Nanostructure and doping engineering of ZnCoP for high performance electrolysis of water

doi:10.1016/j.mtener.2020.100412

Materials Today Energy

Volume 16, June 2020, 100412

https://doi.org/10.1016/j.mtener.2020.100412 Get rights and content

Highlights

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We fabricate Mo doped ZnCoP nanowires for overall water splitting through a two-step approach.
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The as-obtained Mo-ZnCoP-0.5 samples present a HER performance with an overpotential of 153.1 mV at −10 mA cm⁻².
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The samples possess an overall water splitting performance with a cell voltage of 1.51 V.

Abstract

To design rationally bi-functional electrocatalysts with high efficiency and rich defects for overall water splitting is greatly desirable. However, some electrode materials often exhibit slow electrons transportation. To address above issues, Mo doped ZnCoP products with high electrocatalytic activities are fabricated through a convenient hydrothermal and phosphating route. The as-obtained Mo-ZnCoP-0.5 products show HER activities with an overpotential of 153.1 mV at −10 mA cm⁻² and the overpotential of 280 mV at 50 mA cm⁻² for OER. In addition, the as-prepared electrocatalysts present overall water splitting performances with a cell voltage of 1.51 V at 50 mA cm⁻², revealing that Mo element doping can effectively regulate the electronic structures of ZnCoP products due to the formation of many defects.

Graphical abstract

Introduction

With fast-growing environmental pollution and energy shortages, sustainable energy conversion systems are essential to the development of future energy source [[1], [2], [3], [4]]. Among them, electrocatalytic water splitting has been considered an efficient strategy to compensate for traditional fossil fuels, which contains two half reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [[5], [6], [7]]. Hydrogen plays a significant role in green energy and chemical conversion field due to its high purification and environmentally friendly characteristics [8]. The most efficient electrocatalyst for HER is Pt catalyst as it can fast reduce overpotential. However, the scarcity, high price and poor durability seriously restrict its applications [9,10]. In addition, the efficiency of overall water splitting is usually restricted by slow OER [11]. To expedite the kinetics of water splitting, developing highly active and cost-effective electrocatalysts is very essential to improve the energy conversion efficiency. Therefore, highly efficient non-precious metal electrocatalysts for HER and OER are significant to accelerate reaction kinetics and lower overpotential.

Recently, transition metal oxides and hydroxides have been widely studied as electrocatalysts for overall water splitting [[12], [13], [14]]. Among them, layered double hydroxides (LDH) have attracted much attention due to their high activity, low cost and tailoring composition [15,16]. For example, Zhang et al. prepared ZnCo-LDH nanosheets through a facile ultrasonic exfoliation process. The as-fabricated products show an overpotential of 340 mV at 5 mA cm⁻² for OER [17]. However, the intrinsic poor conductivity and small specific surface usually restrict their scalable applications. Transition metal phosphides have obtained widespread attention owing to the unique electronic structures and abundant chemical states, which benefits to improving the catalytic activity through increasing the number of active sites [18]. However, the electrocatalytic performance is still far from satisfactory due to the low intrinsic activity. The electrochemical reactions usually occur to the surfaces of electrode materials [19]. Hence, the electrocatalytic performance can be optimized by surface modification, such as introducing heterogeneous atoms and constructing defects. For instance, Zhang et al. synthesized N-NiCoP materials using a facile method. The as-obtained products exhibit the overpotential of 225 mV for OER and 78 mV for HER at 10 mA cm⁻² [20]. Luo et al. reported nest-like NiCoP samples with the overpotential of 62 mV (HER) at 10 mA cm⁻² and Tafel slopes of 66.5 mV dec⁻¹ in alkaline solution [21].

Herein, we fabricate Mo doped ZnCoP nanowires for overall water splitting through a two-step approach. The as-obtained Mo-ZnCoP-0.5 samples present a HER performance with the overpotential of 153.1 mV at −10 mA cm⁻² and a Tafel slope of 96.0 mV dec⁻¹. As for OER, the samples exhibit the overpotential of 280 mV at 50 mA cm⁻² and excellent cycle stability. In addition, the samples possess an overall water splitting performance with a cell voltage of 1.51 V and superior cycle stabilities.

Section snippets

Experimental section

All chemicals were analytically grade and directly used without further purification. Prior to a typical procedure, a piece of Ni foam (4 × 4 cm²) was pre-treated in 0.5 M HCl solution, absolute ethanol and deionized water under sonication, respectively. After that the Ni foam was dried for overnight. 3.5 mM zinc nitrate, 3.5 mM cobalt nitrate, 8 mM urea, 4 mM NH₄F and a certain amount of Na₂MoO₄·6H₂O were dissolved into 60 ml deionized water. Then, a clean Ni foam and the above solution were

Results and discussion

Mo-doped ZnCoP nanowires are fabricated through an in-situ transformation from Mo-doped ZnCo precursors, as illustrated in Fig. 1. Firstly, Mo doped ZnCo-LDH samples are prepared through a simple hydrothermal process. Then, the as-fabricated samples were transformed into Mo doped ZnCoP nanowires through one-step phosphate process.

Crystal structure of the as-fabricated product is analyzed by XRD, as shown in Fig. 2a. The strong diffraction peaks at 2θ values of 44.4, 51.6 and 76.1° can be

Conclusion

In summary, we have reported novel Mo-ZnCoP electrocatalysts through a simple hydrothermal and phosphating process. The as-fabricated Mo-ZnCoP-0.5 nanowires show a low overpotential and Tafel slope for OER. In addition, the samples present excellent HER performance. When used as the anode and cathode for overall water splitting, Mo-ZnCoP-0.5 products exhibit a low cell voltage. This work provides a new strategy for the design and synthesis of highly active and stable transition metal compounds

CRediT authorship contribution statement

Meizhen Dai: Conceptualization, Methodology, Software, Data curation, Writing - original draft. Depeng Zhao: Conceptualization, Methodology, Software, Data curation, Writing - original draft. Hengqi Liu: Visualization, Investigation. Yongli Tong: Visualization, Investigation. Pengfei Hu: Software, Validation. Xiang Wu: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgment

This project is supported by the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201904SIC), Education department funding of Liaoning province (LJGD2019001) and Funding of Science and Technology Bureau, Shenyang City (No. RC190138).

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Materials Today Energy

Nanostructure and doping engineering of ZnCoP for high performance electrolysis of water

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental section

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgment

Nano Energy

Mater. Today Energy

Nano Energy

J. Colloid Inter. Sci.

Electrochim. Acta

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J. Energy Chem.

Appl. Catal. B Environ.

Chem. Eng. J.

Appl. Catal. B Environ.

Chem. Eng. J.

J. Energy Chem.

Mater. Today Phys.

Chem. Eng. J.

Catal. Today

J. Power Sources

Nano Energy

Nat. Energy

Adv. Mater.

Cryst. Eng. Comm.