Understanding the pseudocapacitive response of transition metal oxide and carbide electrodes
Open Access
- Author:
- Keilbart, Nathan Daniel
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 05, 2019
- Committee Members:
- Ismaila Dabo, Dissertation Advisor/Co-Advisor
Clive A Randall, Committee Chair/Co-Chair
Susan B Sinnott, Committee Member
Ramakrishnan Rajagopalan, Committee Member
Kwadwo Asare Osseo-Asare, Outside Member - Keywords:
- Pseudocapacitor
DFT
Monte Carlo
First-principles
Energy Storage - Abstract:
- The production of renewable energy is an outstanding societal and environmental challenge. One option to help achieve energy sustainability is to interconvert electrical energy into chemical energy using electrochemical cells. At present, one of the predominant electrochemical energy-storage device is the lithium-ion battery whose commercial production began in 1991. Although lithium-ion batteries possess large energy densities, they are intrinsically limited in their ability to deliver high electrical power because of the slow kinetics of lithium intercalation. An alternative electrochemical energy storage device with high power density is the supercapacitor which exploits two separate mechanisms for storing energy: double-layer interfacial charge accumulation and pseudocapacitive ion adsorption. Pseudocapacitive materials are characterized by fast and reversible redox reactions that occur at or near the surface enabling the storage of electrical energy at high rates. Although there has been considerable advances in the conceptual understanding of pseudocapacitive materials, a computational framework for predicting their charge storage capabilities had not been established. The aim of this work is to construct a methodological framework for predicting the energy storage capabilities of diverse pseudocapacitive electrode materials. Electronic structure calculations based on density-functional theory (DFT) are used in combination with a self-consistent continuum solvation (SCCS) model to extract and build a dataset of material properties under realistic conditions. This dataset is incorporated into grand canonical Markov Chain Monte Carlo sampling to predict the adsorption isotherms and ultimately the charge-voltage response. To evaluate the performance of the newly proposed model in predicting the charge storage capabilities of pseudocapacitive electrodes, the prototypical material ruthenia (RuO$_2$) is chosen as a model system for preliminary testing. Initial results of the (110) orientation indicate that the quantum-continuum model contributes an additional stabilizing force to the adsorbing proton at the surface. Calculations that do not include the influence of electrical polarization revealed a linear response of the Fermi energy through low coverage and the predicted intrinsic double-layer capacitances are in qualitative agreement with experiment. The simulated adsorption isotherms are extracted and converted into charge-voltage curves which exhibit an intrinsic pseudocapacitive trend with no modification of the calculated dataset. The charge-voltage response is dependent on the individual surface facets as well as the chemical environment of the subsurface layer. These simulations reveal that small changes in the double-layer capacitance at the electrode--electrolyte interface can induce drastic alterations to pseudocapacitive response. Additionally, although the double-layer capacitance represents a small fraction of the stored charge, it controls to a large extent the pseudocapacitive response. Although ruthenia exhibits superior pseudocapacitive charge-storage capabilities, the intrinsic cost and toxicity make it an undesirable choice for industrial scale applications. The recently discovered MXene compounds overcome these barriers while being relatively cheap and exhibit promising energy storage capabilities. An initial study of the intercalation pseudocapacitance focuses on the formation energies and open-circuit voltages where the possible combinations of the transition metal element, layer thickness, and surface termination are enumerated and computed. A thorough analysis of these compounds establishes that the group IV transition metal elements are the most stable compounds and have the lowest open-circuit voltages. Further studies into the surface redox pseudocapacitance of the Ti$_3$C$_2$O$_2$ MXene structure explores several adsorption schemes while providing a critical assessment of the electrochemical environmental contributions by applying a shift to the chemical potential of the adsorbing species. For proton adsorption, the charge--voltage response exhibits a pseudocapacitive trend under varying conditions but is maximized at higher chemical potential shifts. Lithium demonstrates a transition from a battery-like to capacitor-like response when shifted to higher chemical potentials. Additionally, the coadsorbtion of hydrogen and lithium creates a stabilizing force which recovers a pseudocapacitive response over a small voltage window with no shifts to the chemical potential. The results of this work provide a computational methodology for predicting the pseudocapacitive response of transition metal oxide and carbide electrodes. This methodology can be further extended to optimize the pseudocapacitive response in known materials and to predict future pseudocapacitive materials allowing for the accurate simulation of the charge-voltage capabilities.