Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

A review of polymer electrolyte membrane fuel cell stack testing

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.     

Performance equations for a polymer electrolyte membrane fuel cell with unsaturated cathode feed

A mathematical formulation for the cathode of a membrane electrode assembly of a polymer electrolyte membrane fuel cell is proposed, in which the effect of unsaturated vapor feed in the cathode is considered. This mechanistic model formulates the water saturation front within the gas diffusion layer with an explicit analytical expression as a function of operating conditions. The multi-phase flows of gaseous species and liquid water are correlated with the established capillary pressure equilibrium in the medium. In addition, less than fully hydrated water contents in the polymer electrolyte and catalyst layers are considered, and are integrated with the relevant liquid and vapor transfers in the gas diffusion layer. The developed performance equations take into account the influences of all pertinent material properties on cell performance using first principles. The mathematical approach is logical and concise in terms of revealing the underlying physical significance in comparison with many other empirical data fitting models.     

New materials for polymer electrolyte membrane fuel cell current collectors

Polymer Electrolyte Membrane Fuel cells for automotive applications need to have high power density, and be inexpensive and robust to compete effectively with the internal combustion engine. Development of membranes and new electrodes and catalysts have increased power significantly, but further improvements may be achieved by the use of new materials and construction techniques in the manufacture of the bipolar plates. To show this, a variety of materials have been fabricated into flow field plates, both metallic and graphitic, and single fuel cell tests were conducted to determine the performance of each material. Maximum power was obtained with materials which had lowest contact resistance and good electrical conductivity. The performance of the best material was characterised as a function of cell compression and flow field geometry.     

Numerical investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance

The effect of the cathode catalyst layer's structure and composition on the overall performance of a polymer electrolyte membrane fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of ionomer (Nafion) loading, catalyst (platinum) loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is then used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. It is found that the performance trends are often different at intermediate versus high current densities—the former being governed by agglomerate-scale (or local) losses, while the latter is governed by catalyst layer thickness-scale (or global) losses. The presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-scale mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It is also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance, even if cost is to be disregarded.     

Comparative studies of polymer electrolyte membrane fuel cell stack and single cell

The polymer electrolyte membrane fuel cell (PEMFC) was investigated comparatively as a single cell and a 30-cell stack. Various types of Nafion membranes, such as Nafion 117, 115, 112 and 105, were tested as electrolyte within the single cell and at different temperatures, among which Nafion 112 gave the optimal result. The 30-cell stack was evaluated at different humidities and temperatures. The potential–current and power–current curves, both for single cell and the stack, were analyzed by computer simulation, whereby the kinetic and mass-transfer parameters were calculated. The long-term performance of the stack and the water production during long-term operation were also measured.     

Influence of local porosity and local permeability on the performances of a polymer electrolyte membrane fuel cell

In the literature, many models and studies focused on the steady-state aspect of fuel cell systems while their dynamic transient behavior is still a wide area of research. In the present paper, we study the effects of mechanical solicitations on the performance of a proton exchange membrane fuel cell as well as the coupling between the physico-chemical phenomena and the mechanical behavior. We first develop a finite element method to analyze the local porosity distribution and the local permeability distribution inside the gas diffusion layer induced by different pressures applied on deformable graphite or steel bipolar plates. Then, a multi-physical approach is carried out, taking into account the chemical phenomena and the effects of the mechanical compression of the fuel cell, more precisely the deformation of the gas diffusion layer, the changes in the physical properties and the mass transfer in the gas diffusion layer. The effects of this varying porosity and permeability fields on the polarization and on the power density curves are reported, and the local current density is also investigated. Unlike other studies, our model accounts for a porosity field that varies locally in order to correctly simulate the effect of an inhomogeneous compression in the cell.     

Numerical analysis on performances of polymer electrolyte membrane fuel cells with various cathode flow channel geometries

This paper investigated numerically the effect of cathode channel shapes on the local transport characteristics and cell performance by using a three-dimensional, two-phase, and non-isothermal polymer electrolyte membrane (PEM) fuel cell model. The cells with triangle, trapezoid, and semicircle channels were examined using that with rectangular channel as comparison basis. At high operating voltages, the cells with various channel shapes would have similar performance. However, at low operating voltages, the fuel cell performance would follow: triangle > semicircle > trapezoid > rectangular channel. Analyses of the local transport phenomena in the cell indicate that triangle, trapezoid, and semicircle channel designs increase remarkably flow velocity of reactant, enhancing liquid water removal and oxygen utilization. Thus, these designs increase the limiting current density and improve the cell performance relative to rectangular channel design.     

