Developing hydrogen oxidation electrocatalysts using a frustrated Lewis pair approach

Lawrence, Elliot (2015) Developing hydrogen oxidation electrocatalysts using a frustrated Lewis pair approach. Doctoral thesis, University of East Anglia.

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In order for hydrogen to be utilised as an energy vector, precious metals such as platinum are often employed as electrocatalysts, to enable the efficient conversion of hydrogen into electrical energy. However, the low abundance and high cost of such metals limits their economic viability.
Frustrated Lewis pairs (FLPs) are capable of heterolytically cleaving hydrogen, and have found application in hydrogenation chemistry. In this thesis, the hydrogen-activating ability of FLPs is exploited for the development of new, metal-free electrocatalysts for hydrogen oxidation. In essence, this approach combines FLP hydrogen activation with the electrochemical oxidation of the resulting borohydride.
Initially, a fundamental study into the single-electron reduction of the archetypal Lewis acid, tris(pentafluorophenyl)borane, was undertaken in low donor strength solvents. This allowed pertinent thermodynamic, kinetic and mechanistic information to be obtained.
The redox chemistry of the archetypal FLP system, tris(pentafluorophenyl)borane/tri-t-butylphosphine, was then studied for the first time. FLP pre-activation of hydrogen was found to decrease the required potential for non-aqueous hydrogen oxidation by 610 mV at inexpensive and abundant carbon electrodes. This system was then extended to include platinum electrode materials, where strong surface electrocatalytic effects were exhibited.
The combined electrochemical-frustrated Lewis pair approach was applied to a series of carbene-stabilised borenium cations. Their efficacy towards the electrocatalytic oxidation of hydrogen was assessed. The borenium cation derived from 9-borabicyclo[3.3.1]nonane decreased the required voltage for hydrogen oxidation by 910 mV. This system also exhibited improved catalyst recyclability compared with the original tris(pentafluorophenyl)borane system.
Finally, the cyclic voltammetry of two regioisomers of tris[bis(trifluoro-methyl)phenyl]borane was explored for the first time. Solutions of tris[3,5-bis(trifluoromethyl)phenyl]borane in the donor solvent tetrahydrofuran were highly effective for facile hydrogen cleavage. Replacing the Lewis basic component of conventional FLP systems with donor solvents is a new approach to FLP chemistry, and proffers many advantages for electrochemical-FLP systems.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: Stacey Armes
Date Deposited: 03 Mar 2015 16:30
Last Modified: 31 Jan 2020 01:38


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