Carbon black supported Ag nanoparticles in zero-gap CO2 electrolysis to CO enabling high mass activity

In this study Ag nanoparticles supported on carbon black (Ag/C) were studied as catalysts for the electrochemical reduction of CO₂ to CO. The nanoparticles were synthesized on three carbon supports, namely Super P, Vulcan and Ketjenblack with surface areas from 50 to 800 m² g^-1 using cysteamine as a linker as proposed by Kim et al., J. Am. Chem. Soc., 2015, 137, 13844. Gas diffusion electrodes were fabricated with all three Ag/Cs and then characterized in a zero-gap electrolyzer. All three supported catalysts achieve high voltage efficiencies, mass activities, and faradaic efficiencies above 80% up to 200 mA cm^-2 with Ag loadings of ∼0.07 mg cm^-2. Using an IrO₂ anode, a partial CO current density of 196 mA cm^-2 at 2.95 V and a mass activity of 3920 mA mg^-1 at a cell voltage of 3.2 V was achieved. When changing the electrolyte from 0.1 M KOH to 0.1 M CsOH, it is possible to achieve 90% FE_CO at 300 mA cm^-2. This results in a mass activity up to 5400 mA mg^-1. Moreover, long-term tests at 300 mA cm^-2 with 0.1 M CsOH resulted in FE_CO remaining above 80% over 11 h. The electrochemical performance did not show a dependence on the carbon support, indicating that mass transport is limiting the cathode, rather than catalyst kinetics. It is worth noting that this may only apply to electrodes with PTFE binders as used in this study, and electrodes with ionomer binders may show a dependence on the catalyst support.

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries

To boost the performance of vanadium redox flow batteries, modification of the classically used felt electrodes is required to enable higher cycling performance and longer life cycles. Alternative approaches to the standard thermal oxidation procedure such as wet chemical oxidation are promising to reduce the thermal budget and thus the cost of the activation procedure. In this work we report a rapid 1 hour activation procedure in an acidified KMnO₄ solution. We show that the reported modification process of the felt electrodes results in an increase in surface area, density of oxygenated surface functionalities as well as electrolyte wettability, as demonstrated by N₂-physisorption, XPS, Raman spectroscopy as well as contact angle measurements. The activation process enables battery cycling at remarkably high current densities up to 400 mA cm⁻². Stable cycling at 400 mA cm⁻² over 30 cycles confirms promising stability of the reported activation procedure.

Schematic diagram of the K-GF fabrication process. Step 1: deposition of MnO_x layers onto the P-GF electrode surface using acidified KMnO₄ solutions. Step 2: removal of MnO_x layers using an acidified H₂O₂ solution to produce the K-GF electrode.