GCEP 2015 Progress Report

This project collectively addresses multiple areas of GCEP’s research activities in energy and environment, and aims to develop the fundamental understanding towards, • advanced coal conversion to electricity in a specialized solid oxide fuel cell (SOFC) • electrochemical hydrogen production from coal for energy storage • CO2 mitigation and capture without separation The project aims to achieve these outcomes in a single process chamber using a novel SOFC design, where coal is gasified internally to produce CO, which is oxidized at the anode to CO2 by the oxygen ions stripped from H2O and O2, and transported across the solid electrolyte to the anode. The ultimate goal is to develop a clean and highly efficient coal conversion technology that simultaneously generates electricity and hydrogen, while producing nearly capture-ready CO2. In other words, the project aims to convert dirty coal to clean energy while minimizing the carbon footprint on the environment. As coals contain sulfurous impurities and sulfur is a potent and well-known poison that rapidly deactivates catalytic anodes, work in our laboratory over the past year has focused on sulfur mitigation strategies including the investigation of sulfur sorbent solids as well as the synthesis and development of sulfur tolerant catalytic anode materials primarily based on perovskite-based oxides for the purpose of improving fuel cell lifetime. Also, work continued to investigate the kinetics of CO oxidation at the anode to help improve a comprehensive model we have been developing as part of this project in order to make predictive assessment of the cell performance as well as to gain better insights on the effects of cell operating parameters on overall cell behavior. Important findings for the previous year include: • Electrochemical impedance spectroscopy (EIS) studies on model electrodes for CO oxidation and oxygen reduction kinetics indicated activation energies of 132 kJ/mol and 192 kJ/mol for CO oxidation and oxygen reduction reactions, respectively, where the anode exchange current density was also found to vary as PCO and PCO2. • Studies on solid sorbents for sulfur removal indicated significantly improved cell performance and partial enhancement in anode durability in the presence of the sorbent. • Our comprehensive finite-element planar fuel cell model was extended to a practical tubular fuel cell geometry to predict cell performance metrics as a function of cell design and operating parameters, and indicated that at optimum tube spacing, carbon bed and freeboard heights, it is possible to achieve primary conversion efficiencies up to 70 % with practical power densities. • Candidate materials for sulfur tolerant anodes were identified and synthesis and doping techniques for making and characterizing several compositions of the perovskite family LaxBaySr1-x-yTiO3, including Ba0.5Sr0.5TiO3 and La0.4Sr0.6TiO3 were developed. If successful and widely adopted, this inherently efficient electrochemical conversion technology promises to increase the practical efficiency of coal power generation, resulting in a significant reduction in global CO2 emissions thereby ensuring continued use of our cheapest and most abundant fuel – coal – in an environmentally friendly manner. Introduction This project aims to develop a clean coal technology that simultaneously generates electric power and carbon-free hydrogen, and produces nearly capture-ready CO2 at significantly reduced quantities. Storing coal energy in environmentally benign fuel hydrogen is an attractive proposition. As a clean energy carrier, hydrogen has the potential to become a significant player especially in the transportation sector. By large, molecular hydrogen is generated by steam reforming of methane, which accounts for more than 75% of the current production capacity, but is cost effective only when it is produced centrally and at large scale. Storage and transportation of hydrogen, however, is costly and inefficient because of its low volumetric energy density (~11 MJ/m). An effective hydrogen-based clean transportation economy would therefore require distributed production of carbon-free hydrogen, and coal provides a viable low-cost option. The nested double cell scheme used in this project is built upon the steamcarbon fuel cell concept [1-4] and involves conversion of coal or biomass at the anode, while hydrogen from the reduction of steam is produced at the cathode, shown in Fig. 1. The steamcarbon cell makes up the left hand half of the double cell, while the right hand half is the solid oxide-based carbon fuel cell (SO-CFC). The reaction systems at the common anode and the two cathode chambers are separated physically by impermeable oxide ion conducting yttria stabilized zirconia (YSZ) ceramic membranes. This scheme in essence allows the water gas shift and steam gasification reactions of coal to be achieved in such a way that the anode and cathode reaction streams do not mix Figure 1: Schematic of a coupled steamcarbon and air-carbon fuel cell, and the corresponding oxygen chemical potential gradients (red arrows) across the YSZ electrolyte membranes. YSZ YSZ with each other, minimizing entropic losses. Furthermore, the favorable thermodynamics of this system allows spontaneous and simultaneous production of hydrogen and electrical energy. The downhill gradients (red arrows in Fig. 1) facilitated by the differences in the chemical potential for oxygen across the two ceramic membranes, drive the uphill oxygen abstraction reactions at the two cathodes, namely, from steam on the left hand side compartment of Fig. 1 and molecular oxygen in the air on the right hand side, and then transport the oxide ions via vacancy mechanism across the crystal lattice of the YSZ ceramic membranes towards the common anode where they react with the carbonaceous fuel to form CO2 [5]. The electrons released at the common anode travel through the external circuit to perform useful electrical work. As the carbon and hydrogen streams are physically separated, this novel scheme offers efficient and distributed production of carbon-free hydrogen and electrical power, potentially also from locally available carbonaceous resources including biomass and waste. The goal of the project is to gain mechanistic and operational understanding of this novel fuel cell concept, which represents a game-changing opportunity to achieve highly efficient conversion of coal and biomass in fuel cells on practical scales with simultaneous and spontaneous production of electricity and carbon-free hydrogen. This research effort involves experimental and modeling components. Laboratory experiments are designed to provide the information needed to characterize mass transport and electrochemistry at the cathode and anode of the steam-carbon fuel cell as well as the heterogeneous gasification reactions that occur in the coal bed, producing the synthesis gas that is oxidized at the anode. As sulfurous impurities commonly present in coal and other solid fuels rapidly poison and deactivate anode performance, a two-prong approach is used for mitigating the adverse impact of sulfur on the anode electrocatalyst. Specifically, it involves investigation of catalytic materials that are both good for CO oxidation and tolerant to the sulfur liberated when the carbonaceous fuel is gasified, as well as development of solid sorbent materials for removing sulfur from the coal during gasification before the syngas reaches the electrocatalyst. Models are developed to describe the observed phenomena and to gain operational understanding for system optimization. Understanding challenges in system scaling, thermal management, power management, and membrane electrode assembly (MEA) designs, especially for multi-cell configurations are key concerns. If successful and widely adopted, the technology would greatly reduce greenhouse gas emissions while efficiently producing high-purity hydrogen and electricity from coal and biomass. The impacts of this research project are in the areas of efficient energy conversion (coal) and storage (hydrogen) with reduced carbon footprint, as well as in the education of students in advanced approaches to energy and environment for a sustainable future.