Reticulated polyaniline nanowires as a cathode microporous layer for high-temperature PEMFCs

Traditional dense microporous layers (MPLs) are suitable for low-temperature proton exchange membrane fuel cells (PEMFCs), but they greatly hinder mass transport in high-temperature PEMFCs. Here, we report a novel cathode MPL based on reticulated polyaniline nanowires that were grown on carbon paper via in-situ electropolymerization. The maximum power density of the high-temperature PEMFC based on the new MPL was 476 mW/cm², which was 36% higher than that based on a conventional MPL. Oxygen gain tests and electrochemical impedance spectroscopy showed that the new MPL accelerated oxygen transfer due to its unique pore size distribution, which ultimately improved the performance of the HT-PEMFC.     

Integrating high-temperature proton exchange membrane fuel cell with duplex thermoelectric cooler for electricity and cooling cogeneration

To harvest the waste heat from exothermic reaction processes, a novel hybrid system model mainly incorporating a high-temperature proton exchange membrane fuel cell (HT-PEMFC) and a duplex thermoelectric cooler is conceptualized to theoretically predict the potential performance limit. The duplex thermoelectric cooler is composed of a thermoelectric generator (TEG) and a thermoelectric cooler (TEC), where the TEG harvests the waste heat to generate electricity and the TEC utilizes the generated electricity for cooling production. A mathematical model is established to estimate the proposed system performance from both exergetic and energetic perspectives considering various irreversible effects, from which effectiveness and practicality of the proposed system can be examined. The hybrid system maximal output power density allows 14.1% greater than that of the basic HT-PEMFC, whereas the according destruction rate density of exergy is decreased by 7.7%. The feasibility and effectiveness of the proposed system configuration are verified. Moreover, substantial parametric analyses indicate that the proposed system performance can be improved by elevating the HT-PEMFC operating temperature, inlet relative humidity and doping level while worsened by enhancing the leak current density, electrolyte thickness and Thomson coefficient. The results acquired may be helpful in designing and optimizing such an actual hybrid system.     

温度、压力和湿度对质子交换膜燃料电池性能的影响

以Nafion质子交换膜燃料电池(PEMFC)为对象，通过测量电池的电流—电压、电流—功率和电压—时间曲线，研究了温度、压力和湿度等条件对电池性能的影响，同时也考察了电池的能量转换效率及短期运行时的稳定性，得出了电池较佳的工作条件。实验和计算结果表明：在反应温度为72℃、H2和02压力分别为0．2MPa、进气湿度饱和时，电池最大输出功率可达0．7W．cm^-2；在0．3W.cm^-2～0．7W．cm^-2范围内电池能量转换效率为62％—34％；在大电流密度下电池仍能稳定工作。     

Observation of flooding-induced performance enhancement in PEMFCs

In this study, water removal is observed through a transparent fuel cell, and the relationships between it and the responses of voltage and impedance are analyzed. The water flows in the channel are accompanied by voltage soaring. For all studied cases, water flow appearances increase as GDL flooding is induced by supplying more vapor; for example, under a stoichiometry ratio of 2.0, the number of water flow appearances is 0, 3, and 25 for 1800 s as relative humidity increase to 30, 50, and 100%. The average voltage is calculated, and a positive relationship between it and the frequency of water flows is determined. The results reveal that frequent flooding-induced water removal could be one strategy to enhance fuel cell performance.     

Numerical study of thermal stresses in high-temperature proton exchange membrane fuel cell (HT-PEMFC)

The purpose of this work is to numerically examine the thermal stress distributions in a high-temperature proton exchange membrane fuel cell (HT-PEMFC) based on a phosphoric acid doped polybenzimidazole (PBI) membrane. A fluid structure interaction (FSI) method is adopted to simulate the expansion/compression that arises in various components of a membrane electrode assembly (MEA) during the HT-PEMFC assembly processes, as well as during cell operations. First, three-dimensional (3-D) finite element method (FEM) simulations are conducted to predict the cell deformation during cell clamping. Then, a nonisothermal computational fluid dynamic (CFD)-based HT-PEMFC model developed in a previous study [1] is applied to the deformed cell geometry to estimate the key species and temperature distributions inside the cell. Finally, the temperature distributions obtained from these CFD simulations are employed as the input load for 3-D FEM simulations. The present numerical study provides a fundamental understanding of the stress–temperature interaction during HT-PEMFC operations and demonstrates that the coupled FEM/CFD HT-PEMFC model presented in this paper can be used as a useful tool for optimizing HT-PEMFC clamping and operating conditions.     

