Influence of Lithium Sulfate on the Kinetics of Hydrogen Oxidation in H2so4
Authors: Faral Manon , Sacré Nicolas , Chenitz Régis , Mokrini Asmae , Bibienne Thomas , Laroche Nicolas , Magnan Jean-François , Dollé Mickael 
Published: May 2020

During the last few years, development of lithium ion batteries (LIB) offers innovative solutions for energy storage and has major role in the global energy future. Now used for many purposes, the increase in energy demand is driving the development and the production of LIB towards more environmentally friendly methods while also reducing their production costs.

Nemaska lithium, a Canadian collaborator in this project, is recognized as a new producer of lithium hydroxide, a key precursor for high capacity Li-ion cathode material production. In a constant improvement ambition, Nemaska Lithium decided to study the opportunity to use an Hydrogen Depolarized Anode (HDA) in order to save energy for the direct electromembrane synthesis of high purity Lithium Hydroxide (LH). This new technology is based on hydrogen oxidation instead of oxygen evolution reaction, (H2(g) -> 2H+
(aq) +2e-; E0=0,00V vs NHE), this configuration allows for the decrease of the voltage of the electrolysis cell.

With a structure similar to proton exchange membrane fuel cells (PEMFC) anode, an HDA is a porous electrode, supplied with hydrogen from its gas diffusion side and in contact with the electrolyte in the catalyst coated side. The reaction itself is taking place in a small part of the electrode called the catalytic layer. It consists of a triple phase point; an ionomer (for the proton’s transport), platinum nanoparticles (used as catalyst) and a conductive carbon (for electron transport). This study is focusing on two aspects that can influence the performance of the catalytic layer: the impact of the electrolyte on the reaction kinetics and its composition on its efficiency. In order to further advance our knowledge of the reaction mechanism occurring at the HDA, a study on the reaction kinetic of hydrogen oxidation on platinum was conducted. Usually studied in sulphuric acid, the reaction is monitored in this case in the presence of lithium sulfate. For this study, a rotating disk electrode (RDE) has been used. The advantage of this hydrodynamic method is to reduce the dependence of the system on mass transport in order to isolate kinetic current. Using the Koutecky-Levich equation combined with RDE measurements, a logarithmic representation of Tafel slopes for different lithium concentrations have been determined. Linking overpotential to current density, the Tafel curves are used to determine the influence of lithium sulfate on the kinetic of hydrogen oxidation. In a first step, the study was performed on platinum disk electrode. With this well-established system, it was possible to conclude a significant decrease in reaction kinetic in the presence of the salt. Subsequently, the same technique is conducted on a vitreous carbon electrode, where a porous catalytic layer close to HDA configuration is deposited. This configuration allows to study the impact of ionomer presence close to Pt nanoparticles that would change interface between platinum surface and lithium salt electrolyte.