Optimisation of carbon nanofiber based electrodes for polymer electrolyte membrane fuel cells prepared by a sedimentation method

Tubular carbon nanofibers with an average diameter of 150 nm are investigated as a possible material for the electrodes preparation for polymer electrolyte membrane fuel cells. Well-dispersed platinum particles with an average crystallite size of 4.6 nm are deposited on surface-oxidised fibers to be used as a catalyst support with an electroless plating method. The carbon nanofiber-based electrodes are prepared by a sedimentation method without the use of organic solvents. This method allows an exact setting of the fiber and binder content and the catalyst loading. The electrodes are optimised by varying the thickness of the gas diffusion layer and its binder content as well as the thickness of the active layer. These optimised electrodes show a considerably better performance when compared to carbon black-based electrodes with the same catalyst loading prepared by a spraying process using the same type and amount of electrolyte in the membrane electrode assembly. By reducing the platinum content from 0.7 to 0.2 mg cm⁻², catalyst utilisation is significantly increased.     

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.     

Demonstration and development of a polymer electrolyte fuel cell system for residential use

Fuel cell systems, especially those fed with hydrogen, have reached considerable performance targets in laboratory conditions with constant loads and conservative environmental conditions. However, a check of the potential of such systems in real conditions is necessary, particularly in terms of varying electrical and thermal loads and of more severe climatic conditions.To determine the state of the art of such technology and to develop systems capable of supporting future national energy scenarios within the PNR-FISR project, “Polymeric electrolytes and ceramic fuel cells: demonstration of systems and development of new materials,” the development of fuel cell systems ranging from 1 to 5 kW of power and based on either solid polymer (PEMFC) or solid oxide (SOFC) technology are in progress. In this paper, the demonstration of a pre-commercial PEMFC system fed with hydrogen and developed in cooperation with NUVERA is described. The system has been developed in order to determine its limits and capacity in relation to start-up time, response time, consumption, efficiency, reliability, etc. It has currently reached 1000 working hours of continuous performance with variable loads that simulate those of a typical residential dwelling.     

Fabrication of polymer electrolyte membrane fuel cell MEAs utilizing inkjet print technology

Utilizing drop-on-demand technology, we have successfully fabricated hydrogen–air polymer electrolyte membrane fuel cells (PEMFC), demonstrated some of the processing advantages of this technology and have demonstrated that the performance is comparable to conventionally fabricated membrane electrode assemblies (MEAs). Commercial desktop inkjet printers were used to deposit the active catalyst electrode layer directly from print cartridges onto Nafion^® polymer membranes in the hydrogen form. The layers were well-adhered and withstood simple tape peel, bending and abrasion tests and did so without any post-deposition hot press step. The elimination of this processing step suggests that inkjet-based fabrication or similar processing technologies may provide a route to less expensive large-scale fabrication of PEMFCs. When tested in our experimental apparatus, open circuit voltages up to 0.87 V and power densities of up to 155 mW cm⁻² were obtained with a catalyst loading of 0.20 mg Pt cm⁻². A commercially available membrane under identical, albeit not optimized test conditions, showed about 7% greater power density. The objective of this work was to demonstrate some of the processing advantages of drop-on-demand technology for fabrication of MEAs. It remains to be determined if inkjet fabrication offers performance advantages or leads to more efficient utilization of expensive catalyst materials.     

Modification of hydrocarbon structure for polymer electrolyte membrane fuel cell binder application

The development of novel hydrocarbon polymer membranes needs to be accompanied by catalyst layer hydrocarbon binder research so that the resultant membrane electrode assembly (MEA) can be durable. Hydrocarbon polymers which show high performance levels as membranes, however, are inadequate as catalyst layer binders as they are designed for low fuel penetration. Modification to the hydrocarbon polymer structure of high performing hydrocarbon polymers such as Sulfonated poly(arylene ether sulfone) (SPAES) can take advantage of its high conductivity while increasing gas permeability and maintaining interfacial compatibility with the membrane. The incorporation of a biphenyl fluorene group into the polymer backbone of SPAES successfully increased d-spacing which led to an increase in gas crossover. In the catalyst layer, the modified polymer ionomer showed higher penetration into primary pore volume thus increasing ESA. Higher catalyst utilization due to easier fuel access and ionomer coverage led to higher fuel cell performance. Durability tests revealed that structural modification did not hinder interfacial compatibility as well as performance.     