Graphite oxide-incorporated SnP2O7 solid composite electrolyte for high-temperature proton exchange membrane fuel cells

Proton conductors operating in a high temperature range (over 200 °C) have attracted much interest. The promising candidates for inorganic composite membranes for proton exchange membrane fuel cells (PEMFCs) using solid SnP₂O₇ and SnP₂O₇/GO at temperatures between 150 °C and 250 °C are reported. Graphite oxide (GO) incorporated with SnP₂O₇ improves the proton conductivity and reduces the crossover. The structure and phase stability of the membranes are analysed by X-ray diffraction and thermos-gravimetric analysis (TGA), and the microstructure morphology is analysed by scanning electron microscopy (SEM). The SnP₂O₇/GO composite electrolyte exhibits the proton conductivities of 0.0076 cm^?1 at 225 °C and a peak power density of 18 mW cm^?2 at 220 °C.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

Performance improvement of air-breathing proton exchange membrane fuel cell stacks by thermal management

Air-breathing is known as a way to reduce the weight, volume, and the cost of PEMFCs. In this study, the thermal management of the high-powered air-breathing PEMFC stacks by applying different cathode flow channel configurations is carried out to improve the stack performance. In order to verify the thermal management results, numerical simulation is also performed. The research results show that a combination of the 50% and 58.3% opening ratios in the air-breathing stack reduces the stack temperature and enhances the temperature distribution uniformity, leading to a better and more stable stack performance. In addition, it is found that the stack performance is significantly improved under the assisted-air-breathing condition. Moreover, the simulation results and the experimental data are basically consistent. It is suggested to adopt the average temperature over the cross-sectional flow region from simulation as fitting the simulation results and the measured data.     

Energy and exergy performance assessments of a high temperature-proton exchange membrane fuel cell based integrated cogeneration system

High-temperature proton exchange membrane fuel cell (HT-PEMFC), which operates between 160 °C and 200 °C, is considered to be a promising technology, especially for cogeneration applications. In this study, a mathematical model of a natural gas fed integrated energy system based on HT-PEMFC is first developed using the principles of electrochemistry and thermodynamics (including energy and exergy analyses). The effects of some key operating parameters (e.g., steam-to-carbon ratio, HT-PEMFC operating temperature, and anode stoichiometric ratio) on the system performance (electrical, cogeneration, and exergetic efficiencies) are examined. The exergy destruction rates of each component in the integrated system are found for different values of these parameters. The results show that the most influential parameter which affects the performance of the integrated system is the anode stoichiometric ratio. For the baseline conditions, when the anode stoichiometric ratio increases from 1.2 to 2, the electrical, cogeneration, and exergetic efficiencies decrease by 42.04%, 33.15%, and 37.39%, respectively. The highest electrical power output of the system is obtained when the SCR, operating temperature, and anode stoichiometric ratio are taken as 2, 160 °C, and 1.2, respectively. For this case, the electrical, cogeneration, and exergetic efficiencies are found as 26.20%, 70.34%, and 26.74%, respectively.     

Parametric analysis of simultaneous humidification and cooling for PEMFCs using direct water injection method

The effects of different operating parameters on humidification and cooling for proton exchange membrane fuel cells (PEMFCs) using direct water injection method were experimentally investigated. Experiments with various injection water temperature, operating pressure and relative humidity of cathode side were carried out. In order to quantitatively analyze the performance of direct water injection method, polarization curves and dew point temperatures of cathode outlet gas were measured. Also, the possible mechanisms of the effect of each parameter were discussed. The experimental results showed that elevation of the injection water temperature and relative humidity of cathode side led to the improvement of stack performance. It resulted from humidification and cooling effect by the evaporation of injected water. Operating pressure also had an effect on the performance of direct water injection method. In pressurized operating condition, the evaporation of injected water was difficult to occur, and the effect of direct water injection method decreased. Based on the experimental results, it was demonstrated that the stack performance was remarkably improved because of humidification and cooling effect from direct water injection method.     

Experimental study on spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack

This paper experimentally explores the spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack based on thermal imager and thermocouples inserted in the cathode flow channels. The temperature distribution and evolution during the dynamic process are analyzed in detail. Besides, the effects of air flow rate and load current on the thermal characteristics of the stack are also investigated. The results show that during the start-up, the hot spot first sprouts in the central area and then spreads rapidly to the surrounding area. During the shutdown, the central and lower regions are first cooled, followed by the hydrogen inlet region, and finally the endplates. The temperature during the load stepwise increase is inconsistent with that during the load stepwise decrease, showing a temperature drift phenomenon. Moreover, there is a time lag in the response of temperature and voltage to changes in current.     

