Suppression of H2 bubble formation on an electrified Pt electrode interface in an acidic “water-in-salt” electrolyte solution

Abstract

Electrode-oxidation of H₂ 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 H₂ oxidation and blocks H⁺ flux to an electrode. In this article, we report that the formation of H₂ 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⁺/H₂ redox reaction. Redox reversible voltammograms of H⁺/H₂ 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 H₂ 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 H₂ 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 ∼2 min, and even after this, it increased at a slow rate, which was reflective of suppressed H₂ bubble formation on a Pt electrode in WiSE_LiTFSI. The inhibition of H₂ bubble formation was not because of increased solubility of H₂ 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 H₂. H₂ was distinctively electrode-oxidized in the 6 m WiSE_LiTFSI only when the electrode potential was negatively biased during the injection of H₂ gas into the solution, indicating the crucial role of forming an electrode potential driven IFL for the stabilization of H₂ in its dissolved form in proximity to the Pt electrode. A possible model for H₂ stabilization in the IFL was suggested, and lastly, the electrochemically irreversible nature of H₂ electrode-oxidation on an Au electrode was confirmed in WiSE_LiTFSI, indicating its strong inner sphere characteristics.