Rich P vacancies modulate Ni2P/Cu3P interfaced nanosheets for electrocatalytic alkaline water splitting

https://doi.org/10.1016/j.jcis.2019.12.114Get rights and content

Highlights

  • A two-step strategy is developed to construct Ni2P/Cu3P with rich P vacancies.

  • The synergistic effects of interfaces and P vacancies have been investigated.

  • Ni2P/Cu3P with rich P vacancies exhibits superior activities in HER and OER.

  • A low voltage of 1.60 V is achieved at 10 mA cm−2 for the constructed full-cell.

Abstract

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.

Introduction

Electrocatalytic water splitting has been considered as a promising solution to generate hydrogen in a large scale [1]. In electrocatalytic water splitting, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are key factors for electrocatalytic performances [2], [3]. In general, both HER and OER require electrocatalysts to lower the overpotentials owing to the thermodynamic and kinetic hindrances [4], [5]. Although Pt and RuO2 or IrO2 are the most active catalysts for HER and OER, respectively, the low reserves and high costs of these noble catalysts seriously limit the practical applications [6]. Therefore, developing efficient catalysts with low cost is vital for water splitting.

Up to now, transition metal phosphides (TMPs) are emerging as promising catalysts for overall water splitting, owing to the high electronic conductivity and high activity [7]. For the HER, it has been shown that the Gibbs free energy of hydrogen adsorption (ΔGH*) in single-metal phosphide is far from to zero, indicating the unsatisfactory HER activities [8], [9]. For the OER, TMPs are not stable under high-oxidation OER conditions and in-situ formed oxides or hydroxide are the actual active species [10], [11]. It is common knowledge that the number and the intrinsic activity of active sites are two important factors for electrocatalytic performances [12]. To boost the catalytic properties of TMPs, considerable efforts have been paid to fabricating nanostructured TMPs [13], directly growing TMPs on conductive substrates [14], doping heteroatoms to optimize the hydrogen adsorption kinetic energy [15], and incorporating with other compounds [16]. Importantly, interface modification with bimetal phosphides is an effective strategy to change the valence electron state and enhance the intrinsic activity of active sites [17], [18]. For example, Zhou et al. verified the interfacial electron transfer in Ni2P-NiP2 heterointerfaces to decrease the absolute value of ΔGH* and improve electronic conductivity, thus leading to the enhanced HER properties [19]. Meanwhile, Fu et al. also demonstrated that forming Ni3N-VN and Ni2P-VP interfaced nanosheets effectively activate the intermediates and induce electron redistribution at the interface, leading to the promoted HER and OER dynamics [20]. Meanwhile, the rich defects at the interface also provide abundant active sites and accelerate the surface reaction, resulting in the enhanced activities [12]. It is worth noting that the common defects, such as O, S and Se vacancies, were recently suggested to show a significant impact on HER and OER performances [21], [22], [23], [24]. For example, Feng et al demonstrated that Se vacancies can induce strong electron-electron interaction and optimize the electronic structures, which was beneficial for HER performances [22]. Despite of these progresses, limited researches about transition-metal-interface electrocatalysts with rich P vacancies are reported.

In this work, we construct Ni2P/Cu3P interfaced nanosheets with rich P vacancies, and investigate synergistic effects of interfaces and P vacancies on the catalytic activities. As shown in Scheme 1, starting from NiCu-precursor by the hydrothermal process, Ni2P/Cu3P interfaced nanosheets with rich P vacancies were simultaneously achieved by the phosphation process under the Ar plasma condition. As-made Ni2P/Cu3P with rich P vacancies shows superior HER and OER activities. Further, when applying as bifunctional electrodes, the overall waster splitting device only needs a voltage of 1.60 V at 10 mA cm−2.

Section snippets

Results and discussions

Scheme 1 shows the synthesis process for Ni2P/Cu3P interfaced nanosheets with rich P vacancies. Firstly, NiCu-precursors were synthesized on carbon paper by the hydrothermal process. Figs. S1 and S2 show the scanning electron microscopy (SEM) images of NiCu-precursor with different mass ratio of Ni and Cu. NiCu-precursors composed of nanosheets are uniformly distributed on the surface of carbon paper. However, bulk precursors are formed on the carbon paper when only adding Cu source.

Conclusion

In summary, we successfully construct Ni2P/Cu3P interfaced nanosheets with rich P vacancies directly on carbon paper for overall water splitting. By conducting a series of experiments, we get insight into the synergistic effects of interfaces and P vacancies on the catalytic activities. Notably, the results show the abundant interfaces and rich P vacancies, leading to rich active sites and modified electron interactions, which is important for regulating HER and OER performances. Consequently,

CRediT authorship contribution statement

Jinghuang Lin: Conceptualization, Data curation, Investigation, Writing-orginal draft. Yaotian Yan: Investigation, Data curation. Tianxiong Xu: Investigation, Data curation. Jian Cao: Resources, Funding acquisition. Xiaohang Zheng: Resources. Jicai Feng: Resources. Junlei Qi: Conceptualization, Writing - review & editing, Resources, Funding acquisition.

Acknowledgements

The support from the National Natural Science Foundation of China (Grant Nos. 51575135, 51622503, U1537206 and 51621091) and Natural Science Foundation of Heilongjiang Province of China (YQ2019E023) is highly appreciated. We would like to thank professor Weidong Fei for mechanism for his valuable guidance on the energy conversion mechanism.

Declaration of Competing Interest

The authors declare no conflict of interest.

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