Development of a novel pseudo bipolar piezoelectric proton exchange membrane fuel cell with nozzle and diffuser

Previous studies of a piezoelectric proton exchange membrane fuel cell with nozzle and diffuser (PZT-PEMFC-ND) have shown that a PZT device could solve the cathode flooding problems and improve cell performance. In this study, an innovative design for a PZT-PEMFC-ND bi-cell with pseudo bipolar electrodes is developed to achieve a higher power in the stack design to solve water-flooding problems and improve cell performance. This new design, with a reaction area of 8 cm², contains two cells with two outside anodes and two inside cathodes that share a common PZT vibrating device for pumping air flow. The influence of the varying aspect ratio (AR) of the diffuser elements on the unit cell-flow rate is investigated using a three-dimensional transitional model. The simulation results show that a proper AR value of 11.25 for the diffuser, with a smaller diffuser angle of 5°, could ensure a smoother intake of the air and, thus, better cell performance. The experimental results show that the performance of the PZT-PEMFC-ND bi-cell can be 1.6 times greater than that of the single cell.     

The performance of a novel pseudo-bipolar bi-cell piezoelectric proton exchange membrane fuel cell with a smaller nozzle and diffuser

Previous studies of a bi-cell piezoelectric proton exchange membrane fuel cell with a nozzle and diffuser (PZT-PEMFC-ND bi-cell), using a novel pseudo-bipolar design, have shown that the performance of the bi-cell could be 1.6 times greater than that of the single cell. In this study, this novel design, using a reduced nozzle and diffuser, contains two cells with two outside anodes and two inside cathodes that share a common PZT vibrating device for pumping air flow. The results show that the bi-cell should be operated with a larger stoichiometric ratio of 1.5 and a cell temperature of 50 °C to prevent concentration loss. Furthermore, the performance of the bi-cell using one degraded membrane electrode assembly (MEA) and one normal MEA is investigated to understand the current flow characteristic of the bi-cell. Although an internal current is observed, the bi-cell can still deliver a non-negative power. This finding will help reinforce the viability of using a PZT-PEMFC-ND bi-cell for future stack designs. Moreover, the power consumption of the PZT device is temperature-dependant and this should be taken into consideration when determining the net power of the PZT-PEMFC-ND bi-cell. The maximum net power of the bi-cell is found to be 0.7 W.     

Numerical study of a novel micro-diaphragm flow channel with piezoelectric device for proton exchange membrane fuel cells

Previous studies have shown that the amplitude of the vibration of a piezoelectric (PZT) device produces an oscillating flow that changes the chamber volume along with a curvature variation of the diaphragm. In this study, an actuating micro-diaphragm with piezoelectric effects is utilized as an air-flow channel in proton exchange membrane fuel cell (PEMFC) systems, called PZT-PEMFC. This newly designed gas pump, with a piezoelectric actuation structure, can feed air into the system of an air-breathing PEMFC. When the actuator moves outward to increase the cathode channel volume, the air is sucked into the chamber; moving inward decreases the channel's volume and thereby compresses air into the catalyst layer and enhancing the chemical reaction. The air-standard PZT-PEMFC cycle is proposed to describe an air-breathing PZT-PEMFC. A novel design for PZT-PEMFCs has been proposed and a three-dimensional, transitional model has been successfully built to account for its major phenomena and performance. Moreover, at high frequencies, PZT actuation leads to a more stable current output, more drained water, higher sucked air, higher hydrogen consumption, and also overcomes concentration losses.     

A novel ribbed cathode polar plate design in piezoelectric proton exchange membrane fuel cells

Previous studies have indicated that a novel design for proton exchange membrane fuel cells with a piezoelectric (PZT) device, which is regarded as an actuator for pumping air onto the cathode channel, can offer better performance with higher current generation. These results indicate that piezoelectric proton exchange membrane fuel cells (PZT-PEMFCs) may compress more air into the catalyst layer and thus may enhance electrochemical reactions, resulting in higher current output. At the same time, produced water vapor is pumped out from the cathode channel during the compression process. Previous studies on PZT-PEMFCs without ribs showed the strong effect of ohmic and concentration losses. In this study, a shallow rib is chosen to reduce the aforementioned losses and pressure drop in the cathode channel. The rib design is an important parameter that can be used as the support for the membrane electrode assembly (MEA). A transient three-dimensional model is built to simulate and compare the performance of PZT-PEMFCs both with and without ribs. Water vapor, oxygen, and current density profiles in the PZT-PEMFC are studied in detail. The major operating parameters include the rib width and the PZT vibration frequency. Our results show that the ribbed cathode channel can reduce ohmic losses and double current generation. Moreover, at higher PZT vibration frequency (f = 64 Hz), an air-breathing PZT-PEMFC compresses more oxygen into the catalyst layer and thus enhances the electrochemical reaction, resulting in a higher current output (0.208 A cm⁻²).     

