Mechanistic implications from 'in operando' optical and electrochemical studies of bio-related fuel chemistry in solid oxide fuel cells

Abstract

Solid oxide fuel cells using bio-renewable fuels promise efficient, sustainable, and clean electricity production, and are becoming more attractive sources of electrical power as global consumption of non-renewables accelerates. The excellent efficiencies of solid oxide fuel cells as solid state electrochemical devices arise mainly from the direct conversion of chemical to electrical energy--through oxygen reduction at the cathode, oxide diffusion through the electrolyte, and fuel oxidation at the anode. Some of these processes possess high activation energies, requiring high operational temperatures (generally > 650 °C). These conditions can hasten deleterious carbon accumulation and anode deterioration. These high operating temperatures also pose significant challenges in directly observing chemical reactions responsible for electrochemical oxidation and materials degradation. Yet, these observations are needed to understand fundamental mechanisms responsible for these processes--an understanding necessary to improve the performance, durability and versatility of SOFCs, especially as these devices are required to operate with complex fuels and fuel mixtures. In this work, solid oxide fuel cells constructed with traditional Ni/yttrium stabilized zirconia ceramic-metallic (or cermet) anodes are studied in operando and in situ with several novel optical techniques (Raman vibrational spectroscopy, near infrared thermal imaging and fourier-transform infrared emission spectroscopy) and electrochemical measurements that provide vital insights into mechanisms surrounding bio-related fuel electrochemistry. The first study demonstrated that Ni oxidation is slower than reduction at the anode. The second study quantified electrochemically accessible anode carbon accumulation under methane fuel, while detailing deleterious mechanisms and microstructural changes that accompany cell polarization in the absence of gas phase fuels. The third study explored the kinetics of carbon removal from Ni-based anodes by CO 2, H 2O, and O 2 and detailed mechanistically why they may be different. The fourth study revealed mechanisms associated with carbon formation and current generation from biogas and methane as a function of operational condition. Collectively, these studies have begun to provide the direct, molecularly specific information necessary to empirically evaluate mechanistic descriptions of biorelated fuel electrochemistry and anode degradation.