Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co-N4C Catalyst.

Enhanced oxygen reduction reaction (ORR) kinetics and selectivity are crucial to advance energy technologies like fuel cells and metal-air batteries. Single-atom catalysts (SACs) with M-N4/C structure have been recognized to be highly effective for ORR. However, the lack of a comprehensive understanding of the mechanistic differences in the activity under acidic and alkaline environments is limiting the full potential of the energy devices. Here, a porous SAC is synthesized where a cobalt atom is coordinated with doped nitrogen in a graphene framework (pCo-N4C). The resulting pCo-N4C catalyst demonstrates a direct 4e- ORR process and exhibits kinetics comparable to the state-of-the-art (Pt/C) catalyst. Its higher activity in an acidic electrolyte is attributed to the tuned porosity-induced hydrophobicity. However, the pCo-N4C catalyst displays a difference in ORR activity in 0.1 m HClO4 and 0.1 m KOH, with onset potentials of 0.82 V and 0.91 V versus RHE, respectively. This notable activity difference in acidic and alkaline media is due to the protonation of coordinated nitrogen, restricted proton coupled electron transfer (PCET) at the electrode/electrolyte interface. The effect of pH over the catalytic activity is further verified by Ab-initio molecular dynamics (AIMD) simulations using density functional theory (DFT) calculations.

Reverse Dual-Ion Battery Enabled by Reversing the Cation/Anion Storage Mechanism in an Aqueous ZnCl2 Water-in-Salt Electrolyte.

Dual ion batteries (DIBs) have garnered significant attention from researchers due to their unique ability to store charges using electrolyte-born ions, making them promising candidates for grid storage applications. However, despite extensive efforts to explore DIBs with various electrolytes, such as organic, aqueous, gel polymer etc., challenges such as electrolyte decomposition and poor stability of anode materials in aqueous electrolytes remain unresolved. To address these issues, we report a novel approach utilizing a flip-cum-reverse sequence of anion/cation storage chemistry in a ZnCl2 water-in-salt electrolyte (ZnCl2 -WiSE)-based reverse dual ion battery (RDIB), employing Zn-based Prussian blue analogue i. e., Zn3 [Fe(CN)6 ]2 and ferrocene-carbon composite (FcC) as cathode and anode electrodes, respectively. The RDIB operates in the opposite direction compared to conventional DIBs, offering a fresh perspective. Through our investigations, we discovered that increasing the concentration of ZnCl2 -WiSE [ZnCl2 -WiSE] resulted in a positive shift of 270 mV in the redox potential for cation/anion (de)insertion at the cathode, and a negative shift of 70 mV at the anode, indicating enhanced performance. Remarkably, the RDIB operate in 10 m ZnCl2 -WiSE exhibited an impressive energy density of 23 Wh kg-1 , showcasing the potential of this approach for high-performance energy storage.