High performance proton exchange membrane fuel cell electrode assemblies

The conventional 5-layer membrane electrode assembly (MEA) consists of a proton exchange membrane (PEM) locating at its center, two layers of Pt-C-40 (Pt content 40 wt%) locating next on both surfaces of PEM, and two gas diffusion layers (GDL) locating next on the outer surfaces of Pt-C layers (structure-a MEA). In this paper, we report three modified MEAs consisting of Pt-C-40 (Pt content 40 wt%) and Pt-C-80 (Pt content 80 wt%) catalysts. These are: (1) 7-layer structure-b MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and PEM; (2) 7-layer structure-c MEA with a thin Pt-C-80 layer locating between Pt-C-40 layer and GDL; and (3) 5-layer structure-d MEA with Pt-C-40 and Pt-C-80 mixing homogeneously and locating between PEM and GDL. Under a fixed Pt loading, we find structure-b, -c, and -d MEAs with 20-40 wt% Pt contributed from Pt-C-80 have better fuel cell performance than structure-a MEA consisting only of Pt-C-40. The reasons for the better fuel cell performance of these modified MEAs are attributed to the better feasibility for O₂ gas to reach cathode Pt particles and lower proton transport resistance in catalyst layers of the modified MEAs than structure-a MEA.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

Hydrogen generation from solid NaBH4 with catalytic solution for planar air-breathing proton exchange membrane fuel cells

In the pursuit of the development of alternative mobile power sources with high energy densities, this study elucidated a new hydrogen generation approach from solid NaBH₄ using a new catalyst, sodium hydrogen carbonate (NaHCO₃), which was placed in a small and compact cartridge. A planar air-breathing PEMFC system fitted with the cartridge has been investigated for testing hydrogen generation from NaBH₄ and NaHCO₃. NaHCO₃ allowed the hydrogen cartridge to control hydrogen generation and to improve the power density, fuel efficiency, energy efficiency, and cell response. The cell performance of solid NaBH₄ air-breathing PEMFC system strongly depended on the operating conditions: the feeding rates and concentrations of catalytic solutions for NaBH₄ hydrolysis. In various concentrations (5 - 12 wt %) of NaHCO₃ aqueous solutions, 10 wt % NaHCO₃ aqueous solution exhibited the highest maximum power density of 128 mW cm⁻² at 0.7 V, which was estimated to be a Faradic efficiency of 78.4% and an energy efficiency of 46.3%. The data illustrated that NaHCO₃ was an effective catalyst for hydrogen generation with the solid NaBH₄, which is considered as a hydrogen carrier for air-breathing micro PEMFCs operated without auxiliary hydrogen controller or devices.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

Control of a heat-integrated proton exchange membrane fuel cell system with methanol reforming

This work presents a novel heat-integrated fuel cell stack system with methanol reforming. Its configuration is composed of fuel processing units (FPUs), proton exchange membrane (PEM) fuel cell stack, and heat exchangers (HEXs). Well mixed methanol and oxygen flows in contact with countercurrent flowing water dominates the production of hydrogen at the exit of FPUs and influences the stack temperature. The heat exchange connections can enhance the utilization of energy of FPUs. To ensure the stable steady-state operation, the model-free fuzzy incremental control scheme within the multi-loop feedback control framework is developed. Finally, the proposed system integration and control configuration are verified by closed-loop simulations.     

Dynamic modeling and simulation of air-breathing proton exchange membrane fuel cell

Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications.     

Experimental evaluation of CO poisoning on the performance of a high temperature proton exchange membrane fuel cell

We experimentally studied a high temperature proton exchange membrane (PEM) fuel cell to investigate the effects of CO poisoning at different temperatures. The effects of temperature, for various percentages of CO mixed with anode hydrogen stream, on the current-voltage characteristics of the fuel cell are investigated. The results show that at low temperature, the fuel cell performance degraded significantly with higher CO percentage (i.e., 5% CO) in the anode hydrogen stream compared to the high temperature. A detailed electrochemical analysis regarding CO coverage on electrode surface is presented which indicates that electrochemical oxidation is favorable at high temperature. A cell diagnostic test shows that both 2% CO and 5% CO can be tolerated equally at low current density (<0.3 A cm⁻²) with high cell voltage (>0.5 V) at 180 °C without any cell performance loss. At high temperature, both 2% CO and 5% CO can be tolerated at higher current density (>0.5 A cm⁻²) with moderate cell voltage (0.2-0.5 V) when the cell voltage loss within 0.03-0.05 V would be acceptable. The surface coverage of platinum catalyst by CO at low temperature is very high compared to high temperature. Results suggest that the PEM fuel cell operating at 180 °C or above, the reformate gas with higher CO percentage (i.e., 2-5%) can be fed to the cell directly from the fuel processor.     

