Proton exchange membrane fuel cell degradation under close to open-circuit conditions: Part I: In situ diagnosis

Durability of polymer exchange membrane (PEM) fuel cells under a wide range of operational conditions has been generally identified as one of the top technical gaps that need to be overcome for the acceptance of this fuel cell technology as a commercially viable power source, especially for automotive and portable applications. In this study, a 1200 h lifetime test was conducted with a six-cell PEM fuel cell stack under close to open-circuit conditions. In situ measurements of the hydrogen crossover rate through the membrane, high frequency resistance and electrochemically active surface area of each single cell, in combination with cell polarization curves, were used to investigate the degradation mechanisms. Direct gas mass spectrometry of the cathode exhaust gas indicated the formation of HF, H₂O₂ and CO₂ during the durability testing. The overall cell degradation rate under this accelerated stress testing is approximately 0.128 mV h⁻¹. The cell degradation rate for the first 800 h is much lower than that after 800 h, which may result from the dominance of different degradation mechanisms. For the first period, the degradation of fuel cell performance was mainly attributed to catalyst decay, while the subsequent dramatic degradation is likely caused by membrane failure.     

Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells

Highly active and stable carbon composite catalysts for oxygen reduction in PEM fuel cells were developed through the high-temperature pyrolysis of Co–Fe–N chelate complex, followed by the chemical post-treatment. A metal-free carbon catalyst was used as the support. The carbon composite catalyst showed an onset potential for oxygen reduction as high as 0.87 V (NHE) in H₂SO₄ solution, and generated less than 1% H₂O₂. The PEM fuel cell exhibited a current density as high as 0.27 A cm⁻² at 0.6 V and 2.3 A cm⁻² at 0.2 V for a catalyst loading of 6.0 mg cm⁻². No significant performance degradation was observed over 480 h of continuous fuel cell operation with 2 mg cm⁻² catalyst under a load of 200 mA cm⁻² as evidenced by a resulting cell voltage of 0.32 V with a voltage decay rate of 80 μV h⁻¹. Materials characterization studies indicated that the metal–nitrogen chelate complexes decompose at high pyrolysis temperatures above 800 °C, resulting in the formation of the metallic species. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface doped with nitrogen groups is catalytically active for oxygen reduction.     

Ageing of anode-supported solid oxide fuel cell stacks including thermal cycling,and expansion behaviour of MgO–NiO anodes

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H₂ fuel and 0.35 A cm⁻² (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm⁻² (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to −2 mV per cycle for operation at 0.3 A cm⁻², open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Ω cm² per cycle.     

Electrochemical, structural and surface characterization of nickel/zirconia solid oxide fuel cell anodes in coal gas containing antimony

The interactions of antimony with the nickel-zirconia anode in solid oxide fuel cells (SOFCs) have been investigated. Tests with both anode-supported and electrolyte-supported button cells were performed at 700 and 800 °C in synthetic coal gas containing 100 ppb to 9 ppm antimony. Minor performance loss was observed immediately after Sb introduction to coal gas resulting in ca. 5% power output drop. While no further degradation was observed during the following several hundred hours of testing, cells abruptly and irreversibly failed after 800-1600 h depending on Sb concentration and test temperature. Antimony was found to interact strongly with nickel resulting in extensive alteration phase formation, consistent with expectations based on thermodynamic properties. Nickel antimonide phases, NiSb and Ni₅Sb₂, were partially coalesced into large grains and eventually affected electronic percolation through the anode support. Initial degradation was attributed to diffusion of antimony to the active anode/electrolyte interface to form an adsorption layer, while the late stage degradation was due the Ni-Sb phase formation. Assuming an average Sb concentration in coal gas of 0.07 ppmv, a 500 μm thick Ni/zirconia anode-supported cell is not expected to fail within 7 years when operated at a power output of 0.5 W cm⁻² and fuel utilization above 50%.     

Catalytic dehydrogenation of ammonia borane in non-aqueous medium

Dehydrogenation of Ammonia Borane (NH₃BH₃, AB) catalyzed by transition metal heterogeneous catalysts was carried out in non-aqueous solution at temperatures below the standard polymer electrolyte membrane (PEM) fuel cell operating conditions. The introduction of a catalytic amount (∼2 mol%) of platinum to a solution of AB in 2-methoxyethyl ether (0.02–0.33 M) resulted in a rapid evolution of H₂ gas at room temperature. At 70 °C, the rate of platinum catalyzed hydrogen release from AB was the dehydrogenation rate which was 0.04 g s⁻¹ H₂ kW⁻¹.     

