Controlled development of higher-dimensional nanostructured copper oxide thin films as binder free electrocatalysts for oxygen evolution reaction

The development of cost-effective and highly efficient electrocatalysts for oxygen evolution reaction (OER) is a grave challenge in water splitting catalysis for the clean and viable production of molecular hydrogen (H₂). Herein, we report the fabrication of higher-dimensional thin film CuO nanostructures with controlled morphologies i.e., nanosheets, nanocubes, nanoflowers, and nanoleaves and their relative performance for water oxidation catalysis. Among these nanostructures, CuO nanoflowers exhibit high catalytic activity with an onset potential of 1.48 V and a Tafel slope of 84 mVdec⁻¹ in 1 M KOH solution. Moreover, an overpotential of 270 mV and 400 mV is needed to attain a current density of 10 mAcm⁻² and 100 mAcm⁻² respectively with high Faradaic efficiency. More promisingly, the catalytic performance was found highly dependent upon the nanoscale features and subsequently the improved electrochemically active surface area (ECSA). Such morphology dependent OER performance and binder-free nature of catalyst may provide the high-speed track for electrons transport owing to the inherent catalyst-substrate electronic interconnection and thus making it more promising and high-performance electrocatalyst for OER.     

Paintbrush-like Co doped Cu3P grown on Cu foam as an efficient janus electrode for overall water splitting

In this work, we demonstrated a facile strategy to fabricate paintbrush-like Co Doped Cu₃P architecture grown on porous copper foam (Co-Cu₃P/CF), which was obtained from cation exchange reaction followed by a pyrolysis assisted phosphorization step. Co-Cu₃P/CF showed outstanding electrocatalytic performance for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1 M NaOH solution, affording low overpotential of 270 mV to reach the current density of 50 mA cm⁻² for OER. As for HER, a low overpotential of 200 mV is required to obtain the same catalytic current density. The overall water electrolyzer by using Co-Cu₃P/CF as both anode and cathode showed a low cell voltage of 1.55 V to deliver 10 mA cm⁻². The excellent electrocatalytic performance of Co-Cu₃P/CF could be ascribed to its paintbrush-like hierarchical architecture, offering plentiful of active sites and accelerating electrolyte penetration, the presence of Co dopant also could rationally modify its electronic properties, and thus lead to the synergetic effects.     

Bifunctional iron doped CuS catalysts towards highly efficient overall water electrolysis in the alkaline electrolyte

Efficient catalysts towards overall water electrolysis in alkaline electrolytes were highly desirable for the hydrogen production technology. The surface electronic states of copper in CuS nanocrystal catalysts were modified by iron doping through a simple wet-chemical method. The iron-doped CuS catalysts displayed drastically enhanced catalytic activities for overall water electrolysis in the strong alkaline electrolyte of 1 M KOH after a simple cyclic voltammetry activation. The optimized catalytic performance for overall water electrolysis was achieved in the CuFe_0.6S_1.6 catalyst, which exhibited a low overpotential of ?237 mV for HER and 302 mV for OER to reach 10 mA cm^?2. The high activities for overall water electrolysis in CuFe_0.6S_1.6 were induced by the enhanced charge transfer from Cu to S via iron doping, which not only modified the surface electronic state of copper but also enhanced charge transfer during the electrochemical reactions. Moreover, the catalysts displayed satisfying stability for over 20 h at a high current density of 300 mA cm^?2 for both HER and OER, showing great potential for industrial water electrolysis.     

Facile synthesis of copper selenide with fluffy intersected-nanosheets decorating nanotubes structure for efficient oxygen evolution reaction

Rational nanostructure design is the key point to prepare catalysts with superior catalytic performance, and tedious preparation method limits them large-scale application. Here, a Cu₂Se with fluffy intersected-nanosheets decorating nanotubes structure were prepared by a simple and rapid solution-immersion method at room temperature. The hollow hierarchical structure on a good conductor Cu foam (CF) enlarges surface available sites, enhances the conductivity of electrode materials, then endowing the catalyst with quick charge/mass transportation and favorable oxygen evolution reaction (OER) performance. In alkaline medium, our as-prepared Cu₂Se/CF electrode demonstrates high OER performance, especially for lower overpotential (200 mV at 10 mA cm⁻²) compared with the previously reported Cu-based catalysts. Moreover, the Cu₂Se catalyst could afford galvanostatic test of 10 mA cm⁻² test over 12 h and present superior OER tolerance. These results indicate that the Cu₂Se catalyst via cost efficiency and efficient solution-immersion method could be applied to large-scale efficient OER.     