Simulation of an innovative polymer electrolyte membrane fuel cell design for self-control thermal management

Two novel fuel cell designs attempt to improve efficiency and reduce the balance of plant weight by implementing a square hole through the center of the bipolar plates. Air is forced through the square hole for the purpose of oxygen delivery, water removal, and stack cooling. This study demonstrates, for the two novel designs, a more even temperature distribution and hot spots away from the center of the bipolar plates. This reduces the number and size of components required to effectively run the system, thus reducing the weight of the balance of plant. Four simulations are presented in this paper, with inlet gases and initial cell temperature set to 333 K. The maximum temperature for case 1 without cooling is 347.97 K, case 1 with water cooling is 335.29 K, case 2 with forced air cooling is 339.42 K, and case 3 with forced air cooling is 335.13 K.     

Prediction of local current distribution in polymer electrolyte membrane fuel cell with artificial neural network

In the development process of a fuel cell, understanding the local current distribution is essentially required to achieve better performance and durability. Therefore, many developers apply a segmented fuel cell to observe current distribution under various operating conditions. With the application, experimental data is collected. This study suggests a utilization method for this collected data to develop a local current prediction model. The details of this neural network-based prediction model are introduced, including the pretreatment of the data. In the pretreatment process, current residual values are used for better prediction performance. As a result, the model predicted local current values with a 2.98% error. With the model, the effects of pressure, temperature, cathode relative humidity, and cathode flow rate on local current distribution trends are analyzed. Since the non-uniform current distribution of a fuel cell often leads to low performances or fast local degradation, the optimal operating condition to achieve current uniformity is acquired with an additional model. This model is developed by switching inputs and outputs of the local current prediction model. With the model application, the uniform current distribution is achieved with a standard deviation of 0.039 A/cm² under the current load at 1 Acm^?2.     

阴极流道分流进气对燃料电池性能影响的实验研究

针对高工作电流密度下,燃料电池内局部水淹导致的传质损失问题,本研究提出了一种阴极流道多进口分流进气方式。实验研究了三种典型分流口位置及分流进量对电池性能的影响。研究发现随着分流口远离阴极主进气口,电池性能呈现先上升后下降的趋势,且当分流口靠近主进气口时,增加分流量有助于电池性能提升,但分流量的增加对电池性能的提升存在一个极限值;因此,在对电池进行分流进气优化时需综合考虑分流口位置和分流量的影响。当分流口为SIP-30%且分流量为按化学当量比ξc = 0.75取值时,分流进气方式相比传统进气方式,电池的最大功率密度高出17.8%。     

Performance enhancement of air-cooled open cathode polymer electrolyte membrane fuel cell with inserting metal foam in the cathode side

In this study, a novel way to improve performance of the air-cooled open cathode polymer electrolyte membrane fuel cell is introduced. We suggest using a metal foam in the cathode side of the planar unit fuel cell for the solution to conventional problems of the open cathode fuel cell such as excessive water evaporation from the membrane and poor transportation of air. We conduct experiment and the maximum power density of the fuel cell with metal foam increases by 25.1% compared with the conventional fuel cell without metal foam. The open cathode fuel cell with metal foam has smaller ohmic losses and concentration losses. In addition, we prove that the open cathode fuel cell with metal foam prevents excessive water evaporation and membrane drying out phenomena with numerical approach. Finally, we apply the metal foam to the air-cooled open cathode fuel cell stack as well as the planar unit cell.     

