Characterization of catalysts and membrane in DMFC lifetime testing

The lifetime and performance of a direct methanol fuel cell (DMFC) were investigated to understand the correlation between the structure of catalysts/membrane and cell performance versus time. The cell polarization and performance curves were obtained during the DMFC operation with the time. The catalysts and Nafion^® membrane of the membrane electrode assembly (MEA) from the lifetime test were comprehensively examined by XRD, HRTEM, FTIR and Raman spectroscopy techniques. The results revealed that there was significant performance degradation during the first 200 h operation; while the degradation was slowing down between 200 and 704 h operation. The degradation became worse after 1002 h operation. The increases of the catalyst particle size from both anode and cathode catalysts were observed after the DMFC lifetime test. The changes of microstructure, surface composition, the interfacial structure of the MEA, and the aging of Nafion^® under the DMFC lifetime tests were also observed.     

Analysis of Direct Methanol Fuel Cell (DMFC)‐Performance via FTIR Spectroscopy of Cathode Exhaust

Water and methanol flux through Nafion™ and polyaryl‐blend membranes prepared at ICVT were studied under DMFC operation. The water, methanol, and CO₂ content in the cathode exhaust were measured by FTIR spectroscopy. Both the water and methanol flux turned out to be strongly dependent on the operating temperature and thus on membrane swelling. Apart from this, water flux through the membrane is primarily affected by the gas volume flux on the cathode side. A coupling between water flux and methanol flux was observed, which leads to the conclusion that methanol is transported both by diffusion and by convection caused by the superimposed water flux. Polyaryl‐blend membranes showed a reduced diffusive methanol transport when compared to Nafion™ due to their different internal microstructure. The impact of methanol cross‐over on cathode losses at high current density needs further clarification with respect to the prevailing mechanism of methanol oxidation at the cathode.     

Model for the Degradation Performance of a Single‐Cell Direct Methanol Fuel Cell under Varying Operational Conditions

The effect of varying operating parameters on the degradation of a single‐cell direct methanol fuel cell (DMFC) with serpentine flow channels was investigated. Fuel cell internal temperature, methanol concentration, and air and methanol flow rates were varied in experimental tests and fuel cell performance was chronologically recorded. A DMFC semi‐empirical performance model was developed to predict the polarization curves of the DMFC and validated at different operating conditions. Performance degradation was observed and modeled over time by a linear regression model. Unlike previous studies, the cumulative exposure of the operating factors to the fuel cell was considered in the degradation analysis. The degradation model shows the cell voltage generation capacity does not significantly degrade. However, the Tafel slope of the cell changes with cumulative exposure to methanol concentration and air flow, and the ohmic resistance changes with cumulative exposure to temperature, methanol and air flow.     

AC‐Impedance Investigation of Different MEA Configurations for Passive‐Mode DMFC Mini‐Stack Applications

Membrane‐electrode assemblies (MEAs) characterised by different hydrophobic–hydrophilic properties were investigated in a passive Direct methanol fuel cell (DMFC) monopolar mini‐stack at room temperature. These properties were modulated by varying the amount of Nafion or replacing the ionomer in the catalytic layer with polytetrafluoroethylene (PTFE). Impedance spectroscopy provided valuable information with respect to the limiting processes occurring during fuel cell operation. Methanol crossover, especially in the presence of high methanol concentration, played a major role in determining the overall performance. The development of a methanol impermeable membrane appears crucial for increasing the performance of DMFC devices for portable applications.     

