Membranes made with conductive materials: A novel concept for overcoming mass-transfer in electrochemical CO₂ conversion

A membrane made with conductive materials – a novel electrode for the electrochemical oxidation and reduction – was fabricated and applied for conversion of CO₂ into value-added products. By adapting conductive membrane electrodes, the mass-transfer of CO₂ onto the surface of SnO₂ nanoparticles was significantly improved, thereby resulting in the highest current efficiency ever reported.

Article | Spring 2020

Elecrochemical oxidation and/or reduction technologies have been recently highlighted as a treatment tool of undesirable compounds or as a conversion tool of potential resources into value-added products in various media such as water, air, and soil after the potential combination with renewable energy technologies.

However, when the target compounds exist in low concentrations, the process is merely controlled by the diffusion of target compounds onto the surface of electrodes. In such a way, the delivery of target compounds has become one of the biggest hurdles to achieve highly efficient electrochemical processes during application. For example, it has been reported that the Faradaic efficiency of electrochemical oxidation of micropollutants in water was less than 1% due to the discrepancy between the existing time of OH-radicals generated from electrochemical oxidation (nano-seconds domain) and the delivery of micropollutants to the surface of electrodes by diffusion (mili-seconds domain). To overcome the diffusion-domain transport, the preparation of a hollow-fiber membrane structure made with conductive matters such as carbon nanotubes enables both the convective transport of reactant to the fibrous electrode surface and the incorporation of catalyst on the large surface area of carbon nanotubes. In this structure, the transport of reactants can be controlled by the applied pressure that may determine the permeation velocity. An additional benefit of the conductive hollow-fiber membrane electrode (CHME) is that the product stream is separated as a form of permeate from the reactor and thus will not cause any side reaction with counter-electrodes.

The CHMEs prepared at the Sustainable Water Environment and Energy Technology (SWEET) laboratory in the Department of Civil and Environmental Engineering are being applied as the gas-diffusion electrodes (GDE_s) where CO₂ reduction is achieved by flowing CO₂–containing gas on the catalytic layer of the GDE. The enhanced selectivity of CO over formate for SnO₂-CHME in gas-phase mode was achieved by the free flow of CO₂ in the convective way on to the solution where the reaction took place. It has been also demonstrated that the higher current density in the gas-phase reaction comparing to the liquid-phase would lead to faster and more efficient conversion of CO₂.

 

Although more parameters (.e.g., catalyst loading, and electrolyte pH) must be investigated to unravel the details of the mechanisms, it is noteworthy that current results of SnO₂-CHME operated in gas-phase mode have already exhibited a significantly higher current density for syngas formation compared to previously reported studies. While the concept is simple, this new electrode configuration offers a platform to overcome the mass-transfer limitation of reactants in electrochemical systems. This research was published and selected as the cover article on December 10, 2019 in ACS Sustainable Chemistry & Engineering under the title of “Electrocatalytic CO₂ Reduction via a Permeable CNT Hollow-fiber Electrode Incorporated with SnO₂ Nanoparticles” (doi 10.1021/acssuschemeng.9b05701)