stable electrocatalyst for overall water splitting.

Nitrogen-doped porous carbon encapsulated nickel iron alloy nanoparticles, one-step conversion synthesis for application as bifunctional catalyst for water electrolysis

Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) constitute the water electrolysis toward clean and sustainable hydrogen fuel. Highly active and low cost bifunctional catalysts for both OER and HER are highly desired to fulfill the practical water electrolysis. Herein, we report a low-cost and efficient tactics for preparation of N doped carbon confined nickel iron alloy nanoparticles electrocatalysts via facile pyrolysis of ethylenediamine tetraacetic acid (EDTA) chelated Ni, Fe salts precursor. Benefited by the high electrochemical active surface area and the catalytic effect of the N doped carbon (NC) shell promoted by encapsulated alloy nanoparticles, the optimal Ni2Fe@NC composite with Ni:Fe ratio of 2:1 demonstrates low overpotential of 237 mV at 10 mA cm−2, low Tafel slope of 37 mV dec−1 and robust stability for OER, as well as overpotential of 198 mV at -10 mA cm−2 for HER in alkali electrolyte. Furthermore, the full water electrolysis cell using Ni2Fe@NC as both anodic and cathodic catalytic electrodes can operate with low input voltage (1.81 V, at 10 mA cm−2) and good durability. The current work shows the potential of Ni2Fe@NC composite as OER and HER bifunctional electrocatalyst for water splitting.

Metal-substituted zirconium diboride (Zr1-xTMxB2; TM = Ni, Co, and Fe) as low-cost and high-performance bifunctional electrocatalyst for water splitting

Recent years have witnessed an unprecedented surge in research on earth-abundant and efficient electrocatalysts for the water splitting process. Among those, the development of boron-based advanced catalysts is subject to designing the active and durable compounds, working as bifunctional materials under alkaline medium. In this study, a series of ZrB2-based catalysts with a general formula of Zr(1-x)TMxB2 (x = 0.05, 0.1, and 0.2) (TM = Fe, Co, and Ni) were prepared through a straightforward route and employed as bifunctional electrocatalysts in hydrogen and oxygen evolution reactions (HER and OER). The electrochemical measurements confirmed that the incorporation of Ni into the crystal structure of ZrB2 in the Zr0.8Ni0.2B2 sample led to an onset potential of 1.58 V in OER at a current density of 10 mA cm–2, indicating a remarkable performance with a very low overpotential of 350 mV. Besides, Zr0.8Ni0.2B2 displayed an infinitesimal value of 56.6 mV dec–1 regarding the Tafel slope, which was lesser as compared to the commercial RuO2 (66.2 mV dec–1). For the case of HER, Zr0.8Co0.2B2 showed the best performance compared to other samples with an overpotential of 420 mV and a Tafel slope of 101.6 mV dec–1, following the Volmer mechanism. Both catalysts were examined for their long-term stability, manifesting excellent catalytic durability even after 12 h. Surprisingly, Zr0.8Co0.2B2 exhibited a drop from 420 to 380 mV in the overpotential value after 1000 CV sweeps, providing a promising performance in terms of HER. As-prepared metal-substituted ZrB2 electrocatalysts have great potential to be implemented in various green energy system applications.

Co-doped Pyrrhotite Fe7S8 nanosheets as bifunctional electrocatalysts for water splitting

The development of low-price and highly efficient catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has always been challenging with regard to water splitting. Pyrrhotite Fe7S8 has become a research hotspot due to its abundant reserves, high electrical conductivity and excellent catalytic activity. In general, introducing metal heteroatoms into transition metal sulfides (TMSs) can effectively enhance the activity and stability of TMSs. Herein, Co-doped pyrrhotite Fe7S8 nanosheets were obtained by a straightforward two-step method consisting of hydrothermal treatment followed by subsequent heat treatment. Results from XPS analysis and contact angle measurements showed that doping optimized the chemical environment around iron atoms and enhanced the adsorption of water molecules on the surface of Fe7S8, thereby greatly improving the electrocatalytic efficiency of water electrolysis. Compared with other counterparts, Co0·95Fe6·05S8 exhibited superior OER activity with an overpotential of 311 mV at a current density of 10 mA cm−2, and higher performance towards HER with an overpotential of 284 mV at 10 mA cm−2 in alkaline solution. In sum, Co0·95Fe6·05S8 is promising as a bifunctional catalyst for water splitting under alkaline conditions.

