Surface - Gated Charge Carrier - Selective Nanocontacts in Photoelectrochemical Catalysis
Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
Experimental Condensed Matter Physics
Synthesis and Properties of Functional Materials
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
Photoelectrochemical (PEC) water splitting is a direct and efficient route to store the sunlight’s energy into hydrogen and oxygen. However, so far, all PEC devices lack long-term stability in the electrolyte. In the first half of my DFG-postdoctoral fellowship at the University of Oregon, I investigated the use of nanoscale point contacts in PEC devices. Such contacts are predicted to generate large photovoltages, and hence high performance, due to the surface-gate effect; the semiconductor’s properties can be tuned via a nearby surface layer to improve the efficient charge carrier extraction. Together with a graduate student of the Boettcher group, Forrest Laskowski (now postdoc at Caltech), I studied the surface-gate effect for electrodeposited nickel nanocontacts on n-type silicon. Previously, unexpectedly-high open-circuit voltages have been observed for such contacts, but no consistent mechanistic picture emerged. Here, we used electrochemical atomic force microscopy to study the nanocontacts under electrochemically-relevant conditions. With the help of high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) we were able to show that such contacts indeed exhibit surface-gating; initially, after deposition of the nickel contacts, no increase in the local photovoltage was observed. Only after electrochemical “activation”, i.e. the growth of a catalytically-active NiOOH surface layer, did we observe increasing open-circuit voltages with decreasing contact size. Additional simulations further support a picture where the high work function NiOOH gates the underlying silicon from the contact edges and thereby increases the contact’s performance. These results have been published in Nature Materials. In the second step, I wanted to develop chemically-robust interfaces based on protected nanoscale point contacts. To that end, I explored selective-area atomic layer deposition; I grew metal oxide films on silicon, but not on the metal contact surface, which I covered beforehand with stable ligands. These interfaces showed indeed slightly improved stability, but eventually led to severe degradation and performance loss. Closer inspection showed that the metal was peeling off, starting from the edges. Apparently, the electrolyte can enter microscopic gaps between the protective metal oxide and the metal point contact. This stability impediment forced me to pivot into a new direction for the remainder of my DFG fellowship. Due to my previous work on charge carrier-selective contacts in photovoltaics and photoelectromistry, I got interested in the role of ion-selective membranes in electrochemistry. These membranes are conventionally seen as an efficient way to conduct ions between two electrode reactions. However, in my view, this picture overlooked another important role; ion-selective membranes are essentially an “ionic wire” which can separate ionic processes that usually occur before, during or after complex electrode reactions. The spatial separation allows studying these ionic reactions in isolation and under controlled conditions. I used this approach to study the ionic process of water dissociation (H2O -> H+ + OH-). This reaction can be isolated between two ion-selective layers - one that conducts only OH- and one that only conducts H+ - in what is called a bipolar membrane. Using this approach, I discovered pH- dependent water dissociation activity of metal-oxide catalysts added between the two ion-selective layers. In the next step I confirmed my hypothesis about the role of ion-selective membranes. For example, during the hydrogen evolution reaction in base, H+ need to be extracted from the H2O, because their free concentration in base is very low. When I placed these electrocatalysts into the bipolar membrane, their water dissociation activity directly correlated with their activity as electrocatalysts. These discoveries led me to realize record-breaking bipolar membrane devices that can dissociate water very efficiently. These results have been published in Science. Follow-up work on the water transport through cationic-thin membrane layers has further led to the highest reported current densities for a bipolar membrane device. This work was published in ACS Energy Letters. In hindsight, the stability issue of the nanoscale point contacts turned out to be very fruitful. It forced me to pivot into a new direction, develop new research methods and discover truly new science. This will form the basis for my future research career on ionic interface processes.
Publications
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Nanoscale semiconductor-catalyst interfaces in photoelectrochemistry, Nature Materials 19, 69–76 (2019)
F.A.L. Laskowski, S.Z. Oener, M.R. Nellist, A.M. Gordon, D.C. Bain, J.L. Fehrs and S.W. Boettcher
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Accelerating Water Dissociation in Bipolar Membranes and Electrocatalysis, Science 369, 6507, 1099-1103 (2020)
S.Z. Oener, M. Foster, S.W. Boettcher
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Integrated Reference Electrodes in Anion-Exchange-Membrane Electrolyzers: Impact of Stainless-Steel Gas-Diffusion Layers and Internal Mechanical Pressure, ACS Energy Letters 6, 305-312, 1-8 (2021)
Q. Xu, S.Z. Oener, G.A. Lindquist, H. Jiang, C. Li, S.W. Boettcher
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Potentially Confusing: Potentials in Electrochemistry, ACS Energy Letters 6, 261-266 (2021)
S.W. Boettcher, S.Z. Oener, M.C. Lonergan, Y. Surendranath, S. Ardo, C. Brozek, P.A. Kempler
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Thin Cation-Exchange Layers Enable High-Current-Density Bipolar Membrane Electrolyzers via Improved Water Transport, ACS Energy Letters 6, 1, 1-8 (2021)
S.Z. Oener, L.P. Twight, G.A. Lindquist, S.W. Boettcher