Control-oriented modeling of gas purging process on the cathode of polymer electrolyte membrane fuel cell during shutting down

Gas purging process of cathode side during the shut-down procedure of a polymer electrolyte membrane fuel cell (PEMFC) system is of great importance for a successful cold start. This paper proposes a study on the modeling and control of the cathodic gas purging process, whose main purpose is to remove liquid water in the gas diffusion layer (GDL) and the membrane. The water removal process can be divided into three steps, which are called (a) the through-plane drying of the GDL, (b) the in-plane drying of the GDL, and (c) the vapor-transport from the membrane. A nonlinear model is firstly developed to describe the water removal process in the GDL and the membrane. It includes a one-dimensional three-step purging sub-model and an energy consumption sub-model considering the properties of the air compressor. Experiments are carried out to validate the water-remove model by using the membrane HFR. An optimal constant purging control strategy that minimizes energy consumption during the cathodic purging process is designed based on the model and verified in simulation.     

In-fibre Bragg grating sensors for distributed temperature measurement in a polymer electrolyte membrane fuel cell

A new application of in-fibre Bragg grating (FBG) sensors for the distributed measurement of temperature inside a polymer electrolyte membrane fuel cell is demonstrated. Four FBGs were installed on the lands between the flow channels in the cathode collector plate of a single test cell, evenly spaced from inlet to outlet. In situ calibration of the FBG sensors against a co-located micro-thermocouple shows a linear, non-hysteretic response, with sensitivities in good agreement with the expected value. A relative error of less than 0.2 ° C over the operating range of the test cell (∼20-80 °C) was achieved, offering sufficient resolution to measure small gradients between sensors. While operating the fuel cell at higher current densities under co-flow conditions, gradients of more than 1 ° C were measured between the inlet and outlet sensors. Due to their small thermal mass, the sensors also exhibit good temporal response to dynamic loading when compared with the thermocouple. Design and instrumentation of the graphite collector plate features minimal intrusion by the sensors and easy adaptation of the techniques to bipolar plates for stack implementation.     

Numerical analysis of the manipulated high performance catalyst layer design for polymer electrolyte membrane fuel cell

A two‐dimensional, multiphase, non‐isothermal numerical model was used to investigate the effect of the high performance catalyst layer (CL) design. Microstructure‐related parameters were studied on the basis of the agglomerate model assumption. A conventional CL design (uniform Pt/C composition, e.g., 40 wt%) was modified into two sub‐layers with two different Pt/C compositions (in this study, 40 and 80 wt%). The performance of sub‐layers with different CL designs is shown to be different. Simulation results show that substituting part of the Pt/C 40 wt% with Pt/C 80 wt% increases the cell performance. It was found that factors including proton conductivity, open circuit voltage, and sub‐layer thickness have a significant impact on overall cell performance. Different water distribution for different membrane electrode assembly designs was also observed in the simulation results. More liquid water accumulation inside the membrane electrode assembly is seen when the Pt/C 80 wt% sub‐layer is next to the gas diffusion layer. Finally, several key design parameters for the proposed high performance CL design including agglomerate radius, Nafion thin film thickness, and the Nafion volume fraction within the agglomerate in terms of CL fabrication were identified on the basis of our simulation results. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of porous carbon foam polymer electrolyte membrane fuel cell

In order to prove the feasibility of using porous carbon foam material in a polymer electrolyte membrane fuel cell (PEMFC), a single PEMFC is constructed with a piece of 80PPI (pores per linear inch) Reticulated Vitreous Carbon (RVC) foam at a thickness of 3.5 mm employed in the cathode flow-field. The cell performance of such design is compared with that of a conventional fuel cell with serpentine channel design in the cathode and anode flow-fields. Experimental results show that the RVC foam fuel cell not only produces comparative power density to, but also offers interesting benefits over the conventional fuel cell. A 250 h long term test conducted on a RVC foam fuel cell shows that the durability and performance stability of the material is deemed to be acceptable. Furthermore, a parametric study is conducted on single RVC foam fuel cells. Effect of geometrical and material parameters of the RVC foam such as PPI and thickness and operating conditions such as pressure, temperature, and stoichiometric ratio of the reactant gases on the cell performance is experimentally investigated in detail. The single cell with the 80PPI RVC foam exhibits the best performance, especially if the thinnest foam (3.5 mm) is used. The cell performance improves with increasing the operating gauge pressure from 0 kPa to 80 kPa and the operating temperature from 40 °C to 60 °C, but deteriorates as it further increases to 80 °C. The cell performance improves as the stoichiometric ratio of air increases from 1.5 to 4.5; however, the improvement becomes marginal when it is raised above 3.0. On the other hand, changing the stoichiometric ratio of hydrogen does not have a significant impact on the cell performance.     