Induction of Co2P Growth on a MXene (Ti3C2Tx)-Modified Self-Supporting Electrode for Efficient Overall Water Splitting.

It still is a challenge to create a superior and easily coupled bifunctional electrocatalyst for water splitting impelled by a low voltage. In this work, the controlled growth of Co2P NAs on the surface of a MXene (Ti3C2Tx)-modified self-supporting electrode is demonstrated as a competent and reliable bifunctional electrocatalyst for efficient water splitting. The heterointerface in Co2P@Ti3C2Tx with an optimized adsorption free energy of H*, H2O, and better conductivity can give enhanced HER (hydrogen evolution reaction) activity, with a low overpotential (42 mV) at 10 mA cm-2. Additionally, the OER (oxygen evolution reaction) activity has also been similarly strengthened by the synergy of Co2P and MXene with an overpotential of 267 mV to arrive at 10 mA cm-2. Furthermore, the excellent bifunctional electrode (Co2P@Ti3C2Tx∥Co2P@Ti3C2Tx) exhibits efficient engineering water-splitting performance (1.46 V@10 mA cm-2) in alkaline solution. This simple design can propose a promising approach to exploit precious-metal-free catalysts for energy conversion.

A Co3O4/CuO composite nanowire array as low-cost and efficient bifunctional electrocatalyst for water splitting

A Co3O4/CuO composite nanowire array as low-cost and efficient bifunctional electrocatalyst for water splitting

Metallic two-dimensional metal-organic framework arrays for ultrafast water splitting

This work reports a universal dissolution-recrystallization method to synthesize two-dimensional (2D) conductive metal-organic framework (MOF) arrays. The MOF arrays have exhibited excellent structural characteristics for electrochemical reactions, including ultrathin architecture, metallic conductivity, and hierarchical macro-/micro-porosity. Particularly, 2D nickel, iron-MOF arrays act as bifunctional electrocatalysts for water splitting, which deliver a current density of 500 mA cm−2 at an overpotential of 305 mV for anodic oxygen evolution reaction (OER), 359 mV for hydrogen evolution reaction (HER), and ~640 mV for overall reactions. The electrode also shows prolonged stability against bulk water splitting (up to 20 h at 500 mA cm−2). In addition, the water splitting system has been further coupled with commercial solar cells to simulate natural water photolysis, demonstrating a high solar-to-hydrogen efficiency of 17.69%. Mechanism study thorough density function theory (DFT) computations shows strong synergistic effect between Ni and Fe active sites that can reduce the Gibbs energy barrier of hydroxyl dissociation step (OH* → O*) during OER and H* desorption step during HER simultaneously. These promising results open up enormous opportunities of designing versatile low-dimensional conductive MOF architectures for renewable energy applications.

Ni-Mo based mixed-phase polyionic compounds nanorod arrays on nickel foam as advanced bifunctional electrocatalysts for water splitting

Abstract The exploration of low-cost, high-efficient and robust bifunctional electrocatalysts for water splitting is urgently desired for developing clean hydrogen energy conversion technology. In this work, we present a facile and effective one-step molten salt synthesis tactics to prepare self-supported 1D Ni-Mo based mixed-phase polyionic compounds nanorod arrays (NMNAs). Due to the exposed more active sites of this nanostructure and accelerated charge transfer derived from modulated electronic structures between Ni-Mo, the as-fabricated NMNAs electrodes deliver remarkable bifunctional electrocatalytic water splitting performance, with overpotential values of 234.2 mV at 200 mA cm−2 and 191.2 mV at 100 mA cm−2 in 1 M KOH for OER and HER, respectively. Furthermore, the alkaline electrolyzer composed of NMNAs needs a low overall-water-splitting cell voltage of 1.423 V to drive a current density of 10 mA cm−2. This work will shed light on the preparation of other related self-supporting nanostructured bifunctional electrocatalysts with excellent performance.