Hierarchical structure of amorphous bimetallic hydroxide modified Co-metal organic framework catalyst efficient and robust for oxygen evolution reaction

OER is a four-electron-proton coupling reaction, generating a high kinetic barrier and suffering from high overpotential and low efficiency. Developing long-time operational stability the oxygen evolution electrocatalysts under large catalytic current densities is a crucial step for efficient OER. Herein, an amorphous NiCo(OH)_x modified Co-MOF octahedron assembled by ultra-thin nanosheets catalysts grow on nickel foam (labelled as NiCo(OH)_x@Co-MOF/NF) with hierarchical structure is synthesized by a mild interfacial nucleation strategy. The obtained material exhibits an excellent catalytic activity and stability toward OER, a low overpotential of only 210 and 531 mV can be required at 10 and 1000 mA cm ^?2 current density with remarkable durability for >24 h and a smaller Tafel slope of 43 mV dec ^?1 in alkaline aqueous solution. The experimental studies demonstrate that the modification of amorphous NiCo(OH)_x improves the electrical conductivity of Co-MOF. In addition, the hierarchical structure assembled by ultra-thin nanosheets possess a larger surface area and expose number of active sites, enhancing the charge transfer rate and catalytic performance.     

Thin-film iron-oxide nanobeads as bifunctional electrocatalyst for high activity overall water splitting

Developing only Fe derived bifunctional overall water splitting electrocatalyst both for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) while performing at low onset overpotential and with high catalytic stability is a rare instance. We present here the first demonstration of unique iron-oxide nanobeads (FeO_x-NBs) based electrocatalyst executing both OER and HER with high activity. Thin-film electrocatalytic FeO_x-NBs assembly is surface grown via simple spray coating (SC). The unique SC/FeO_x-NBs propels OER initiating water oxidation just at 1.49 V_RHE (η = 260 mV) that is the lowest observable onset potential for OER on simple Fe-oxide based catalytic films reported so far. Catalyst also reveals decently high HER activity and competent overall water splitting performance in the FeO_x-NBs two-electrode system as well. Catalyst also presents stable kinetics, with promising high electrochemically active surface area (ECSA) of 1765 cm², notable Tafel slopes of just 54 mV dec^1? (OER) and 85 mV dec^1? (HER), high exchange current density of 1.10 mA cm^2? (OER), 0.58 mA cm^2? (HER) and TOF of 74.29s^1?@1.58V_RHE, 262s^1?@1.62V_RHE (OER) and 82.5s^1?@-0.45V_RHE, 681s^1?@-0.56V_RHE (HER).     

Engineering FeNi-based electrocatalysts conjoined with Mo2C grown on carbon spheres toward efficient water oxidation via structure and electronic modulation

The development of active non-noble electrocatalysts for oxygen evolution reaction (OER) is urgently desired to accomplish high-performance electrocatalytic water splitting. Here, we report a unique structure electrocatalyst composed of FeNi and Mo₂C heterojunction as spherical shell supported on carbon sphere as core for efficient OER. With the aid of Mo₂C incorporation, FeNi–Mo₂C@C shows increased specific surface area, more electrocatalytic active sites, improved surface water adsorption, and reduced energy barrier. Benefiting from the synergy of shell-core and heterojunction sturcture, the optimized FeNi–Mo₂C@C exhibits superior activity for OER with an overpotential of 283 mV at 10 mA cm⁻² as well as Tafel slope of 29.2 mV dec⁻¹, which is comparable to that of the benchmark ruthenium oxide. The feasible bond between structural design and electronic alteration enhances the charge transfer efficiency, conductivity, and catalytic kinetics, thus intrinsically boost the electrocatalytic performance. This study hence supports a viable strategy to develop highly-efficient non-precious OER electrocatalysts through structural and electronic modification.     

Interface regulation of Pt quantum dots doped nickel phosphide and cobalt hydroxide to promote electrocatalytic overall water splitting

The transition metal phosphates are earth-abundant minerals that have been shown to perform well in electrocatalytic water splitting, whereas these catalysts still tend to have excessively high overpotentials and slow kinetics in HER and OER processes. In the present work, hybrid catalysts consisting of Pt quantum dots doped NiP (NiP-Pt) nano-embroidery spheres and Co(OH)₂ nanosheets were successfully prepared by two-step electrodeposition method. The excellent catalytic performance of the catalyst relies principally on the synergistic interaction between NiP and Pt quantum dots. Additionally, the NiP-Pt exhibits strong electronic interactions at the interface with Co(OH)₂. Consequently, the catalyst has a strong catalytic performance in terms of HER and OER catalytic performance. In terms of HER, an overpotential of only 40 mV is required when the current density reaches 10 mA cm^?2, corresponding to a Tafel slope of 49.85 mV·dec^?1. At the same time, the catalyst also performs well at OER, with a current density of 10 mA cm^?2 at an overpotential of 186 mV and a Tafel slope of 53.049 mV·dec^?1 much less than most electrocatalysts. This study involving electrodeposition and doping of quantum dots provides a new idea for the efficient synthesis of fundamental HER and OER bifunctional catalysts.     