Effect of cathode catalyst layer thickness on methanol cross-over in a DMFC

The effects of cathode catalyst layer (CCL) thickness on the detrimental effect of methanol cross-over in a direct methanol fuel cell (DMFC) under various operating conditions are studied. Three membrane electrode assemblies (MEAs) with different CCL thicknesses but identical catalyst loading and identical anode catalyst layer are used. The results show that, when a thicker CCL, approximately twice the thickness of the base case, is used, the fuel cell performance increases significantly. The increase in performance with a thicker CCL is attributed to the oxidation of the methanol crossed-over in part of the catalyst layer and leaving the rest of the catalyst layer free from methanol contamination, leading to mitigations of the effects of mixed potentials. The results of electrochemical impedance spectroscopy (EIS) show that the charge transfer resistance for the fuel cell with twice the thickness of CCL is 30% lower compared to that for the base case, indicating that the active catalyst area available for oxygen reduction reaction (ORR) is indeed greater. The results of the electrochemical active surface areas (ECA) show that without methanol contamination, the ECA of the thicker CCL is actually lower, indicating that the better performance and the lower charge transfer resistance are not caused by a higher original ECA, but a higher active area for ORR. A much thicker CCL, about 5 times of that for the base case, is also used and the cell performance is also higher than that for the base case. The experimental results show that there exists an optimum cathode catalyst layer thickness and the thickness of cathode catalyst layer has a significant effect on DMFC performance.     

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

A porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N₂, O₂, CO and under mixed reactant O₂/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.     

A glue method for fabricating membrane electrode assemblies for direct methanol fuel cells

We present a simple glue method for fabricating membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFC). Rather than the conventional “dry” hot-pressing method that relies solely on hot-pressing at a high pressure and temperature to form a MEA, the “wet” method developed in this work introduces a binding agent, consisting of Nafion^® solution, between a polymer electrolyte membrane (PEM) and an anode/cathode. The introduced binding agent can provide a better adhesion and stronger binding force between a membrane and an electrode, thereby facilitating a better interfacial contact between the electrode and the Nafion^® membrane, which has been proved by scanning electron microscopy (SEM) analyses to the cross-sectional morphology of the MEA after long-term operation. The cell performance characterization showed the MEA fabricated by the glue method was more stable in cell performance than that fabricated by the conventional hot-pressing method. Cyclic voltammetry (CV) results also demonstrated the MEA fabricated by the glue method exhibited a higher electrochemical surface area (ESA) as a result of the improved interfacial contact between the Nafion^® membrane and the electrodes. Finally, the DMFC with the MEA fabricated by the glue method was characterized by the electrochemical impedance spectroscopy (EIS).     

Shunt current protocol to eliminate reversible degradation of polymer electrolyte membrane fuel cells during constant load operations

The objective of this study is to determine the durability of polymer electrolyte membrane fuel cells (PEMFCs) in constant current operation incorporated with regular recovery protocol for eliminating reversible performance loss of membrane electrode assemblies. Effects of operation ‘shunt current protocol’ on PEMFC durability are studied through analyses of the main degradation mechanism based on results of electrochemical characterizations and post-mortem investigations. The voltage of the protected cell using the shunt current protocol is stably preserved under the applied current density for 700 h with less degradation (4.2% of decay ratio), while the performance of the unprotected cell steadily decreased with time (15.1% over 700 h). The substantial performance deterioration of the unprotected cell is mainly attributed to morphology deterioration of the cathode catalyst layer with oxidation of Pt catalysts and chemical degradation of ionomers caused by generation of excessive water from electrochemical oxygen reduction reactions under high-humidity operating conditions. In contrast, the shunt current protocol plays an important role in sustaining high oxygen activity at the cathode catalyst surface, detaching partially covered OH⁻ species on Pt active sites from water oxidation during cell operation caused by periodically applied shunt current for a very short period of 1 s every 5 h. We hope to provide insight into the operation protocol to extend the lifetime of PEMFCs, minimizing conversion from recoverable performance loss to irreversible (permanent) degradation during operation.     

Effect of methanol crossover on the cathode behavior of a DMFC: A half-cell investigation

A half-cell consisting of a normal direct methanol fuel cell (DMFC) cathode and a membrane that contacts with an electrolyte solution was developed to investigate the effect of methanol crossover on the cathode behavior. Open circuit potentials, cyclic voltammetry profiles, polarization curves and electrochemical impedance spectroscopy (EIS), resulting from the oxygen reduction reaction (ORR) with/without the effect of methanol oxidation reaction (MOR), were measured. The transient measurements indicated that both current and open circuit potential of the electrode exhibited significant oscillations when the anodic MOR was superposed on the cathodic ORR, which explain the instabilities that may be encountered in the practical DMFC operation. The steady-state results confirmed that the presence of methanol at the cathode led to a significant poisoning effect on the ORR, especially when the DMFC operates at higher methanol concentrations and discharges at lower potentials. More importantly, the half-cell was proved to be ideal for the EIS study of DMFC electrodes because the system not only facilitates an accurate potential control but also reflects the actual mass transport process that occurs in practical DMFCs.     

