Materials for Electrochemical Energy Conversion and Storage

Abstract

Photoelectrochemical systems are a promising method for the conversion of solar energy to storable chemical fuels or charge carriers. Limitations on these systems include high overpotentials for driving redox reactions, which reduce the efficiency of energy conversion, and difficulties in long-term storage of charge carriers. The work described in this thesis addresses both issues. First, high overpotentials can be mitigated by the addition of an electrocatalyst. Heterogeneous molecular electrocatalysis is promising but reports in photoelectrochemical systems are limited. This thesis describes the use of reduced graphene oxide (RGO) thin films to immobilize molecular electrocatalysts on electrodes. Second, charge carriers used to store electrochemical energy are subject to poisoning by oxygen. This thesis presents the use of host-guest chemistry to stabilize a radical species in the presence of oxygen.

First, a novel method for the fabrication of RGO thin films is demonstrated. This method is the first report using dissolved outer-sphere reductants to reduce graphene oxide to RGO. As a result, these RGO films are reduced without heteroatom doping or over-reduction, common problems with previously reported inner-sphere reductions. Furthermore, this method is exceedingly gentle and can be performed with a variety of underlying substrates including soft organic, non-conducting, and chemically sensitive materials.

RGO thin films deposited on electrode surfaces are shown to adsorb the proton reduction electrocatalyst cobalt(III) bis(dichlorobenzenedithiolate), presumably through π-stacking interactions. The retention of the electrocatalyst under turnover conditions is poor on smooth films but excellent on rough films. It is therefore hypothesized that the π-stacking interactions are weak and may become disrupted during turnover, leading to fast loss of the catalyst from smooth surfaces. However, on rough RGO films, catalyst intercalation is possible, leading to mechanical trapping that prevents it from diffusing away during electrocatalysis. This work has implications for the field of small molecule surface modifications that employ π- π interactions. Specifically, these interactions may be weak, but intercalation within a graphitic material can lead to enhanced retention of adsorbed species.

Taking advantage of these findings, I then devised a new approach where catalyst and GO are co-deposited, and I studied the kinetics of electrocatalytic proton reduction in the resulting RGO films that are embedded with the electrocatalyst. This is the first time that fundamental kinetics of an electrocatalytic RGO thin film have been investigated. It is shown that different processes are limiting depending on the thickness of the film. For films thinner than 200-500 nm, diffusion processes limit the current, whereas for thicker films, electrical conductivity of the film likely plays a role. These conclusions are relevant for maximizing current in electrochemical energy conversion systems, especially since RGO is commonly used as a catalyst support in such systems.

The second part of this thesis describes interactions of two bis-viologen species with the cage molecule cucurbit[8]uril (CB[8]). A unique viologen oxidation state is identified in the presence of CB[8], identifiable by its absorption spectrum. This species possesses extended stability in the presence of oxygen. Computations suggest that the presence of a buried SOMO is the origin of this enhanced stability. This work has relevance in energy storage systems such as redox flow batteries and solar redox batteries, where trace oxygen can poison solutions of reduced viologens. Extending the stability of viologens by entrapment within cage complexes is a promising method for improving the shelf-life of these species.