Chelating agent assisted heat treatment of carbon supported cobalt oxide nanoparticle for use as cathode catalyst of polymer electrolyte membrane fuel cell (PEMFC)

Cobalt-based catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cell (PEMFC) have been successfully incorporated cobalt oxide (Co₃O₄) onto Vulcan XC-72 carbon powder by thermal decomposition of Co-ethylenediamine complex (ethylenediamine, NH₂CH₂CH₂NH₂, denoted en) at 850 °C. The catalysts were prepared by adsorbing the cobalt complexes [Co(en)(H₂O)₄]³⁺, [Co(en)₂(H₂O)₂]³⁺ and [Co(en)₃]³⁺ on commercial XC-72 carbon black supports, loading amount of Co with respect to carbon black was about 2%, the resulting materials have been pyrolyzed under nitrogen atmosphere to create CoO_x/C catalysts, donated as E1, E2, and E3, respectively. The composite materials were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). Chemical compositions of prepared catalysts were determined using inductively-coupled plasma-atomic emission spectroscopy (ICP-AES). The catalytic activities for ORR have been analyzed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The electrocatalytic activity for oxygen reduction of E2 is superior to that of E1 and E3. Membrane electrode assemblies (MEAs) containing the synthesized CoO_x/C cathode catalysts were fabricated and evaluated by single cell tests. The E2 cathode performed better than that of E1 and E3 cathode. This can be attributed to the enhanced activity for ORR, in agreement with the composition of the catalyst that CoO co-existed with Co₃O₄. The maximum power density 73 mW cm⁻² was obtained at 0.3 V with a current density of 240 mA cm⁻² for E2 and the normalized power density of E2 is larger than that that of commercial 20 wt.% Pt/C-ETEK.     

Experimental study on dual recirculation of polymer electrolyte membrane fuel cell

Durability and start-up ability in sub-zero environment are two technical bottlenecks of vehicular polymer electrolyte membrane (PEM) fuel cell systems. With exhaust gas recirculation on the anode and cathode side, the cell voltage at low current density can be reduced, and the membrane can be humidified without external humidifier. They may be helpful to prolong the working lifetime and to promote the start-up ability. This paper presents an experimental study on a PEM fuel cell system with anodic and cathodic recirculation. The system is built up based on a 10 kW fuel cell stack, which consists of 50 cells and has an active area of 261 cm². A cathodic recirculation pump and a hydrogen recirculation pump are utilized on the cathode and anode side, respectively. Key parameters, e.g., stack current, stack voltage, cell voltage, air flow, relative humidity on the cathode side, oxygen concentration at the inlet and outlet of the cathode side, are measured. Results show that: 1) with a cathodic recirculation the system gets good self-humidification effect, which is similar to that with an external humidifier; 2) with a cathodic recirculation and a reduction of fresh air flux, the cell voltage can be obviously reduced; 3) with an anodic recirculation the cell voltage can also be reduced due to a reduction in the hydrogen partial pressure, the relative humidity on the cathode side is a little smaller than the case with only cathode recirculation. It indicates that, for our stack the cathodic recirculation is effective to clamp cell voltage at low current density, and a self-humidification system is possible with cathodic recirculation. Further study will focus on the dynamic model and control of the dual recirculation fuel cell system.     

Humidification of polymer electrolyte membrane fuel cell using short circuit control for unmanned aerial vehicle applications

In the present study, a short circuit controller for use in the humidification of polymer electrolyte membrane fuel cells was developed for unmanned aerial vehicles (UAVs). Fuel cells (FCs) require an external humidifier to avoid drying up. Particularly in UAV applications, humidity control is more important because the boiling point of water decreases with increase in flight altitude. In this study, overcurrent was generated by short-circuiting an FC to humidify the electrolyte membrane and improve the electric power output. An FC controller incorporating a short circuit unit was developed, and a battery was hybridized with the FC to compensate the power when the latter was short-circuited. The performance of the FC was evaluated for the interval (period) and duration of short circuit. Using this method, the power output was improved by 16% when short circuit control was operated at the optimal condition.