High Temperature Electrocatalysis
Various applications in high temperature electrocatalysis are being studied, including sulfur- and coke-resistant solid oxide fuel cells, reduction of carbon dioxide and water to produce syngas, and oxidative dehydrogenation of lower alkanes to olefins. Reaction experiments are performed using an electrocatalytic cell, which is sealed onto a reaction chamber, and heated to temperatures between 500-700 °C. Challenges in this research include the reactor design and sealing procedure, development of the catalysts which make up the cell’s electrodes, and characterization of the electrocatalysts.1-11
Reduction of Carbon Dioxide and Water to Syngas
Due to the detrimental effect of CO2 as a greenhouse gas, its capture and storage has become an active area of research. One promising way of converting CO2 into value added products is through high temperature co-electrolysis of steam and CO2 in a solid oxide electrolysis cell (SOEC) to produce syngas, which can subsequently be turned into liquid fuels by Fischer-Tropsch synthesis. Reduction of CO2 is possible at low temperature also, but requires the usage of highly active and costly noble metal catalysts for achieving suitable kinetics. At a high temperature, voltage requirement for electrolysis decreases and a faster reaction rate can be achieved.
In an SOEC, H2O and CO2 simultaneously get reduced at the cathode and produce oxide ions; these ions then travel through an oxygen ion conducting electrolyte to the anode where they combine to form molecular oxygen. The main challenge in the construction of an efficient SOEC lies in the development of a suitable cathode catalyst. A cathode catalyst should be coke resistant, thermally stable and have high electrical and ionic conductivity. Ni-YSZ, which is commonly employed as cathode in SOECs, suffers from material degradation in oxidative environments. Perovskite oxides of the form ABO3 (typically containing an alkaline earth, alkali, or rare earth metal at the A-site and a transition metal at the B-site) are being studied for this application due to their stability at high temperatures, and ability to conduct both oxygen ions and electrons.
Oxidative Dehydrogenation of Lower Alkanes to Olefins
Olefins are major chemical building blocks, which are used to produce nearly all consumer products. Oxidative dehydrogenation (ODH) is a method of producing light olefins like ethylene and propylene from widely available hydrocarbon sources such as ethane and propane. Typically, olefins are produced through steam cracking in petrochemical facilities. However, one of the shortcomings of traditional ODH is the limited ability to prevent the further oxidation of the formed olefins to carbon oxides, in addition to being dependent on fossil fuels.
Novel reactors using an oxide ion-conducting membrane offer a potential solution to control the selectivity to the desired olefins by regulating the availability of oxygen to the alkane, and make use of a cheap hydrocarbon source, which is becoming more widely available as natural gas is developed. These reactors are also similar to the setup of solid oxide fuel cells, but instead of generating power, an external voltage/ current is applied to control the oxygen supply to the alkane. The external current pushes oxygen ions through the cell so that they may react with the alkane at the anode. In this scenario, the cell is operating in ion pump mode. This can potentially help regulate the extent of alkane conversion and enhance the selectivity towards partial oxidation products. Because the ODH reaction occurs on the anode electrocatalyst’s surface, that component is of highest interest. Perovskite oxide catalysts are also being considered for this application.
Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFCs) are high temperature fuel cells (700-1000 °C) that could be used for stationary energy generation devices and auxiliary power systems on buses and commercial trucks. Previous work in our group has focused on oxygen surface interactions over perovskite-type cathode materials for low temperature SOFCs, as well as development of coke and sulfur tolerant anode materials.
SOFCs show great promise for generating clean power from a variety of fuels. A major roadblock against their implementation is a large cathodic resistance, which causes high operating temperatures, insufficient power densities and high fabrication costs. The large cathodic resistance is caused by slow oxygen activation kinetics and oxide ion transport of the current manganite-based cathode. Thus, the development of highly active and ionically conductive materials suitable for use as cathodes is needed to help SOFCs realize their wide-scale application. Our research endeavored to determine the factors that control the activity of the cathode for the oxygen reduction reaction, including oxygen adsorption, oxygen dissociation, and oxygen diffusion and the relation of these parameters to the nano-structure of the cathode material. While many scientific studies focus on either the material preparation or electrochemical aspects of the ceramic components of the SOFC, our research work focused on the catalysis of the oxygen reduction process.
SOFCs are also used directly with hydrocarbon fuels and as on-board reformers in other energy systems. The current Ni-YSZ anode catalysts exhibit very good catalytic activity towards hydrocarbon reforming and good compatibility with the rest of the SOFC system, but are highly susceptible to poisoning due to sulfur in the fuel. Previous research focused on studying the poisoning mechanism of the anode catalysts by sulfur in the fuel, as well as developing materials that not only match the activity of the Ni-YSZ catalyst, but also have tolerance to sulfur and coking. Cerium-doped strontium cobalt ferrite perovskite materials were studied for this application.