Birss Group
Solid Oxide Fuel/Electrolysis Cells
As part of world-wide efforts to identify new energy conversion systems, there is great interest in the development of reversible fuel cells, particularly reversible solid oxide fuel cells (RSOFCs). RSOFCs can run in both the electrolysis mode (SOEC) to electrolyze H2O to H2 or co-electrolyze CO2 and H2O to syngas, when excess electricity is available, and then run in the fuel cell mode (SOFC) to convert H2 or syngas to electricity and heat when electricity is needed. In our group, we develop and study SOFCs and RSOFCs from conventional Ni-YSZ catalysts to developing novel and state-of-the art materials. We work towards understanding the reaction and degradation mechanisms with electrochemical methods.
Materials development for highly efficient electrolyte-supported symmetrical RSOFCs
The primary goal of our work has been to develop several materials for highly efficient electrolyte-supported RSOFCs, based on La0.3Sr0.7Cr0.3Fe0.7O3-δ (LSFCr) and La0.3Ca0.7Cr0.3Fe0.7O3-δ (LCFCr) mixed ionic-electronic conducting (MIEC) perovskite electrodes, operating at 600-800 oC. In addition to the excellent performance of this family of perovskites as an RSOFC electrode material, a key concept behind our work is that LSFCr and LCFCr can be employed as both the fuel and air electrodes in both the SOFC and SOEC modes, thus significantly lowering the cost by decreasing the number of materials used and also simplifying the manufacturing process. We have shown that LSFCr is a very good air and H2 electrode and an excellent catalyst for CO2 reduction/CO oxidation, and that LCFCr is also a very good candidate as a reversible air electrode. As part of this work, we determine the kinetics and mechanism of these electrochemical reactions using CV, EIS, and galvanostatic/ potentiostatic studies. We are also able to determine fundamental parameters, such as the exchange current density (i0) and transfer coefficient (α), as well as the reaction orders associated with the electrode processes.
FIGURE. Performance data for symmetrical RSOFC with a configuration of LSFCr/GDC/YSZ/GDC/LSFCr, operated at 800 C, showing (a) OCP EIS data at 800 and (b) V–I plot acquired at 1 mV s–1 scan rate. From P. K. Addo, B. Molero-Sanchez, M. Chen, S. Paulson and V. Birss, Fuel side CO/CO2 studies of high performance La0.3Sr0.7Fe0.7Cr0.3O3 -δ RSOFC electrodes, Fuel Cells, (2015), 15, 689-696.
Our group is also very interested in understanding the degradation mechanisms in RSOFCs, such as air electrode delamination during electrolysis and sulfur tolerance of the fuel electrode. We carry this work out using as-prepared as well as post-mortem cells by SEM imaging, and as advanced High resolution TEM and EELS mapping. This work is showing that our LSFCr and LCFCr materials are very durable and thus highly promising for use in RSOFCs.
FIGURE. HRTEM images of LCFCr crystals along the [1 0 1] zone axis and the corresponding diffraction patterns and performance data for symmetrical RSOFC with a configuration of LSFCr/GDC/YSZ/GDC/LSFCr, operated at 800 C, showing EIS data at 800, I-V plot acquired at 5 mV s–1 scan rate. From: B. Molero-Sanchez, Prado-Gonjal, J., Ávila-Brande, D., Birss, V. and Morán, E., Ceramics International, 2015 41, 8411-8416. and B. Molero-Sanchez, P. Addo, A. Buyukaksoy S. Paulson, and V. Birss, Faraday Discussions, 2015, 182, 159-175.
New generation Ni/YSZ anodes produced using infiltration methods
Our interests are also focused on a new micro-solid oxide fuel cell (μ-SOFC) design that is based on anodically grown ZrO2 nanotubular (NT) films in collaboration with Dr. Etsell, University of Alberta. The very desirable features of ZrO2 NTs, such as their high surface area-to-volume ratio, can be exploited by electrocatalyst infiltration into the NTs. In this way, extremely long triple phase boundaries (TPBs) can be achieved and the NT base (15–25 nm thickness) could be utilized as the ultra-thin electrolyte, yielding high power densities at low temperatures (< 600 °C). As a first step, we have carried out a thorough electrical characterization of the anodically grown ZrO2 NTs using impedance spectroscopy and obtained a conductivity of 1.6 × 10−6 S cm−1 at 600 °C in N2, approximately twice that reported for dense ZrO2 films measured at the same temperature in air. Our ongoing efforts focus on the enhancement of the ionic conductivity of the ZrO2 NT films by ion doping and device development.
Figure - SEM image of a 3-layer slip casted tubular YSZ scaffold sintered at 1350 °C for 3 h prior to Ni nitrate solution infiltration at two magnifications, (a) 500x and (b) 10000x [P. Keyvanfar, A. R. Hanifi, P. Sarkar, T. H. Etsell, and V. I. Birss, ECS Transactions, 68 (1) 1255-1263 (2015)].
We also study the long-term stability of infiltrated Ni/YSZ anodes, as instability of infiltrated Ni particles at SOFC operating temperature (800 °C) is quite challenging (Fig. 2a). For that purpose, factors, such as the amount of infiltrated Ni, ex-solution of Ni (after infiltration and heat treatment at high temperatures), using ethylene-glycol-based nitrate infiltration solution instead of aqueous-based nitrate solution, addition of a second ceramic phase infiltration and also YSZ scaffold microstructure on the long-term stability of infiltrated Ni/YSZ anodes have been examined by our group. Each of these factors can affect the connectivity of infiltrated Ni particles at high temperatures and consequently, change the long-term stability of infiltrated Ni/YSZ anodes.
FIGURE-Area-corrected EIS results obtained from symmetrical tubular cells that were vacuum-infiltrated with Ni, (a) with the first 12 infiltrations based on aqueous Ni nitrate containing Triton-X, at 90 ⁰C, and then the last 3 infiltrations involving the use of aqueous Ni nitrate containing urea (18±1 wt% Ni). From: P. Keyvanfar, A. R. Hanifi, P. Sarkar, T. H. Etsell, and V. I. Birss, ECS Transactions, 68 (1) 1255-1263 (2015).
μ-SOFCs based on metal oxide nanotubes
Our interests are also focused on a new micro-solid oxide fuel cell (μ-SOFC) design that is based on anodically grown ZrO2 nanotubular (NT) films. The very desirable features of ZrO2 NTs, such as their high surface area-to-volume ratio, can be exploited by electrocatalyst infiltration into the NTs. In this way, extremely long triple phase boundaries (TPBs) can be achieved and the NT base (15–25 nm thickness) could be utilized as the ultra-thin electrolyte, yielding high power densities at low temperatures (< 600 °C). As a first step, we have carried out a thorough electrical characterization of the anodically grown ZrO2 NTs using impedance spectroscopy and obtained a conductivity of 1.6 × 10−6 S cm−1 at 600 °C in N2, approximately twice that reported for dense ZrO2 films measured at the same temperature in air. Our ongoing efforts focus on the enhancement of the ionic conductivity of the ZrO2 NT films by ion doping and device development.