A group of scientists led by China’s Wenzhou University and Guangxi University have proposed a novel approach to improve the performance of polysulfide-iodide–based redox flow batteries (SIRFBs).
Redox flow batteries (RFBs) are a type of battery that stores energy in chemicals that undergo redox reactions. Redox stands for reduction-oxidation, a chemical reaction where one substance loses electrons and another gains electrons.
“The multistep charge transfer reactions within the S2-/Sx2- and I-/I3- couples on electrode results in elevated polarization resistance and poor kinetic reversibility, which induce slow adsorption behavior, limited operational lifespan, and diminished energy efficiency (EE), thereby impeding the widespread adoption of SIRFBs,” the group said. “Therefore, designing efficient and stable catalytic electrodes to accelerate the transformation of S2-/Sx2- and I-/I3- couples remains challenging but crucial for practical deployment of SIRFBs.”
To address this issue, the team proposed using a two-dimensional molybdenum disulfide (MoS2) nanosheet, as demonstrated in previous literature, to accelerate the redox reaction. However, its intrinsic activity is still far from the high-power demand of industrial RFBs, which led the scientists to suggest single atoms anchoring of cobalt (Co) onto the MoS2 surface.
They also explained that single-atom catalysts (SACs) have recently emerged as a potential solution to the above-mentioned issue, due to their maximal atom utilization, low-coordination environment, and abundant unoccupied orbitals, which are key factors regulating the defective structure and electronic property of MoS2 substrate.
The introduction of CoSA to MoS2 is intended to trigger the formation of sulfur vacancies (SVs) defects. With that, the team has reached a design of CoSA-doped sulfur vacancies-containing MoS2 (CoSA-VS/MoS2) for the SIRFB system. The CoSA-VS/MoS2 nanosheets were synthesized by the team using the one-step solvothermal in situ growth strategy, followed by an annealing activation process.
“Consequently, CoSA and Vs sites synergistically optimized the interface electronic structure, promoted the reactant adsorption capacity, and accelerated the kinetics of S2-/Sx2- and I-/I3- redox couples, simultaneously,” the researchers said. “The derived SIRFB achieved an energy efficiency of 87.9% at 20 mA cm2, which is higher than that of the reported CoS2/CoS (71.6%), Cu2CoGeS4 (77.2%), Cu7S4 (78.5%), CuFeS2 (79.6%)18.”
In addition, the novel battery showed a peak power density of 95.7 mW cm2 and an average energy efficiency of 76.5% at 30 mA cm2 within 50 cycles. It demonstrated a cycling life of approximately 850 cycles during continuous operation at 10 mA cm2 with a 10% state of charge (SOC) and a low overpotential of 113 mV at 20 mA cm2. “Signally, the initial EE of 93.1% could be almost fully recovered after refreshing the electrolytes (200th and 600th cycles),” the academics added.
The battery was described in “Synergy of single atoms and sulfur vacancies for advanced polysulfide–iodide redox flow battery,” published in Nature Communications. The research group included scientists from Lanzhou Jiaotong University, the University of Electronic Science and Technology of China, Yunnan University and Tsinghua University.
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