FeCoP2 Nanoparticles Embedded in N and P Co-doped Hierarchically Porous Carbon for Efficient Electrocatalytic Water Splitting.

The design and synthesis of low-cost and efficient bifunctional electrocatalysts for water splitting are critical and challenging. Hereby, a bimetallic phosphide embedded in a N and P co-doped porous carbon (FeCoP2@NPPC) material was synthesized by using sustainable biomass-derived N- and P-containing carbohydrates and non-noble metal salts as precursors. The obtained material exhibits good catalytic activities in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. The bimetallic alloy phosphide (FeCoP2) is the active site for electrocatalysis. Theoretical calculation indicates that the sub-layer Fe atoms and top-layer Co atoms in FeCoP2 exhibit a synergistic effect for enhanced electrocatalytic performance. The carbon matrix around the FeCoP2 can prevent the corrosion during the catalytic reactions. The hierarchically porous structure of the FeCoP2@NPPC material can promote the transfer of electrons and electrolyte, and increase the contact area of the active sites and electrolytes. N- and P-containing functionalities improve the wetting and conductivity properties of the porous carbon. Due to the synergistic effects, FeCoP2@NPPC requires a low overpotential of 114 and 150 mV at the current density of 10 mA cm-2 for HER in 0.5 M H2SO4 and 1.0 M KOH, and an overpotential of 236 mV for OER in 1.0 M KOH solution, which are much lower than those of FeP@NPPC and CoP@NPPC. Based on the density functional theory calculation, FeCoP2 yields the smallest Gibbs free energy change of rate-determining step among the samples, which leads to better electrochemical performances. In addition, when FeCoP2@NPPC was used as a bifunctional catalyst in water splitting, the electrolyzer needed a low voltage of 1.60 V to deliver the current density of 10 mA cm-2. Furthermore, this work provides a strategy for preparing sustainable, stable, and highly active electrocatalysts for water splitting.

Theoretical and experimental investigations of Co-Cu bimetallic alloys-incorporated carbon nanowires as an efficient bi-functional electrocatalyst for water splitting

Application of noble metal-free electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) during electrocatalytic water splitting is crucial for clean energy conversion and has drawn extensive attention. However, the development of highly active and low cost electrocatalysts is a considerable challenge. Herein, Co-Cu alloy nanoparticles-incorporated carbon nanowires electrocatalyst was synthesized and evaluated for both OER and HER. The nanomaterials were fabricated by facile electrospinning of sol-gel composed of cobalt acetate, copper acetate, and poly(vinyl alcohol) followed by calcination in an inert environment. Adjusting the composition of the metallic counterpart was found to significantly enhance electrochemical properties of the catalyst. Furthermore, the unique nanowire morphology and structural properties of incorporated Co-Cu alloy, the (Co0.95Cu0.05@CNWs) composition exhibits good electrocatalytic performance for both OER and HER in the alkaline medium. Physicochemical characterizations using X-ray diffraction, X-ray photoelectron spectroscope, scanning electron microscopy, and transmission electron microscopy have confirmed the formation of alloy structure and nanowire morphology. The optimum composition (Co0.95Cu0.05@CNWs) requires small overpotential, ɳ10 of ∼285 mV for oxygen evolution reaction (OER) and ∼160 mV for hydrogen evolution reaction (HER) with the corresponding Tafel slope of 92 mV dec−1 and 172 mV dec−1 versus the reversible hydrogen electrode, respectively. In addition, only negligible loss in activity was observed after 1000 cycles and prodeces cell voltage of 1.58 V at current of 10 mA/cm2 and 1.72 V at current density of 50 mA/cm2 in two electrode system. Density Functional Theory (DFT) calculations were employed to verify experimental results. Electronic density of states (DOS) results reveal an increase in electronic states near the Fermi level upon Co-Cu heterojunctioning with CNWs. This is indicative of improved catalytic activity and more favorable binding energies of HER and OER intermediates. Reaction coordinate diagrams for HER and OER were developed, which aided in identifying thermodynamically limiting steps. This work may provide a feasible approach for incorporating other transition metals to design low-cost and high-performance bifunctional electrocatalysts for overall water splitting.

