Research Article: Bifunctionality from Synergy: CoP Nanoparticles Embedded in Amorphous CoOx Nanoplates with Heterostructures for Highly Efficient Water Electrolysis

Date Published: July 13, 2018

Publisher: John Wiley and Sons Inc.

Author(s): Jie Yu, Yijun Zhong, Xinhao Wu, Jaka Sunarso, Meng Ni, Wei Zhou, Zongping Shao.


Hydrogen production from renewable electricity relies upon the development of an efficient alkaline water electrolysis device and, ultimately, upon the availability of low cost and stable electrocatalysts that can promote oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Normally, different electrocatalysts are applied for HER and OER because of their different reaction intermediates and mechanisms. Here, the synthesis of a heterostructured CoP@a‐CoOx plate, which constitutes the embedded crystalline cobalt phosphide (CoP) nanoclusters and amorphous cobalt oxides (CoOx) nanoplates matrix, via a combined solvothermal and low temperature phosphidation route is reported. Due to the presence of synergistic effect between CoP nanoclusters and amorphous CoOx nanoplates in the catalyst, created from the strong nanointerfaces electronic interactions between CoP and CoOx phases in its heterostructure, this composite displays very high OER activity in addition to favorable HER activity that is comparable to the performance of the IrO2 OER benchmark and approached that of the Pt/C HER benchmark. More importantly, an efficient and stable alkaline water electrolysis operation is achieved using CoP@a‐CoOx plate as both cathode and anode as evidenced by the obtainment of a relatively low potential of 1.660 V at a 10 mA cm−2 current density and its marginal increase above 1.660 V over 30 h continuous operation.

Partial Text

Hydrogen, a zero carbon emission energy carrier, is considered as one of the most environmentally benign and renewable alternatives to fossil fuels.1 However, hydrocarbon reforming is currently the main pathway to produce hydrogen, which is associated with high fossil fuel consumption and significant amount of CO2 release.[[qv: 1e]] To this end, alkaline water electrolysis, one of the most facile processes for hydrogen production, serves as an attractive technology route to produce high‐purity hydrogen from electricity that comes from renewable energy resources.2 It essentially combines two half‐cell reactions, i.e., hydrogen evolution reaction (HER, 2H2O + 2e− → H2 + 2OH−) and oxygen evolution reaction (OER, 4OH− → O2 + 2H2O + 4e−).3 Currently, one of the main challenges toward widespread use of water electrolysis is how to reduce the energy consumption and the cost, primarily pursued via the research and development of alkaline water electrolysis technology on its several aspects, i.e., the electrodes (containing electrocatalysts), the electrolytes, the ionic transport, and the bubble formation.[[qv: 3d]] Electrocatalysts, in particular, can facilitate charge transfer or chemical reaction; providing the reduced activation energy of the reaction as reflected by the reduced overpotential of either or both of the two half reactions.[[qv: 3d,4]] Thus, the design and synthesis of highly efficient electrocatalysts for the HER and OER under an alkaline condition is critical to decrease the overall energy losses in alkaline water electrolysis.3, 4 To this end, low cost yet highly active HER and OER catalysts have been sought upon from different alternative family of materials such as transition metal oxides, (oxy)hydroxides, phosphides, carbides, nitrides, sulfides, their hybrids, pure carbon materials, and carbon‐based hybrids.[[qv: 2b,3–5]] Among them, cobalt phosphide in particular displayed high catalytic potential for water electrolysis; the performance of which has been further enhanced via morphology control, coupling with conductive carbon, and doping with other metals such as Fe, Mn, and Ni.6 Despite the significant progress, the existing works on these alternative materials have nevertheless been performed exclusively in an acidic electrolyte for HER and in an alkaline electrolyte for OER,[[qv: 4b,6a]] which do not reflect the practical water electrolysis scenario where the coupling of HER and OER catalysts should occur in a single electrolyte.

The synthesis procedure of CoP@a‐CoOx plate is depicted in Scheme1 (see the Experimental Section for the details). First, CoCo layered double hydroxides precursors (CoCo‐LDH plate) were prepared via a one‐pot solvothermal route by refluxing cobaltous acetate in ethylene glycol (EG) media at 200 °C. Subsequently, the resultant CoCo‐LDH plate was phosphidated at 300 °C using phosphine vapor from sodium hypophosphite. Following such phosphidation, the color of the CoCo‐LDH plate changed from pink to black, suggesting the formation of phosphides in the final CoP@a‐CoOx plate.

In summary, we have reported the formation of heterostructure in the CoP@a‐CoOx plate where the crystalline CoP nanoclusters were embedded within the amorphous CoOx matrix, which displayed very high OER activity in addition to high HER activity in an alkaline media. Such outstanding bifunctional activities were also accompanied by high operational stability; suggesting its potential for use in practical alkaline water electrolysis device. The CoP@a‐CoOx plate was synthesized via combined solvothermal and low temperature phosphidation route. We attributed the significant enhancement in OER and HER catalytic activities of the CoP@a‐CoOx plate over those of the CoCo‐LDH‐Ar plate and the CoP b‐plate to the presence of synergy between the two phases as created by strong interactions between them in such heterostructure, resulting in increased ECSA, enhanced charge transfer rate, and more importantly, the modified electronic configuration. We also demonstrated an efficient and stable alkaline water electrolysis operation using the CoP@a‐CoOx plate as both the cathode and anode components. In addition to these superior electrocatalytic properties, its low cost, natural abundance, and environmental compatibility increase its attractiveness for use in large scale industrial electrolysis application.

Catalysts Synthesis: Synthesis of CoCo‐LDH plate: The CoCo‐LDH plate was prepared via a solvothermal route according to a previous report.13 In brief, 0.5 g of cobaltous acetate was added to 36 mL of EG. Following dissolution by ultrasonication, the solution mixture was heated at 200 °C for 5 h under continuous stirring. The solution became turbid at the end of the reaction due to the gradual formation of CoCo layered double hydroxides precursors (CoCo‐LDH plate). The mixture was then cooled down naturally to room temperature. The pink precipitate was recovered by suction filtration, rinsed with de‐ionized (DI) water and ethanol several times, and finally dried at 60 °C for overnight.

The authors declare no conflict of interest.