Study Of Anti-flooding Gas Porous Electrode

At present, despite the great advances in proton exchange membrane fuel cells (PEMFCs) technology over the past two decades through intensive research and development activities, their large-scale commercialization is still hampered by the higher materials cost and lower reliability and durability. In this context, proper water management is of vital importance to achieve maximum performance and durability from PEMFCs. However, when the liquid water generation rate at the cathode, by electro-osmotic drag and the ORR, exceeds the liquid water removal rate from the cathode by back diffusion to the anode, evaporation, water vapor diffusion and liquid water capillary transport through the GDL, the water flooding appears. With water accumulation and the cathode flooded, oxygen starvation,and even oxygen depletion would occur at the cathode. In this case, protons H+ reduction reaction (PRR) carries on at the cathode of PEMFC rather than oxygen reduction reaction (ORR). The potential of PRR is 1.23V less than that of ORR; it would cause a remarkable decline of cathode potential as ORR was replaced by PRR. The output voltage of a single cell with oxygen starvation would be likely reversed. This phenomenon was defined as"voltage reversal effect"(VRE) in this paper, which will lead to a significant, sometimes catastrophic, decrease in cell performance. Over the last two decades, extensive research work has been carried out on water flooding, including prediction through numerical modeling, detection by experimental measurements. The flooding mitigation strategies are mainly focused on system design and engineering such as the design of cell components and the manipulation of operating conditions, which is often accompanied by significant parasitic power loss. However, there are few reports that aimed to overcome water flooding happening in the pores of a porous electrode so far, which is just the headstream of water flooding.In fact, since water flooding happens within the membrane electrode assemblies (MEA), a simpler approach for water management through material design and engineering of the components of the MEA is preferred because it does not usually have an associated parasitic load. Thus, as a new attempt of material design and engineering of the cathode and/or anode to address water management in the PEMFCs, modification of the microstructures of the catalyst layer has been carried out in this paper.Firstly, a MnO₂–Pt/C composite electrode was designed to solve the VRE caused by oxygen starvation, which was based upon the fact that the electrochemical reduction of MnO₂ has almost the same Nernstian potential as the ORR. It has been found that the introduction of MnO₂ into the Pt/C catalyst not only can alleviate, to a certain extent, the problem VRE in the case of oxygen starvation, but also play a synergistic role with Pt/C in catalysis of the ORR in the case of oxygen rich conditions. The impedance spectra of the MnO₂–Pt/C and Pt/C electrodes further confirm that MnO₂ in the composite electrode does substitute for oxygen as an electron-acceptor in the case of oxygen starvation. The discharged MnO₂ can recover to its initial state regardless of oxygen rich or oxygen starvation conditions. Thus, it may be possible to apply the proposed composite MnO₂–Pt/C electrode for practical use.Secondly, an anti-flooding electrode (AFE) was prepared by introduction of water-proof oil, dimethyl-silicon-oil (DMS) into the conventional Pt/C electrode. The experiments results indicate that the DMS mainly distributes in the pores with diameter ranging from 20 to 70 nm. The single PEMFC cell with the AFE cathode displays much better power output not only in the case of water flooding but also in the case of a well-designed operational condition than the cell with the conventional cathode. The novel anti-flooding electrode displays outstanding anti-flooding capability, especially in the case of a large current density and over-humidification. The success of the AFE in anti-flooding lies in that (1) it solves the water flooding to the porous electrode itself rather than the water accumulation in the gas channels of the bipolar plate, and (2) it solves the water flooding of the pores with a diameter of 20 to 70 nm, in which water flooding frequently happens and is not easy to remove by the routine ways.In the part three, electrochemical impedance spectroscopy (EIS) studies have been carried out in order to evaluate the anti-flooding capacity of the AFE. From the dependences of the EIS on the overpotentials, the impedance properties were analyzed in terms of the thin film/flooded-agglomerate dynamics in the catalyst layer. The EIS study demonstrated that the excellent anti-flooding capability of the AFE in the case of completely flooding lies in that AFE alleviates the thin film diffusion effect in the case of flooding due to the DMS has occupied partial pores in the agglomerate before they are flooded by product water, thus prevents the conversion of oxygen diffusion type from faster agglomerate diffusion to slower thin film diffusion to some extent, and therefore provides the unoccupied channels with high solubility of oxygen in such DMS-filled channels for oxygen transportation.In the part four, an ordered anti-flooding oxygen electrode was prepared by adding water-proof oil DMS into the conventional MnO₂/C electrode, thus the channels for oxygen transportation and OH- ions are orderly allotted between the channels/pores occupied by DMS and electrolyte, which makes the oxygen and OH- hold their own fixed and stabile transport channels, respectively. The success of the ordered anti-flooding oxygen electrode in anti-flooding lies in that it solves the water flooding to the catalyst layer of oxygen electrode used in AFE and metal/air batteries due to the PTFE degradation and physical wetting phenomenon in alkaline electrolyte.Finally, A novel anode for preventing liquid sealing effect in the DMFC was invented by adding water-proof oil DMS into the conventional PtRu/C electrode. The novel electrode displays outstanding capability in preventing liquid sealing effect. The performance of DMFC was increased from 42.2 to 51.6 mW cm-2 with substitution of the PLSE for the CPRE. The success of the novel anode structure lies in that the solubility and diffusion coefficient of CO₂ in the water-proof oil DMS are higher than in methanol-water solution. Then, the hydrophobic DMS supplies the unoccupied channels for CO₂ transportation, which are separated from the chnnels for methanol solution diffusion. Therefore, the introduction of MDS effectively prevents the Liquid Sealing Effect (LSE) to CO₂.