A Simple Pressurized SOFC Test Rig for Measurements of Cell Performance, Impedance, and Various Overvoltages

Nowadays, our modern society is facing many serious environmental and energy shortage problems. Delay of action will jeopardize the ability of future generations to meet their needs. Thus, how to develop highly efficient power generation systems with very low emissions is an urgent need for our modern society. Probably, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, which has the highest efficiency up to 70%, is the best candidate. The development of such hybrid power system helps to the consolidation of the fuel cell industry, the expansion of the gas turbine industry, and the improvement of the environment. As such, the hybrid PSOFC-MGT power system has become an emerging topic in the 21^th century with wide interest, as evidenced by a large volume of literature and several demonstration projects. For examples, Siemens demonstrated a 220 kW PSOFC-MGT hybrid system in 2002, the Korea Institute of Energy Research displayed a 30 kW (5 kW PSOFC integrated with 25 kW MGT) hybrid system in 2006, and Mitsubishi Heavy Industries run a 200 kW hybrid PSOFC-MGT combined-cycle power plant in 2011. Indeed, considerable progresses have been made over the past two decades in optimization of geometry and development of new materials and in understanding of mass and heat transports together with electrochemical reactions in porous microstructures. However, there are still technical challenges remaining to be solved step by step with limited but controllable parameters before a stable operation among different components of the hybrid PSOFC-MGT system can be assured and the status of the development can be then elevated from a module level. One of the challenges is the detail electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to design a simple efficient high-pressure full/half button cell testing platform for measurements of the effect of pressurization and increasing temperature on EIS and various overvoltages of PSOFC.

The testing platform has several parts, from inside out including a specially thread-designed housing carrier for the full/half button cell, a serpentine heating pipe system for uniform heating of the supplied fuel and air gases, a temperature-controlled furnace, and a large high-pressure vessel together with measuring devices and their associates. Hence, current-voltage curves and AC impedance spectra of any full/half button cells can be measured. In this study, we present power-generating characteristics and EIS data of an anode-supported full button cell over wide ranges of pressure (p = 1~5 atm) and temperature (T = 700~850 ^oC). We apply constant gas flow rates, 200ml min^-1 H₂ in anode and 200ml min^-1 air in cathode, for all experimental events.

Results show that, for any given values of T or p, power densities increase with increasing p or T at any fixed current densities. Such enhancement on power densities is more sensitive to the increase of T than to the increase of p. These results are explained by the corresponding EIS data. The latter shows that both high and low frequency arcs of impedance spectra decrease with increasing p, resulting in a reduction of the total polarization resistance. Specifically, the high frequency arcs decrease rather weakly with increasing p as compared to that of the low frequency arcs. Further, we found that the characteristic frequencies of high and low frequency arcs occur around 100 ~ 1000 Hz and at about 10 Hz, respectively. The former may be attributed to the cathode activation polarization and the latter may be due to the diffusion processes in the anode electrode. Hence, pressurization can simultaneously decrease the cathode activation polarization and the anodic diffusion concentration polarization.

In addition to the aforesaid power-generating characteristics and EIS results, this study reports analyses of activation and concentration overvoltages using the Butler-Volmer equation and the related concentration overvoltage equation based on a 1D diffusion model. We found that both activation and concentration overvoltages decrease with increasing p. Two important parameters, the exchange current density and the anodic effective diffusion coefficient, are also calculated by applying the present measured data. Results show that pressurization enhances the exchange current density leading to an increase of the electrochemical reaction rate. Though pressurization reduces slightly the anodic effective diffusion coefficient, it can increase the molar concentration and thus the overall gas-phase diffusion rate in porous electrode. These overvoltage results will be compared with previous numerical and experimental data to enhance our understanding of the effect of pressurization on the polarization mechanisms in SOFCs.