Advance Electrochemical CO2 Utilization Towards Higher Industrial Readiness: Across Multiple Scales from Catalyst Design, Electrode Decoration, to Reactor Engineering
Authors: Lu Xu 
Published: May 2020

The challenges in electro-catalytic mitigation of CO2 press innovations across multiple scales going from catalyst structure at microscale to the complex multiphase thermofluids network at system scale. In this presentation, I will demonstrate a molecular electrocatalyst active for CO2 to methanol conversion, a pioneering catalyst restructuring technique to realize 12-day CO2-formate inter-conversion, a polymer-molecular catalyst hybrid gas diffusion electrode to reduce CO2 in presence of O2 just like the industrial flue gases, an alkaline based micro-flow reactor to boost the CO2 electro-reduction performance, and an unprecedented in-situ platform to reveal the near-electrode local environment in a flow reactor.

At microscale: catalyst design. An integration combining the functions of a CO2 electrolyzer and a formate fuel cell is a new option for carbon‐neutral energy storage but entails rapid, reversible and stable interconversion between CO2 and formate over a single catalyst. We develop a new catalyst with such functionalities based on a Pb–Pd alloy system that reversibly restructures its phase, composition, and morphology and thus alters its catalytic properties under controlled electrochemical conditions. Under cathodic conditions, the catalyst is relatively Pb‐rich and is active for CO2‐to‐formate conversion over a wide potential range; under anodic conditions, it becomes relatively Pd‐rich and gains stable catalytic activity for formate‐to‐CO2 conversion. The bifunctional activity and superior durability of our Pb–Pd catalyst leads to the first proof‐of‐concept demonstration of an electrochemical cell that can switch between the CO2 electrolyzer/formate fuel cell modes and can stably operate for 12 days.

At mesoscale: electrode design. The electrochemical reduction of CO2 in the presence of O2 would allow the direct valorization of flue gases from fossil fuel combustion and of CO2 captured from air. However, it is a challenging task because O2 reduction is thermodynamically favored over that of CO2. Typically, 5% O2 in the CO2 feed gas is sufficient to completely inhibit the CO2 reduction reaction. We developed an O2-tolerant catalytic CO2 reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with > 95% Faradaic efficiency (FE) for CO2 reduction to CO coated with a polymer of intrinsic microporosity that works as a CO2-selective layer with a CO2/O2 selectivity of ~ 20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO2 feed gas containing 5% O2 exhibits a FECO of 75.9% with a total current density of 27.3 mA/cm2 at a cell voltage of 3.1 V. A FECO of 49.7% can be retained when the O2 fraction increases to 20%. Stable operation for 18 h is achieved. The electrochemical performance and O2 tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.

At macro scale: reactor system design and probing the cathode surface environment. The promise and challenge of electrochemical mitigation of CO2 calls for innovations on both catalyst and reactor levels. Enabled by high-performance and earth abundant CO2 electroreduction catalyst materials, we developed alkaline microflow electrolytic cells for energy-efficient, selective, fast, and durable CO2 conversion to CO and formate. With the above-mentioned cobalt phthalocyanine-based cathode catalyst, the CO-selective cell starts to operate at a 0.26 V overpotential and reaches a Faradaic efficiency of 94% and a partial current density of 31 mA/cm2 at a 0.56 V overpotential. With a SnO2-based cathode catalyst, the formate-selective cell starts to operate at a 0.76 V overpotential and reaches a Faradaic efficiency of 82% and a partial current density of 113 mA/cm2 at a 1.36 V overpotential. In contrast to previous studies, it was found that the overpotential reduction from using the alkaline electrolyte is mostly contributed by a pH gradient near the cathode surface, which was validated by direct observations in a Raman-enhanced novel in-situ platform.