Most generally, due to concentration overpotentials leading to a limiting current at the cell level, to which a non-homogeneous distribution of gases into the stack can be added, steam conversion rates of 50% or below are reported. Then large flows of steam need to be overheated up to the operating temperature, which impacts the overall efficiency of the system.
The present paper presents the methodology undertaken to develop a stack design able to operate at high steam conversion rates (80% or above). Starting from a reference stack design made of 25 cathode supported cells and ferritic stainless steel interconnects, modifications of the design have been made to improve the gas distribution to the cells and its homogeneity in the stack. This improvement has first been validated at the scale of a SRU (single repeat unit, the cell active area being equal to 100 cm²). I-V curves have been recorded at 800°C, with a cathodic inlet flow constituted of 90%H2O/10%H2and an anodic flow made of air, the total cathodic flow rate varying between 9 and 30 NmL/min/cm².
It has been shown that with these design modifications, i-V curves can be recorded up to a steam conversion rate of 100%. For an inlet flow rate very low, equal to 9 Nml/min/cm², the 100% of steam conversion rate is reached for a current density of 1.15 A/cm² at 1.23V (figure 1). For an inlet flow rate of 12 Nml/min/cm², the 100% of steam conversion rate is reached for a higher current density of 1.56 A/cm² at 1.40V. In both cases, a limiting current appeared only far on the curve, for a current density of 1.06 A/cm² in the first case and 1.45 A/cm² in the second case. I-V curves have been plotted with several other inlet flow rates, and it has been shown that the lower is the inlet flow rate, the higher is the maximum steam conversion rate. However, even for the highest flow rates tested (24 and 30 NmL/min/cm²), the maximum reachable steam conversion rates remain respectively at values of 90 and 80%. These results validate the design modification, which was then implemented at the scale of a 25-cell stack.
The reference stack design, prior any modification, reaches, at 800°C, a current density of 0.85 A/cm² at 1.2V in average per cell, for a steam conversion rate of 55% and a corresponding inlet flow rate of 12 NmL/min/cm². For this value of steam conversion rate, a limiting current starts to appear. On the contrary, with the design modification, no limiting current appeared on the i-V curve. For the highest current density recorded, equal to 1.0 A/cm², corresponding to an voltage of 1.17 V/cell, the steam conversion rate was of 64% with a corresponding inlet flow rate of 12 NmL/min/cm². Therefore it can be concluded that thanks to the design modifications, the limiting current is shifted to values well above 65%, and that the stack can be operated at high steam conversion rate.
Several 25-cell stacks have been produced with either the reference design or the modified design. With the reference stacks, the voltage scattering among the 25 cells of the stack is around 15%, which is acceptable. Thanks to the design modification, the scattering decreases to a value around 5%. It shows that the design modification not only allows to reach higher steam conversion rates, but also improves the gas distribution homogeneity into the stack.
Finally, a 25-cell stack with the modified design has been integrated into a SOE system including all BoP components. It has been possible to operate this stack, at the system level, up to a value of steam conversion of 80%.