Fundamental study determines Pt surface hydronium pKa and reveals correlation with HER kinetics

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Prof. Xiangfeng Duan

A research collaboration between Professor Xiangfeng Duan’s group in the UCLA Department of Chemistry & Biochemistry, Professor Yu Huang’s group in the UCLA Department of Materials Science and Engineering, and Professor William A. Goddard III’s group at the California Institute of Technology, experimentally determined the interfacial pKa of hydronium ions (H3O+) on platinum surface equals 4.3, a value distinct from that in the bulk (0).

Hydrogen evolution reaction (HER) represents an essential reaction for green hydrogen production through water electrolysis (H2O-> H2 +O2), a subject of growing interest in the green hydrogen economy. This electrochemical reaction is facilitated by catalysts, among which platinum is considered as the champion catalyst. Thus, gaining fundamental understanding of the reaction kinetics on the Pt surface is critical to both researchers and stakeholders to optimize hydrogen production. 

The importance of the Pt-water interaction in electrochemistry is evidenced by the striking differences in its performance of electrocatalysis, making the pH effect on HER kinetics very important. This has intrigued many researchers for a long time, however there has not been a way to measure or calculate these properties at the Pt surface. Consequently, the behaviors of water molecules on catalyst surfaces remains poorly understood.

As a turning point, in the recently published UCLA-Caltech collaboration, researchers were able to track experimentally Pt-surface dynamics in response to adsorbate molecules using a unique home-developed electrical transport spectroscopy (ETS).

“The ETS signal is exclusively sensitive to surface adsorbates,” said Huang. “Depending on the molecular structure of the surface adsorbates, it will interact with the surface differently. Larger size of atoms like O in water scatter conduction electrons of Pt more and give a lower reading in conductance than smaller atoms such as H would.”

ETS was first developed by Duan and Huang group in 2015 and has been used for interfacial research ever since. “There have been many efforts attempting to probe surface water pKa, but unfortunately most of the studies are highly convoluted with the interference from water in the bulk solution to date. The ETS developed by the UCLA group provides a very unique approach to the problem”, said Goddard.

Goddard said that these results motivated his group to use quantum mechanics methods to predict pKa to clarify the mechanism underlying the dramatic effect measured at UCLA. The problem is that to describe the Pt nanowires used in the experiments requires calculations on ~10,000 atoms for nanoseconds of molecular dynamics whereas, while quantum mechanics (QM), the Gold standard for predictions of such steps, is limited to ~300 atoms. Here the Caltech team used the ReaxFF reactive force field developed to retain the accuracy of QM for systems with 10,000 to a million atoms. They tested ReaxFF against QM for simple Pt facets, showing that ReaxFF agrees well with QM and then applied ReaxFF to the UCLA Pt nanowires where the predicted pKa agreed well with the experiment. These calculations validated that the experiments indeed did measure the pKa and showed in addition that the effect changed dramatically with distance, dropping to just pKa=1 just 0.2 nm from the surface layer. This validation of ReaxFF by experiment opens the door to QM accuracy on model of the full fuel cell catalyst including carbon support, electrolyte and anode for optimizing fuel cell design.

Figure 1. Schematic illustration of ETS. Au-based source and drain creates a signal pathway to measure the conductance of platinum nanowires (PtNWs) upon molecules adsorption to its surface at different electrochemical potentials.

With ETS, the research team observed a distinct pH-dependent signal with a sharp switching at a critical point that closely resembles a typical titration curve, which allows them to derive the pKa value based on Henderson-Hasselbalch equation (Figure 2a).

“We quantified that the pKa of hydronium on the platinum surface is 4.3, which is higher than bulk H3O+ far away from the surface (pKa = 0). Our collaborative theoretical study also confirms hydronium enrichment near the Pt surface leads to higher pKa.” Commented Aamir Hassan Shah, co-author of the paper and a 4th year PhD candidate in Duan group.

The experimentally determined pKa  for surface hydroniums is well corroborate by ab initio molecular dynamics (AIMD) calculations on and 10,000-atom ReaxFF reactive molecular dynamics simulations (RMD).  The higher than bulk pKa values lead to the high protonation state and enrichment of hydroniums on Pt surface (Figure 2b), which could have profound effect on relevant reactions.

The team further applied the determined surface hydronium pKa to interpret the distinct pH-dependent HER kinetics (Figure 2a). It is well recognized the HER process following different reaction pathway in acidic or basic conditions with distinct kinetics. However, the exact switching point of such kinetics and the underlying molecule picture has been elusive for the field. To this end, the determined surface hydronium pKa gives a precise molecular-level explanation of the switch of in the reaction mechanism at different pH.

At solution pH 0 to 2, well below the surface pKa, there are abundant protonated surface hydroniums as proton source for hydrogen adsorption. Thus, the reaction follows the typical acidic pathway with the Tafel step as the rate-determining step (RDS) and a Tafel slope of 30 mV per decade. At solution pH 5 to 7, well above the surface pKa, the surface hydroniums are fully deprotonated with water molecules as the dominant species to supply protons; thus, the Volmer step becomes rate determining, resulting in a large Tafel slope of 120 mV per decade. In intermediate pH, there are finite number of hydroniums near Pt surface, leading to a mixed reaction pathway depending on the potential applied.

Figure 2. (a) pH-dependent ETS signal correlated well with pH dependent Tafel slope. It helps researchers determine the rate limiting step of HER pathway the underlying molecular pathway. (b) The statistical distribution of H3O+ averaged from 50 independent ReaxFF MD simulations, each averaged over 100 snapshots.

“Our studies provide molecular insights into pH effects on the surface water structure and their critical role in the relevant electrochemical reactions. These results suggest further studies to fully understand the implications of our model on electrocatalysis on various relevant reactions”, said Duan. “Since a water molecule can act as a base or an acid, further deprotonation of the Pt–water interface is quite possible and worthy of serious examination”, commented further by Goddard.

The UCLA research was supported by National Science Foundation and New Hydrogen Inc.

The Caltech research was supported from the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.

About the Lead UCLA Authors

Professor Xiangfeng Duan joined the chemistry and biochemistry faculty in 2008. His group’s research interests include nanoscale materials, devices and their applications in future electronics, energy technologies and biomedical science.

Guangyan Zhong received his B.S. degree in Chemistry from Peking University, Beijing, China in 2016. Guangyan joined Professor Xiangfeng Duan’s group at UCLA in 2016 and conducted research related to the solution-catalyst interface based on electrical transport spectroscopy. He was awarded UCLA Chemistry and Biochemistry Excellence in Research Fellowship in the year 2020 and Dr. Myung Ki Hong Dissertation Award in the year 2021. He joined Applied Materials Inc. as Process engineer since year 2021.

Aamir Hassan Shah received his undergraduate and master degree in physical chemistry from Quaid-i-Azam University, Islamabad, Pakistan. He is now a fourth year Ph.D. candidate in Prof. Duan’s group. His research at UCLA focused on the fundamental understanding of electrode-electrolyte interface in water splitting reactions using on-chip Electrical transport spectroscopy (ETS). Aamir recently received the department’s 2022 College of Letters and Science Stone Fellowship.

Article by Zhuoying Lin (Duan Group), UCLA Department of Chemistry & Biochemistry, Lin is a chemistry graduate student and science writer who joined our program in Fall 2021.  Read more of Lin’s UCLA Chemistry & Biochemistry articles here.