Boron and nitrogen co-doped graphene aerogels: Facile preparation, tunable doping contents and bifunctional oxygen electrocatalysis

Abstract

Rational design of low-cost and highly-efficient bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is currently critical for renewable energy storage and conversion devices. Herein, we represent a series of three-dimensional porous B and N co-doped graphene aerogels (BN-GAs) using NH₄B₅O₈ as precursor for B and N source by hydrothermal method and freeze-drying process. In this facile strategy, we can tune doping contents of B and N configurations in BN-GAs by simply changing the synthesis conditions. Linear sweep voltammetry results confirm that the increasing doping contents of pyridinic N and BC₃ phases in BN-GAs can boost ORR activity, which may be because the enhanced synergy that arises from combining pyridinic N and BC₃ phases can greatly improve ORR activity. The proposed BN-GAs not only show similar ORR activity to commercial Pt/C, but also more superior stability and excellent methanol tolerance than commercial Pt/C. Remarkably, the resultant BN-GAs also exhibit considerable OER activity and achieve desired performance as an air cathode for a rechargeable zinc-air battery device. Notably, this work may assist in a new insight to feasible preparation and molecular design of more advanced graphene-based catalyst, and can be extended to other catalytic system.

Introduction

With the development of renewable energy storage and conversion devices including metal-air and fuel cells, and etc., low-cost and highly-efficient bifunctional oxygen electrocatalysts are highly required to significantly improve the overall electrochemical performance of such devices [[1], [2], [3], [4]]. Many multifunctional catalysts including transition metal oxides [5], carbonaceous materials [6,7], and metal oxide-nanocarbon hybrid materials [8,9], have been constantly employed for application towards electrocatalysis and energy storage. Remarkably, the state-of-the-art nonmetal heteroatom-doped (such as B, N, S, P and I) carbonaceous materials, greatly improving their performance for electrocatalysis because of their electroneutrality broken and charge modulation, have been developed into a research direction [[10], [11], [12], [13], [14]].

Recently, co-doping carbonaceous materials with B and N simultaneously are becoming one of the main trends in creating charged sites for desired applications [[15], [16], [17]]. Especially, the difference in electronegativity of N (3.04), C (2.55), and B (2.05) can generate a unique electronic structure for an enhanced synergetic effect between heteroatom dopants and such co-doped carbonaceous materials can own much more active sites than most singly doped carbonaceous materials [[18], [19], [20], [21]]. For example, Wang et al. [18] developed a new method by pyrolysis of melamine diborate to prepare vertically aligned B and N co-doped nanotubes for the first time and their half-wave potential for oxygen reduction reaction (ORR) with positive shifts of 250 mV and 50 mV as compared with B-doped nanotubes and N-doped nanotubes, respectively. After that, they [19] further synthesized B and N co-doped graphene (BNG) by thermal annealing method. The onset potential of the resultant BNG was close to that of the commercial Pt/C catalyst due to the enhanced synergetic effect between co-doping B and N atoms. In this regard, B and N co-doped carbonaceous materials can provide a possibility for further application in reversible energy conversion systems owing to low cost and highly catalytic efficiency.

With the deepening of the research, the coexistence of B and N elements in the sp² carbon leads to different B−C−N configurations including bonded B−N phase or segregated N−C and B−C phases, which are related to different electronic structure and eventually result in distinct ORR performance [22]. A typical example is that Zheng et al. [23] synthesized BNG with coexistence of segregated N−C and B−C species by a two-step synthesis strategy. This structure can promote the synergetic effect that greatly enhanced the ORR activity as compared with bonded B−N phase in the co-doped graphene. More importantly, they also carried out theoretical calculations to verify that various types of N species have different functions with the incorporation of B atoms, for example, pyridinic N phase can boost ORR activity of B dopants while graphitic N phase decreases its activity. Thus, rationally regulating the doped N and B configurations in BNG can provide the enhanced catalytic capabilities for electrocatalytic reactions.

Graphene aerogels (GAs), a new kind of porous and ultralight carbonaceous materials, have aroused interest in electrocatalysis [24,25]. As compared with two-dimensional (2D) graphene, GAs are more capable of facilitating the electron transport and ion diffusion due to three-dimensional (3D) conductive networks, high specific surface area and hierarchical porous architecture, thereby significantly boosting catalytic activity [[26], [27], [28]]. Recently, Xu et al. [29] proposed a two-step synthesis method containing a thermal treatment process, which was up to 1000 °C, to prepare B and N co-doped GAs (BN-GAs) possessing an outstanding ORR activity. Wu et al. [24] prepared BN-GAs as high-performance supercapacitors by hydrothermal method and freezing-drying process through using NH₃BF₃ as B and N source, which is toxic and corrosive. Additionally, many studies pay attention on ORR activity of BN-GAs, but oxygen evolution reaction (OER) activity and further applications in batteries are rarely reported. Thus, exploring a facile method in the synthesis of BN-GAs by using nontoxic and low-cost precursor to apply for bifunctional electrocatalysts is still a great challenge.

Herein, we report that by facile hydrothermal method and freezing-drying process, a series of BN-GAs with 3D porous structure can be simply prepared in controlled reaction conditions with tunable contents of B and N configurations. Linear sweep voltammetry (LSV) results demonstrated that the doping contents of B and N configurations in BN-GAs have a great impact on ORR activity and the increasing doping contents of pyridinic N and BC₃ phases result in boosting ORR activity. The proposed BN-GAs with highest contents of pyridinic N and BC₃ phases showed outstanding ORR activity, which is similar to commercial Pt/C. In addition to superior ORR performance of BN-GAs, their considerable OER activity lead them to be the desired bifunctional catalysts in rechargeable zinc-air batteries.