Impact of defective single cell on the operation of polymer electrolyte membrane fuel cell stack

Potential inversion of a single cell in a PEMFC stack reduces its performance and can lead to severe damage in the MEAs. Numerous causes of potential inversion can be considered, from MEA degradation to insufficient feed of reacting gases. The first part of the paper deals with experiments carried out to investigate whether the potential inversion could be suppressed by increasing the inlet gas flow. Then, the defective cell resistances were measured and compared to those exhibited by the other cells in the 23-cell stack. We also studied the influence of this cell on the whole stack resistances. Finally, the defective cell was dismantled and analysed, in order to explain where the defect comes from and whether it has affected each part of the cell.     

Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method

A proton exchange membrane fuel cell (PEMFC) electrode having a modified morphology of conventional Teflon (PTFE) bonded electrodes was studied using the AC impedance method. The electrode differs from other types of electrodes in the presence of a thin catalyst-supporting layer between the gas diffusion backing and the catalyst layer. The thickness and composition of the supporting layer were optimized on the basis of the information from AC impedance measurements. The optimal thickness of the supporting layer and its PTFE content turned out to be approximately 3.5 mg cm⁻² and 30 wt.%, respectively. The catalyst layer was cast on top of the supporting layer, from solution that has the proper ratio of ionomer Nafion and Pt/C catalyst. The optimal amount of the ionomer in the catalyst layer was approximately 0.8 mg cm⁻² when Pt loading was kept at 0.4 mg cm⁻². These values are rationalized in terms of the catalyst active area and the transport of the involved species for the electrode reaction.     

Towards deep computer vision for in-line defect detection in polymer electrolyte membrane fuel cell materials

Polymer Electrolyte Membrane (PEM) fuel cells are a promising source of alternative energy. However, their production is limited by a lack of well-established methods for quality control of their constituent materials like the membrane-electrode assembly during roll-to-roll manufacturing. One potential solution is the implementation of deep learning methods to detect unwanted defects through their detection in scanned images. We explore the detection of defects like scratches, pinholes, and scuffs in a sample dataset of PEM optical images using two deep learning algorithms: Patch Distribution Modeling (PaDiM) for unsupervised anomaly detection and Faster-RCNN for supervised object detection. Both methods achieve scores on performance metrics (ROC-AUC and PRO-AUC for PaDiM and AP for Faster-RCNN) that are comparable to their scores on benchmark datasets. These methods also have the potential to detect a wider range of defects compared to IR thermography and previous optical inspection methods. Overall, deep learning shows promise at detecting relevant defects of interest and has the potential to achieve real-time defect detection.     

Simulated start-stop as a rapid aging tool for polymer electrolyte fuel cell electrodes

The corrosion stability of supported catalysts as employed in state of the art intermediate temperature polymer electrolyte fuel cells has been studied by means of simulated start-stop cycling (150 cycles). The carbon dioxide formation from the air electrode has been monitored during repeated cycling runs and the loss of catalyst support has been correlated with performance drops. Degradation effects have been studied at different current densities in order to differentiate between kinetic and mass transport effects. Finally, correlations of this accelerated aging tool with a more realistic durability test over 4000 h and 157 start-stop cycles have been made and the good agreement between simulated and realistic approaches has been confirmed, demonstrating the high value of the experimental approach and analysis.     

Optimal catalyst layer structure of polymer electrolyte membrane fuel cell

In a membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells, the structure and morphology of catalyst layers are important to reduce electrochemical resistance and thus obtain high single cell performance. In this study, the catalyst layers fabricated by two catalyst coating methods, spraying method and screen printing method, were characterized by the microscopic images of catalyst layer surface, pore distributions, and electrochemical performances to study the effective MEA fabrication process. For this purpose, a micro-porous layer (MPL) was applied to two different coating methods intending to increase single cell performances by enhancing mass transport. Here, the morphology and structure of catalyst layers were controlled by different catalyst coating methods without varying the ionomer ratio. In particular, MEA fabricated by a screen printing method in a catalyst coated substrate showed uniformly dispersed pores for maximum mass transport. This catalyst layer on micro porous layer resulted in lower ohmic resistance of 0.087 Ω cm² and low mass transport resistance because of enhanced adhesion between catalyst layers and a membrane and improved mass transport of fuel and vapors. Consequently, higher electrochemical performance of current density of 1000 mA cm^-2 at 0.6 V and 1600 mAcm⁻² under 0.5 V came from these low electrochemical resistances comparing the catalyst layer fabricated by a spraying method on membranes because adhesion between catalyst layers and a membrane was much enhanced by screen printing method.     

