One significant contribution to the anodic potential during aluminum electrolysis is the formation of CO₂ bubbles that screen the anode surface. This effect creates an additional ohmic resistance as well as an increased reaction overpotential, hyperpolarization, as the effective surface area decreases. This work aims to improve the understanding of how anode properties - including isotropy at the optical domain level, wettability (toward electrolyte), surface roughness and porosity - affect bubble evolution. Pilot anodes, made with single source coke types varying in isotropy, were used to study bubble evolution by electrochemical methods. In order to retain bubbles during experiments, anodes were designed to have only horizontal surface area. Bubble formation and release were monitored at different current densities, and were tracked by measuring the oscillations in anode potential and series resistance. Anodes made from different cokes were found to have different bubble evolution properties, possibly due to variation in the density of nucleation sites at the surface of each anode and varying anode-electrolyte wettability.

Although the anode process during aluminum electrolysis has a substantial overpotential that increases the energy demand and production cost of aluminum, properties of the coke that can influence the electrochemical reactivity in the industrial anode itself have not been well documented. In this work the electrochemical performance of anodes fabricated from single source (anisotropic and isotropic) cokes, including an ultrapure graphite as reference material, was determined, and compared to the material properties of the cokes and baked anodes. Anodes were characterized for air and CO₂ reactivity and optical texture, and cokes and anodes characterized for presence of oxygen surface groups and microstructure (fractions of basal, edge and defect sites on the surface and pore volume below 16 nm). Results show that anodes made from more isotropic cokes (increasing optical texture fineness) had a slight improvement in electrochemical performance compared to those made from more anisotropic cokes. For all anodes, electrochemical reactivity correlated with the electrochemically-wetted surface area, as determined by the double layer capacitance. This appears to be related to microstructure and possibly also to differences in surface chemistry, rather than differences in surface roughness and porosity as determined by optical techniques (i.e. on a μm-scale).

Availability of anode raw materials in the growing aluminum industry results in a wider range of petroleum cokes being used to produce carbon anodes. The boundary between anode grade cokes and what previously was considered non-anode grades are no longer as distinct as before, leading to introduction of cokes with higher sulfur and higher trace metal impurity content in anode manufacturing. In this work, the chemical nature of sulfur in five industrial cokes, ranging from 1.42 to 5.54 wt pct S, was investigated with K-edge XANES, while the reactivity of the cokes towards CO₂ was measured by a standard mass loss test. XANES identified most of the sulfur as organic sulfur compounds. In addition, a significant amount is identified (16 to 53 pct) as S-S bound sulfur. A strong inverse correlation is observed between CO₂-reactivity and S-S bound sulfur in the cokes, indicating that the reduction in reactivity is more dependent on the amount of this type of sulfur compound rather than the total amount of sulfur or the amount of organic sulfur.