Solid Oxide Cells and Stacks for Reversible Operation at High Current Densities

Abstract

Renewable energy sources such as solar and wind power are fluctuating by nature, and energy supply is therefore not fully controllable. Certain countries and communities have already incorporated significant share of fluctuating renewable energy in their energy system e.g. reaching a share of electricity supply from wind of 43% as reported for the Danish energy grid for 2017, which in turn also lead to periods where the supply of wind power exceeds the electricity consumption. In that perspective there is already at present a starting need for efficient, flexible and durable energy conversion and storage solutions, and the need for increased capacity of energy conversion and storage will increase steadily on the path to an energy supply aiming for substituting all fossil fuels and turning to an energy supply based 100% on renewable energy. Among a variety of energy conversion technologies reversible solid oxide cells (SOC) are a good candidate to tackle this challenge of efficient energy conversion and storage. Their capability to operate reversibly, i.e. in ‘electrolysis mode (SOEC)’ or ‘fuel cell mode (SOFC)’, allows using the surplus energy of wind and solar farms to produce H2 or syn-gas (which then can be stored) and re-using the gasses to generate electricity (and possibly heat) on demand. However for solid oxide cells to be a future viable and flexible energy conversion and storage solution it is required to develop, test and demonstrate cells and stacks optimized for reversible operation with the capability to operate at high current densities and 85% fuel utilization in both operating modes. In this study, we investigate performance and durability of optimized SOC at cell and stack level. The cells and stacks have been targeted to operate at high current densities and are operated in both constant SOFC and SOEC mode and in load cycling mode at a temperature 700°C. This in turn means reversible operation of SOC at high overpotentials and daily shift exothermic and endothermic operation. Figure 1 shows an example of cell voltage development during single cell testing of two types of cells and the analysis of cell degradation is supplemented by analysis of electrochemical impedance spectra showing that the dominating degradation of the cell originates from the degradation of the fuel electrode. Furthermore, the tested cells have been investigated by SEM and low-voltage in-lens SEM to investigate the microstructure after load cycling test. In line with the single cell test; stack test (25 cells) applying similar cells have been conducted and results will be reported. Figure 1: Long-term degradation testing of two types of single cells. SOFC operation: fuel side with dry H2, oxygen side with air, current density of 0.6 A/cm2 corresponding to a H2 utilization of 85%; SOEC mode: fuel side with gas composition of H2O/H2:90/10, oxygen side with air, current density of -1.2 A/cm2 corresponding to a H2O utilization of 85%. Cycling between modes was 16/8 hours at SOFC/SOEC. Temperature of 700 ˚C. Figure 1

Reversible Metal Supported Solid Oxide Cells

“…In addition, they can be compared to degradation rates at constant conditions in either modes as first approach. The trends of decreasing degradation rates over time and the higher degradation rates in EC compared to FC mode have been observed before on SoA cells, even though those tests were carried out at temperatures higher than 650 o C as in this work and on other specific cells (different cell suppliers) (2).…”

“…Solid oxide cells (SOCs) excel by high efficiencies in fuel cell as well as electrolysis modes, and by being able to operate in both modes as a reversible cell. The reversible concept can be combined with fluctuating electricity production from sustainable sources such as wind or solar: SOCs can produce both electricity and heat from a green fuel in fuel cell mode and store electricity as gas or make it available for other sectors in electrolysis mode (examples for demonstration of the reversible SOC at system level are (1,2). The SOC in those examples are ceramic based concepts: electrolyte supported and fuel electrode supported.…”

Development of an Efficient rSOC Based Renewable Energy Storage System

“…After testing those improvements at single cell level in terms of performance (5), two optimized cells were selected for further durability tests (6) in rSOC conditions still at single cell (53x53 mm 2 ) level. Following the results obtained, and taking into account the ability of the cells to be produced in large amounts for the needs of the project and beyond, one of the two cells was considered as Generation 2 (G2) cell for the project.…”

“…Then test #2 was performed alternating from SOEC to SOFC by daily cycles of respectively 8h and 16h. Further details about the testing conditions can also be found in (5)(6)(9)(10). In addition, the stack performance has been tested in SOFC considering CH4 as a fuel (considered as representative of natural gas which will be considered for the real system in-field).…”

Development of an Efficient rSOC Based Renewable Energy Storage System

“…Then test #2 was performed alternating from SOEC to SOFC by daily cycles of respectively 8h and 16h. Further details about the testing conditions can also be found in (5)(6)(9)(10). In addition, the stack performance has been tested in SOFC considering CH4 as a fuel (considered as representative of natural gas which will be considered for the real system in-field).…”