Characterizing CO2 Reduction Mechanism Using Advanced Synchrotron-Based X-Ray Spectroscopy Techniques
Authors: Farmand Maryam , Drisdell Walter , Wu Chenghao , Yano Junko , Feaster Jeremy T , Jaramillo Thomas F , Mehta Apurva , Davis Ryan , Favaro Marco , Resasco Joaquin , Bell Alexis T. , Guo Jinghua , Prendergast David , Goddard William A 
Published: February 2020

Generation of solar fuels through photo-catalytic reduction of CO2 could simultaneously address both the need for sustainable and affordable energy sources and the necessity of reducing global CO2 footprint. Discovering and designing new catalysts that have both high activity and selectivity for CO2 reduction is the critical barrier to overcome in order for solar fuel generation processes to commercialize. So far, copper is the sole material known to efficiently catalyze conversion of CO2 to considerable amounts of C2 hydrocarbon products over time. However, CO2 reduction on Copper exhibits a high overpotential plus the added disadvantage of producing a mixture of several products including hydrogen.1,2 Thus, it is of vital importance to understand the so far ambiguous and poorly understood catalytic CO2 reduction mechanism on copper. In this work, a combination of various in-situ X-ray spectroscopy techniques are utilized to probe the electrode/electrolyte interface and provide insight into the reduction reaction.X-ray absorption spectroscopy (XAS) provides an element-specific probe of the conduction band via a core-level excitation into unoccupied electronic states and can reveal both oxidation state and details of the electronic structure.

Operando Grazing incidence XAS at beamline 11-2 of SSRL is utilized to study the electrode surface at the metal K- and L-edges. A specially designed 3D printed flow cell maintainsa 300 micron liquid layer above the catalyst surface, enabling XAS characterization of the catalyst surface during electrochemistry in grazing incidence mode. The grazing incidence geometry provides a probe depth of about 5 nm into the catalyst, hence providing a highly surface sensitive spectroscopic tool. Preliminary measurements were conducted on the relatively inert surface of AuPd alloy catalyst. This is the first step in determining the feasibility of the study before examining the more complicated copper system. Distinct reversible shifts are observed in spectroscopic features with applied potential, which are believed to be due to H+ intercalation into Pd phase. (Figure 1)
Further, operando soft X-ray spectroscopy measurements at C-edge are conducted at beamline 8.0.1 of ALS to detect intermediate carbon species. Electron yield soft XAS data are collected at a depth of 1 nm above the electrode surface in a specially designed in-situ electrochemical flow cell3. Strong 1s to π* transition in C edge spectra provides fingerprint for different products. First principles DFT calculations are conducted to characterize features obtained through these XAS measurements. (Figure 2)
Combination of these spectroscopic techniques with electrochemical and theoretical calculations will paint a comprehensive picture of the exact CO2RR mechanism on Cu surface, which then can be used to design next generation photo-catalysts.

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Figure 1