Multiphase nanosheet-nanowire cerium oxide and nickel-cobalt phosphide for highly-efficient electrocatalytic overall water splitting
Graphical Abstract
CeO2-NiCoPx, a hybrid structure made of nanosheets and nanowires, is synthesized in situ on Ni-Co foam (NCF) as an efficient bi-functional electrocatalyst for water splitting. The CeO2-NiCoPx/NCF catalyst exhibits excellent catalytic activity, stability and sustainability, as well as very high H2/O2 yield for both HER and OER at industry-relevant current densities. The high catalytic activity is confirmed by the low overpotentials of 39 and 205 mV for HER, and 260 and 455 mV for OER at j10 and j500 in alkaline medium, respectively, as well as with the small Tafel slope of 67 (HER) and 72 (OER) mV dec−1. Moreover, the stability and sustainability over 100 h at j100 is achieved.
Introduction
Electrochemical splitting of water can not only provide clean hydrogen energy, but also help reduce environmental pollution in the process of energy generation [1]. Usually, water electrolysis proceeds via the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). Owing to the very different catalytic mechanisms, electrocatalysts with better HER activity are often unsuitable for OER, and vice versa [2], [3]. Currently, the benchmarked catalysts for HER and OER are the platinum (Pt) based materials and the Ruthenium/Iridium (Ru/Ir) oxides, respectively [4]. From the point of view of commercialization, the economic pressure is not only the high cost of precious metal catalysts, but also the additional costs caused by the complexity of cross contamination of different cathode-anode materials [5]. Therefore, it is of great significance to exploit and prepare lower-cost, productive and stable bifunctional catalysts.
In these years, transition metal phosphides (TMPs) have attracted major attention because of their low cost, earth abundance, and environmental friendliness [6], [7]. Nevertheless, both the electroactivity and sustainability of TMPs for HER or OER are a long way off the industrial translation for renewable energy applications. For increasing the catalytic activity of electrocatalysts, engineered multiple-phase compounds containing rich heterointerfaces are especially promising [8], [9]. Noticeably, the close contact of different active species at the interface can cause a strong synergistic effect and improve the adsorption and activation energy of intermediates, which normally is not achieved by a single-component catalyst [10]. Therefore, interfacial design for engineering heterojunction structures is a viable and attractive approach to fabricate high-performance electrocatalysts for water splitting [11].
Recently, based on the unique physico-chemical properties (e.g., flexible oxidation states transition between the Ce3+ and Ce4+, reversible surface oxygen ion exchange, good electronic/ionic conductivity, high oxygen-storage capacity), cerium oxide (CeO2) as a rare-earth metal oxide has been widely utilized as a promoter for the both anodic and cathodic electrocatalytic reactions [12], [13]. Meanwhile, the plentiful active sites of CeO2 can effectively capture oxygen species [14], [15]. In addition, the CeO2 can notably enhance the electrocatalytic stability of the catalyst because of its mechanical resistance and outstanding anticorrosion ability [16], such as the developed FeOOH/CeO2 heterolayered nanotube electrocatalysts [17] and the CeO2 nanocubes anchored Co3O4 nanosheets (Co3O4/CeO2) [18]. These results demonstrate the validity of CeO2 and advanced interface engineering for improving electrocatalytic performance. However, tremendous efforts are still needed to improve the electrocatalytic performance of low-cost catalysts.
Based on the previously reported works [19], [20], the NiCo-based compounds are considered as highly-efficient electrocatalysts for HER and OER, especially the Ni-Co phosphides which have rich adsorption sites and a low adsorption energy for H species [21]. Herein, we develop an innovative hydrothermal synthesis and low temperature phosphorization method to in situ synthesize CeO2-NiCoPx on the NCF matrix containing the incorporated Ce atoms. Featuring the hybrid nanosheet and nanowire morphology, the resulting CeO2-NiCoPx/NCF catalysts show high electrocatalytic performance for both H2 and O2 evolution reactions. By a series of characterization analysis, the CeO2 wrapped on the surface of NiCoPx can not only modify the interface, increase the surface defects, but also change the surface electronic valence of phosphides, which greatly increases the electrocatalytic activity and stability of NiCoPx. The overpotentials of nanosized CeO2-NiCoPx/NCF for transferring j10 in alkaline electrolyte are about 39 mV (HER) and 260 mV (OER). Noted that both HER and OER processes can last for more than 100 h and remain stable during the operation at a high current density (j100). Meanwhile, the potential of CeO2-NiCoPx/NCF||CeO2-NiCoPx/NCF cell delivering j10 only needs 1.49 V, which is not only better than that of Pt/C||RuO2 (1.52 V) industry standard, but also much lower than for most of the reported advanced transition metal bifunctional electrocatalysts. The complementary DFT calculations and in-situ Raman spectroscopy reveal that the nickel and cobalt atoms positioned at the heterostructure interface are the active centers for HER and OER, and the formed metal oxides promote the dissociation and adsorption of water, thus causing the fast generation of H2 and O2.
Section snippets
In-situ fabrication of NiCoCe(OH)x/NCF and powders of NiCoCe(OH)x and CoCe(OH)x
Firstly, the NCF (1.5 × 2 cm) was rinsed with acetone, pure ethanol, and deionized water in an ultrasonic device for 15 min, respectively. Afterwards, the clear NCF was placed in a 50 ℃ oven for several hours. The NiCoCe(OH)x/NCF precursor was grown on the NCF using an one-step hydrothermal method. In brief, 3 mmol of Ce(NO3)2.6 H2O and 5 mmol urea were evenly dispersed in 25 mL of deionized water through a magnetic stirring device. Subsequently, the NCF and the fresh uniform solution were
Materials engineering and characterization
The detailed process of engineering CeO2-NiCoPx electrode material is presented in Fig. 1. The CeO2-NiCoPx electrocatalyst was successfully fabricated by combining in situ hydrothermal and low-temperature phosphating. For achieving the optimal conditions, the hydrothermal reaction parameters of NiCoCe(OH)x/NCF were firstly explored step by step by adjusting the process parameters. Fig. S1-S6 shows that the Ni/Co substrate ratios, Ce(NO3)3 concentration, hydrothermal process temperature and time
Conclusions
In summary, the CeO2-NiCoPx/NCF electrode material with a hybrid nanosheet and nanowire structure was in situ prepared by a custom-designed process involving the hydrothermal and low-temperature phosphating steps. The thus synthesized CeO2-NiCoPx/NCF catalyst possesses more dense active centers due to the heterojunction interfaces formed between the CeO2 and NiCoPx, which ensures very high catalytic activity in alkaline medium. For the HER and OER processes, the CeO2-NiCoPx/NCF catalyst affords
Credit author statement
Herein, we ensure that all our experimental contents are from our original data without any manmade treatments, and our results are scientific and repeatable. Meanwhile, all authors have performed some works in this paper.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Many thanks to financial supports, such as the National Natural Science Foundation of China (Grants 52177162, 22061002, 52067002, 51862001, 51877184), the Zhejiang Natural Science Foundations of China (No. LZ22E070003, LQ22E020006), the Jiangxi Provincial Natural Science Foundation (Grants No. 20212ACB211001). K.O. acknowledges partial support from the Australian Research Council (ARC) and QUT Centre for Materials Science.
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These authors contributed equally to this work.