A computational model for assessing impact of interfacial morphology on polymer electrolyte fuel cell performance

A two-dimensional, non-isothermal, anisotropic numerical model is developed to investigate the impact of the interfacial morphology between the micro-porous layer (MPL) and the catalyst layer (CL) on the polymer electrolyte fuel cell (PEFC) performance. The novel feature of the model is the inclusion of directly measured surface morphological information of the MPL and the CL. The interfacial morphology of the MPL and the CL was experimentally characterized and integrated into the computational framework, as a discrete interfacial layer. To estimate the impact of MPL|CL interfacial surface morphology on local ohmic, thermal and mass transport losses, two different model schemes, one with the interface layer and one with the traditionally used perfect contact are compared. The results show a ∼54 mV decrease in the performance of the cell due to the addition of interface layer at 1 A cm⁻². Local voids present at the MPL|CL interface are found to increase ohmic losses by ∼37 mV. In-plane conductivity adjacent to the interface layer is determined to be the key controlling parameter which governs this additional interfacial ohmic loss. When the interfacial voids are simulated to be filled with liquid water, the overpotential on the cathode side is observed to increase by ∼25 mV. Local temperature variation of up to 1 °C is also observed at the region of contact between the MPL and the CL, but has little impact on predicted voltage.     

Simulation of nanostructured electrodes for polymer electrolyte membrane fuel cells

Aligned carbon nanotubes (CNTs) with Pt uniformly deposited on them are being considered in fabricating the catalyst layer of polymer electrolyte membrane (PEM) fuel cell electrodes. When coated with a proton conducting polymer (e.g., Nafion) on the Pt/CNTs, each Pt/CNT acts as a nanoelectrode and a collection of such nanoelectrodes constitutes the proposed nanostructured electrodes. Computer modeling was performed for the cathode side, in which both multicomponent and Knudsen diffusion were taken into account. The effect of the nanoelectrode lengths was also studied with catalyst layer thicknesses of 2, 4, 6, and 10 μm. It was observed that shorter lengths produce better electrode performance due to lower diffusion barriers and better catalyst utilization. The effect of spacing between the nanoelectrodes was studied. Simulation results showed the need to have sufficiently large gas pores, i.e., large spacing, for good oxygen transport. However, this is at the cost of obtaining large electrode currents due to reduction of the number of nanoelectrodes per unit geometrical area of the nanostructured electrode. An optimization of the nanostructured electrodes was obtained when the spacing was at about 400 nm that produced the best limiting current density.     

Estimation of crack propagation in polymer electrolyte membrane fuel cell under vibration conditions

In transportation applications, the main reasons of mechanical damage in polymer electrolyte membrane fuel cell (PEMFC) are road-induced vibrations and impact loads. The most vulnerable place of these cells is the interface between membrane and catalyst layer in the membrane electrode assembly (MEA). Hence, studies on mechanical strength of PEMFC should focus on that interface. The objective of present study lies in the fact that employing a prediction method to investigate the damage propagation behavior of vibration applied PEMFC using artificial neural network (ANN). The data available in the literature are used to constitute an ANN model. Three-layer model; input, hidden and output, are used for construction of ANN structure. Initial delamination length (a), amplitude (A), frequency (ω) and time (t) are used as input neurons whereas delamination length is output. Levenberg–Marquardt algorithm is selected as learning algorithm. On the other hand, number of hidden layer neuron is decided with the use of different neuron numbers by trial and error method. It is concluded that prediction capability of ANN model is in allowable limits and model can be suggested as efficient way of delamination length estimation.     

Longevity test results for reformate polymer electrolyte membrane fuel cell stacks

Polymer electrolyte membrane fuel cell (PEMFC) stacks offer a great potential for combined heat and power (CHP) applications because of their good performance and technical maturity of the key components. Nonetheless, some developmental issues have remained open. Among those are the long-term stability with respect to performance degradation and sudden death phenomena like membrane rupture.In a development program for domestic CHP systems, PEMFC stacks intended for long-term operation on reformate were developed. Development targets were high performance, high media utilization, good longevity and low degradation rates. In this paper, results on long-term performance tests of these stacks are reported. Operating times of more than 15,000 h with degradation rates of approx. 10 μV h⁻¹ have been achieved.     

Improvement of water management in polymer electrolyte membrane fuel cell thanks to cathode cracks

The role of cathodic structure on water management was investigated for planar micro-air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The electrical results demonstrate the possibility to decrease, with the same structure, both cell drying and cell flooding according to the environmental and operation conditions. Thanks to a simultaneous study of internal resistance and scanning electronic microscope (SEM) images, we demonstrate the advantageous influence of the presence of crack in cathodic catalytic layer on water management. On the one hand, the gold layer used as cathodic current collector is in contact with the electrolyte in the cracked zones which allows water maintenance within the electrolyte. It allows to decrease the cell drying and thus strongly increase the electrical performances. For cells operated in a 10% relative humidity atmosphere at 30 °C and at a potential of 0.5 V, the current density increases from 28 mA cm⁻² to 188 mA cm⁻² (+570%) for the cell with a cathodic cracked network. On the other hand, the reduction in oxygen barrier diffusion due to the cathodic cracks allows to improve oxygen diffusion. In flooding state, the current densities were higher for a cell with a cracked network. For cells operating in a 70% relative humidity atmosphere at 30 °C and at a potential of 0.2 V, a current density increase from 394 mA cm⁻² to 456 mA cm⁻² (16%) was noted for the cell with a cathodic cracked network. Microscopic observations allowed us to visualize water droplets growth mechanism in cathodic cracks. It was observed that the water comes out of the crack sides and partially saturates the cracks before emerging on cathodic collector. These results demonstrate that cathode structuration is a key parameter that plays a major role in the water management of PEMFCs.     

