Designed synthesis of cobalt nanoparticles embedded carbon nanocages as bifunctional electrocatalysts for oxygen evolution and reduction

Abstract

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are fundamental for many renewable energy conversion and storage technologies. Developing economic and durable bifunctional ORR and OER electrocatalysts as substitute for noble-metal based catalysts, are urgently expected to reduce the industrial cost. Herein, a strategy towards synthesizing cobalt nanoparticles embedded N-doped carbon hollow nanocages (Co@NPC-H) by ion exchanging is reported. The Co@NPC-H nanocage structures integrate the high activity of heteroatoms doped catalytic active sites with the hollow structure for high exposure ratio of the active sites, exhibiting superior electroctalytic performance to noble-metal catalyst for both ORR and OER with the application potential of 860 mV when used as bifunctional catalyst, much smaller than other reported electrocatalysts, making Co@NPC-H a promising candidate as efficient air electrode in Zn-air batteries.

Introduction

During the process of society evolution from traditional fossil fuel dominant to renewable energy rise, electrochemical methods work a lot in clean energy produce and utilization [1]. Among the electrochemical methods, electrochemical reduction and oxidation process, such as the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), provides an efficient, economical and clean means to realize the energy utilization by dissociating water into oxygen and then reducing oxygen back to water [[2], [3], [4], [5]]. These two O₂ reactions work in a variety of energy storage and conversion devices, such as rechargeable metal (Zn or Li)-air batteries and fuel cells [[6], [7], [8]]. Such reversible electrochemical devices exhibite a series of merits such as the high theoretical specific capacity, improved overall potential and significant reduction of emissions over traditional fossil fuel [9,10]. However, during the practical application of unitized regenerative and rechargeable devices, one of the obstacle to realize high energy conversion efficiency is the sluggish kinetics of ORR and OER, which is influenced by the electron/proton transfer, leading to not only a high over-potential but low round-trip efficiency [2,11]. Thus, the development of economic and sustainable electrocatalysts with high activity as bifunctional catalyst to fulfill the demands in practical use is highly desirable.

Up to now, the efficient electrocatalysts for ORR or OER are mainly recognized as the noble metal catalysts such as Pt-, Ru-, and Ir-based materials [[12], [13], [14], [15], [16], [17]]. However, Pt and Pt-based alloys exhibite high efficient electrocatalytic activity for ORR but poor for OER, and, Ru, Ir and their oxides exhibit an absolutely contrarily electrocatalytic performance. Moreover, their supply scarcity, high cost and inferior durability become an obstacle to realize the large-scale commercialization [[18], [19], [20], [21], [22], [23], [24]]. Thus, successfully designing and realizing the bifunctional active sites in a single catalyst efficient for both ORR and OER as the substitutes for noble metal electrocatalysts in a facile and economical method is still in great urgency [[25], [26], [27], [28], [29], [30]]. Recent investigations demonstrate that transitional metal (or metal oxide, sulfide, etc.)/N-doped carbon hybrids (M-N-C, M = Fe, Co, Ni, etc.) hold great potential to be the alternate to replace noble metal catalysts in pH-universal medium [[31], [32], [33]]. Among currently reported strategies, direct pyrolysis of zeolitic imidazolate frameworks (ZIFs), which are constructed by the coordinated bonds between atomically dispersed central metal (Co, Zn, Fe, etc.) and imidazole based organic linkers, provides an efficient approach for producing nanostructured M-N-C hybrid catalysts [[34], [35], [36], [37], [38]].

Most recently, ZIF-67, which consists of inorganic cobalt cation and organic 2-methylimizazole ligands, has become one of the most typical candidate precursor for preparing N, Co-dual doped nanocarbon materials. However, the electrocatalytic activity of bulk carbons synthesized by directly pyrolyzing ZIF-67 nanocrystals often suffer from the irreversible collapse of the tunnels, fusion of the carbon skeleton and aggregation of cobalt nanoparticles, leading to the insufficient active surface and limited charge transfer. To address these issues, the performance of ZIF-67 derived electrocatalysts could be improved by rational structural engineering design. In this regard, hollow nanostructures are of great interests due to their low density, large surface area, multiple interfaces, and reduced diffusion lengths for mass transport, which endows them with better catalytic activities than their corresponding bulk counterparts [35,[39], [40], [41]].

In this investigation, we demonstrated a method of synthesizing hollow carbon nanocubes with cobalt embedded into the N-doped porous carbon shells (Co@NPC-H) by directly pyrolyzing hollow ZIF-67 nanocubes (ZIF-H). The ZIF-H is precisely synthesized from Co₃[Co(CN)₆]₂ nanocubes in aqueous solution via the induction of n-butylamine. The as-prepared Co@NPC-H is used as the electrocatalyst for ORR and OER. Interestingly, the resulting Co@NPC-H exhibits more excellent stability, competitive catalytic activity and superior methanol tolerance compared to the noble metal-based catalysts.