Initial Testing of Solutions to Redox Problems with Anode-Supported SOFC

Abstract

The most commonly used solid oxide fuel cell (SOFC) anode material is a two phase nickel- and yttria-stabilized zirconia (Ni/YSZ) cermet. During typical fuel cell operation, this material remains a cermet; however, in a commercial SOFC system seal leakage, fuel supply interruption, or other situations may cause the anode to reoxidize. The cyclic reduction and oxidation (redox) of nickel results in large bulk volume changes, which may have a significant effect on the integrity of interfaces within the fuel cell and thus may cause significant performance degradation. Following initial study of the degradation mechanisms and redox kinetics, a number of solutions have been devised. These can be broadly categorized into system solutions and unit cell or materials solutions. The former includes purge gas, other means to produce a reducing gas, cell reversal, and passive solutions such as hydrides, oxygen getters, and steam purging. The latter includes alternative anode materials, microstructural modification of anode functional layers (AFLs), graded porosity, and composition of AFL and redox barrier layers. This paper discusses the pros and cons of each approach and some initial feasibility testing in short-stack repeat units.

Durability of Ni anodes during reoxidation cycles

“…The authors concluded that there is an optimum composition and microstructure for substrates and anodes which yield a minimum redox strain and lead to mechanical degradation. Considering the various approaches in this field and to some extent inconsistent results, further investigations on optimizing the microstructure are required [1,9]. The influence of particle sizes, particle size ratios, porosity, and pore size distribution and composition on the redox tolerance of substrates and anodes has yet to be clarified as does the potential for a technological solution of the redox problem by optimizing the microstructure.…”

“…The reoxidized nickel can be re-reduced, but various investigations have revealed that the structure of substrate and anode cannot be restored [2][3][4][5][6][7][8]. The structural changes in the substrate and anode upon reoxidation lead to dimensional changes that generate stresses in substrate, anode and other cell components, potentially promoting damage in all layers of the cell and therefore degrading cell performance or even causing complete failure of the cell [1,3,5,6,9,10].…”

“…In real fuel cell power systems emergency (shut down) procedures, eventually including inert gas purging, have to be implemented in the control system and they automatically start in case of accidental failures. They are needed to comply with safety standards, but they may be also useful to protect the electrochemical module from further damages [9,12].…”

Redox Tolerant SOFC Anodes with High Electrochemical Performance

“…Although it has been shown that some improvement of the redox stability is possible by optimisation of the microstructure [3,8,9], it remains a challenge to construct an anode cermet that is really tolerant to redox cycling. Especially for anode supported cells, tensile stresses associated with the expansion of the substrate due to nickel oxidation are often accomodated by cracks in the electrolyte and cathode layer, which leads to gas cross-over and thereby efficiency loss, and efforts to improve the redox tolerance had only limited success [10][11][12].…”

A possible solution to the mechanical degradation of Ni–yttria stabilized zirconia anode-supported solid oxide fuel cells due to redox cycling

“…Maintenance solutions to the redox cycling problem currently include purging with N 2 gas, but this is not always feasible in remote locations [12]. System solutions to the redox cycling problem are generally very costly due to the high operating temperature [13,14], are being investigated, no Ni-free anodes have yet been proven to be adequate in terms of performance or longterm operation.…”