Using surface science to understand interfaces for solar fuel devices
The development of a technology that uses water, sunlight, and CO2 to efficiently generate liquid fuel (artificial photosynthesis) is one of the most important goals of the science and engineering community today. The global population, whose energy consumption increases dramatically every year, relies heavily on stored liquid fuel for transportation, heating, and electricity generation. Solar-driven liquid fuel production is considered a sustainable, viable near- and mid-term solution for the world's energy problems because it permits energy storage compatible with existing infrastructure. However, the high stability of CO2 means that developing this technology will require strong interdisciplinary basic science efforts. Specifically, there are important kinetic and thermodynamic obstacles for its chemical transformation that are not understood on the atomic and molecular levels.
Once a CO2 molecule is chemically activated, the complexity of carbon chemistry takes over and often yields a plethora of branching reaction pathways that compete for the available energy in the system. For example, in a recent analysis of the electrochemical reduction of CO2 using copper metal as the active electrode, Jaramillo and coworkers identified 16 different reaction products in electrochemical operating conditions.1 Although promising results have been achieved with more optimized catalysts, most practical materials give low product selectivity.
In natural photosynthesis, multiple electrons and protons are transferred in a concerted matter (a process called proton-coupled electron transfer). For the reaction to yield the desired liquid products and use only small energy inputs (as can be provided by solar energy), electrochemical CO2 reduction must be designed to mimic this characteristic. Therefore, we use molecular catalysts that optimize both the rate and selectivity of the relevant reduction reactions to improve efficiency. Heterogeneous processes, relying at least in part on the reaction of surface-bound species, are technologically promising for the catalytic reactions required for fuel generation.
Surface science is ideally suited to uncover mechanistic details of heterogeneous catalytic reactions relevant to solar fuel generation. We are conducting a series of surface science experiments to provide experimental data that will generate mechanistic information for catalyzed CO2 reduction reactions. Figure 1 shows a schematic describing our basic approach for investigating these reactions. We focus on the role of electrode surfaces, especially photoactive gallium phosphide (GaP), in N-heterocycle-catalyzed CO2 reduction, which has been shown to selectively produce methanol and formic acid in a number of recent investigations.2–8
Surface-sensitive spectroscopies and microscopies can describe the composition, structure, and energetics of species at the active interfaces. We present results describing the composition and thermal stabilities of the surface-bound layer on GaP(110) upon interaction with water.9 Using synchrotron-based ambient pressure photoelectron spectroscopy, we established experimentally over 10 orders of magnitude of pressure that the interaction of water with GaP(110) induces the formation of gallium hydroxide, Ga-H2O, and phosphorus hydride (P-H) species. Figure 2 provides representative results, including a high-resolution x-ray photoelectron spectroscopy measurement of the GaP(110) P 2p level in ultrahigh vacuum, as well as an ambient pressure photoelectron spectroscopy measurement of the O 1s level at 0.9 torr H2O. The formation of the negatively charged hydride on surface phosphorus atoms is notable, given the important role of hydride transfer in CO2 reduction reactions. We hypothesize that the high stability of the surface P-H species contributes to the high selectivity for methanol production observed for this system,2 since it should be associated with the material's high overpotential for the competing hydrogen evolution reaction.9
Scanning tunneling microscopy is used to probe the preferential adsorption sites and bonding interactions of reactants and catalysts with the surface of GaP(110). Specifically, we consider the interaction of pyridine with surface gallium sites, and discuss the implications of this bonding on the electrocatalysis of this system: see Figure 2(c). We find evidence supporting the claim that adsorbed pyridine is susceptible to nucleophilic attack, which may be an important characteristic for catalysts serving to shuttle hydrides to CO2.10 In addition, by performing measurements at 5K, we also are able to stabilize and establish with atomic resolution the preferred adsorption sites of weakly physisorbed CO2 on this surface.
The adsorption states of reacting species influence the energetics of the system and therefore the most favorable reaction pathway. Thus, with the baseline measurements described here, we can begin to build a molecular-level understanding of the reaction mechanisms that are relevant to heterogeneous catalysis for solar-driven CO2 reduction. Ultimately our goal is to understand the role of surfaces in these reactions. This will improve our mechanistic understanding of the processes governing selectivity, which will drive forward the technology needed to synthesize storable fuels using solar energy.
This work is supported by the US Department of Energy Office of Science, Office of Basic Energy Sciences, under award number DE-SC0012455.
Princeton University
Coleman Kronawitter is a postdoctoral research associate. His research concerns the study of materials, surfaces, interfaces, and molecular catalysts for energy applications. He particularly focuses on efficiency in processes for solar energy conversion.
Bruce Koel is a professor of Chemical and Biological Engineering at Princeton University, a member of the associated faculty of the Department of Chemistry, Department of Mechanical and Aerospace Engineering, the Princeton Institute for the Science and Technology of Materials, and a National Spherical Torus Experiment collaborator at Princeton Plasma Physics Laboratory. His research addresses surface chemistry and interfacial phenomena in photoelectrocatalysis, heterogeneous catalysis, plasma-surface interactions, batteries, and nanomaterials. He has published 275 peer-reviewed journal articles and 11 book chapters.
