Controllable construction of CoP nanoparticles anchored on a nitrogen-doped porous carbon as an electrocatalyst for highly efficient oxygen reduction in Zn-air batteries

Abstract

Exploring cost-efficient and highly-efficient noble metal-free catalysts for the oxygen reduction reactions (ORRs) involved in sustainable energy devices remains a great challenge. Transition-metal phosphides supported on heteroatom-doped carbons have shown potential as alternative candidates for precious metals because of their tunable electronic structures and higher catalytic performance. Phosphating was used to construct CoP nanoparticles (NPs) anchored on a nitrogen-doped porous carbon framework (CoP@NC) from Co NPs loaded on NC, using PH3 gas released from NaH2PO2 during heat treatment. The dodecahedral structure of Co NPs was retained in their transformation to CoP NPs. The CoP@NC electrocatalyst shows a remarkable ORR activity with a half-wave potential up to 0.92 V under alkaline conditions, which is attributed to the combined coupling between the well dispersed CoP nanoparticles on the nitrogen-doped carbon and the efficient mass transport in the porous structure. Zinc-air batteries assembled with the CoP@NC electrocatalyst as a cathode have a high open-circuit voltage of 1.51 V and power density of 210.1 mW cm−2. This work provides a novel strategy to develop low-cost catalysts with an excellent ORR performance to promote their practical use in metal-air batteries.

1. Introduction

The oxygen reduction reaction (ORR) is a key cathodic reaction in next-generation renewable energy conversion devices such as metal-air batteries[1–4]. However, the main challenge ORR face is lacking excellent catalysts, mainly due to the high energy barriers and sluggish kinetics caused by the four-electron/proton transfer pathway and intermediates adsorption/desorption in ORR[5–8]. Current commercially available platinum (Pt)-based catalysts hinder the large-scale and sustainable application of these devices by their low natural abundance, high-cost, and poor durability[2,9,10]. Therefore, the researchers focused their efforts on cost-effective precious-metal-free ORR catalysts to promote the commercial application of clean-energy devices [11–14].

Among various alternatives of Pt-based electrocatalysts, transition-metal phosphide (TMP) catalysts have attracted extensive interests due to their good conductivity, excellent stability, adjustable electronic structures and abundant bonding types[15–18]. Recent studies have shown that the synergy between the various active components can improve the catalytic activity of TMP catalysts[19–24]. For example, a composite of Co2P nanoparticle decorated N, P co-doped defective carbon materials (Co2P@CoNPG) exhibits excellent catalytic activity attributed to the synergistic effects of Co2P and Co-Nx active sites[25–29]. In addition, the powerful synergistic coupling between phosphides and defect-abundant carbon matrix can also enhance the catalytic activity of TMP catalysts[30]. CoP particles loaded on N, P-doped necklace-like carbon show excellent ORR activity due to the synergistic effects between CoP nanoparticles and carbon matrix[31]. Moreover, encapsulation of TMPs into carbon materials has been proved to further boost electrocatalytic activity and prevent the chemical corrosion[32–35]. Li et al. prepared a multifunctional Co-NC@CoP-NC catalyst, and its better ORR performance and stability are due to the high efficient of CoP and the complete protection of cobalt by the nitrogen-doped carbon layer [19]. Nevertheless, it has been reported that different crystalline phases (CoP and Co2P) of cobalt phosphide exist in TMP catalysts, which show different catalytic preferences for different catalytic reactions. Scott M. Geyer et al. compared the catalytic performance of well-dispersed CoP and Co2P nanocrystals, confirming that Co2P is preferred species for oxygen evolution reaction (OER) due to the easy formation of abundant Co2P@COOH heterogeneous structures and CoP for ORR due to abundant P sites[36]. Therefore, effective and versatile synthetic methodology for the controllable fabrication of TMP catalysts should make full use of the advantages of structural and compositional properties to maximize the ORR performance.

In this work, a high-efficient CoP@NC catalyst (CoP nanoparticles wrapped by nitrogen-doped carbon shells supported on porous carbon) was synthesized via a phosphating strategy. The catalyst exhibits outstanding ORR activity and stability, and its application in Zn-air batteries demonstrates high performance, providing a novel approach for developing low-cost, high-performance ORR catalysts.