Numerical evaluation of various thermal management strategies for polymer electrolyte fuel cell stacks

Thermal management is one of the key factors required to ensure good performance polymer electrolyte fuel cell (PEFC) stacks. The choice of the thermal management strategy depends on the specific application, size, weight, design, complexity, and cost. In this work, we investigate various alternative thermal management strategies for PEFC stacks, e.g., forced convection in specially design cooling plate/channel with either (i) liquid or (ii) air as the coolant; (iii) edge-air cooling with fins and; combine oxidant and coolant flow (open-cathode) with (iv) forced and (v) natural convection air cooling. A three-dimensional two-phase model, comprising of the equations of conservation of mass, momentum, species, energy and charge, is employed to quantify the performance of various cooling strategies. The results demonstrate that thermal management is essential to ensure good stack performance. Liquid cooling, as expected, performs the best compared to air cooling, whereas natural convection cooling is just marginally able to maintain a stack with large number of cells from steep drop in performance. Finally, results presented in this paper can provide useful design guidelines for selection of a suitable thermal management strategy for a PEFC stack and its near-to- or optimum cooling condition.     

Modification of carbon aerogel supports for PEMFC catalysts

Nitrogen enriched carbon aerogels and Co-based non-noble metal catalysts supported on carbon aerogels have been synthesized and tested using XPS, HRTEM, XRD and RDE techniques. XPS spectra of unmodified carbon aerogels indicated a presence of two oxygen O(1s) groups and five carbon C(1s) groups in deconvoluted spectra. XPS spectra of chemically modified samples indicated nitrogen N(1s) introduced in the carbon aerogel structure by acidic (HNO₃) or basic (NH₄OH) chemical treatment.Synthesis of aerogel supported Co catalysts performed by using Co-methoxy-tetra-phenylporfirin as a macrocyclic compound incorporated into the aerogel structure, and sintered at 700 and 900 °C in N₂, revealed the presence of Co-metal nano-particles with 20 nm diameter. HRTEM and diffraction patterns show a β-Co FCC structure with many {111}<110> micro-twins in the Co nano-particles. The electrochemical properties of the synthesized catalysts in O₂-free and O₂-saturated sulfuric and perchloric acid solutions, evaluated by a rotating disc electrode (RDE) technique, demonstrated catalytic activity in hydrogen oxidation and oxygen reduction reactions.     

A hybrid model combining a support vector machine with an empirical equation for predicting polarization curves of PEM fuel cells

A hybrid model was proposed by combining a support vector machine (SVM) model with an empirical equation for more accurate prediction of the polarization curves of a PEM (polymer electrolyte membrane) fuel cell under various operating conditions. Operational data were obtained from designed experiments for a PEM fuel cell for training, testing, and validating the hybrid model, and a model training procedure was presented for determining the model coefficients and hyper-parameters of the hybrid model. The predictive performance of the hybrid model was compared with that of a SVM model. The SVM model showed somewhat poor performance, especially yielding large prediction errors in the high voltage ranges of the polarization curves as reported in the literature. In contrast, the hybrid model exhibited almost perfect matches between the predicted and measured polarization curves, resulting in significantly lower root-mean-square errors of 1.7–4.4 mV which correspond to only 14–21% of those obtained from the SVM model.     

Determination of the pore size distribution of micro porous layer in PEMFC using pore forming agents under various drying conditions

In this paper, the effect of the pore size distribution of a micro-porous layer (MPL) on the performance of polymer electrolyte membrane fuel cells (PEMFC) was investigated using self-made gas diffusion layers (GDLs) with different MPLs for which the pore size distribution was modified using pore forming agents under different drying conditions. When MPL dried at high temperature, more macro pores, approximately 1,000–20,000 nm in diameter, and less micro pores, below 100 nm, were observed relative to when MPL was dried at low temperature. Self-made GDLs were characterized by a field-emission scanning electron microscope (FE-SEM), mercury porosimetry and self-made gas permeability measurement equipment. The performance of the single cells was measured under two different humidification conditions. The results demonstrate that the optimum pore size distribution of MPL depended on the cell operating humidification condition. The MPL dried at high temperature performed better than the MPL dried at low temperature under a low humidification condition; however, MPL dried at low temperature performed better under a high humidification condition.     

Design of thermal management subsystem for a 5 kW polymer electrolyte membrane fuel cell system

High-efficiency thermal management subsystem has a key role on the PEM fuel cell performance and durability. In this study, design of thermal management subsystem for a 5 kW PEM fuel cell system is investigated. A numerical model is presented to study the cooling flow field performance. The number of parallel channels in parallel serpentine flow field is selected as the design parameter of the flow field and its optimum value is obtained by compromising between the minimum pressure drop of coolant across the flow field and maximum temperature uniformity within the bipolar plate criteria. The optimum coolant flow rate is also determined by compromising between different criteria. Test results of a 5-cells short stack are presented to verify the numerical simulation results.     

