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.