Atomic rhodium catalysts for hydrogen evolution and oxygen reduction reactions

Highlights

• Rh@NG shows ORR performance with E_1/2 of ∼0.848 V vs RHE in 0.1 M KOH.

• Rh@NG shows ORR performance with E_1/2 of ∼0.74 V vs RHE in 0.1 M HClO₄.

• Rh@NG shows HER activity with overpotential of 29 mV at 10 mA cm⁻² in 0.5 M H₂SO₄.

• Rh@NG shows HER activity with overpotential of 45 mV at 10 mA cm⁻² in 0.1 M KOH.

• The efficient active sites for HER and ORR might be RhN₃ and RhN₄, respectively.

Abstract

High specific activity of single-atom catalysts shows great application potentials in water splitting and fuel cell systems. Herein, we report a facile pyrolysis method to fabricate an atomically dispersed rhodium catalyst (Rh@NG). The Rh@NG exhibits outstanding electrochemical activities and kinetics for oxygen reduction reaction in alkaline medium, which can compare to Pt/C catalysts, along with excellent durability and good tolerance to methanol and carbon monoxide. The half-wave potential on Rh@NG in 0.1 M KOH and 0.1 M HClO₄ is 0.848 and 0.74 V vs RHE, respectively. Additionally, the Rh@NG displays superior performance for hydrogen evolution reaction, offering a low overpotential of only 29, 33, and 45 mV at 10 mA cm⁻² in 0.5 M H₂SO₄, 1.0 M KOH, and 0.1 M KOH, respectively. Theoretical calculations reveal that the efficient active sites for HER and ORR might be RhN₃ and RhN₄ moiety embedded into the N-doped graphene, respectively.

Introduction

The imminent global energy and environmental crisis has created keen research on energy conversion and storage systems, including electrocatalytic water splitting, metal–air batteries and fuel cells [[1], [2], [3], [4], [5]]. The electrocatalytic oxygen reduction reaction (ORR) is the core step in metal–air batteries and fuel cells [6], whereas the electrocatalytic hydrogen evolution reaction (HER) plays a crucial part in electrochemical water splitting [7]. Although Pt-based catalysts can efficiently catalyze both ORR and HER with low overpotentials, the scarcity, high price and chemical susceptibility severely limit their large-scale applications [8]. Therefore, development of high-performance and cost-efficient electrocatalysts for ORR and HER is an urging task for regenerative fuel cells and water splitting.

Recently, a large number of experiments and density functional theory (DFT) calculations demonstrated that metal and nitrogen codoped carbon (M-N-C, M = Fe or Co) materials exhibited excellent ORR performance under alkaline conditions [9]. The ORR activity is significantly influenced by the coordination number, and the electronegativity of metal ion and its coordination atom [10]. The active site with M-N₄ configuration might be the catalytic center for ORR [11]. In addition, Fe-N₅ [12], Co-N₂ [13], Cu^I-N₂ [14], ScN₃O and ScN₂O [15] sites were also reported as the efficient ORR active centers. However, considering the stability issues of the cathode of fuel cells and electrolyzers operated in acidic media, nonprecious metal electrocatalysts are a long way from being practical applications in fuel cell and water splitting systems.

Rhodium metal nanoparticles and phosphides showed excellent HER performance [[16], [17], [18]]. Compared with other metal doped SrTiO₃, the Rh-doped one showed better photocatalytic hydrogen evolution performance [19,20]. Moreover, compared with Pt-based catalysts, ultrathin wavy Rh nanowires exhibited higher electrocatalytic performance in methanol oxidation reaction [21]. In addition, Rh-based catalysts can electrocatalytically activate C–H bonds [22,23]. The addition of Rh to the Pt and Ir systems can promote the electrochemical activity for ammonia oxidation [24,25]. Downsizing nanoparticles can expose more surface metal atoms and increase the defects, thus improving the catalytic activity. However, as a member of the Pt group, rhodium is very rare and expensive, which force us to reduce the dosage and the cost of the catalyst to the utmost extent [26]. The most efficient strategy in size reduction is to form a single-site heterogeneous catalyst, where metal ions are atomically dispersed onto a support. Graphene materials have excellent physical-chemical properties, such as large specific area, high electrical conductivity, adjustable surface electronic structure, and outstanding chemical stability, which are suitable for support in fuel cell and water splitting systems [27]. Nevertheless, the atomic rhodium catalysts have seldom been reported for ORR and HER.

In this work, we adopted a rational preparation method for fabrication of Rh-N-C catalysts by simple annealing of a mixture of graphene oxide (GO) and rhodium salt under an ammonia atmosphere. The oxygen-rich functional groups on the surface of GO can coordinate with the Rh³⁺ ions for providing high dispersion. The Rh-N bonds can be generated through a high temperature nitriding strategy. The resultant atomic rhodium catalyst shows high performance and stability toward both ORR and HER in both acidic and alkaline media with extraordinarily low overpotentials.