Realising a reference electrode in a polymer electrolyte fuel cell by laser ablation

This work presents a new concept for realising a reference electrode configuration in a PEM fuel cell by means of laser ablation. The laser beam is used to evaporate a small part of the electrode of a catalyst-coated membrane (CCM) to isolate the reference electrode from the active catalyst layer. This method enables the simultaneous ablation of the electrodes on both sides of the CCM because the membrane is transparent for the laser beam. Therefore, a smooth electrode edge without electrode misalignment can be realised. A test fuel cell was constructed which together with the ablated CCM enables the separation of the total cell losses during operation into the cathode, anode and membrane overpotentials in PEFC as well as in DMFC mode. The methanol tolerance of a selenium-modified ruthenium-based catalyst (RuSe _x ) was investigated under real fuel cell conditions by measuring polarisation curves, electrochemical impedance spectroscopy (EIS) and current interrupt measurements (CI).     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

Raman Spectroscopy of an Aged Low Temperature Polymer Electrolyte Fuel Cell Membrane

The cost and durability of the membrane electrode assembly (MEA) are today limiting factors for large‐scale commercialisation of the polymer electrolyte membrane fuel cell (PEMFC). The MEA durability in a real working fuel cell (FC) is closely linked to specific operating conditions such as temperature, gas humidity, load dynamics, etc. This often results in both chemical and mechanical degradation of the ion‐conducting membrane and subsequent operation failure of the FC. In this study, Raman spectroscopy is used to identify and distinguish between two different degradation processes for a 1,500 h in situ aged FC membrane. The primary process is due to the loss of proton conducting sulphonic acid end groups over the entire membrane. The secondary process is a degradation of the fluorinated backbone concentrated to the cathode interface; making possible the collapse of carbon into the resulting voids of the membrane. Using spatially resolved Raman spectroscopy we can unambiguously observe both the localisation and the state of the carbon inside the membrane; being similar/identical to the microporous layer (MPL).     

SAXS Analysis of Catalyst Degradation in High Temperature PEM Fuel Cells Subjected to Accelerated Ageing Tests

The loss in performance during fuel cell operation is one of the critical factors that hamper fuel cells commercialization. This paper presents a research activity related to high temperature polymer electrode membrane fuel cell (HT‐PEMFC) degradation. The aim of the study is to investigate catalyst degradation of membrane electrode assemblies (MEAs) subjected to load cycles. Two HT‐PEM MEAs have been subjected to accelerated ageing tests based on load cycling. The cycles profile has been chosen in order to enhance catalyst degradation. Both the tests show a fuel cell performance loss lower than 30 mV after 100,000 cycles at 600 mA cm⁻². In order to analyze the catalyst evolution, synchrotron small angle X‐ray scattering (SAXS) has been employed. The catalyst degradation of the two conditioned samples has been compared with the data obtained from a new MEA that has been used as reference sample. The SAXS results showed a mean size increase of the platinum nanoparticles up to the 100%.     

Operation Characteristic Analysis of a Direct Methanol Fuel Cell System Using the Methanol Sensor‐less Control Method

The application of methanol sensor‐less control in a direct methanol fuel cell (DMFC) system eliminates most of the problems encountered when using a methanol sensor and is one of the major solutions currently used in commercial DMFCs. This study focuses on analyzing the effect of the operating characteristics of a DMFC system on its performance under the methanol sensor‐less control as developed by Institute of Nuclear Energy Research (INER). Notably, the influence of the dispersion of the methanol injected on the behavior of the system is investigated systematically. In addition, the mechanism of the methanol sensor‐less control is investigated by varying factors such as the timing of the injection of methanol, the cathode flow rate, and the anode inlet temperature. These results not only provide insight into the mechanism of methanol sensor‐less control but can also aid in the improvement and application of DMFC systems in portable and low‐power transportation.     

