Role of Water Structure in Alkaline Water Electrolysis

A universal activity descriptor for catalytic alkaline hydrogen evolution reaction (HER) was unavailable, though metal-hydrogen binding energy can be considered as a good such descriptor in acidic medium. Herein, with the help of experimental and first principles density functional theory (DFT) based studies, we have shown that structural changes in the water coordination in electrolytes having high alkalinity can be a possible reason for the reduced catalytic activity of platinum (Pt) in high pH. Studies with polycrystalline Pt electrodes indicate that electrocatalytic HER activity reduces in terms of high overpotential required, high Tafel slope, and high charge transfer resistances in concentrated aqueous alkaline electrolytes (say 6M KOH) in comparison to that in low alkaline electrolytes (say 0.1M KOH), irrespective of the counter cations (Na+, K+ or Rb+) present. The changes in the water structure of bulk electrolytes with concentration are established using Raman, infrared, and 1H NMR based spectroscopic analyses. The changes in the interfacial water structure are also studied using in situ Raman scattering experiments where the changes in the coordination of water from tetrahedral to trihedral to free water are observed as the potential goes more cathodic towards HER. DFT based studies show enhanced water dissociation energy required for tetrahedrally coordinated water followed by trihedral, and then free water having the least dissociation energy for the Volmer process. But the water structure seems to be unaffected in anodic potentials. Hence the study paves new ways in studying the HER process in terms of the water structure near the electrode-electrolyte interface.