Numerical analyses on oxygen transport resistances in polymer electrolyte membrane fuel cells using a novel agglomerate model

Characterizing oxygen transport resistances in different components of a polymer electrolyte membrane fuel cell (PEMFC) is essential to achieve better cell performance at high current under low Pt loading. In this work, a macroscopic three-dimensional model, together with a novel agglomerate model was proposed to analyze impacts of operating conditions on these resistances via limiting current strategy. By introducing a focusing factor obtained with lattice Boltzmann method at mesoscopic level, the structure-dependent local transport resistance in ionomer thin-film of the electrode was comprehensively captured and validated by existing experimental studies. Contributions of the cell components to the total transport resistance were dissected. Results show that the present agglomerate model could well reproduce the local transport behaviors of oxygen in catalyst layer by fully considering the detailed nanoscale diffusion and adsorption processes. A small mass fraction of oxygen was favored to minimize the relative deviation of the local transport resistance from its intrinsic one due to the water production and heat generation, which can reach 7% for the mass fraction of oxygen of 1%. Contribution of the in-plane diffusion of oxygen in the inactive electrode is around 1%. The total transport resistance increased with the absolute pressure, mainly due to the dominated molecular diffusion mechanism in gas channel and gas diffusion layer. Gas convection accounted for 26% of the oxygen transport resistance originated from gas channel. The transport resistance of catalyst layer increased significantly with the reduction of Pt loading, and decreased with relative humidity and operating temperature, particularly at high Pt loading.     

Determination of liquid-solid mass transfer coefficients for a spinning disc reactor using a limiting current technique

Liquid-solid mass transfer performance on a 30 cm diameter spinning disc reactor is determined by use of the limiting current technique for copper deposition at different radial locations. Values for the local mass transfer coefficient are determined for a range of liquid flow rates and rotational speeds. The experimental data is compared with a model based on diffusion into a laminar flowing film and the enhancement in performance over this model is examined.     

Mechanism of improving oxygen transport resistance of polytetrafluoroethylene in catalyst layer for polymer electrolyte fuel cells

Oxygen transport resistance of catalyst layer (CL) has significant impact on the performance for polymer electrolyte fuel cells (PEFCs). Nano-Polytetrafluoroethylene (PTFE) particles are added into CL to improve the oxygen transport resistance. The CV curves indicate that PTFE do not reduce the utilization of Pt. The IV polarization curves suggest that the performance incorporated PTFE in CL gradually improve at high current densities and the output is 0.57 V at 1.8 A cm^?2, 70 mV higher than that without PTFE. The water contact angle for CL with 20 wt% PTFE shows that continuous hydrophobic network may not be formed at 150 °C heat treatment temperature. The total transport resistance of CL with PTFE decreases about 2.5% at 70 °C and 250 kPa, mainly caused by the reduction of pressure-independent resistance (R_other). In the R_other reduction, the Knudsen diffusion resistance reduction in CL account for 74%. The pore size distributions reveal that the porosity increases 29% and the proportion of pores at around 100 nm increases for primary pores in CL with PTFE. This finding indicates that not the hydrophobicity of PTFE but the porous structure conducive to Knudsen diffusion for CL plays the predominant role in improving the performance.     

Out-of-cell measurements of H2-H2O effective binary diffusivity in the porous anode of solid oxide fuel cells (SOFCs)

The effective binary diffusivity of H₂ and H₂O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800 °C using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective binary diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H₂ partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.     

Effects of property variation and ideal solution assumption on the calculation of the limiting current density condition of alkaline fuel cells

A computational study of the electrochemical hydrodynamic process in an alkaline fuel cell was conducted. The computation relaxed the ideal solution assumption, accounted for thermodynamic solubility of the reactants, and allowed for property variations due to temperature and concentration effects. The results showed that the ideal solution assumption is not adequate for calculation of the transport process of the concentrated electrolyte considered, 7 M. The ideal solution formulation resulted in a lower limiting current density condition by about 50% than that predicted by the non-ideal solution formulation. The study also showed that the thermal condition is important to the calculation of the limiting current density condition. The calculated limiting current density increased by about 30% when the boundary condition was changed from isothermal to adiabatic. The computational results suggest that maintaining a uniform KOH concentration in the electrolyte (for example, at design point of 7 M) be an effective measure to increase the limiting current density condition.     

Factors affecting limiting current in solid oxide fuel cells or debunking the myth of anode diffusion polarization

Limiting current densities for solid oxide fuel cells were measured using both button cells and a flow-through cell. The cell anodes were supplied with mixtures of humidified hydrogen and various inert gasses. It was demonstrated that the true limiting current in flow-through cells is reached when either: the hydrogen is nearly or completely depleted at the anode-electrolyte interface near the outlet; or when the concentration of steam at that interface becomes high enough to interfere with adsorption or transport of the remaining hydrogen near the triple-phase boundaries. Choice of inert gas had no effect on limiting currents in the flow-through tests, indicating that diffusion within the porous anode had no significant effect on cell performance at high currents. In the button cells, the apparent limiting currents were significantly changed by the choice of inert gas, indicating that they were determined by diffusion through the bulk gas within the support tube. It was concluded that the apparent limiting currents measured in button cells are influenced more by parameters of the experimental setup, such as the proximity of the fuel tube outlet, than by the physical properties of the anode.     

