Suppression of H-2 bubble formation on an electrified Pt electrode interface in an acidic "water-in-salt" electrolyte solution

Abstract

Electrode-oxidation of H-2 in aqueous solutions is of great interest as a powerful and clean electrochemical fuel, but it suffers from bubble formation, which acts as an insulator that impedes the electrode rate of H-2 oxidation and blocks H+ flux to an electrode. In this article, we report that the formation of H-2 bubbles can be kinetically suppressed on an electrified Pt electrode in an acidic "water-in-salt" electrolyte solution (WiSE), which leads to the reversible voltammetric features associated with H+/H-2 redox reaction. Redox reversible voltammograms of H+/H-2 were observed in acidic 6 m LiTFSI and LiOTf solutions, while those in both LiCl and LiNO3 solutions were redox irreversible on the Pt electrode. The apparent rate constants of H-2 bubble formation were estimated in the four different types of 6 m electrolyte solutions, and the formation of bubbles was inhibited most severely in the LiTFSI medium. The suppression of H-2 bubble formation during the electrode-reduction of H+ in an acidic "water-in-6 m LiTFSI" electrolyte solution (WiSE(LiTFSI)) was macroscopically monitored, and it was also interrogated through electrochemical quartz crystal microbalance (EQCM) measurements. In acidic 6 m LiCl and LiNO3 solutions, the abrupt generation of bubbles and rapid arrivals at the saturated surface coverages by the bubbles were monitored during the electrode-reduction of H+, which were almost analogous to that observed in an acidic solution without concentrated lithium salts. In a 6 m LiOTf acidic solution, smaller bubbles form and more compactly cover an interface on Pt than those formed in LiCl and LiNO3 media. In an acidic solution with the same molal concentration of LiTFSI, the frequency in the EQCM was not increased within similar to 2 min, and even after this, it increased at a slow rate, which was reflective of suppressed H-2 bubble formation on a Pt electrode in WiSE(LiTFSI). The inhibition of H-2 bubble formation was not because of increased solubility of H-2 in bulk WiSE(LiTFSI) but because of an interfacial layer (IFL) that formed on the electrified Pt electrode. This observation was validated through a voltammetric investigation in solutions containing different types of 6 m electrolytes with saturated H-2. H-2 was distinctively electrode-oxidized in the 6 m WiSE(LiTFSI) only when the electrode potential was negatively biased during the injection of H-2 gas into the solution, indicating the crucial role of forming an electrode potential driven IFL for the stabilization of H-2 in its dissolved form in proximity to the Pt electrode. A possible model for H-2 stabilization in the IFL was suggested, and lastly, the electrochemically irreversible nature of H-2 electrode-oxidation on an Au electrode was confirmed in WiSE(LiTFSI), indicating its strong inner sphere characteristics.