Synchronous Electrocatalytic Design of Architectural and Electronic Structure Based on Bifunctional LDH-Co3 O4 /NF toward Water Splitting.

The rational design of highly efficient bifunctional electrocatalysts for water splitting is extremely urgent for application in sustainable energy conversion processes to alleviate the energy crisis and environmental pollution. In this work, through simple deposition of layered double hydroxides (LDH) on Co3 O4 /NF (NF=nickel foam) nanosheets arrays, hierarchical Co3+ -rich materials based on LDH-Co3 O4 /NF are prepared as highly active and stable electrocatalysts for water splitting. The NiFe-LDH-Co3 O4 /NF demonstrates excellent electrochemical activity with an overpotential of 214 mV for the OER and an overpotential of 162 mV for the HER at 10 mA cm-2 . Such a performance is attributed to the optimized electronic states with a high concentration of Co3+ , which improves the intrinsic activity, and the sheet-on-sheet hierarchical structure, which increases the number of active sites. The unique synchronous design of both the architectural and electronic structure of nanomaterials can simultaneously accelerate the reaction kinetics and provide a more convenient charge transfer path. Therefore, the strategy reported herein may open a new pathway for the design of excellent electrocatalysts for water splitting.

Integrating Co3O4 nanoparticles with MnO2 nanosheets as bifunctional electrocatalysts for water splitting

Developing high-efficiency and low-cost electrocatalyst is significant for the application of water splitting technology. Herein, Co3O4 nanoparticles and MnO2 nanosheets are separately synthesized and subsequently assembled into a unique 0/2-dimensional heterostructure via van der Waals interactions. The consequent composites expose abundant accessible active sites and expedite the reaction kinetics, which can be testified by the superiorities in Tafel slope, exchange current density and double-layer capacitance, only requiring overpotentials of 355 and 129 mV for oxygen and hydrogen evolution reactions in 1.0 M KOH at 10 mA cm−2, respectively. Moreover, a cell voltage of 1.660 V can drive the electrolyzer at 10 mA cm−2. Benefitted from robust integration, the original aggregation and restacking of individual materials have been overcome, thereby leading to superior elelctrocatalysis durability. This facile and universal strategy may inspire the researchers on the design and construction of advanced functional composites.

Self-Supported Phosphorus-Doped Vertically Aligned Graphene Arrays Integrated with FeCoNiP Nanoparticles as Bifunctional Electrocatalysts for Water-Splitting Over a Wide pH Range

Self-supported non-noble metal based bifunctional electrocatalysts with high catalytic activity and long-term stability in a wide pH range are highly essential for the production of hydrogen and oxygen, remains a great challenge. Herein, a bifunctional electrocatalyst is synthesized via electroless plating of FeCoNiP nanoparticles on self-supported phosphorus-doped vertically aligned graphene arrays (FeCoNiP/P-VG). FeCoNiP/P-VG exhibits an exceptionally high catalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a wide pH range, with an overpotential of 81 and 141 mV for HER, and 240 and 409 mV for OER, in 1.0 M KOH and 0.5 M H2SO4 respectively, at current density of 10 mA. It also performs quite low Tafel slope value of 40 mV·dec−1 for HER and 69 mV·dec−1 for OER in 1.0 M KOH. More importantly, it shows prominent stability in acidic and alkaline electrolytes. This study may open a new avenue for the design and fabrication of self-supported bifunctional electrocatalysts for water splitting.