Low temperature decal transfer method for hydrocarbon membrane based membrane electrode assemblies in polymer electrolyte membrane fuel cells

Decal transfer is an effective membrane electrode assembly (MEA) fabrication method known for its low interfacial resistance and suitability for mass processing. Previously decal transfer for hydrocarbon membranes was performed at temperatures above 200 °C. Here a novel low temperature decal transfer (LTD) method for hydrocarbon membranes is introduced. The new method applies a small amount (2.2 mg cm⁻²) of liquid (1-pentanol) onto the membrane separator before decal transfer to lower the T_g of the membrane and achieves complete decal transfer at 110 °C and 6 MPa. Nafion binder amount in the catalyst layer and catalyst layer annealing temperature is controlled to optimize the fuel cell performance. Compared to conventional decal transfer (CDT), the novel LTD method shows enhancement in energy efficiency, simplicity in the process scheme, and improvement in fuel cell performance.     

Experimental analysis of internal gas flow configurations for a polymer electrolyte membrane fuel cell stack

The internal gas distribution system utilised for supplying fresh reactants and removing reaction products from the individual cells of a fuel cell stack can be designed in a parallel, a serial or a mixture of parallel and serial gas flow configuration. In order to investigate the interdependence between the internal stack gas distribution configuration and single cell as well as overall stack performance, a small laboratory-scale fuel cell stack consisting of identical unit cells was subject to operation with different gas distribution configurations and different operating parameters. The current/voltage characteristics measured with the different gas distribution configurations are analysed and compared on unit cell- as well as on stack-level. The results show the significant impact of the internal stack gas distribution system on operation and performance of the individual unit cells and the overall stack.     

Improvement of cell performance in catalyst layers with silica-coated Pt/carbon catalysts for polymer electrolyte fuel cells

Carbon-supported Pt catalysts (Pt/Cs) for use of cathode catalyst layers (CLs) for PEFCs were covered with silica layers in order to improve performance. CLs with low ratio of ionomer to carbon (I/C) for Pt/C and silica-coated Pt/C were fabricated using an inkjet printing (denoted as Pt/C(IJ) and SiO₂-Pt/C(IJ)) to reduce oxygen diffusion resistance. Compared to Pt/C(IJ), SiO₂-Pt/C(IJ) ink maintained good dispersion and high stability under the lower I/C. The performance of SiO₂-Pt/C(IJ) was significantly higher than Pt/C(IJ) at 0.6 V under all humidity conditions. In particular, the performance of SiO₂-Pt/C(IJ) under low humidity conditions showed noticeable improvement regardless of current density area. From FIB-SEM, it was confirmed that the morphologies and porosities of both catalysts were the same. Thus, these results indicate that oxygen diffusion resistance, related to structure of CLs, hardly affects the performance, whereas improved performance is attributed to increased proton conductivity by silica layers containing hydrophilic groups.     

Electrochemical impedance diagnosis of micro porous layer in polymer electrolyte membrane fuel cell electrodes

In the present study, Electrochemical Impedance Spectroscopy (EIS) is used to evaluate two different types of gas diffusion electrodes for Polymer Electrolyte Membrane Fuel Cells (PEMFC). The over potential losses due to the components are determined without any reference electrode using symmetric gas supply of hydrogen or oxygen at various conditions viz., open circuit potential (OCP), various load up to 0.7 A cm⁻², and various humidity conditions. Though it is very clear that the cathode impedance is a major contributor for voltage loss, it is observed that the two type of electrodes with different micro-porous layers (DSGDL and SSGDL), show different behavior with respect to operating conditions like dry gas operation, humid condition etc., The DSGDL is favorable for operating the cell at higher temperature and relative humidity while SSGDL is favorable under dry gas operation. However at higher current density and with humidity, Nernst diffusion plays a major contribution. The Nernst diffusion coefficient decreases with increasing current density for DSGDL and increasing for SSGDL, suggesting that the gas diffusion electrodes need to be engineered depending on the operating conditions and current density.     

Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers

Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials.