Effects of anode orientation and flow channel design on performance of refuelable zinc-air fuel cells

The effects of anode orientation (whether an anode is located above or under a cathode) and flow channel design (parallel or serpentine flow channel) on the performance of refuelable zinc-air fuel cells (RZAFC) continuously fed with KOH electrolyte were investigated. The performance test was conducted at different electrolyte flow rates of 2, 4, and 6 ml h^?1. A polarization test of the cell was conducted at the initial stage of operation, followed by a long-term current discharge test in potentiostatic mode. The spent zinc powders were characterized by a scanning electron microscope and X-ray diffraction. The experimental results revealed that the anode-bottom orientation in the cell performed much better than the anode-top orientation with 11.4 times higher zinc utilization. The performance reduction of the anode-top orientation cell was caused by the cathode overpotential, due to the flooding of the cathode by water crossover from the anode, which was induced by the gravity force. For the flow channel design effects, there was an optimum electrolyte flow rate, to yield a maximum current discharge capacity, of 4 ml h^?1 in this study. At this optimum flow rate, the total charge per gram of zinc delivered from the anode serpentine cell was 1.75 times higher than that from the anode-parallel one.     

Promotional roles of Ru and Sn in mesoporous PtRu and PtRuSn catalysts toward ethanol electrooxidation

The roles of Ru and Sn in mesoporous PtRu and PtRuSn alloys for ethanol electrooxidation reaction were investigated. The catalyst samples were prepared via co-reduction of metal precursors in an aqueous domain of lyotropic liquid crystalline phase of a nonionic surfactant. The crystallite sizes, obtained from x-ray diffractograms of mesoporous PtRu and PtRuSn catalysts, were approximately the same at 3.6 nm. There was a good agreement between the measured lattice parameter and the one calculated from a modification of Vegard's Law suggesting that the ternary PtRuSn alloy had formed. From XPS analysis, the surface species of Ru in both catalysts is in metallic form while Sn in PtRuSn is in SnO phase. The electrochemical measurements in ethanol solution revealed that PtRuSn had exhibited a lower onset potential by about 0.1 V, and also produced a significantly higher oxidation current density than had PtRu. In addition, the chronoamperometry tests demonstrated a lower poisoning rate for ethanol oxidation on the PtRuSn surface at the low potential of 0.3 V vs RHE. However, the adsorbed poisoning species had been effectively oxidized from the surface of PtRu, and consequently shown to be the most poison-tolerant catalyst at a high potential of 0.6 V and a high temperature (60 °C). As a result, Ru and Sn addition exhibited different promotional effects for ethanol oxidation. The addition of Sn promoted dissociative adsorption of ethanol molecules, while the addition of Ru activated the water molecules, which was followed by the oxidation of the strongly adsorbed CO. The added Ru and Sn had enhanced the overall ethanol oxidation on the PtRuSn catalyst.     

A comparison of direct methanol fuel cell degradation under different modes of operation

The degradation test of direct methanol fuel cells (DMFCs) under on–off operation with variable load profile, known as a variable load on-off operation (VLOO) mode, was compared to continuous operation with a constant load, known as a constant load continuous operation (CLCO) mode. It was found that the performance loss in the VLOO mode was higher than that under the CLCO mode in terms of open circuit voltage drop (3.6 times higher) and the maximum power density reduction rate (1.23 times greater). Under both operation modes, the anode catalyst was deteriorated by Ru dissolution. However, the faster decay of cell performance under VLOO mode was mainly caused by the increase of cathode polarization due to higher methanol crossover and the decrease of Pt activity to the oxygen reduction reaction induced by Ru deposition.     

The fading behavior of direct methanol fuel cells under a start-run-stop operation

Stability tests of direct methanol fuel cells (DMFCs) were conducted under two different operational modes, start-run-stop (SRS) and long-running (LR) modes, to investigate the difference in performance decay of the cells. Frequency response analysis (FRA), cyclic voltammetry (CV), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) were used to identify the causes of cell degradation. The cell performance test results showed that the fading behavior of the cell under the SRS operation was greater than that under the LR operation. The maximum power density was reduced approximately 20% and 32% of the initial value after operating under the LR and the SRS mode, respectively. This result was corresponded with the anode catalyst agglomeration data obtained from both XRD and TEM analysis. The increase of PtRu particle size under the SRS operation was higher than that under the LR operation. The FRA spectra showed that the anode reaction resistance increased from the initial value of 0.26 Ω cm² to 0.30 Ω cm² after life-testing under SRS mode for 45 h. A right-shift of the methanol oxidation peak and a 5.0% reduction of electrocatalyst surface area observed from the cyclic voltammograms also supported this finding. Finally the decay of cell performance was due to the Ru crossover, as verified by EDX results.