Synthetic Optimization and Electrochemical Studies of Delafossite CuCrO2 for Solar Energy Conversion and Storage Applications

Abstract

Metal oxides have been of great importance to the development of new energy conversion and storage technologies including heterojunction solar cells and Li-ion batteries. P-type delafossite CuCrO2 is a metal oxide which is of interest for such applications due to its wide band gap and relatively high valence band edge. However, defects in p-type materials result in poor performance for solar cell devices compared to alternative metal oxides due to inferior charge separation at the metal oxide interface as well as possible increases in electron hole recombination. On the other hand, reports have been made on defect-induced CuCrO2 having improved performance for ion battery application due to increased hole density, higher conductivity, and faster charge extraction which can improve ion intercalation. Researching the fundamental electrochemical properties of CuCrO2 and the effects defects can have on those properties can aid in understanding the impact of surface states as recombination centers and redox capabilities, and how solar cell and battery applications can be impacted. Electrochemical impedance spectroscopy (EIS) is one of the most utilized methods to characterize these electrodes in the context of energy applications. The utility of EIS stems from its ability to differentiate multiple interfaces (i.e. solid/electrolyte, solid/solid) within devices based on their frequency response to a modulated potential and the subsequent decoupling of resistive and capacitive circuit components. In Chapter 2, examples are covered from the literature where EIS has been particularly important in the understanding of electronic properties related to metal oxide electrodes within energy storage devices, specifically ion batteries. Previous CuCrO2 studies used a hydrothermal synthetic route which commonly leads to byproduct formation, making electrochemical characterization challenging. An important aspect of this work is the consideration of Cr3+ as the reductant used to reduce Cu2+ to Cu+, discussed in Chapter 3. This was confirmed by detection and quantification of CrO42- as a product of hydrothermal synthesis in addition to the fact that CuCrO2 purity was maximized at a ratio of 4:3 Cr:Cu, consistent with the proposed stoichiometric reaction: 4Cr3+ + 3Cu2+ + 20 OH- → 3CuCrO2 + CrO42- + 10 H2O. Using a 4:3 ratio of Cr:Cu starting materials and allowing the synthesis to proceed for 60 hours eliminates the presence of CuO beyond detection by powder X-ray diffraction (pXRD). Furthermore, washing the solid product in 0.5 M NH4OH removes Cu2O and Cr2O3 impurities, leaving behind the isolated CuCrO2 product as confirmed using powder X-ray diffraction and inductively coupled plasma mass spectrometry (ICP-MS). In Chapter 4, CuCrO2 was used to fabricate mesoporous thin films to study its electrochemical properties, where a strong Li+ dependence was observed. A shift in Cu2+/+ redox E1/2 was observed as [Li+] in the electrolyte was increased from 0.03 V to 0.14 V vs Fc+, in addition to the growth of a new redox feature at E1/2 = -0.43 V vs Fc+/0, attributed to Li+ occupation in Cu+ vacancy sites and the ability for Li+ to affect the Helmholtz layer due to increased surface charge. A 4% Cu+ deficiency was determined by ICP-MS. Based on chronoamperometry experiments in which a series of electrolyte solutions were used, the growth in surface charge as [Li+] in electrolyte increased fit to a Langmuir binding isotherm where an equilibrium constant K was determined to be 0.057 M-1 and a maximum surface charge of 15.5 mC. An increase in the surface charge from chronoamperometry as well as current from cyclic voltammetry as [Li+] increases is also observed. Comparing these results to those of CuGaO2 films in which the aspect ratio is larger and the particles preferentially stack in which terminal redox active Cu–O is less exposed compared to those in CuCrO2, it is conclusive that the morphology of delafossite particles plays an important role in electrochemical performance. Chronopotentiometry experiments on CuCrO2 reveal a 7.8 mA h g-1 charge capacity at cycle 2, but with a % cycling efficiency from 83% to 91% over 10 cycles. This and multiple cycling cyclic voltammetry (CV) experiments reveal the degradative behavior of the film, which was found to be related to the loss of Li-coupled redox activity. Nonetheless, the significant increase in surface charge and capacity with increased Li+ can be used to further our understanding of surface defects and their effect on hole recombination in solar cell devices, as well as how Li+ occupies surface sites for electrochemical energy storage application. Finally, Chapter 5 provides insight into future research directions based on conclusions drawn from previous chapters. An emphasis is made on preliminary results in which CuCrO2 powder is washed in a pH 1 HCl solution to study the resulting electrochemical properties. Acid washing CuCrO2 films for 12 – 336 hours was shown to lead to consistent increases in both surface charge and capacities compared to their base washed counterpart, with films made from CuCrO2 washed for 96 hours lead to overall optimal electrochemical performance. These results are related to possible Cu+ induced vacancies and morphological changes in the particles. Other interesting topics in Chapter 5 include but are not limited to effects of varying the cation source in electrolyte solution, morphological changes, and doping, on the resulting electrochemical properties and stability of CuCrO2.