Electrochemical characterization of few-atom heterogeneous catalysts: And other electrochemical investigations

The transition from a society dependent on fossil oil, gas and coal to a sustainable one, relies, among other things, on the ability to convert and store energy for long periods of time. Electrolysis can be used to turn electricity into storable chemicals with high specific energy, such as hydrogen, ammonia, hydrocarbons and more. Fuel cells can turn these chemicals back to electricity. These chemicals can also replace oil as chemical feedstock in a variety of Industries. Many of the losses in the conversion between these chemicals and electricity stem from theoretically inferior catalysts.

This thesis investigates electrocatalysts that are relevant for energy conversion technologies in the following reactions: The hydrogen evolution reaction (HER), the hydrogen oxidation reaction (HOR), the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR), the CO₂ reduction reaction (CO₂RR) and the CO reduction reaction (CORR). The first chapter of the thesis motivates these reactions and the technologies behind, in a societal perspective. The second chapter goes through fundamental theory in electrocatalysis and surface physics, and the third chapter discusses experimental methods and considerations, when measuring electrocatalysts with the techniques used in this project.

Current distribution in ECMS: Chapter four marks the first original project. This is a theoretical project, concerned with accurately evaluating kinetic measurements made in one of the setups used in this project. The setup can measure volatile products made on the electrode surface, but falls victim to high electrolyte resistance, which results in large electrolyte potential gradients over the electrode. The results from this project can be used assess how large these gradients are. For a typical electrolyte of 0.1 M perchloric acid and a typical average current density of 0.5 mA/cm², this results in a total overpotential change of 60 mV over the electrode.

Copper nanoparticles for CORR: Chapter five is concerned with measuring copper nanoparticles for the CORR with the sizes of 2.5, 3.75, 5, 7.5 and 10 nm. It is found that larger nanoparticles generally have an earlier ethylene onset potential, but this tendency is not without exception. The earlier ethylene onset might be correlated with an increased polycrystallinity and grain boundary density, where smaller particles are more likely to single crystal.

Few-atom heterogeneous electrocatalysts: Chapter six describes properties of few-atom heterogeneous electrocatalysts such as single-, dual- and triple-atom catalysts. These are predicted to have special properties, that can take their activity and selectivity beyond what can be achieved with traditional catalysts. This chapter is parted in four subprojects where the first subproject is a review of dual- and triple-atom catalysts, the second describes measurements done on a specific molecular organic framework with a palladium trimer, and the last two subprojects are concerned with single-atom doping of carbon and gold, respectively, for the CO₂RR. The review gives an overview of results, and highlights fallacies done when synthesizing, characterizing and measuring these catalysts. The organic framework is unstable in the electrochemical environment considered in this project, but it seems to have some activity towards HOR, which is an important result in itself. It might be better suited for reaction in milder environments. The doped carbon shows altered activity, but only towards HER. The doped gold shows activity similar to undoped gold.

Of these projects, the review on dual- and triple-atom electrocatalysts is published (article appended), and the results from the current distribution and the copper nanoparticle study are close to publication (drafts appended). Though no new commercial catalyst is found, the results aid in understanding what makes some catalysts work, and some not.