Solid Oxide Based Electrodes for CO2 Reduction

Abstract

The conversion of CO2 to fuels can be instrumental in reducing future dependence on fossil fuels. High temperature (800oC) electrochemical reduction of CO2 to CO using a solid oxide electrolysis cell (SOEC) provides an energy-efficient pathway to achieve the same. The CO thus obtained can be either directly used or channeled into higher hydrocarbons via the well-known Fischer-Tropsch process. Popular cathode catalysts such as Ni, CeOx, and perovskites like LaSryMn1-yOx suffer from inherent low kinetic activity and deactivation phenomena such as coking. Some of these catalysts (e.g., CeOx, LSM) can function within pure CO2 streams, but others (e.g., Ni) require the presence of safe gases such as H2 and CO to function effectively. New catalytic materials are necessary to develop cathodes that can provide good stability and energy-efficient CO2 conversion. These catalysts function under multifactorial environments. Factors such as reactant and product compositions, temperature, and electronic conductivity significantly influence the activity and stability of catalysts. Developing new catalysts that can function under such complex parameter space is challenging due to the intertwined behavior of materials and processes. Development requires both quick and deep chemical insights. Operando spectroscopy using a confluence of Raman, electrochemical impedance, and online mass spectroscopy has the potential to provide direct and instantaneous chemical information that can significantly enhance the speed of discovery and development. We have developed a novel two-chamber cell that can carry out operando Raman/optical spectroscopy in combination with electrochemical impedance and online mass spectroscopy. This setup allows us to study the catalysts in action in an environment which is very similar to that of a real solid oxide cell (800oC, separate anode and cathode chambers). We have used it to track catalyst structure during the reaction and correlate it with activation/deactivation phenomena. These insights lead to new key performance indicators for catalyst activity and stability. In certain cases, we have been able to mitigate such challenges through catalyst modification via adding additional metal atoms. In other cases, process limits that can define sustained performance were established. We have studied two classes of electrodes Ni{M}x-YSZ and Ce{M}Ox-YSZ, where doped Ni and doped ceria were evaluated under pure CO2 and CO2 (+ H2) streams. Traditionally, Ni-YSZ electrodes have been believed to deactivate under pure CO2 streams via oxidation of Ni and the formation of coke on the catalyst surface. By applying the operando methods described above, we have shown that Ni-YSZ electrodes can, in fact, be operated within certain limits for electrolysis within pure CO2 streams. Our measurements reveal that under operation conditions, Ni-YSZ, in fact, oxidizes to NiOx-YSZ and is operational for CO2 reduction via an oxide-mediated mechanism. Ironically, the electrode deactivation actually coincides with strongly reducing conditions where the NiOx is reduced to metallic Ni. Cu-infiltration into Ni-YSZ architecture was demonstrated to mitigate the deactivation issue by modulating the Ni-O-Ni bond strengths and forming a more stable oxide on Ni. This oxide with higher stability to reduction continued to carry out CO2 reduction under strongly reducing conditions. The new electrode also demonstrated improved kinetics and stability against carbon deposition. Under CO2 feed containing H2 (~5%), a thermochemical reverse water gas shift (rWGS; CO2 + H2 CO + H2O) reaction was active under conditions of no applied bias. Application of bias resulted in electrolysis of both CO2 and H2O. Effects of temperature, applied bias, and concentration of reactants were explored. Cu infiltration resulted in improved electrochemical kinetics in general. CeOx-YSZ electrode was evaluated in both pure CO2 and CO2 (+H2) feeds. This electrode is known to perform in either environment. In pure CO2, operando studies revealed that, upon application of the bias, CO production correlated with the reduction of Ce4+ sites to Ce3+ sites as evidenced by decay in the intensity of the Raman band at 450 cm-1 (α). The electronic conductivity increased with applied bias due to an increase in Ce3+ sites. The addition of H2 (~5%) resulted in rWGS reaction and an additional band at 480 cm-1 (φ). The CO formation was more facile at a lower bias compared to the pure CO2 stream. The φ band is likely to be associated with the formation of Ce4+-H- (hydridic) sites. The electronic conductivity shows a slight dip at low biases (5% H2) due to the reoxidation of Ce3+ sites to Ce4+-H- (hydridic) sites. Over a large range of bias, the correlation between the decay of the α band and the increase in conductivity is retained. The addition of Pr/Gd into ceria resulted in a significant enhancement of electrode performance in CO2 (+ H2). The electrodes were not functional in a pure CO2 environment. The electrode resistance was found to reduce with increasing bias due to its MIEC character. Besides the standard band observed in ceria, the doped electrodes showed an additional band at ~2100 cm-1 (θ). CO production correlated with the reduction of Ce4+ centers to Ce3+ with progressive application of bias as evidenced by decay in the intensity of α band. At higher concentrations of CO2, a strong bias resulted in coke formation without affecting the general activity of the catalyst. Onset of Raman bands associated with coke strongly correlated with the complete disappearance of α bands (associated with the complete disappearance of Ce4+ sites) and the onset of decay of θ band associated with the metal carbonyl sites. A coking mechanism based on the coupling of CO molecules adsorbed on adjacent Ce3+ sites (“Ce3+-- CO”) has been proposed.