Ultrasound-assisted transformation from waste biomass to efficient carbon-based metal-free pH-universal oxygen reduction reaction electrocatalysts

Highlights

• Ultrasonic treatment of biochar and nitrogen dopant prior to chemical activation.

• Bio-derived metal-free carbon-based catalysts show pH-universal ORR activity.

• An optimal catalyst presents superior ORR activity to Pt/C in a wide pH range.

• Coconut palm leaves are first-time employed to make cathode catalysts of fuel cells.

Abstract

Efficient carbon-based nitrogen-doped electrocatalysts derived from waste biomass are regarded as a promising alternative to noble metal catalysts for oxygen reduction reaction (ORR), which is crucial to fuel cell performance. Here, coconut palm leaves are employed as the carbon source and a series of nitrogen-doped porous carbons were prepared by virtue of a facile and mild ultrasound-assisted method. The obtained carbon material (ANDC-900-10) conveys excellent pH-universal catalytic activity with onset potentials (E_onset) of 1.01, 0.91 and 0.84 V vs. RHE, half-wave potentials (E_1/2) of 0.87, 0.74 and 0.66 V vs. RHE and limiting current densities (J_L) of 5.50, 5.45 and 4.97 mA cm⁻² in alkaline, neutral and acidic electrolytes, respectively, prevailing over the commercial Pt/C catalyst and, what's more, ANDC-900-10 displays preeminent methanol crossover resistance and long-term stability in the broad pH range (0–13), thanks to its abundant hierarchical nanopores as well as effective nitrogen doping with high-density pyridinic-N and graphitic-N. This work provides sonochemical insight for underpinning the eco-friendly approach to rationally designing versatile metal-free carbon-based catalysts toward the ORR at various pH levels.

Introduction

The overuse of conventional fossil energy brings about global energy crisis and environmental pollution, doing harm to human health and natural environment [1]. Fuel cells are part of efficient and clean energy conversion applications that can effectively reduce the load of conventional fossil energy [2]. It is of great significance to develop electrocatalysts toward the oxygen reduction reaction (ORR) with high activity and low price, so as to boost its inherently sluggish kinetics at cathodes in terms of the commercial applications of fuel cells [3]. Platinum (Pt) and its alloys have been considered to be outstanding catalytic materials toward the ORR [4]. Nevertheless, there are some shortcomings of Pt that cannot be ignored, such as high price, low abundance, poor tolerance to impurities [5], [6], [7]. For the sake of solving these bottleneck problems of noble metals, many efforts have been made for decades to study non-precious metal catalysts [8]. So far, different sorts of non-precious metal electrocatalysts have been explored, including metal-nitrogen doped carbon materials [9], [10], transition metal oxides [11], [12], carbides and nitrides [13], and nonmetallic heteroatom-doped carbon-based materials [14], [15].

Among the above-mentioned electrocatalysts, metal-free heteroatom-doped carbon materials are one of the most promising materials for the ORR due to their superiorities of low price, controllable pore structure, high electronic conductivity, and high tolerance to impurities [16], [17]. To create porous carbon materials, various synthetic methods have been adopted, such as hydrothermal carbonization and template method [18], [19]. However, such conventional preparation methods are high-cost, energy-wasting, time-consuming, and environmentally unfriendly [20], [21]. Further, an ultrasound-assisted method has many advantages exemplified by simplicity, cleanness, high-efficiency and flexibility [22]. The physical and chemical effects of ultrasound involve cavitation and nebulization and, during the ultrasonic process, the growth and rupture of small bubbles will occur because of the pressure fluctuations in liquids, which will affect the surface and interface properties of materials [23]. Despite this, few studies have thus far focused on the ultrasonic method to prepare metal-free heteroatom-doped carbon materials for the ORR.

At present, plenty of metal-free heteroatom-doped carbon materials, possessing superior ORR catalytic activities to Pt-based catalysts in basic electrolytes, have been synthesized [24], [25]. However, the catalytic ability of carbon-based materials in acidic and neutral media is inferior to in alkaline media [26]. Acidic and neutral media are required for many low-temperature fuel cells [27], [28]. In order to cater to the practical applications of cathode catalysts in different types of fuel cells, developing high-active ORR electrocatalysts which can work efficiently in alkaline, neutral and acidic electrolytes is of great significance. However, it is still a challenge to prepare efficient non-precious-metal-based pH-universal ORR electrocatalysts which are comparable to Pt.

Carbon materials derived from biomass have aroused extensive attentions [29]. Biomass is not only abundant and renewable, but also contains special channels and microstructures for transporting and storing water and nutrients, which are highly beneficial to the fabrication of nanopores [30]. Up to now, a great deal of biomass has been successfully applied to the preparation of functional porous carbon, such as eichhornia crassipes [31], enoki mushroom [32], egg white [33], chlorella [34], soybean shells [35], and malachium aquaticum [36]. Coconut palm trees are widely distributed in tropical areas. In previous studies, coconut-based materials have been used as the carbon source to prepare ORR electrocatalysts [37], [38], [39], but seldom have coconut palm leaves been employed for fuel cell applications.

Herein, a suite of efficient metal-free carbon-based nitrogen-doped pH-universal electrocatalysts toward the ORR have been prepared by an ultrasonic treatment of coconut palm leaves, followed by mild chemical activation. The resultant catalysts possess abundant micro/mesopores and high-content pyridinic-/graphitic-N. The optimal carbon catalyst owns the high ORR catalytic activity as well as good methanol tolerance and stability in the alkaline, neutral and acidic electrolytes, reaching or even exceeding the benchmark Pt/C electrocatalyst. This work offers a scalable and practical approach to obtaining the metal-free carbon-based ORR catalysts that can work within a broad pH range.