Experimental study of commercial size proton exchange membrane fuel cell performance

Commercial sized (16 × 16 cm² active surface area) proton exchange membrane (PEM) fuel cells with serpentine flow chambers are fabricated. The GORE-TEX® PRIMEA 5621 was used with a 35-μm-thick PEM with an anode catalyst layer with 0.45 mg cm⁻² Pt and cathode catalyst layer with 0.6 mg cm⁻² Pt and Ru or GORE-TEX® PRIMEA 57 was used with an 18-μm-thick PEM with an anode catalyst layer at 0.2 mg cm⁻² Pt and cathode catalyst layer at 0.4 mg cm⁻² of Pt and Ru. At the specified cell and humidification temperatures, the thin PRIMEA 57 membrane yields better cell performance than the thick PRIMEA 5621 membrane, since hydration of the former is more easily maintained with the limited amount of produced water. Sufficient humidification at both the cathode and anode sides is essential to achieve high cell performance with a thick membrane, like the PRIMEA 5621. The optimal cell temperature to produce the best cell performance with PRIMEA 5621 is close to the humidification temperature. For PRIMEA 57, however, optimal cell temperature exceeds the humidification temperature.     

About internal currents during start-up in proton exchange membrane fuel cell

This paper reports about an experimental and numerical study of the internal currents that occur during fuel cell start-up under open-circuit conditions. The internal currents were measured in a segmented cell specifically designed for this purpose and it was found that they could reach values higher than 1 A cm⁻². They result from the potential that appears at the inlet of the anode compartment while hydrogen pushes oxygen, air, or possibly nitrogen that was introduced for purging water toward the outlet. For a short time, a fraction of the channels is filled with hydrogen while the other part is still occupied by the gas initially present. The model presented in the paper demonstrates that the occurrence of internal currents can be explained mostly by capacitive effects. Carbon oxidation occurs probably simultaneously but its contribution to the internal currents is by all appearances negligible. The model also explains the transient voltage rise (over the steady state open circuit voltage) that is sometimes observed experimentally shortly after the fuel cell start-up.     

Modeling and simulation of a proton exchange membrane fuel cell using computational fluid dynamics

An isothermal, three dimensional, single phase model was presented to evaluate a proton exchange membrane fuel cell (PEMFC) with serpentine flow. The mass, momentum and electrochemical equations were solved simultaneously for the steady state condition using computational fluid dynamics (CFD) software based on the finite element method. The model considered reactions as mass source/sink terms, and electron transport in the catalyst layers and GDLs. To validate the model, the numerical results were compared to the experimental data collected from the fabricated membrane electrode assemblies. The exchange current density parameter of the catalysts was fitted by the model to calibrate the results. The model showed good agreement with experimental data and predicted a higher current density for the catalyst with a higher surface area and Pt content. The oxygen, hydrogen and water mass fraction distribution, velocity magnitude and pressure distribution were estimated by the model. Moreover, the effect of pressure and temperature, as two important operating conditions, on the current density was predicted by the validated model.     

Effect of orthohydrogen–parahydrogen composition on performance of a proton exchange membrane fuel cell

The effect of orthohydrogen–parahydrogen concentration on the performance of a proton exchange membrane fuel cell is calculated and experimentally investigated. Gibbs free energy and reversible cell potential calculations predict that parahydrogen at room temperature produces a voltage 20 mV/cell higher than normal hydrogen and a 1.6% increase in efficiency over normal hydrogen. Experimental data based on a 1 kW proton exchange membrane fuel cell rapidly switched between normal and parahydrogen did not show a statistically significant difference in performance. Variations due to stack humidity and anode purging are found to dominate fuel cell output. The experimental results confirm that, as anticipated, parahydrogen concentration has a negligible impact on fuel cell performance for the majority of practical applications.     

