Abstract
The development of cost-effective, highly efficient, and durable electrocatalysts has been a paramount pursuit for advancing the hydrogen evolution reaction (HER). Herein, a simplified synthesis protocol was designed to achieve a self-standing electrode, composed of activated carbon paper embedded with Ru single-atom catalysts and Ru nanoclusters (ACP/RuSAC+C) via acid activation, immersion, and high-temperature pyrolysis. Ab initio molecular dynamics (AIMD) calculations are employed to gain a more profound understanding of the impact of acid activation on carbon paper. Furthermore, the coexistence states of the Ru atoms are confirmed via aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). Experimental measurements and theoretical calculations reveal that introducing a Ru single atom site adjacent to the Ru nanoclusters induces a synergistic effect, tuning the electronic structure and thereby significantly enhancing their catalytic performance. Notably, the ACP/RuSAC+C exhibits a remarkable turnover frequency (TOF) of 18 s-1 and an exceptional mass activity (MA) of 2.2 A mg-1, surpassing the performance of conventional Pt electrodes. The self-standing electrode, featuring harmoniously coexisting Ru states, stands out as a prospective choice for advancing HER catalysts, enhancing energy efficiency, productivity, and selectivity.
1. Introduction
Hydrogen energy has gained significant attention as a most promising alternative to fossil fuels, offering advantages, including zero carbon emissions and sustainability.[1-4] One particularly noteworthy approach is electrochemical water-splitting, specifically through the cathodic hydrogen evolution reaction (HER) by use of electrocatalysts.[5-9] Among the platinum (Pt)-group metals, ruthenium (Ru) has been considered as a potential substitute for Pt in the HER process due to its comparable hydrogen affinity and lower price.[10,11] Single-atom catalysts (SACs) possessing a metal-nitrogen-carbon (M-N-C) structure have emerged as a promising frontier for optimizing catalytic performance. Each individual metal atom is highly dispersed and immobilized onto various supports, serving as an active site.[12] Owing to their distinctive structures, SACs exhibit nearly 100% atomic utilization efficiency, as well as excellent catalytic activity and selectivity.[13]However, a major challenge arises with Ru metal, as it tends to aggregate due to its high cohesive energy.[14]This aggregation hampers optimal metal loading, ultimately leading to a low density of active sites, compromising SACs activity. To address this challenge, studies on the introduction of nanoclusters or nanoparticles have been steadily conducted to optimize the adsorption/desorption behavior of intermediates on the metal center.[15,16] For example, Lu et al. reported that the coexistence of Fe single atoms and Fe nanoparticles cooperatively accelerated the electrochemical oxygen reduction reaction (ORR).[17] Besides, Liu et al. demonstrated that the copresence of single-atomic Fe sites with Fe clusters greatly boosted the ORR activity through interaction with each other. Inspired by these findings, our research delved into catalytic behavior of Ru electrocatalysts, which are present in various forms.[18]
The development of self-standing electrodes has proven to be an effective strategy for overcoming challenges related to the reproducibility and scalability of the complex process involved in fabricating powder-type SACs into electrodes.[19]Among the self-supporting substrates, carbon paper (CP) is an ideal support due to its good conductivity, flexibility, and mechanical strength. These types of electrodes eliminate the need for additional binders, which typically result in reduced electrical conductivity and increased interfacial resistance between the electrocatalysts and substrate.[20,21]
Herein, we propose a rational construction of a self-standing electrode using commercial CP as a self-standing conductive substrate. This approach incorporated an additional activation step via acid treatment. It enables achieving a higher atomic loading, even with the same quantity of Ru atom doping, providing cost-effective benefits. The impact of the acid treatment on CP was further substantiated using sophisticated theoretical simulation models. Binder-free, self-standing electrodes were successfully fabricated via a simple acid treatment by directly incorporating Ru atoms into activated carbon paper (ACP). To stabilize the Ru single atoms, dicyandiamide (DCD), which consists of abundant nitrogen atoms, was introduced as a nitrogen source through high-temperature pyrolysis. The resulting self-standing electrode, which contained both single-atom Ru sites and Ru nanoclusters (ACP/RuSAC+C), demonstrated efficient catalytic activity towards HER, requiring 87 mV of overpotential to deliver a current density of 10 mA cm−2. It also exhibited adequate stability, showing no significant activity degradation for H2 production over nearly 50 h. Notably, ACP/RuSAC+C demonstrated superior turnover frequency (TOF) values across a wide range of overpotential (3.96 s−1 at 100 mV, 9.7 s−1 at 150 mV, 18 s−1 at 200 mV), outperforming ACP/RuCand many other advanced HER electrocatalysts recently reported in the literature. In addition, ACP/RuSAC+C exhibited an exceptionally high mass activity (MA), surpassing that of Pt and ACP/RuC. These enhanced HER performances can be attributed to the strong synergistic effects arising from Ru single atoms and Ru nanoclusters, which accelerated H*adsorption, as validated by experiments and theoretical calculations.