Design and Mechanism Study of Electrocatalysts for Alkaline Hydrogen Evolution

Abstract

The hydrogen evolution reaction (HER) is one of the most important and fundamental electrochemical processes in sustainable photo/electrochemical energy generation technologies. However, even after decades of study, the HER mechanism in neutral/alkaline media is still unclear and the slow kinetics of the reaction remains an ongoing problem. Consequently, most of the alkaline HER catalysts, especially the highly cost-effective non-noble metal catalysts, are still inefficiently designed based on trial and error methods. This lack of knowledge surrounding the alkaline HER has not only affected the development of hydrogen-based catalysis, but also obstructed the proliferation of energy conversion technologies (e. g. fuel cell, sea water splitting etc.). Understanding the cause of the sluggish alkaline HER kinetics and developing efficient design protocols for highly active catalysts is now urgently required by both academia and industry. To this end, work contained in this thesis aims to contribute to meeting these needs. The first two chapters of this thesis provide a systematic review on the current understanding and progress on alkaline HER catalysts. These chapters have introduced the latest opinions on the reaction mechanism of alkaline HER, concluded the design strategies of almost all the current catalytic materials for the reaction and give outlook on the future development of the field. The last three chapters of the thesis are specially focused on noble metal catalysts, which are the most representative candidates for the alkaline HER. Here, the thesis presents a series of Pt-based catalysts in two parts. The first part of the thesis focuses on the unique alkaline HER mechanism on nanostructured electrocatalysts. So far, almost all studies on the alkaline HER mechanism are carried out using single/poly crystalline Pt as a model catalyst. However, it has been found that some of the mechanistic understandings drawn from the bulk model catalysts are inapplicable to highly active nanostructured catalysts in practice. Therefore, an alternative alkaline HER reaction mechanism for nanostructured catalysts was proposed based on the finding of a new reactive intermediate that has not been reported before on the model catalysts. In situ-Raman and a series of electrochemical characterizations were carried out to confirm that in a high-pH environment, a large amount of hydronium ions (H3O+) are generated on the surface of nanostructured catalysts during the HER process. The H3O+ forms an acid-like environment on the surface of the catalysts which improve the overall activity by reducing the activation energy of the reaction. Such phenomenon distinct on nanostructured catalysts provides a comprehensive explanation to the observed differences in catalytic behavior between bulk and nanostructured catalysts. In the second part of the thesis, the origin of the high activity on nanostructured catalysts are studied from the perspective of the material. Firstly, a catalyst design strategy is proposed to break the activity limitation or ‘volcano plot’. A volcano plot is a relationship between the catalytic performance of a series of catalysts and a certain descriptor, which in the current case is the hydrogen adsorption ability. By investigating a group of typical nanostructured catalysts, a volcano plot was built up for Pt-based bimetallic materials, and some unique dealloyed samples were found to go beyond the limitation of the volcano plot and represent much higher activity than theoretical predictions. Thermodynamic and kinetic characterizations indicate that the reason for the unusual performance of these samples is that dealloying can selectively optimize the H and OH binding energy on Pt sites, promoting the overall activity. Secondly, well-defined RuPt alloy and core-shell (Ru@Pt) nanoparticles were studied to demonstrate the contribution of the electronic and geometric effects of a nanostructured catalyst towards alkaline HER performance. The two groups of nanoparticles have similar electronic structures but differing surface strain. A comparison between the catalytic activity of the two materials indicated that the strain effect has a dominant role in determining the intrinsic alkaline HER activity of the catalyst. In particular, the compressive surface strain of Ru@Pt nanoparticles provided the catalyst with weakened hydrogen binding and improved interaction with hydroxyl species, leading to enhanced apparent activity.