Enhanced electrocatalytic activity of PtRu/nitrogen and sulphur co-doped crumbled graphene in acid and alkaline media

Abstract

The low mass activity and high price of pure platinum (Pt)-based catalysts predominantly limit their large-scale utilization in electrocatalysis. Therefore, the reduction of Pt amount while preserving the electrocatalytic efficiency represents a viable alternative. In this work, we prepared new PtRu₂ nanoparticles supported on sulphur and nitrogen co-doped crumbled graphene with trace amounts of iron (PtRu₂/PF) electrocatalysts. The PtRu₂/PF catalysts exhibited enhanced electrocatalytic performance and stability for the hydrogen evolution reaction (HER) at pH = 0. Moreover, the prepared PtRu₂/PF electrocatalyst displayed higher HER activity than commercial 20% Pt/C. The PtRu₂/PF catalyst achieved a current density of 10 mA cm⁻² at an overpotential value of only 22 mV for HER, performing better activity than many other Pt-based electrocatalysts. Besides, the PtRu₂/PF revealed a good performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline media. The PtRu₂/PF catalyst recorded a current density of 10 mA cm⁻² at an overpotential of only 270 mV for OER in KOH (1.0 M) solution and an onset potential of 0.96 V vs. RHE (at 1 mA cm⁻²) for ORR in KOH (0.1 M) solution.

Introduction

Platinum on carbon (Pt/C) is a widely used electrocatalyst for hydrogen evolution reaction (HER) due to its low overpotential and enhanced durability in harsh environment [1], [2], [3]. Despite these favorable properties, the extremely high price and low natural abundance have severely constrained Pt large-scale industrial application. In order to make fuel cells commercially viable, several strategies have been attempted through the reduction of Pt amount or using non-noble metal electrocatalysts without sacrificing their performance [4], [5], [6]. The best performing non-noble metal electrocatalysts reported often contain a transition metal cation such Fe [7], [8], Co [9], [10] or Ni [11], [12]. Unfortunately, their Tafel slopes (thus kinetics) and overpotential values are higher than those of common Pt-based catalysts for HER. Additionally, to achieve low Tafel slopes and small overpotentials, high loadings of these non-precious catalysts are required [13], preventing their widespread usage in fuel cells.

Reduction of Pt amount while preserving the electrocatalytic efficiency is another viable alternative. Over the past decade, countless attempts have been devoted to research for the reduction of the amount of Pt, which in turn allows reducing the cost [14]. From the literature survey, introducing one or several different metals to contact with single-phase Pt nanomaterials has brought great hope to alleviate the above challenges, owing to their dramatic enhancement of the electrocatalytic properties and stability, as compared to single element Pt catalyst. For example, Zhang et al. demonstrated the enhanced performance of CoFePt alloys towards the HER with an overpotential of 18 mV at 10 mA cm⁻² [15]. Liang et al. described an effective way to improve alkaline HER performance of Pt-Ni alloys through tuning their H and OH adsorption abilities [16]. Liu et al. prepared a Pt monolayer catalyst supported on complex 3D structures with improved HER [17], [18], [19]. In general, in most of these methods, a similar strategy was adopted, consisting of Pt loading onto large specific surface area materials and highly dispersible nanomaterials with the aim to reduce the Pt amount [20], [21], [22], [23], [24]. However, these approaches necessitate strict technical requirements and are associated with expensive production costs. Additionally, the low stability of these catalysts also restricts their performance for fuel cell applications, because this could eventually lead to a substantial catalytic activity loss, meaning that these catalysts need to be replaced soon upon their utilization. In order to increase the efficiency, the electrocatalyst should exhibit good stability, high activity, and low cost. Alternatively, coupling Pt with Ru in various configurations such as core–shell and alloys represents a viable approach to prepare electrocatalysts with improved catalytic properties and durability for the oxygen evolution reaction (OER) [25], HER [26], [27], [28], [29] and hydrazine oxidation reaction [29].

In this study, we describe the synthesis of high-performance platinum-ruthenium (Pt-Ru) nanoparticles loaded on sulphur- and nitrogen-co-doped crumbled graphene derived from polyethylenedioxythiophene with trace amount of Fe (PF) [30]. These catalysts have several merits as the amount of Pt used was reduced to a minimum, and Ru being relatively cheap (~4% of Pt) and displaying a comparable bond strength for hydrogen as Pt [31]. PF was investigated as the support, owing to its corrosion resistance and high surface area (Fig. 1). Interestingly, the PtRu₂/PF catalyst, with a small amount of Pt, displayed improved performance than the catalyst with higher Pt loading (PtRu/PF) for HER in acidic media. Additionally, the catalyst also had an excellent performance for oxygen reduction reaction (ORR) and OER in alkaline media compared to many reported noble-metal catalysts.