Maximum power cold start mode of proton exchange membrane fuel cell

Successful and fast cold start is important for proton exchange membrane (PEM) fuel cell in vehicular applications in addition to the desired maximum power in any case. In this study, the maximum power cold start mode is investigated in details and compared with other cold start modes based on a multiphase stack model. It is found that for the maximum power cold start mode, the current density is generally kept at high levels, and the performance improvement caused by the membrane hydration and temperature increment may not be observable. Therefore, before the melting point, the performance drops continuously. The maximum power cold start mode could better balance the heat generation and ice formation, leading to improved cold start survivability than that in the constant voltage and constant current modes, with a fast start-up generally guaranteed. Once the survivability can be ensured, the initial water content needs to be higher for fast cold start, suggesting that over purging should be avoided. The maximum power mode is suggested to be optimal for PEM fuel cell cold start based on the modeling results.     

Simulation studies of the membrane exchange assembly of an all-liquid,proton exchange membrane fuel cell

A model has been designed and constructed for the all-liquid, sodium borohydride/hydrogen peroxide fuel cell under development at the University of Illinois at Urbana-Champaign. The electrochemical behavior, momentum balance, and mass balance effects within the fuel cell are modeled using the Butler–Volmer equations, Darcy's law, and Fick's law, respectively, within a finite element modeling platform. The simulations performed with the model indicate that an optimal physical design of the fuel cell's flow channel land area or current collector exists when considering the pressure differential between channels, and the diffusion layer permeability and conductivity. If properties of the diffusion layer are known, the model is an effective method of improving the fuel cell design in order to achieve higher power density.     

Graphene nanosheet–CNT hybrid nanostructure electrode for a proton exchange membrane fuel cell

This work demonstrates two-step growth of graphene nanosheets (GNS), in which carbon nanotubes (CNTs) are grown directly on a carbon cloth. GNS are subsequently constructed on the CNT surface, revealing the stand-up structure of the GNS–CNT hybrid nanostructure. The GNS–CNT hybrid nanostructure shows Nernstian and fast electron-transfer kinetics for electrochemical reactions of Fe(CN)₆^3−/4−. A 0.1 mg cm⁻² Pt/GNS–CNT is used in the cathode of a proton membrane exchange fuel cell, in which the maximum power density is 1072 mW cm⁻² at 80 °C under H₂/O₂. In addition to a low-resistance electron-transfer pathway, the GNS–CNT hybrid nanostructure also provides numerous edge planes with strong electrochemical activity, ultimately enhancing electrochemical activity and fuel cell performance.     

The development of air-breathing proton exchange membrane fuel cell (PEMFC) with a cylindrical configuration

A small air-breathing proton exchange membrane fuel cell with a cylindrical configuration (Cy-PEMFC) and a helical flow-channel was developed to provide a uniform contact pressure to the membrane electrode assembly (MEA) with a thin cathode current collector. A comparison of the contact pressure and performance of the Cy-PEMFC and general planar PEMFC was performed to determine the effect of the cylindrical configuration. For the contact pressure comparison, numerical analysis was performed using commercial software. Numerical analysis showed that the Cy-PEMFC has its own structural advantage of changing the applied clamping pressure to a uniformly distributed contact pressure. The actual pressure measurements were carried out with pressure-sensitive film to support results of numerical analysis. These results also showed that the Cy-PEMFC had a uniformly distributed contact pressure, whereas the planar PEMFC did not. The polarization curves of both PEMFCs were measured to determine the performance variations caused by the uniform contact pressure and better mass transfer. The maximum power density of the Cy-PEMFC was 220 mW/cm², which was approximately 24% higher than the planar PEMFC.     

Experimental study and mitigation of abnormal behavior of cell voltage in a proton exchange membrane fuel cell stack

The aim of this study is to investigate the abnormal behavior of cell voltage in a proton exchange membrane fuel cell stack and a mitigation strategy. The proposed strategy is simple and requires only a three‐way solenoid valve to replace the direct way solenoid valve of the original system. It is applied to a proton exchange membrane fuel cell stack with a dead‐ended anode to verify its validity. The behavior of the cell voltages in the stack is discussed in detail, especially the cell reversal process. The results show that the proposed strategy can significantly reduce the severity of hydrogen starvation. And the maximum power of the stack is increased by 10.67%. It is a sudden increase related to cell reversal mitigation. Uneven hydrogen distribution is the cause of low cell voltage and cell reversal. This strategy increases the cell voltage by increasing the hydrogen content in the anode flow channel downstream. It also significantly reduces the fluctuations in cell voltage and improves the uniformity of the cell voltage. This experimental study contributes to mitigate hydrogen starvation in cells of proton exchange membrane fuel cell stacks in application.