Improving energy storage properties of carbon felt electrodes for vanadium redox flow batteries via ZIF modifications

• Fe-, Co–, Ni-, Cu-, and Zn-based ZIFs were deposited onto commercial carbon felts. • Layer-by-layer and hydrothermal methods were tested for deposition. • ZIF-modified felts effectively catalyzed the reactions in VRFB. • VRFB assembled with ZIF-modified felts exhibited lower polarization. • Capacity was increased by 33 %, and energy efficiency improved by 29 %. In this study, we successfully enhanced the electrochemical energy storage properties of commercial carbon felts by modifying their surface with metal–organic frameworks (MOFs) of the zeolitic imidazolate framework (ZIF) type, incorporating Fe, Co, Ni, Cu, and Zn as metal centres. These modifications were achieved through two distinct processes: layer-by-layer deposition and a hydrothermal synthesis method. The resulting materials were thoroughly characterized using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), inductively coupled plasma spectroscopy (ICP), and cyclic voltammetry (CV) in a three-electrode cell. Our findings indicate that the materials synthesized via the hydrothermal process exhibited superior electrochemical performance compared to those obtained through the layer-by-layer method. In light of the findings, the study progressed to the device stage, specifically a single-cell vanadium redox flow battery. In this stage of the study, the modified electrodes were characterized using two key techniques: galvanostatic charge–discharge and electrochemical impedance spectroscopy. This characterization revealed that electrodes modified with ZIF structures displayed significantly reduced polarization compared to those fabricated with the unmodified commercial felt. The ZIFs that exhibited the most significant enhancements in electrocatalytic performance were those based on Zn, Cu, and Ni (in this order), as these metals demonstrated higher deposition levels on the carbon felt electrodes and exhibited superior dispersion. The enhancements resulted in significant performance improvements, with energy efficiency increases of up to 29 % and accessible capacity improvements of up to 33 %. This research demonstrates the potential of ZIF-modified carbon felt as a highly effective electrode material for vanadium redox flow batteries, paving the way for more efficient and scalable energy storage systems. Despite the minimal metal content present in the MOFs, our results demonstrate a significant enhancement in electrode performance, highlighting the efficiency of this approach and its potential to optimize the electrochemical activity of VRFB electrodes with minimal material usage.