Electronic/mass transport increased hollow porous Cu3P/MoP nanospheres with strong electronic interaction for promoting oxygen reduction in Zn-air batteries

https://doi.org/10.1016/j.apcatb.2021.120415Get rights and content

Highlights

  • The Cu3P/MoP@C catalyst is fabricated by impregnation and phosphating treatment.

  • The catalyst exhibits highly active ORR performance than commercial Pt/C in alkaline solution.

  • The Cu3P/MoP@C displays a superb stability over 231 h superior to Pt/C in Zn-air battery.

  • The porous structure and synergy between Cu3P and MoP dominate the excellent performance.

Abstract

Developing high-efficient non-noble metal-based catalysts for oxygen reduction reaction (ORR) is an inevitable way to improve Zinc-air batteries' performance (ZAB). Herein, we report a Cu3P/MoP electrocatalyst (Cu3P/MoP@C) supported by hollow-porous-carbon nanospheres displaying high electrocatalytic activity and exceptional durability in alkaline media. The optimized ORR catalyst outperformed the benchmark Pt/C (20 wt%) and most recently reported Cu-/Mo-based catalysts in various aspects. X-ray photoelectron spectroscopy reveals that the excellent ORR performance originates from the charge transfer between of Cu3P and MoP species. Besides, the large specific surface area of Cu3P/MoP@C with mesoporous structure is particularly advantageous for the inner surface contact with electrolyte to accelerate ORR kinetics. Moreover, Cu3P/MoP@C also exhibits a large power density of 156 mW cm−2, a high round-trip efficiency, and superb stability over 231 h superior to Pt/C in the ZAB. This work offers a scalable and promising approach for the rational fabrication of hollow porous structure materials.

Graphical abstract

The Cu3P/MoP catalyst with hollow porous carbon nanospheres as the support was successfully constructed as an ORR electrocatalyst through high temperature phosphating. The excellent electrocatalytic activity is the result of the strong electronic interaction between Cu3P and MoP components, as well as the hollow porous structure promotes rapid mass transfer.

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Introduction

Growing energy demands and environmental deterioration issues have compelled the pursuit of renewable energy conversion-storage technologies, for instance, fuel cells and metal-air batteries [[1], [2], [3]]. Especially, Zinc-air batteries (ZAB) have obtained considerable attention due to their high theoretical energy density of 1086 Wh kg−1, good safety and environmental benignity [[4], [5], [6]]. However, practical and large-scale commercial implementation of ZAB is lagged owing to their performance is severely affected by the sluggish kinetics of ORR [7,8]. Although Pt-based catalysts are the state-of-the-art ORR electrocatalyst to date, there is high cost, natural scarcity, poor stability, and vulnerability to methanol poisoning greatly restrict their widespread applications [[9], [10], [11]]. As a result, it is imperative to develop cost-effective, high-efficient and earth-abundant alternative Pt-based catalysts to living up to the high-performance requirement of ZAB and promote ORR activity.

In recent years, transition metal phosphides (TMPs) and their compounds are emerging as a promising alternative to Pt-based catalysts due to their natural abundance and cost-efficient. Importantly, previous reports have demonstrated that their ORR properties can be enhanced significantly through rational design [[12], [13], [14], [15]]. However, as compared to other cathodic electrocatalysts, The ZAB of based TMPs is rarely reported because of the low specific surface area, insufficient electrical conductivity, and aggregation [16], but when TMPs such as Cu3P is combined simultaneously with other carbon based substrates to form a composite material, the carbon component in the material improves conductivity; also, the synergy between various components enhances the ORR activity [17,18]. In catalytic processes, there is a consensus that electrocatalytic activity can be improved significantly by exposing more active sites [19]. In this regard, it is necessary to design and synthesize high surface area electrocatalysts with a high porosity conducive to mass transport and increase the availability of active sites, which are pivotal for enhancing the electrocatalytic performance of ORR.

In this work, we reported a rational design of the hollow porous spherical structure of Cu3P/MoP@C by high-temperature phosphating using carbon spheres as support under argon flow. The optimized catalyst exhibits excellent ORR catalytic activity under alkaline conditions with a half-wave potential (E1/2) of 0.90 V (vs. RHE) superior to commercial Pt/C (0.84 V vs. RHE) and a majority of Cu-/Mo-based ORR electrocatalysts (the RHE calibration was shown in Fig. S1). Furthermore, Cu3P/MoP@C also demonstrates outstanding long-term stability compared to Pt/C with a slight current attenuation of 2% after 40000 s tests. The detailed characterizations indicate that the superior activity is attributed to a strong interaction between Cu3P and MoP species, high BET specific surface area, and porous structure facilitated mass transport. More importantly, the ZAB based on Cu3P/MoP@C achieves a high open-circuit voltage (OCV) of 1.51 V, a peak power density of 156 mW cm−2, and the Cu3P/MoP@C + RuO2 based ZAB has little decay after 231 h durability testing at a current density of 5 mA cm−2, certifying its promising application in advanced energy conversion-storage devices.

Section snippets

Chemicals and reagents

Ethanol, formaldehyde solution (CH₂O), tetraethyl silicate (TEOS), resorcinol (C6H6O2), 1,3,5-benzenetricarboxylic acid (BTC), and zinc acetate dihydrate (Zn(Ac)2) were purchased from Xilong Chemical Co., Ltd. Phosphomolybdic acid hydrate (PMo12), copper (Ⅱ) acetate monohydrate (Cu(Ac)2·H2O), and l-glutamic acid (C5H9NO4) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). Ammonia solution (NH4OH), potassium hydroxide and red phosphorus (P) were got from Aladdin Industrial

Synthesis and structural analysis of catalysts

The Cu3P/MoP@C composite was obtained by impregnation and high-temperature phosphating methods to load the hybrid Cu3P/MoP on hollow porous carbon spheres, as shown in Fig. 1a. Briefly, carbon spheres were ultrasonically dispersed in a mixed aqueous Cu(Ac)2·H2O, PMo12, and C5H9NO4, then the ethanol solution of BTC was added dropwise into the above solution under vigorous stirring. Finally, C6H6O2, CH₂O, and NH4OH were added in sequence after 6 h. The ammonia solution acted as a catalyst for

Conclusions

To summarize, we have innovatively adopted carbon spheres as a template, combined with impregnation and high-temperature pyrolysis to co-anchor Cu3P and MoP on a hollow porous carbon support. The resulting Cu3P/MoP@C exhibit outstanding ORR activity in alkaline solution compared with monometallic phosphide. XPS and BET analysis showed that high ORR activity is due to the strong synergistic effect between Cu3P and MoP and the porous structure with a large specific surface area. The aqueous ZAB

CRediT authorship contribution statement

Man Guo: Writing - original draft. Meijiao Xu: Investigation, Methodology. Yuan Qu: Data curation, Conceptualization. Chuan Hu: Investigation. Puxuan Yan: Data curation, Methodology. Tayirjan Taylor Isimjan: Writing - review & editing. Xiulin Yang: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (no. 21965005), Natural Science Foundation of Guangxi Province (2018GXNSFAA294077, 2021GXNSFAA076001), Project of High-Level Talents of Guangxi (F-KA18015), and Guangxi Technology Base and Talent Subject (GUIKE AD18126001, GUIKE AD20297039).

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