Highly active and durable FexCuyNi1-x-y/FeOOH/NiOOH/CuO complex oxides for oxygen evolution reaction in alkaline media

Oxygen evolution reaction (OER) catalyzed by Ru/Ir-free electrocatalysts is pivotal for preparing oxygen in efficient way, yet our understanding of the relationship between microphysical properties and OER performance is still insufficient. Here we report on 41 kinds of Fe_xCu_yNi_1-x-y/FeOOH/NiOOH/CuO complexes (FCN-x) to investigate the Cu and Fe induced electronic perturbation and what it brings to OER performance. As result, the activity mapping of FCN-x shows an optimal composition of 1:2:7 (FCN-7) showing a comparable overpotential potential of 170.3 mV, Tafel slop of 75.9 mV dec^?1 and durability of 24 h (～29% activity loss) to that of mainstream Ru/Ir-free catalysts. Such enhancement could be attributed to the role of alloying contribution of Fe/Cu, electronic perturbation and surface modification of surface oxides. Additionally, the incompletely oxidized Fe_xCu_yNi_1-x-y not only provide a platform for electron conduction, but also work as a sacrificial material to forming fresh oxides to maintain the content of surface oxides, which is a key driver of the excellent durability of FCN-7. This synthetic strategy may give an effective way to design and screen Ru/Ir-free OER catalysts.     

Interfacial interaction between NiMoP and NiFe-LDH to regulate the electronic structure toward high-efficiency electrocatalytic oxygen evolution reaction

Development of low-cost, high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is challenging, even though it is critical for the overall electrochemical splitting. Herein, we report a NiMoP@NiFe-LDH heterostructure electrode supported on nickel foam. The study shows that the electrocatalytic activity for the OER can be improved by coupling NiMoP and NiFe-LDH. The resulting NiMoP@NiFe-LDH heterostructure exhibited remarkable catalytic performance with an ultralow overpotential of only 299 mV at a current density of 150 mA cm^?2 and a Tafel slope of 23.3 mV dec^?1 in 1.0 M KOH solution. Electron transfer from NiFe-LDH to NiMoP at the nanointerface reduces the energy barrier of the catalytic process, thus improving the OER activity performance. Thus, high-efficiency electrocatalysts can be utilised by constructing heterojunctions to regulate the electronic structure at the interface of the electrocatalysts.     

Autologous growth of Fe-doped Ni(OH)2 nanosheets with low overpotential for oxygen evolution reaction

Highly active and durable electrocatalysts for oxygen evolution reaction (OER) play a vital role in water splitting. Despite numerous efforts, the strategies to prepare durable and effective electrocatalysts via scalable methods still remain a great challenge. In this work, we fabricated Fe-doped Ni(OH)₂ ultrathin nanosheets (Fe–Ni–OH/Ni) via autologous growing of Ni(OH)₂ from Ni foam, and in situ electrochemical-assisted doping Fe into Ni(OH)₂. Benefiting from the unique structure with large surface areas and strong coupling effects between Fe and Ni, the optimal Fe–Ni–OH electrodes exhibit remarkable catalytic performance toward OER, which requires an overpotential of 220 mV to achieve a current density of 10 mA cm⁻² with a Tafel slope of 48.3 mV dec⁻¹. The Fe–Ni–OH electrodes also possess high stability even under a high current density of 500 mA cm⁻² for 600 h with an ultralow overpotential of 290 mV. Using Ni–Fe–OH electrodes as both anode and cathode for overall water splitting, only a small overpotential of 1.57 V is required to reach a current density of 10 mA cm⁻². Moreover, the high catalytic performance and scalable preparation method can meet the emergency needs for the practical application.     

