The world has a potential global energy crisis that is expected to erupt before 2050. To combat these issues, there needs to be a development of alternative, solar-based energy technologies for the reduction of CO2 and hydrogen evolution. The key to this problem lies in understanding the energy conversion and storage technologies through the reduction of CO2 or hydrogen evolution. These processes only becomes practical if reaction efficiency is improved and production costs are minimized. To increase the efficiency of these reactions, catalysts are often used to control efficiency and selectivity to various products for these reactions. These catalysts are often used in a process of photoelectrocatalysis, where selectivity to various products can be controlled using potential bias as well as UV-visible light to overcome thermodynamic and kinetic barriers. Metal oxides are cost-effective catalysts for these reactions; however, they suffer from issues with activity and stability in CO2 reduction conditions. The addition of a second metal to a metal oxide has been used to tune structural and electronic properties in the material toward more efficient photoelectrolysis. Delafossite metal oxides containing two metals tend to be promising photocathodes for these reactions because of their greater stability, low-cost, and small band-gaps needed to drive hydrogen evolution and CO2 reduction. However, the activity needed for practical application of these materials is still lacking. Additionally, bulk and surface defects can complicate studies on the photoactivity of these materials. Therefore, understanding the role of these lattice defects and their impact on catalysis is critical to improving design of photoelectrodes.
CuFeO2 is a delafossite material that primarily exhibits three main defect types: O atom interstitials, Cu vacancies, and the formation of heterojunctions with CuO. However, recent research is unclear which defect, if any, is responsible for the enhancement of photoactivity for CO2 reduction and hydrogen evolution. Toward a thorough investigation of these defects, various Cu-Fe mixed oxide samples were made by electrodeposition and annealed under ambient conditions. Catalysts of Cu: Fe compositions deposition solution metal ratios (1:1, 1:3, 1:6) were used along with different annealing temperatures (unannealed, 350, 450, 550, and 650 0C) in air to correlate phases present with possible bulk defects in the material. We demonstrate that our catalyst is Fe-rich for all deposition conditions even for a 1:1 Cu: Fe deposition ratio. By increasing Fe concentration in the deposition solution, hematite and the CuFe2O4 phase form. However, even under these deposition conditions, CuO is present in the Cu-Fe oxide mixed catalyst. While the identity of phases present does not change as a function of annealing temperature for the 1:3 Cu: Fe metal ratio (except for the 650 0C air annealed sample), the weight % of CuO to CuFeO2 does. Because of metal stoichiometry determined by ICP results, it can be concluded that the presence of CuO indicates the presence of bulk lattice defects such as O interstials and Cu vacancies in the material. These results are then correlated with photoelectrochemical characterization, showing the presence of CuFeO2 and CuO in the 550 0C air sample was the most photoactive. This photoactivity can therefore be correlated with the presence of these bulk defects in CuFeO2.
CO2 reduction to energy-dense products is inherently difficult. This is due to a number of factors including side reactions and kinetic barriers. However, we have developed a catalyst capable of producing a C2 product. We demonstrate the ability to convert CO2 to acetate with 80% Faradaic efficiency, using a mixed Cu-Fe oxide catalyst at −0.4 V bias vs Ag/AgCl during visible light illumination. Analysis shows that the selective catalyst is a mixed-phase material consisting of CuFeO2 and CuO. By varying the Cu:Fe atomic fraction from 0.6 to 10 as determined by ICP, it is possible to tune the selectivity for CO2 reduction from primarily acetate to primarily formate. These results identify a new low-cost, Earth-abundant material capable of synthesizing an energy-dense liquid directly from CO2 and show that selectivity for CO2 reduction can be tuned by controlling catalyst composition in mixed Cu-Fe oxide catalysts.
To better understand the complex defect chemistry and role of impurity phases in this material and their effect on the photochemical performance, visible light transient absorption spectroscopy and DFT calculations were employed to investigate the electron dynamics in electrochemically deposited Cu-Fe oxide thin films annealed in air (350, 450, 550, 650 0C). Kinetic analysis of carrier lifetime shows a fast, sub-ps contribution to relaxation followed by persistence of a long-lived state to time delays greater than 2 ns. Increasing amplitude of the long-lived state is shown to correlate with the rate of fast initial relaxation, and this is explained in terms of a competition between charge carrier trapping and charge separation. Charge separation in CuFeO2 occurs via hole thermalization from O 2p to Cu 3d valence band states leading to segregation of electrons and holes across layers in the CuFeO2 lattice. Correlation between transient absorption measurements and DFT calculations suggest that Cu vacancies enhance photochemical performance by facilitating charge separation kinetics. In contrast, O interstitials are predicted to switch the relative positions of O 2p and Cu 3d valence band states, which would inhibit charge separation by inter-band hole thermalization. Finally, the results show no evidence for electron injection from CuFeO2 to CuO suggesting that charge separation at this heterostructure interface does not play a role in the carrier lifetime or photochemical performance of the catalysts studied in this work.