Pore‐forming technology and performance of MEA for direct methanol fuel cells

BACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent. RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm^?2 to 167.42 mW cm^?2 at 55 °C and 0.2 MPa O₂. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes. CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry     

A study on the dissymmetrical microporous layer structure of a direct methanol fuel cell

The effect of carbon type, carbon loading and microporous layer structure in the microporous layer on the performance of a direct methanol fuel cell (DMFC) at low temperature was investigated using electrochemical polarization techniques, electrochemical impedance spectroscopy, scanning electron microscope and other methods. Vulcan XC-72 carbon was found to be most suitable as a microporous layer for low temperature DMFC. Maximum fuel cell performance was obtained utilizing a microporous layer with carbon loading of 1.0 mg cm⁻² when air was used as an oxidant. A membrane electrode assembly with 1.0 mg cm⁻² Vulcan XC-72 carbon with 20 wt.% Teflon in the cathode and no microporous layer in the anode showed a maximum power density of 36.7 mW cm⁻² at 35 °C under atmospheric pressure. The AC impedance study proved that a cell with a dissymmetrical microporous layer structure had lower internal resistance and mass transfer resistance, thus obtaining better performance.     

Investigation on cathode degradation of direct methanol fuel cell

The cathode degradation of a direct methanol fuel cell (DMFC) was investigated after a 240 h discontinuous galvostatic operation at 80 °C. The catalyst coated membrane (CCM) and the cathode diffusion layer were not combined so as to isolate electrochemical and mass transport processes. It was indicated by the EDS and SEM tests that the loss of the cathode electrochemical surface area (ESA) was associated with the decays of the Pt/C catalyst and the interfacial contact. Furthermore, Ru crossover and higher methanol crossover resulting from the anode failure aggravated the degradation of the cathode. On the other hand, the change of the pore structure led to a higher wettability of the cathode microporous layer. Therefore, the oxygen transport was suppressed due to the decrease of hydrophobic passages.     

Accelerated durability test of DMFC electrodes by electrochemical potential cycling

Durability of direct methanol fuel cell electrodes was evaluated by electrochemical potential cycling and we observed the degradation phenomena during the performance decay. An individual potential measurement of anode and cathode with built-in reversible hydrogen electrode revealed that the anode and cathode performance contributions are almost of the same order of magnitude to the entire performance loss, although the anode degradation is relatively bigger, due to the dominating effect of ruthenium dissolution, corresponding loss of electrocatalytic activity. On the contrary, it was apparent that the electrochemical active surface area of Pt cathode decreased significantly with potential cycling under methanol crossover condition, which is not clearly reflected on the performance loss due to the initial decrease of interfacial resistance between membrane and cathode catalyst layer. Impedance studies could reinforce the current–voltage polarization by more comprehensive information.     

Polymer electrolyte-type electrochemical ozone generator with an oxygen cathode

A new electrochemical ozone generator which has a proton exchange polymer membrane as an electrolyte, pure water as a source material and an oxygen diffusion electrode as a cathode was proposed to eliminate hydrogen evolution on the cathode and also to reduce electrolytic power consumption. The device was evaluated under varied operating conditions changing current density, temperature and oxygen flow rate. The water permeable high performance gas electrode enabled current density operation over 1.0 A cm⁻² without generating hydrogen gas in the cathode side outlet gas. The power consumption was successfully reduced to two thirds of that of the hydrogen evolution type electrolyzer over a period of three months' operation.     

Fuel cell performance of polymer electrolyte membrane based on hexafluorinated sulfonated poly(ether sulfone)

The potential-current fuel cell characteristics of membrane electrode assemblies (MEAs) using hexafluorinated sulfonated poly(ether sulfone) copolymer are compared to those of Nafion^® based MEAs in the case of proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). The hexafluorinated copolymer with 60 mol% of monosulfonated comonomer based acid form membrane is chosen for this study due to its high proton conductivity, high thermal stability, low methanol permeability, and its insolubility in boiling water. The catalyst powder is directly coated on the membrane and the catalyst coated membrane is used to fabricate MEAs for both fuel cells. A current density of 530 mA cm^?2 at 0.6 V is obtained at 70 °C with H₂/air as the fuel and oxidant. The peak power density of 110 mW cm^?2 is obtained at 80 °C under specific DMFC operating conditions. Other electrochemical characteristics such as electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry are also studied.