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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.

Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices that directly transform the chemical energy of fuel into electrical energy. They generate electricity far more efficiently and with fewer emissions per megawatt-hour compared to any combustion-based power generation system. More remarkably, SOFCs can directly use hydrocarbon fuels without requiring external fuel reforming, employing low-cost Ni catalyst instead of noble-metal catalysts used for low-temperature fuel cells. However, the conventional SOFCs using Ni-based anodes fed with carbon-containing fuels have one pitfall; the carbon produced by hydrocarbon cracking is deposited on the Ni surface, thereby precluding the surface of the Ni-based anodes from being available for further fuel oxidation and consequently impeding SOFC operation. This dissertation focuses on overcoming this critical drawback to allow for the simultaneous use of Ni-based anodes and hydrocarbon fuels. Further work focuses on improving SOFC performance to provide the highest efficiencies possible. To boost the power densities of SOFCs, a novel, facile approach to modify the surface structure of anode powders and thereby enlarge the three-phase boundary (TPB) regions of anodes is presented. One such powder preparation method based on the electric charge variation of oxides depending upon the pH of the solution results in significantly extended TPB regions and thus a remarkable increase in power densities of SOFCs. Another method involves the formation of Ce₁₋[subscript x]Gd₁₋[subscript y]Ni[subscript x+y]VO₄₋[subscript delta] at the phase boundaries between NiO and Ce₀.₈Gd₀.₂O₁.₉ (GDC) by V⁵⁺-incorporation onto NiO surface; this method improves the microstructure of Ni-GDC-based anodes and considerably lowers GDC electrolyte sintering temperature, thereby enhancing the SOFC performance. With these high performance anodes, natural gas-fueled SOFCs are studied through two strategies to alleviate coking: incorporation of catalytic materials onto the Ni surface and the introduction of catalytic functional layers (CFLs) to the outer surface of an anode-supported single cell. Hydrogen tungsten bronze and hydroxylated Sn formed on the Ni surface provide hydroxyls for the deposited solid carbon, removing it from the anodes as CO₂. Moreover, the use of hydrophilic Sn or Sb-incorporated Ni-GDC CFLs prevents the anode from being exposed directly to hydrocarbon fuels and controls the solid carbon accumulation similarly to the former strategy.