Abstract
Integrated photoelectrochemical devices rely on the synergy between components to efficiently generate sustainable fuels from sunlight. The micro- and/or nanoscale characteristics of the components and their interfaces often control critical processes of the device, such as charge-carrier generation, electron and ion transport, surface potentials, and electrocatalysis. Understanding the spatial properties and structure-property relationships of these components can provide insight into designing scalable and efficient solar fuel components and systems. These processes can be probed ex situ or in situ with nanometer-scale spatial resolution using emerging scanning-probe techniques based on atomic force microscopy (AFM). In this Perspective, we summarize recent developments of AFM-based techniques relevant to solar fuel research. We review recent progress in AFM for (1) steady-state and dynamic light-induced surface photovoltage measurements; (2) nanoelectrical conductive measurements to resolve charge-carrier heterogeneity and junction energetics; (3) operando investigations of morphological changes, as well as surface electrochemical potentials, currents, and photovoltages in liquids. Opportunities for research include: (1) control of ambient conditions for performing AFM measurements; (2) in situ visualization of corrosion and morphological evolution of electrodes; (3) operando AFM techniques to allow nanoscale mapping of local catalytic activities and photo-induced currents and potentials.
Highlights
We review recent progress in atomic force microscopy (AFM) for (1) steady-state and dynamic light-induced surface photovoltage measurements; (2) nanoelectrical conductive measurements to resolve charge-carrier heterogeneity and junction energetics; (3) operando investigations of morphological changes, as well as surface electrochemical potentials, currents, and photovoltages in liquids
The photoelectrochemical (PEC) generation of fuels from sunlight is a promising approach to the production of sustainable, energy-dense chemical fuels without the net release of CO2.1 An integrated PEC solar fuel device consists of three major components: (1) semiconducting light-absorbers that generate separated electrons and holes; (2) catalysts that facilitate multi-electron transfer fuelforming reactions; and (3) an ion-exchange membrane separating the anode and cathode compartments that contain the oxidized and reduced products, respectively
We focus on the following aspects: (1) lightillumination configurations integrated with AFM systems; (2) lightinduced Kelvin Probe Force Microscopy (KPFM) measurements of semiconductors in air to give spatially resolved measurements of surface photovoltages (SPVs); (3) spatially resolving the charge-carrier characteristics upon light excitation and junction energetics based on Conductive AFM (C-AFM) in air; (4) liquid AFM measurements of morphological changes and electrical/potential signals of semiconductors/catalysts operating in electrolytes
Summary
The photoelectrochemical (PEC) generation of fuels from sunlight is a promising approach to the production of sustainable, energy-dense chemical fuels without the net release of CO2.1 An integrated PEC solar fuel device consists of three major components: (1) semiconducting light-absorbers that generate separated electrons and holes; (2) catalysts that facilitate multi-electron transfer fuelforming reactions; and (3) an ion-exchange membrane separating the anode and cathode compartments that contain the oxidized and reduced (i.e., fuel) products, respectively. To prevent pH changes, the membrane must facilitate charge transport of protons or hydroxide ions between the two compartments. These components often operate in highly acidic or alkaline electrolytes to minimize resistive losses, making the susceptibility of each component toward corrosion a key consideration in device design. Protective coatings can be used to prevent or mitigate semiconductor corrosion and as a secondary benefit, may allow tuning of the band-edge positions relative to the solution redox potentials.. Measurements under operating conditions using C-AFM necessitate the use of an insulated nanoelectrode with only the end of the tip exposed to allow electrical contact with the surface.. Use of the peakforce SECM probe as the fourth electrode allows sensing the local electrochemical potential on the electrode surface.38,41,42 In this Perspective, we summarize recent developments of these emerging scanning-probe techniques for studying photoelectrodes in solar fuel devices. We focus on the following aspects: (1) lightillumination configurations integrated with AFM systems; (2) lightinduced KPFM measurements of semiconductors in air to give spatially resolved measurements of surface photovoltages (SPVs); (3) spatially resolving the charge-carrier characteristics upon light excitation and junction energetics based on C-AFM in air; (4) liquid AFM measurements of morphological changes and electrical/potential signals of semiconductors/catalysts operating in electrolytes
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