Electrical Double Layer Effects on Alkaline Hydrogen Reactions on Platinum

Alkali metal cations (AM⁺) and hydroxide anions (OH^-) intricately influence Pt-catalyzed hydrogen evolution and oxidation reactions (HER/HOR) in alkaline media, a topic of ongoing debate. Here, we systematically investigate their effects using twenty-eight electrolytes with independently varied Na⁺ and OH^- concentrations ranging from 0.001 to 1.0 M. Our studies reveal a strong correlation between electrical double layer (EDL) thickness and HER/HOR rates. Increasing the OH^- concentration positively shifts the potential of zero free charge and enhances the negative surface charge on Pt within the HER/HOR potential regime. This reduces the EDL thickness, strengthens interfacial electric fields, and facilitates water dissociation during HER and H_ad/OH^- recombination during HOR, boosting the corresponding reaction rates. At a fixed pH, increasing Na⁺ concentrations initially reduces the EDL thickness and enhances HER/HOR activity. However, further increasing Na⁺ concentrations beyond 0.1 M paradoxically increases the EDL thickness and suppresses the HER/HOR rates. This counterintuitive behavior is attributed to the formation of ion pairs at the outer Helmholtz plane under high Na⁺ concentration conditions, which weakens the surface electric field and slows the reaction kinetics. At the highest pH 14, the even stronger interfacial electrical field induces partial dehydration of the secondary hydration shell, which adversely impacts the interfacial water structure and suppresses the HER/HOR activities despite a significant decrease in EDL thickness. This study elucidates the intricate effects of Na⁺ and OH^- concentrations on EDL thickness and establishes the critical role of EDL thickness and the surface electric field in modifying the surface water structure and thus the HER/HOR kinetics, providing valuable insights for the design of next-generation electrochemical systems.