Numerical study for in-plane gradient effects of cathode gas diffusion layer on PEMFC under low humidity condition

A numerical study about in-plane porosity and contact angle gradient effects of cathode gas diffusion layer (GDL) on polymer electrolyte membrane fuel cell (PEMFC) under low humidity condition below 50% relative humidity is performed in this work. Firstly, a numerical model for a fuel cell is developed, which considers mass transfer, electrochemical reaction, and water saturation in cathode GDL. For water saturation in cathode GDL, porosity and contact angle of GDL are also considered in developing the model. Secondly, current density distribution in PEMFC with uniform cathode GDL is scrutinized to design the gradient cathode GDL. Finally, current density distributions in PEMFC with gradient cathode GDL and uniform cathode GDL are compared. At the gas inlet side, the current density is higher in GDL with a gradient than GDL with high porosity and large contact angle. At the outlet side, the current density is higher in GDL with a gradient than GDL with low porosity and small contact angle. As a result, gradient cathode GDL increases the maximum power by 9% than GDL with low porosity and small contact angle. Moreover, gradient cathode GDL uniformizes the current density distribution by 4% than GDL with high porosity and large contact angle.     

The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method

We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen. When high loading Pt/C catalysts (45.5 wt.%) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS). This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance. In addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes. MEA performance was more sensitive to flow rates under conditions of higher ionomer content. Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt.% ionomer content at the cathode (25 wt.% at the anode) was shown to yield the best performance.     

Implementation of discrete wavelet transform-based discrimination and state-of-health diagnosis for a polymer electrolyte membrane fuel cell

This research investigates a new approach based on the discrete wavelet transform (DWT) that suitable for analyzing and evaluating output terminal voltage signal (OTVS) for discrimination analysis of a polymer electrolyte membrane fuel cell (PEMFC). Due to its ability for extracting information from the non-stationary and transient phenomena simultaneously in both time and frequency domain, the OTVS can be applied as source data in the DWT-based approach. By using the wavelet decomposition including the multi-resolution analysis (MRA) using the Daubechies wavelet (dB) as mother wavelet, the information on the electrochemical characteristics of a PEMFC can be extracted from the OTVS over a wide frequency range. Thus, the cells that have similar electrochemical characteristics can be eventually discriminated. In particular, this present research develops these investigations one step further by showing low-frequency components (approximation A_n) and high-frequency components (detail D_n) extracted from variable single cells with different electrochemical characteristics. Experimental results show that DWT-based approach is clearly appropriate for the reliable SOH diagnosis for a PEMFC.     

A hybrid multi-variable experimental model for a PEMFC

A hybrid model composed of a least square support vector machine (LS-SVM) model and a pressure-incremental model is developed to dispose operation conditions of current, temperature, cathode and anode gas pressures, which have major impacts on a proton exchange membrane fuel cell's (PEMFC) performance. The LS-SVM model is built to incorporate current and temperature and a particle swarm optimization (PSO) algorithm is used to improve its performance. The optimized LS-SVM model fits the experimental data well, with a mean squared error of 0.0002 and a squared correlation coefficient of 99.98%. While a pressure-incremental model with only one empirical coefficient is constructed to for anode and cathode pressures with satisfactory results. Combining these two models together makes a powerful hybrid multi-variable model that can predict a PEMFC's voltage under any current, temperature, cathode and anode gas pressure. This black-box hybrid PEMFC model could be a competitive solution for system level designs such as simulation, real-time control, online optimization and so on.     

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.     

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.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

Hydrogen circulation system model predictive control for polymer electrolyte membrane fuel cell-based electric vehicle application

Polymer electrolyte membrane fuel cell (PEMFC) is one of the promising solutions overcoming future energy crisis and environment pollution in the automotive industry. However, PEMFC is vulnerable to the circulation of hydrogen mass flow rate and pressure, which may cause the degradation of the PEMFC's anode components and reduction of output performance over time. Thus, the control of the hydrogen supply system draws attention currently and is critical for the durability and stability of the PEMFC system. In this study, a model predictive control (MPC) approach for hydrogen circulation system is developed to regulate the hydrogen flow circulating. A model of the hydrogen supply system that contains a flow control valve, a supply manifold, a return manifold and a hydrogen circulating pump is firstly developed to describe the behavior of the hydrogen mass flow dynamics in the PEMFC. Subsequently, a hydrogen circulating pump MPC scheme is designed based on the piecewise linearized model of hydrogen circulation as well as the switched MPC controllers. By predicting the pressure of the return manifold and the angle velocity of the pump, the proposed MPC approach can manipulate the hydrogen circulating pump to achieve efficient and stable operation of the PEMFC.     