Preparation of water management layer and effects of its composition on performance of PEMFCs

A membrane electrode assembly (MEA) with a novel water management layer (WML) used in proton exchange membrane fuel cell (PEMFC) was prepared. The so called WML, which was located between the carbon paper and the catalyst layer, was a sublayer composed of carbon and hydrophobic PTFE. Various parameters of the WML, including carbon loading, PTFE content and species, sintering time and temperature and pore formers, were investigated in this study. As demonstrated in our experimental results, the performance of the membrane electrode assembly (MEA) PEMFC could be significantly improved by WML in the condition of operation with dry reactive gases. The MEA with the WML exhibited more stable performance than the situation of MEA without WML during a long time running period.     

Effects of heat treatment time on electrochemical properties and electrode structure of polytetrafluoroethylene-bonded membrane electrode assemblies for polybenzimidazole-based high-temperature proton exchange membrane fuel cells

To improve cell performance, the effects of heat treatment time on the electrochemical properties and electrode structure of PTFE-bonded membrane electrode assemblies for PBI-based high-temperature proton exchange membrane fuel cells are investigated. The cell performance is observed to decrease in the high-current-density region rather than in the low-current-density region with increasing heat treatment time at 350 °C from 1 to 30 min. Microscopic studies reveal remarkable differences in the electrode structure by the agglomeration of dispersed PTFE and adjacent catalyst particles, depending on the heat treatment time. As the heat treatment time increases, only the large pore (secondary pore) volume in the electrode decreases, resulting in increase in mass transport resistance and concentration overpotential in the high-current-density region. Cell performance is not measured without heat treatment because the electrodes are not formed. When the electrodes are heat treated for 1 min at 350 °C, the best cell performance is obtained, 0.67 V at 200 mA cm⁻².     

An efficient high-temperature PEMFC/membrane distillation hybrid system for simultaneous production of electricity and freshwater

A new hybrid system model mainly consisting of a high-temperature proton exchange membrane fuel cell and a direct contact membrane distillation is proposed. According to thermodynamics and electrochemistry theories, key performance indicators of the proposed system are formulated, from which feasibilities and effectiveness as well as performance features of the proposed hybrid system are verified. Numerical calculation results indicate that the hybrid system's maximum power density, the according energetic efficiency and exergetic efficiency are, respectively, 5580.3 W m^?2, 49.4% and 23.3%, which are, respectively, 51.8%, 110.9% and 112.0% greater than that of a single fuel cell system. Simultaneously, the corresponding exergy destruction rate density is decreased by 28.1%. Direct contact membrane distillation can be regarded as an efficient waste heat recovery technology. Furthermore, extensive parametric studies show that feed water temperature, flow velocities of both feed water and permeate water, convective heat transfer coefficients of both feed side and permeate side as well as porosity of hydrophobic membrane have positive effects on the hybrid system performance, while hydrophobic membrane thickness and permeate water temperature have negative effects on the hybrid system performance. The results obtained may be beneficial to design and run such a real hybrid system.     

Numerical analysis of effects of gas crossover through membrane pinholes in high-temperature proton exchange membrane fuel cells

Durability is a major issue in the widespread commercialization of proton exchange membrane fuel cells (PEMFCs). Various failure modes have been identified over their long runtime. These mainly originate from membrane and catalyst layer failures. One of the most common failure modes in PEMFCs is due to pinhole formation in the membrane and resultant reactant gas crossover through the membrane. Gas crossover induces several critical problems in PEMFCs, including severe reactant depletion in the downstream regions, mixed potential at the electrodes, and formation of local hot spots by hydrogen/oxygen catalytic reaction, which indicates that the cell performance decreases with increasing gas crossover. In this study, we numerically investigate the effects of gas crossover on the performance of a high-temperature PEMFC based on a phosphoric-acid-doped polybenzimidazole (PBI) membrane. In contrast to previous gas-crossover studies 1 and 2 in which uniform gas crossover throughout the entire membrane has been simply assumed, our focus is on examining the impacts of localized gas crossover due to membrane pinholes. Numerical simulations are carried out via arbitrarily assuming pinholes in the membrane. The simulation results clearly show that the presence of pinholes in the membrane significantly disrupts the species, current density, and temperature distributions. Our findings may improve the fundamental and detailed understanding of localized gas-crossover phenomena through the membrane pinholes and the influence of these phenomena on high-temperature PEMFC operation.     