Constructing Ni-Modified Co-FcDA nanosheets for enhancing electrocatalytic oxygen evolution reaction

Electrocatalytic water splitting is an emerging technology for the development of maintainable hydrogen energy. It remains challenging to manufacture a stable, efficient, and cost-effective electrocatalyst that can conquer the slow reaction kinetics of water electrolysis. Herein, A metal-organic framework (MOF) based material is manufactured and productively catalyze the oxygen evolution reaction (OER). The introduction of elemental nickel enhances the catalytic activity of Co-FcDA. The results show that single Ni was well doped in the CoNi-FcDA catalysts and the doping of Ni has a great influence on the OER activity of CoNi-FcDA catalysts. CoNi-FcDA displayed a low overpotential of 241 mV to arrive at the benchmark current density (10 mA cm^?2) with a remarkably small Tafel slope of 78.63 mV dec^?1. It surpassed the state-of-the-art electrocatalyst for OER, that is, RuO₂ (260 mV and 97.26 mV dec^?1) in efficiency as well as instability. Density functional theory (DFT) calculations show that suitable Ni doping at the same time can increase the density of states of the Fermi level, resulting in excellent charge density and low intermediate adsorption energy. These discoveries provide a practical route for designing 2D polymetallic nanosheets to optimize catalytic OER performance.     

In-situ growth of hierarchical CuO@Cu3P heterostructures with transferable active centers on copper foam substrates as bifunctional electrocatalysts for overall water splitting in alkaline media

To fulfill the growing demand for green H₂ fuels, low-cost, efficient, and stable bifunctional electrocatalysts must be developed. Herein, a hierarchical CuO@Cu₃P/CF nanowire core-shell heterostructure with transferable active centers was developed for a bifunctional electrocatalyst with high activity. In this system, the transfer of electrochemically active centers between Cu₃P and CuO is used to facilitate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Particularly, Cu₃P acts as the active center for HER, while the active center shifts to CuO for OER reaction, and Cu₃P acts as a co-catalyst to improve the conductivity of the system. Benefit from superhydrophilicity's high electrochemical surface area and synergistic effect of CuO core and Cu₃P shell, and CuO@Cu₃P/CF shown significant catalytic activity for hydrogen or oxygen evolution, requiring low overpotentials of 144 and 267 mV to achieve a current density of 10 mA cm^?2. In addition, the assembled CuO@Cu₃P/CF-based electrolyzer exhibit excellent overall water splitting performance with a low operating voltage of 1.75 V at 10 mA cm^?2 and a negligible decrease in catalytic activity. This gives encouraging evidence for the utility of our catalysts in application areas.     

Electrodeposition of NiFe layered double hydroxide on Ni3S2 nanosheets for efficient electrocatalytic water oxidation

The oxygen evolution reaction (OER) plays a vital role in various energy conversion applications. Up to now, the highly efficient OER catalysts are mostly based on noble metals, such as Ir- and Ru-based catalysts. Thus, it is extremely urgent to explore the non-precious electrocatalysts with great OER performance. Herein, a simple electrodeposition combined with hydrothermal method is applied to synthesize a non-precious OER catalyst with a three-dimensional (3D) core-shell like structure and excellent OER performance. In our work, NiFe layered double hydroxide (LDH) was electrodeposited on Ni₃S₂ nanosheets on nickel foam (NF), which exhibits a better performance compared with RuO₂, and a low overpotential of 200 mV is needed to reach the current density of 10 mA/cm² in 1 M KOH. Notably, the Ni₃S₂/NiFe LDH only need an overpotential of 273 mV to reach the current density of 200 mA/cm².     

Cu2S/Ni3S2 ultrathin nanosheets on Ni foam as a highly efficient electrocatalyst for oxygen evolution reaction

It is an inevitable choice to find efficient and economically-friendly electrocatalysts to reduce the high overpotential of oxygen evolution reaction (OER), which is the key to improve the energy conversion efficiency of water splitting. Herein, we synthesized Cu₂S/Ni₃S₂ catalysts on nickel foam (NF) with different molar ratios of Ni/Cu by a simple two-step hydrothermal method. Cu₂S/Ni₃S₂-0.5@NF (CS/NS-0.5@NF) effectively reduces the overpotential of OER, displaying small overpotentials (237 mV@100 mA cm^?2 and 280 mV@500 mA cm^?2) in an alkaline solution, along with a low Tafel slope of 44 mV dec^?1. CS/NS-0.5@NF also presents an excellent durability at a relatively high current density of 100 mA cm^?2 for 100 h. The excellent performance is benefited by the prominent structural advantages and desirable compositions. The nanosheet has a high electrochemical active surface area and the porous structure is conducive to electrolyte penetration and product release. This work provides an economically-friendly Cu-based sulfide catalyst for effective electrosynthesis of OER.     

