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Title:Interrogating electrocatalytic mechanisms and developing nano-porous catalysts for energy conversion reactions: I. Oxygen evolution reaction, and II. Carbon dioxide reduction reaction
Author(s):Hoang, Thao Thi Huong
Director of Research:Gewirth, Andrew A.
Doctoral Committee Chair(s):Gewirth, Andrew A.
Doctoral Committee Member(s):Rodríguez-López, Joaquín; Nuzzo, Ralph G.; Kenis, Paul J. A.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Oxygen Evolution Reaction
Carbon Diioxide Reduction Reaction
Abstract:The objectives of my thesis were to interrogate electrocatalytic mechanisms and develop new nano-porous catalysts for energy conversion reactions including the oxygen evolution reaction (OER) and carbon dioxide reduction reaction (CRR). First, I examined the oxygen evolution reaction in basic electrolytes using in situ electrochemical surface stress measurements. Second, I developed a new electrolyte additive-controlled electrodeposition method for the preparation of porous films of Ni and NiFe catalysts with high OER activity. Third, I exploited the additive-controlled electrodeposition method to synthesize Cu and CuAg films with high surface area and tunable morphology for high activity and selectivity of CRR to ethylene. In Chapter 1, I provide background information to the electrochemical energy conversion reaction and lay out the challenges and potential approaches at present in the field. In Chapter 2, I describe our effort to determine the relationship between changes in the OER catalyst surface and activity. In situ electrochemical surface stress measurements were utilized to interrogate oxide formation before and during OER on several common catalysts, including Ir, Ni, Co, Au, and Pt. The stress measurements report directly on changes in oxidation state and phase of the electrode material as the potential is varied. Hysteresis observed in the potential-dependent stress with Ir, Au and Pt electrodes is associated with irreversible composition and roughness changes in the electrode. The stress data also quantitatively reports on the in-plane change in strain developing in bonding during oxide oxidation. The magnitude of the surface stress is nearly identical to that the predicted from bond strains obtained from reported XAS data. Interestingly, there is a rough linear relationship between the change in stress and the amount of oxide formed. More importantly, the stress data shows that metals with higher activity exhibit larger stress and more oxide formation. The origin of this relationship could be explained by differences in conductivity and porosity of different oxides. In Chapter 3, I focus on developing a stable and effective OER catalyst using an additive-controlled electrodeposition. We find that 3,5-diamino-1,2,4-triazole (DAT) acts as a deposition inhibitor that dramatically changes Ni morphology resulting in black Ni films, a phenomenon indicative of small particle formation. Ni films electrodeposited with DAT (NiDAT) exhibit much higher active surface areas with fractal-like behavior. Correspondingly, NiDAT films show a much larger oxidation wave and higher OER rates compared to the Ni film deposited without the DAT additive. Co-electrodeposition of Ni and Fe in the presence of DAT (NiFeDAT) is also explored as Fe is known to increase the OER activity from Ni films. NiFeDAT films are very active toward OER exhibiting 100 mA/cm2 with high stability > 72 hours at 1.50V vs RHE in 1 M NaOH. These metrics make NiFeDAT among the most active OER electrocatalyst reported to date. Equally important, the high activity can be tuned to nearly any arbitrary value by altering the amount of NiFe electrodeposited without film degradation. In Chapter 4, I present electrochemical measurements that examine the effect of deuteration on the OER with Ni and Co catalysts, and an effort to identify the rate-determining step (RDS) of these intricate electrocatalytic reactions involving multiple proton-coupled electron transfer (PCET) processes. The OER Tafel slope and kinetic isotope effect (KIE) calculated from electrochemical data shows that both Ni and Co exhibits inverse secondary KIE, which is never observed before in an electrochemical experiment. These results contribute to a more complete understanding of the OER mechanism and allow for the future development of improved nonprecious-metal catalysts. In Chapter 5, I discuss exploiting the additive-controlled electrodeposition method to synthesize Cu films with high surface area and tunable morphology for high activity and selectivity of CRR to ethylene. Electrodeposition of Cu films from plating baths containing DAT (CuDAT) as an inhibitor exhibit high surface area and high CO2 reduction activities. By changing pH and deposition current density, the morphologies of the Cu films are varied to exhibit wires, dots, or amorphous structures. Among these Cu films, the CuDAT-wire samples exhibit the best CO2 reduction activity with a Faradaic efficiency (FE) of the C2H4 product formation reaching 41% at -0.47 V vs. RHE, a FE for C2H5OH formation reaching 22% at -0.55 V vs. RHE, and a mass activity for CO2 reduction at -0.65V vs. RHE of ~ 720 A/g. In Chapter 6, I present our strategy to enhance C2 production from CO2 electroreduction by doping low Ag contents (<10%) into Cu-wire film. The CuAg-wire catalyst with nanoporous structure and homogenous mixed of Cu and Ag atoms was fabricated by additive-controlled electrodeposition method using DAT. The CuAg-wire catalyst exhibits large active surface and high selectivity of CO2 reduction to C2H4 (~60% Faradaic efficiency - FE) and C2H5OH (~25% FE) at relatively low overpotential (~ -0.7V vs RHE).
Issue Date:2016-11-30
Rights Information:Copyright 2016 Thao Hoang
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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