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

Electrocatalysis is vital towards the construction of future hydrogen economy, which exerts the critical significance on the macro preparation of nanostructured electrocatalysts. Herein, a fast micro-nano liquid layer induced construction (MLC) strategy is proposed, ensuring a tenfold increase on electrode size with almost negligible heat consumption. As a model-of-concept application, the cations coupled FeOOH nanosheets of thousands of square centimetres are synthesized within 20 minutes at room temperature for water splitting. The density functional theory (DFT) calculations further reveal that the modulation of binary cations (Ni, Zr) leads to the charge accumulation around Fe sites. This results in a more balanced proton adsorption for HER, as well as a lowered energy barrier for OER. As proposed strategy paves a technique for the scaled-up synthesis of nanostructured electrocatalysts, and also provides a platform for exploring more functional nanostructured compounds.

Exploring earth-abundant and effective electrode material for electrocatalytic water splitting is vital for energy conversion devices. In this study, S doped NiCoP nanosheet arrays with rich P vacancies (A-NiCoP|S) are developed through plasma treatment. Starting from growing NiCoP through hydrothermal and phosphating processes, S dopant and P vacancy are simultaneously introduced into NiCoP by Ar plasma treatment. The synergic effect of S dopant and P vacancy leads to increased exposed active sites, optimized surface chemical states and improved electrical conductivity. Consequently, A-NiCoP|S shows superior activities with low overpotentials of 88 mV at 10 mA cm−2 for hydrogen evolution reaction (HER) in 1 M KOH, as well as a low Tafel slope of 73.7 mV dec−1. Further, theoretical calculations demonstrate that the synergistic effects of S dopant and P vacancy can modify the electronic structure and lower free energy of hydrogen adsorption in NiCoP, thus promoting the reaction mechanism. Moreover, the water-splitting configuration by A-NiCoP|S only needs 1.60 V to attain 10 mA cm−2.

Constructing well-defined interfaces is vital to improve the electrocatalytic properties, but the studies on transition-metal-interface electrocatalysts with rich vacancies are rarely reported. Here, rich P vacancies to modulate Ni2P/Cu3P interfaced nanosheets for overall water splitting is demonstrated. We conduct a series of experimental parameters to adjust the nanostructures of Ni2P/Cu3P, and to get insight into the synergistic effects of interfaces and P vacancies on the catalytic activities. Notably, Ni2P/Cu3P with rich P vacancies shows the lowest overpotential requirements of 88 and 262 mV at 10 mA cm−2 towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The good activity is ascribed to abundant electroactive sites, electric field effect at the interfaces and tuning the electron structure by P vacancies. In addition, as bifunctional electrode, Ni2P/Cu3P with rich P vacancies allows for a low water-splitting voltage of 1.60 V at 10 mA cm−2. This work may open up a new route for efficient electrocatalysts through the synergistic effects of interfaces and vacancies.

Transition metal oxides show great potential as electrocatalysts, owing to the low cost and rich chemical states. However, the limited surface areas, low intrinsic activity and poor hydrogen evolution reaction (HER) activity greatly restrict the application for overall water splitting. Herein, we have constructed S doped NiCo2O4 nanosheet arrays by Ar plasma (Ar-NiCo2O4|S) to enhance active sites and boost catalytic kinetics. Consequently, the Ar-NiCo2O4|S shows the improved performances for HER and oxygen evolution reaction (OER). Further, as bifunctional electrocatalysts, Ar-NiCo2O4|S exhibit a voltage of 1.63 V at 10 mA cm−2, as well as good stability.

Dual cation doping engineering can achieve high-performance electrocatalysts through the synergic effects between the two cation dopants. Here, we report a dual cation (Fe, Zn)-doped CoP core–shell nanostructure (Fe/Zn-CoP) derived from a metal–organic framework as a robust electrocatalyst. In this strategy, the introduction of Fe not only changes the morphology to provide abundant active sites, but also modifies the electronic structure of CoP to improve intrinsic activity. Furthermore, the Zn element as a sacrificial dopant can expose more active sites to boost the catalytic reaction. Consequently, the Fe/Zn-CoP core–shell arrays exhibit low overpotentials of 75 mV for the hydrogen evolution reaction (HER) and 267 mV for the oxygen evolution reaction (OER) at 10 mV cm−2 in an alkaline medium. When employed as both the anode and the cathode, Fe/Zn-CoP needs a low cell voltage of only 1.59 V to achieve 10 mV cm−2. This work highlights a feasible method for combining nanostructure design and dual cation doping to prepare highly efficient electrocatalysts.

Herein, NiS/VS heterostructures are designed as efficient electrocatalysts for overall water splitting. Vanadium modifies the electronic structure and induces a large amount of active sites. Strong interactions between the interfaces of the heterostructure lead to lower charge transfer resistance and enhance the intrinsic activity of active sites. As a result, NiS/VS only needs a potential of 158 mV for the HER and 240 mV for the OER to achieve 10 mA cm−2. When used in a two-electrode electrolyzer, NiS/VS can achieve 10 mA cm−2 at a low voltage of 1.64 V. Our research may provide a new approach for constructing bifunctional heterostructure electrocatalysts for highly efficient overall water splitting.

Constructing heterointerfaces in heterostructures could effectively enlarge the electroactive sites and enhance the interfacial charge transfer, and thus improve the electrocatalytic performances. Herein, free-standing porous Ni2P-Ni5P4 heterostructured arrays are successfully prepared through in situ phosphating Ni(OH)2 arrays by simply tuning the reaction temperatures. Contributing from the interfacial coupling effects of two phases, large surface areas, highly conductive support of carbon cloth substrates and unique free-standing arrays, Ni2P-Ni5P4 heterostructured arrays show the enhanced kinetics and electrocatalytic performances for the hydrogen evolution reaction, oxygen evolution reaction and overall water splitting. Our research might offer insight into constructing heterophase junctions for efficient overall water splitting.

Vacancy engineering or element doping has been extensively developed to tune the electrocatalytic activity of electrocatalysts. However, it is challenging to simultaneously generate doping and defects, and insights into the structure–property relationship in vacancy-rich and doped catalysts are lacking. Herein, a V-doped NiCoP electrocatalyst is developed through utilizing the V-doping effect enhanced by P vacancies via plasma treatment. The P vacancies can enhance the V doping effect to increase the electroactive sites and regulate the electron density of the active site center. Thus, an enlarged electrochemical active area, accelerated electron transportation and promoted electronic coupling effects can be achieved. As a result, the overpotentials at a current density of 10 mA cm−2 are 58 and 246 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, respectively. When serving as a bifunctional electrode for overall water splitting, V-doped NiCoP with P vacancies can achieve a low voltage of 1.55 V at 10 mA cm−2. This work highlights a feasible strategy to explore efficient electrocatalysts via the heterogeneous atom doping effect enhanced by vacancies.

Developing high-efficiency, low-cost electrocatalysts for water splitting is important but challenging. Two-dimensional nanosheet manganese dioxide (MnO2) arrays are promising candidates for the design and development of advanced catalysts because of their large surface area. Here, a feasible solution to improve the catalytic activity of MnO2 materials via decorating the active sites on the surface is proposed. With the help of plasma engineering, we successfully enabled surface activity of the MnO2 nanosheets by decorating P or Fe species together with rich vacancies on the surface. The decorated P (P-MnO2) or Fe (Fe-MnO2) species were highly beneficial for the absorption of protons and OH− respectively, and rich oxygen vacancies induced the formation of stable Mn3+, which contributed to electron and charge transfer. Thus, increased electrochemically active specific areas, accelerated charge transfer, and a proper surface electronic structure could be achieved. On the basis of this activation strategy, the fabricated P-MnO2 and Fe-MnO2 showed excellent catalytic performance for the hydrogen evolution and oxygen evolution reactions. To our knowledge, the performance of P-MnO2 and Fe-MnO2 outperformed most MnO2-based electrocatalysts in the field of electrocatalytic water splitting. Surface activation of two-dimensional MnO2 materials by decorating active species via plasma treatment can provide a feasible route for modulating the performance of earth-abundant electrocatalysts for practical applications.

Highlights Freestanding Mo-doped NiCoP nanosheets are designed as bifunctional electrocatalysts for overall water splitting.Remarkable electrocatalytic performances are achieved by Mo doping, where a low-water-splitting voltage of 1.61 V at 10 mA cm−2 is obtained. Electronic supplementary material The online version of this article (10.1007/s40820-019-0289-6) contains supplementary material, which is available to authorized users., Rational design of efficient bifunctional electrocatalysts is highly imperative but still a challenge for overall water splitting. Herein, we construct novel freestanding Mo-doped NiCoP nanosheet arrays by the hydrothermal and phosphation processes, serving as bifunctional electrocatalysts for overall water splitting. Notably, Mo doping could effectively modulate the electronic structure of NiCoP, leading to the increased electroactive site and improved intrinsic activity of each site. Furthermore, an electrochemical activation strategy is proposed to form Mo-doped (Ni,Co)OOH to fully boost the electrocatalytic activities for oxygen evolution reaction. Benefiting from the unique freestanding structure and Mo doping, Mo-doped NiCoP and (Ni,Co)OOH show the remarkable electrochemical performances, which are competitive among current researches. In addition, an overall water splitting device assembled by both electrodes only requires a cell voltage of 1.61 V to reach a current density of 10 mA cm−2. Therefore, this work opens up new avenues for designing nonprecious bifunctional electrocatalysts by Mo doping and in situ electrochemical activation. Electronic supplementary material The online version of this article (10.1007/s40820-019-0289-6) contains supplementary material, which is available to authorized users.

Constructing well-defined nanointerfaces is vital to enhance the catalytic performances through electronic coupling effects from different components. However, it is still a challenge to construct metal selenides-based interfacial nanomaterials with satisfactory oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performances for overall water splitting. Herein, sandwich-like structured NiSe2/Ni2P@FeP nanosheet arrays are rationally constructed, which starts from NiSe2 arrays directly on carbon cloth as backbones, followed by coating FeP nanoparticles on the backbones through the phosphorization process. Further, abundant nanointerfaces with rich defects and disordered structure are constructed during the phosphorization process, which can boost the charge transfer rate and provide rich electroactive sites. Consequently, as-synthesized NiSe2/Ni2P@FeP nanosheet arrays exhibit good HER and OER performances with small overpotentials, low Tafel slopes and good stability. Further, the overall water splitting device built by NiSe2/Ni2P@FeP exhibits a voltage of 1.554 V to attain 10 mA cm−2. Our current work may provide some new insights on rationally constructing nanointerfaces with rich defects to boost the catalytic performances for overall water splitting.