A single polymer electrolyte fuel cell has been directly hybridized to a stack of three supercapacitors: the system formed has been investigated in operation in the fuel cell dynamic load cycle, which emulates the energy demand in transported applications. Comparison with regular, non‐hybridized fuel cell operation was analyzed in terms of hydrogen consumption in the case that the gas flow rates are directly controlled by cell current during the cycles with constant gas stoichiometric factors: the smoothening effect of the supercapacitors in the overall circuit leads to more even profiles of the cell current and voltage in the cycle, which allows safer and better hydrogen consumption management in this regime: the average H2 consumption per cycle could be reduced by 16% without change of the overall energy produced. Besides, the runs were conducted over more than 1,300 hours with evaluation of the fuel cell performance and capacity at regular intervals, with or without hybridization. A moderate positive effect of hybridization was observed in the time variations of the voltage‐current curves and the fuel crossover. However, the resistances for ohmic, charge transfer and diffusion phenomena, were not so much improved by the hybridization, in spite of less sharp voltage.
Operation of PEM fuel cells at very low voltages can occur in the case of direct hybridization with supercapacitors, depending on their charge level: in such cases, the cell acts as reliable current sources that can be used with low voltage devices. In spite of the growing importance of this electrical mode aging phenomena for such conditions are still unknown. A single 100 cm2 cell was operated for more than 2,000 hours at short circuit. The current delivered by the cell provided with an MEA after suitable maturation, was shown to be approx. 98% of that corresponding to the injected hydrogen flow rate. The state‐of‐health of the cell was followed by impedance measurements and determination of the electrochemical surface area of the cathode and the fuel crossover. Significant, regular decay rate of the cell voltage after the low‐voltage run was observed to range from 30 to 100 µV h−1 depending on the current density. The features of the cell were little‐to‐moderately affected by the particular regime: 43% of ECSA was lost which affects only little the FC performance, and hydrogen crossover remains unchanged. Only the diffusion resistance was increased up to fourfold at high current density.
CO2 sequestration by reaction with abundant, reactive minerals such as olivine has often been considered. The most straightforward, direct process consists in performing the reaction at high temperature and CO2 pressure, in view to producing silica, magnesium and iron carbonates and recovering the traces of nickel and chromite contained in the feedstock mineral. Most of direct processes were found to have an overall cost far larger than the CO2 removal tax, because of incomplete carbonation and insufficient properties of the reaction products. Similar conclusions could be drawn in a previous investigation with a tubular autoclave. An indirect process has been designed for high conversion of olivine and the production of separate, profitable products e.g. silica, carbonates, nickel salts, so that the overall process could be economically viable: the various steps of the process are described in the paper. Olivine particles (120 μm) can be converted at 81% with a low excess of acid within 3 h at 95°C. The silica quantitatively recovered exhibits a BET area over 400 m2 g-1, allowing valuable applications to be considered. Besides, the low contents of nickel cations could be separated from the magnesium-rich solution by ion exchange with a very high selectivity.
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