Isotope-Labeling Studies in Electrocatalysis for Renewable Energy Conversion

Abstract

This thesis presents methods and results for isotope-labeling studies in oxygen evolution reaction (OER) electrocatalysis. The OER is an essential reaction for a transition to a fossil-fuel-free society. The OER is the main source of e ciency loss in the production of hydrogen by water electrolysis. Hydrogen from water electrolysis, in turn, is key for storing wind and solar energy and for using wind and solar electricity to decarbonize other sectors such as industry and transport. The rst chapter of this Thesis puts this technological motivation in the context of the urgent need to mitigate climate change. The second chapter describes and demonstrates the tools used in the isotope-labeling electrocatalysis studies. The primary tool is electrochemistry-mass spectrometry (ECMS). The version of EC-MS used in this Thesis involves a silicon microchip to make the interface between the high vacuum of the mass spectrometer and the wet ambient environment of the electrochemistry experiment. The advantages of this technique, chip EC-MS, are high sensitivity, well-characterized mass transport, and the ability to dose reactant gases. Isotope labeling studies are introduced with two examples. The rst is an attempt to directly measure the hydrogen evolution exchange current density on platinum by electrochemical H-D exchange, which is however demonstrated to be mass-transport limited. The second is a set of CO stripping and CO oxidation experiments in labeled electrolyte (H218O), which lead to a new way to probe the kinetics of the reaction of CO2 and H2O to form carbonic acid. The third chapter is devoted to oxygen evolution electrocatalysis. The two main water electrolyzers, alkaline electrolyzer cells (AEC) and polymer electrolyte membrane electrolyzer cels (PEMEC), are brie y discussed in the context of the OER catalysts required. Then, the importance of measuring O2 is demonstrated with two examples in which the electrochemical current would overestimate the OER activity. This motivates the study with EC-MS of oxygen evolution on RuO2, one of the only materials (together with IrO2) that can catalyze the OER in the acidic environment of a PEMEC . Using isotope-labeled electrolyte to increase sensitivity, I measured the O2 produced by a series of RuO2 lms and Ru foams down to a record low 1.29 V vs RHE. All of these samples follow approximately the same trend of turn-over-frequency (TOF) vs potential with a very strong potential dependence at low overpotentials. The involvement of lattice oxygen in the oxygen evolution mechanism has received a lot of research attention in recent years. This is investigated by preparing an OER catalyst with one isotope of oxygen (16O or 18O) and measuring the isotopic composition of the O2 evolved in an electrolyte with a di erent isotopic composition than the catalyst. I present a comprehensive comparison of these studies, with views on the advantages and disadvantages of the methods employed. Using RuO2 and IrO2 samples as examples, and coupling the high sensitivity of chip EC-MS with dissolution measurements by inductively coupled plasma - mass spectrometry (ICP-MS) and surface isotopic characterization by ion scattering spectroscopy (ISS), I show that lattice oxygen evolution does not necessarily mean lattice oxygen exchange. In other words, an isotope signal in the oxygen evolved from a labeled OER catalyst does not necessarily imply that lattice oxygen plays an important catalytic role. Non-catalytic evolution of lattice oxygen is demonstrated to be the case for sputter-deposited Ru18O2. In the last experiments presented in this thesis, CO oxidation is used as a probe for lattice oxygen reactivity. Under the right conditions, isotope-labeled oxygen from the catalyst is incorporated in the CO2 produced. These experiments can also be used as an in-situ proof that there is labeled oxygen at the surface of the electrocatalyst, for example after a negative result for lattice oxygen evolution in OER. The nal chapter ties the studies presented in this Thesis back to the motivation by estimating the amount of CO2 emissions avoided by a marginal improvement in electrolyzer e ciency. Using a simple model and literature-based assumptions about the future European energy system, I nd that to achieve a one-year payback time on the CO2 costs of my PhD project by 2030 only requires that the results present here lead to an 0.03 mV improvement in the OER overpotential of electrolysis cells. The Chapters of this thesis present a mix of published and as-of-yet unpublished results, and only a subset of the work done during my PhD project. The articles to which work from my PhD project have contributed are attached.