Experimental study on non-uniform arrangement of 3D printed structure for cathodic flow channel in PEMFC

Enhancing mass transfer capability of flow channel is important subject to improve fuel cell performance. In this study, an experimental study about non-uniform arrangement of metallic structures in cathodic flow channel on polymer electrolyte membrane fuel cell is conducted. Metallic 3D printer is used to manufacture 3D mesh with complex geometry. Channel width and bottom-rear channel depth are selected to change the geometry of structure. Assuming that accelerating flow velocity and reinforcing flow direction to gas diffusion layer can improve fuel cell performance, eight basic arrangements are inserted into cathodic carbon bipolar plate to measure fuel cell performance. With I–V curve, the unit cells with non-uniform arrangements of width and tapered structure show performance enhancement of 12.8% in maximum power density, compared with conventional parallel flow channel. According to electrochemical impedance spectroscopy results, performance enhancement by non-uniform arrangement is mainly occur in high current density operation due to low mass transfer resistance.     

Specific approaches to dramatic reduction in stack activation time and perfect long-term storage for high-performance air-breathing polymer electrolyte membrane fuel cell

Air-breathing polymer electrolyte membrane fuel cell (PEMFC) systems without the humidifier and air blower have been developed to overcome the cost and complexity of balance of plants (BOPs). Until now, there has been no specific way to improve the stack's initial performance through the specific activation protocol and maintain the initial performance for a very long time. Herein, we studied a technique for finishing the total activation within 1 h by using a pre-activation process (i.e., soaking the stack in a DI-water reservoir) and applying current at 0.65 V. The pre-activation procedure significantly increased the swelling of the polymer membrane and the Nafion binder in the catalyst layer, reducing the total activation time. Also, we showed that long-term storage using humidified N₂ gas in a closed box did not hinder the electrocatalytic activity of the Pt and the drying of the polymer membrane for 60 days.     

Hierarchical fault diagnostic method for a polymer electrolyte fuel cell system

This study proposes a hierarchical method for on-line fault detection and diagnosis (FDD) of a stack and balance of plants (BoPs) in a polymer electrolyte fuel cell (PEFC) system. Because the fuel cell system consists of various subsystems with different characteristics, we have developed a multi-stage structure with subsystem-level FDD. In the first stage, faults were diagnosed at the subsystem level. In the next step, component-level faults were identified in the corresponding subsystem. The model-based approach in this study is composed of process estimation, residual generation, and FDD. Supervised machine learning methods were applied to train models for regression and fault classification. Residuals, the difference between analytic redundancies and measured results, were employed as fault indicators, i.e., residuals were used to detect faults and to generate fault patterns. Analytic redundancies were calculated using regression models. Several abrupt and performance degradation faults were considered. Because long-term performance degradations were difficult to introduce in the experimental system, the proposed method was evaluated using test data obtained by artificially decreasing the performance or sensor readings for a short period of time. This study focuses primarily on subsystem-level FDD and demonstrates one scenario of second level FDD. The experimental results verified the accuracy of the model-based approach and demonstrated that the proposed multi-stage hierarchical method effectively diagnosed faults in a PEFC system.     

Development of a novel hydrophobic/hydrophilic double micro porous layer for use in a cathode gas diffusion layer in PEMFC

In this study, a gas diffusion layer (GDL) was modified to improve the water management ability of a proton exchange membrane fuel cell (PEMFC). We developed a novel hydrophobic/hydrophilic double micro porous layer (MPL) that was coated on a gas diffusion backing layer (GDBL). The water management properties, vapor and water permeability, of the GDL were measured and the performance of single cells was evaluated under two different humidification conditions, R.H. 100% and 50%. The modified GDL, which contained a hydrophilic MPL in the middle of the GDL and a hydrophobic MPL on the surface, performed better than the conventional GDL, which contained only a single hydrophobic MPL, regardless of humidity, where the performance of the single cell was significantly improved under the low humidification condition. The hydrophilic MPL, which was in the middle of the modified GDL, was shown to act as an internal humidifier due to its water absorption ability as assessed by measuring the vapor and water permeability of this layer.     

Robust fault diagnosis and fault tolerant control for PEMFC system based on an augmented LPV observer

In order to improve the safety and reliability of proton exchange membrane fuel cell system, this paper proposes a novel robust fault observer for the fault diagnosis and reconstruction of the PEMFC air management system. First, considering the complexity and large computation of the nonlinear PEMFC system, a linear parameter-varying (LPV) model is introduced to describe the system behavior and reduce the computation cost. Then, an augmented state observer based on the LPV model is proposed for simultaneously estimating the internal states and component faults. The robustness is guaranteed by taking the system disturbances and measurement noises into consideration when designing the observer gain. The observer design is transformed into a process of solving a set of linear inequality matrices. According to the results, the augmented robust observer can accurately estimate the system states and faults under different conditions. Moreover, to realize the fault tolerant control of the air supply, the oxygen stoichiometry estimator is designed taking consideration of system fault information and a corresponding controller is employed for air compressor voltage following the net power maximization strategy.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.