Delucidazione dell'effetto elettrolitico sull'attività di un elettrocatalizzatore in oro policristallino mediante la tecnica del transitorio di corrente indotto dal laser.

The investigation of electrified solid-liquid interfaces is of critical importance to developing efficient, sustainable, and safe energy storage and conversion devices. Among aqueous systems, water electrolysis is a promising approach to generating hydrogen fuel for portable applications by storing energy from renewable electricity in chemical bonds. The design of the electrocatalyst governs the performance of redox reactions, such as the hydrogen evolution (HER) from water splitting. In addition, the electrolyte pH and composition have demonstrated a significant influence on the properties of the electric double layer formed at the electrode-electrolyte interface. A crucial interfacial parameter is the net orientation of the water molecules, described by the potential of maximum entropy (PME). This PME is defined as the potential at which the disorder of the interfacial water layer reaches its maximum, consequently lowering energy barriers of the surface exchange and facilitating charge transfers. The effect of different alkali metal cations on the PME of a polycrystalline gold (Aupc) electrode is studied, varying the composition of 0.1M XOH electrolytes (X = Li+, Na+, K+, Rb+, Cs+). Experimentally, the PME is obtained by the laser-induced current transient (LICT) technique which is based on the quick temperature jump of high-power nanosecond laser pulses. The current relaxation responses after excitation by the laser beam indicate the net orientation of the water dipoles and, hence, reveal the PME by measuring the potential at which the net surface charge density changes sign. The results prove a strong cation effect on the PME of the double layer formation. Weakly hydrated cations, namely K+ and Cs+, exhibit PMEs nearer the onset potential of hydrogen evolution compared to more strongly hydrated cations. In parallel, activity measurements in the same alkaline electrolytes showed that large, weakly hydrated cations enhance the HER rates of Aupc at moderate overpotentials. The electrolyte composition thereby determines the interfacial layer’s degree of order which, in turn, affects the catalytic performance. Moreover, two cation-dependent PMEs were discovered for each Li+, Na+, K+ and Cs+-containing electrolyte. This may explain the observed inversion of the HER rates at high overpotentials. The correlation between the hydrated cation effect and HER activity suggests that determining the PME helps predict the catalytic performance and elucidate key redox processes. The LICT technique thus stands as a powerful tool to understand fundamental electrochemical phenomena and optimize energy systems.