An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.     

Proton exchange membrane fuel cell from low temperature to high temperature: Material challenges

Proton exchange membrane fuel cell (PEMFC) is considered as one promising clean and highly efficient power generation technology in 21st century. Current PEMFC operating at low temperatures (<80 °C) encounters several difficulties, such as CO tolerance, heat rejection, which can be, to a great extent, surmounted at higher temperatures (120–150 °C). However, the higher temperature conditions are much more challenging to implement, particularly with regards to the durability of the cell component materials. This paper overviews the drivers behind the interest in high-temperature PEMFC, and the challenges in developing novel materials to enable high-temperature PEMFC, including cell component durability (catalysts, polymer, bipolar plates, etc.), candidate polyelectrolytes for the electrode catalyst layer, and material compatibility in novel membrane electrode assembly (MEA), and provides an insight into the material research and development for PEMFC.     

Numerical investigation of different loads effect on the performance of planar electrode supported SOFC with syngas as fuel

Reducing greenhouse gas emission such as carbon dioxide is the goal of each country. As a developing country with coal as the dominant energy source, China confronts the pressure of saving energy and reducing emission by using coal efficiently and cleanly. Integrated gasification fuel cell (IGFC) hybrid power generation system is one of optimal system for clean coal utilization. In this work, the three-dimensional mathematical models for planar solid oxide fuel cell (SOFC) with syngas as fuel was constructed, and its performance was numerically investigated at different loads. Under all calculating conditions, the optimal power density is obtained at current density of 4700 A m⁻², where the output voltage is 0.57 V and the power density is 2671.01 W m⁻². With the increasing of current density, the temperature increase as well. And also the difference of max- and min-temperature in SOFC enhances. But the ohmic over-potential changes unobvious. Furthermore, the change rate of species is nearly linear with the increment of current density.     

Optimizing the relative humidity to improve the stability of a proton exchange membrane by segmented fuel cell technology

The segmented fuel cell technology was applied to investigate the effects of the humidification conditions on the internal locally resolved performance and the stability of the fuel cell system. It was found at certain operating conditions, the time-dependent oscillation of current at potentio-static state appeared. The appearance of positive spikes of current indicated a temporary improved performance, while the negative current spikes indicated a temporary decreased performance. The periodic build-up and removal of liquid water in the cell caused unstable cell performance. Through the analyses of the evolution of the locally resolved current density distributions, the reasons for the positive or the negative spikes of current peaks with respect to a stationary value were found, which might be due to the drying-out of the membrane or the flooding of the membrane. The contour of the current density mapping differed to each other at the period of current peaks up or down, which might be due to different effect of the drying-out or flooding on the membrane. Through optimizing the relative humidity of anode (RHa) or cathode (RHc) of the fuel cell, the oscillation of the current disappeared and the performance of the cell became stable. RHc affects the performance of fuel cell much more obviously than RHa. The stability of the fuel cell system is also dependent on the imposed voltage. With the cell voltage decreased, the amplitude and the frequency of positive spikes of current increased.     

Effect of catalytic ink on sub-freezing endurance of PEMFCs

Effect of catalytic ink on sub-freezing endurance of proton exchange membrane fuel cells (PEMFCs) was investigated in this paper. By direct spraying method, a catalyst-coated membrane (CCM) was fabricated with isopropyl alcohol as organic solvent (CCM-A), and CCM-B was fabricated with isopropyl alcohol and butyl acetate. The hydrophobicity of the two CCMs was similar proved by contact angle tests, and CCM-B showed larger pore volume demonstrated by mercury intrusion tests. Initial cell performance and relevant electrochemical characteristics of the two CCMs were measured and compared. CCM-B showed better performance and larger electrochemical active surface area (ECA). By analyzing the electrochemical impedance spectra (EIS) at low current densities, the ionic resistances of the catalyst layers were calculated. Results indicated that adding butyl acetate to the catalytic ink benefited the ionic resistance. Then, the fuel cells with the two CCMs were subzero stored at −20 °C with saturated residual water. After 20 freeze–thaw cycles, the CCM prepared with isopropyl alcohol and butyl acetate showed less degradation in terms of polarization curves and EIS. And the ionic resistances of the both CCMs decreased to a certain extent.