In-situ doping of Co in nickel selenide nanoflower for robust electrocatalysis towards oxygen evolution

The state-of-the-art catalytic materials for oxygen evolution reaction (OER) are Ru and Ir precious metals. Despite of high activity in OER, large-scale production and application of these materials are restricted by low abundance and high price. Exploring inexpensive and efficient electrocatalytic materials for OER is a hot spot of research. Herein, Co-doped nickel selenide (Co–NiSe) nanoflowers are solvothermally in-situ grown on nickel foam (NF), which acts as a robust catalytic electrode for OER. In 1.0 M KOH electrolyte, the required overpotential is 380 mV to reach a current density of 100 mA cm⁻². The Tafel slope is 111.0 mV dec⁻¹. A long-term catalytic stability for OER is obtained. Cobalt doping not only produces more catalytically active sites for OER, but also promotes charge transport through electronic interaction with NiSe.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

Co–NiFe layered double hydroxide nanosheets as an efficient electrocatalyst for the electrochemical evolution of oxygen

The development of non-precious metal catalysts for the electrochemical oxygen evolution reaction (OER) is especially important for the water electrolysis process. Herein, a two-dimensional (2D) ultrathin hybrid Co–NiFe layered double hydroxide (LDH) is synthesized via a facile hydrothermal method. In 1.0 M KOH electrolyte, Co–NiFe LDH exhibits remarkable activities for OER. At the current density of 10 mA cm⁻², it only needs an overpotential of 278 mV, which is ca. 50 mV and 20 mV lower than those for NiFe LDH (328 mV) and RuO₂ catalysts (298 mV), respectively. In addition, Co–NiFe LDH also shows impressive long-term stability for OER. Besides the stable morphology and crystal structure, the potential is always kept at 1.50 V and shows almost no attenuation during the 20 h of durability test. Changes in the electronic structure of LDH due to introduction of Co ions, as well as the large specific surface area facilitate the mass/electron transfer and the oxygen bubbles release, and thus lead to the enhanced catalytic properties for OER. This work can be informative not only for understanding the role of physical and electronic structures on OER but also for designing high-performance non-precious metal OER electrocatalysts.     

Nanoporous NiFeMoP alloy as a bifunctional catalyst for overall water splitting

High performance bifunctional catalysts for water splitting are very promising. Transition metal phosphides as catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted considerable attention due to their high performance. However, the catalyst with excellent properties still remains a significant challenge. Herein, the nanoporous NiFeMoP(np-NiFeMoP) ribbon was prepared by quenching and dealloying method. It was found that np-NiFeMoP showed excellent HER and OER performance in 1 M KOH. The overpotential of OER is as low as 197 mV at a current density of 20 mA·cm⁻². When the current density is 10 mA·cm⁻², the overpotential of HER is 223 mV. Moreover, np-NiFeMoP only needs a cell voltage of 1.41 V when current density is 10 mA·cm⁻² for water splitting. Our current work may provide some new insights on rationally constructing nanoporous structure with rich active sites to boost the catalytic performances for overall water splitting.     

One-step hydrothermal synthesized 3D P–MoO3/FeCo LDH heterostructure electrocatalysts on Ni foam for high-efficiency oxygen evolution electrocatalysis

Low-cost yet high-efficiency oxygen evolution reaction (OER) catalysts have attracted ardent attention to speed up the development of water electrolysis. Recent researches have shown that layered double hydroxides (LDH) are promising candidates towards OER, but further improvement is still highly demanded for its large-scale practical application in water splitting. Herein, we report a 3D P-doped MoO₃/FeCo LDH/NF (P–MoO₃/FeCo LDH/NF) ultrathin nanosheet heterostructure electrocatalyst with an extremely low overpotentials of 225 mV for delivering a current density of 10 mA cm^?2 for OER and a great durability for at least 80 h by a simple one-step hydrothermal method. Extraordinarily, the P–MoO₃/FeCo LDH catalyst achieves a high current density of 300 mA cm^?2 and even 350 mA cm^?2 at an extremely low overpotential of 297 mV and 302 mV, respectively, which is crucial for the water electrolysis industry. The remarkable performance may be attributed to that the heterostructure between P–MoO₃ and FeCo LDH not only optimizes electronic structure, thus inducing electron transfer from P–MoO₃ to FeCo LDH and then realizing fast electron transfer rates, but also produces more catalytic active sites. Moreover, the synergetic effect between MoO₃ and FeCo LDH also plays an essential role for enhancing the catalytic performances. This work explores the effect of phosphomolybdic acid on the structure, composition and performances of FeCo LDH catalysts, and also provides a simple and cost-effective way to prepare high-efficiency and low-cost layered double hydroxide electrocatalysts for OER.