Pore-scale study of effects of different Pt loading reduction schemes on reactive transport processes in catalyst layers of proton exchange membrane fuel cells

Reducing the Platinum (Pt) loading while maintaining the performance is highly desired for promoting the commercial use of proton exchange membrane fuel cells (PEMFCs). Different methods have been adopted to fabricate catalyst layers (CLs) with low Pt loading, including utilizing lower Pt/C catalysts (MA), mixing high Pt/C catalysts with bare carbon black particles (MB), and reducing CL thickness while maintaining high Pt/C ratio (MC). In this study, self-developed pore-scale model is adopted to investigate the performance of the three Pt reduction methods. It is found that MA shows the best performance while MB shows the worst. Then, effects of Pt dispersion are further explored. The results show that denser Pt sites will result in higher local oxygen flux and thus higher local transport resistance. Therefore, MA method, which shows the better Pt dispersion, leads to improved performance. Third, CLs with quasi-realistic structures are investigated. Higher tortuosity resulting from the random pores produces higher bulk resistance along the thickness direction, while MA still exhibits the best performance. Finally, improved CL structures are investigated by designing perforated CL structures. It is found that adding perforations can significantly reduce the bulk transport resistance and can improve the CL performance. It is demonstrated that CL structure plays important roles on performance, and there are still huge potentials to further improve CL performance by increasing Pt dispersion and optimizing CL structures.     

Pulsed current water splitting electrochemical cycle for hydrogen production

A pulsed current 3 D MnO₂ electrode water splitting electrochemical cycle is being proposed for hydrogen production. In 3D MnO₂ electrochemical cycle, the reactions take place at the solid/liquid and solid/gas two phase boundaries. Also, this electrochemical cycle should be able to generate hydrogen and oxygen gas separately at different periods of time. Here, we applied an interrupted pulsed current to reduce the overpotential caused by diffusion layers in conventional direct current electrolysis. The pulsed current, which disturbs the formation of the ion diffusion layer in the vicinity of the electrodes, is observed to be effective above 50 Hz. The best electrolysis performance was recorded at a current density of 0.2 A cm^?2, and the observed cell voltage was 1.69 V at 25 °C for a pulse frequency of 500 Hz, which is less than the corresponding conventional alkaline electrolysis.     

Liquid and oxygen transport in defective bilayer gas diffusion material of proton exchange membrane fuel cell

In this paper, pore network simulations are carried out to explore the effects of micro porous layer (MPL) and its crack location on the liquid and oxygen transport in the gas diffusion material (GDM) of proton exchange membrane fuel cell (PEMFC). The constructed network is composed of cubic pores connected by throats of square cross section. The GDM is partially screened by the land, and the MPL is assumed to have a crack. When the MPL crack is considered under the land in the model, the predicted results agree with experimental findings regarding the effect of MPL on the liquid saturation and distribution in the GDM. This indicates that the liquid may prefer to flow through the MPL crack under the land. The role of MPL in the fuel cell performance is revealed to be dependent on the oxygen effective diffusivity of MPL and GDL. Therefore, caution should be taken before employing the MPL to improve the cell performance. Based on the present studies, some guidelines are gained for the GDM design and optimization.     

Converting rice husk activated carbon into active material for capacitor using three-dimensional porous current collector

We have successfully applied rice husk activated carbon (RHAC) as an active material for the electric double layer capacitor using a three-dimensional (3D) porous current collector. The capacity and cycle stability were evaluated in a 1.0 mol dm⁻³ tetraethylammonium tetrafluoroborate/propylene carbonate solution in the range of 0-2.5 V. The specific capacity of the RHAC was about 14 mAh g⁻¹ at the 50 mA g⁻¹ discharge rate, corresponding to 19 F g⁻¹ under the present conditions. The RHAC cell using the 3D porous current collector possessed a lower internal resistance and better high-rate discharge properties than the RHAC cell using a conventional aluminum (Al) foil collector. After 5000 cycles of charging and discharging, the RHAC cell with the 3D current collector maintained 95% of its initial capacity, while the capacity of the one with the Al foil collector dropped to only 30%.     

Development of an experimentally validated semi-empirical fully-coupled performance model of a PEM electrolysis cell with a 3-D structured porous transport layer

A semi-empirical non-isothermal model incorporating coupled momentum, heat and mass transport phenomena for predicting the performance of a proton exchange membrane (PEM) water electrolysis cell operating without flow channels is presented. Model input parameters such as electro-kinetics properties and mean pore size of the porous transport layer (PTL) were determined by rotating disc electrode and capillary flow porometry, respectively. This is the first report of a semi-empirical fully coupled model which allows one to quantify and investigate the effect of the gas phase and bubble coverage on PEM cell performance up to very high current densities of about 5 A/cm². The mass transport effects are discussed in terms of the operating conditions, design parameters and the microstructure of the PTL. The results show that, the operating temperature and pressure, and the inlet water flowrate and thickness of the PTL are the critical parameters for mitigating mass transport limitation at high current densities. The model presented here can serve as a tool for further development and scale-up effort in the area of PEM water electrolysis, and provide insight during the design stage.