Proceedings of nanoGe Fall Meeting19 (NFM19)
Publication date: 18th July 2019
Water splitting in near-neutral pH buffered conditions is getting great attentions for the artificial photosynthesis in which light-absorbing semiconducting materials are immersed in electrolyte solutions. It is essential to identify the origin of overpotential, either kinetics on electrode materials or mass-transport in electrolyte solutions, to guide our research direction for the improvement of efficiency. Although hydrogen evolution reaction (HER) on Pt is one of the most studied electrochemical reactions, its reaction mechanism and exchange current density are not fully understood because mass-transport has a severe influence on the observed electrochemical response due to the facile kinetics on Pt.
In the present study, HER in various buffered conditions (pH and morality) has been investigated using a Pt rotating disk electrode (RDE) configuration to perform a quantitative assessment. Strong concentration dependent overpotentials were observed in pure KH2PO4 (pH = 4.3) and K2HPO4 (pH = 9.5) solutions in which dense buffer conditions (≥1.5 M) are preferable. Meanwhile, between the above mentioned pHs, the concentration dependence got weak and 0.5 M is sufficient to minimize overpotential optimized to 27 mV at −10 mA cm−2 and 3600 rpm.
An advanced model in which dual mass-transport of proton donors and hydrogen is fully covered was developed to quantitatively describe the observed overpotentials. First of all, the kinetic overpotential on Pt electrode has a negligible contribution in the observed overpotential. The rest of overpotential, that is concentration overpotential, was further distinguished between proton donors and product hydrogen. Severe local pH shift and the resultant concentration overpotential from pH gradient were identified in pH level far from pKa while local pH shift was effectively suppressed close to the pKa, which can be explained by the titration curve of buffering action. After minimizing the local pH shift, the concentration overpotential from hydrogen becomes predominant, which appears significantly in dense viscous buffered condition. The present dual mass-transport model effectively describes the observed optimal condition (0.5 M K1.5H1.5PO4) and reveal the great importance of electrolyte engineering under near-neutral pH buffered condition, i.e., pH, molarity and viscosity.
The research reported in this work was supported by the King Abdullah University of Science and Technology.
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