Scale-up of a high temperature polymer electrolyte membrane fuel cell based on polybenzimidazole

A high temperature PEM fuel cell stack with a total active area 150 cm² has been studied. The PEM technology is based on a polybenzimidazole (PBI) membrane. Cast from a PBI polymer synthesised in our lab, the performance of a three-cell stack was analysed in static and dynamic modes. In static mode, operating at high constant oxygen flow rate (QO₂>1105 ml O₂/min) produces a small decrease on the stack performance. High constant oxygen stoichiometry (λO₂>3) does not produce a decrease on the performance of the stack. There are not differences between operating at constant flow rate of oxygen and constant stoichiometry of oxygen in the stack performance. The effect of operating at high temperature with a pressurization system and operating at higher temperatures are beneficial since the performance of the fuel cell is enhanced. A large shut-down stage produces important performance losses due to the loss of catalyst activity and the loss of membrane conductivity. After 150 h of operation at 0.2 A cm⁻², it is observed a very high voltage drop. The phosphoric acid leached from the stack was also evaluated and did not exceed 2% (w/w). This fact suggests that the main degradation mechanism of a fuel cell stack based on polybenzimidazole is not the electrolyte loss. In dynamic test mode, it was observed a rapid response of power and current output even at the lower step-time (10 s). In the static mode at 125 °C and 1 atm, the stack reached a power density peak of 0.29 W cm⁻² (43.5 W) at 1 V.     

Impacts of air bleeding on membrane degradation in polymer electrolyte fuel cells

A long-term accelerated test (4600 h) of a 25 cm² single cell with excess air bleeding (5%) was carried out to investigate the effects of air bleeding on membrane degradation in polymer electrolyte fuel cells. The rate of membrane degradation was negligibly low (fluoride-ion release rate = 1.3 × 10⁻¹⁰ mol cm⁻² h⁻¹ in average) up to 2000 h. However, membrane degradation rate was gradually increased after 2000 h. The CO tolerance of the anode gradually dropped, which indicated that the anode catalyst was deteriorated during the test. The results of the rotating ring–disk electrode measurements revealed that deterioration of Pt–Ru/C catalyst by potential cycling greatly enhances H₂O₂ formation in oxygen reduction reaction in the anode potential range (∼0 V). Furthermore, membrane degradation rate of the MEA increased after the anode catalyst was forced to be deteriorated by potential cycling. It was concluded that excess air bleeding deteriorated the anode catalyst, which greatly enhanced H₂O₂ formation upon air bleeding and resulted in the increased membrane degradation rate after 2000 h.     

Design of an optical thermal sensor for proton exchange membrane fuel cell temperature measurement using phosphor thermometry

Internal temperatures in a proton exchange membrane (PEM) fuel cell govern the ionic conductivities of the polymer electrolyte, influence the reaction rate at the electrodes, and control the water vapor pressure inside the cell. It is vital to fully understand thermal behavior in a PEM fuel cell if performance and durability are to be optimized. The objective of this research was to design, construct, and implement thermal sensors based on the principles of the lifetime-decay method of phosphor thermometry to measure temperatures inside a PEM fuel cell. Five sensors were designed and calibrated with a maximum uncertainty of ±0.6 °C. Using these sensors, surface temperatures were measured on the cathode gas diffusion layer of a 25 cm² PEM fuel cell. The test results demonstrate the utility of the optical temperature sensor design and provide insight into the thermal behavior found in a PEM fuel cell.     

Effects of coal syngas and H2S on the performance of solid oxide fuel cells: Part 2. Stack tests

The performance of two-cell planar solid oxide fuel cell stacks using coal syngas, with and without hydrogen sulfide (H₂S), was studied. All cells were tested at 850 °C with a constant current load of 15.2 A (current density of 0.22 A cm⁻² per cell) and 30% fuel utilization. The H₂S injection immediately and significantly affected the power degradation of the stack system regardless of the carrier fuel. Results for the test with only H₂ and N₂ in the presence of H₂S (119–120 ppm) indicated that the power decay and area-specific resistance (ASR) degradation values were lower than those for the tests where simulated syngas containing CO and increased water content was used. The results indicate that contact points in the stack contributed to the power degradation of the system. Other factors, including contamination from the upstream fuel gas tubing, may have contributed to the higher degradation under simulated syngas conditions. In general the data confirm previous results for single cell testing, and showed that for this specific short stacks (two-cells) arrangement both a fast and a slow response to H₂S injection that eventually stabilized.     

Manufacturing and performance assessment of stamped,laser welded,and nitrided FeCrV stainless steel bipolar plates for proton exchange membrane fuel cells

A manufacturing and single-cell fuel cell performance study of stamped, laser welded, and gas nitrided ferritic stainless steel foils in an advanced automotive bipolar plate assembly design was performed. Two developmental foil compositions were studied: Fe–20Cr–4V and Fe–23Cr–4V wt.%. Foils 0.1 mm thick were stamped and then laser welded together to create single bipolar plate assemblies with cooling channels. The plates were then surface treated by pre-oxidation and nitridation in N₂–4H₂ based gas mixtures using either a conventional furnace or a short-cycle quartz lamp infrared heating system. Single-cell fuel cell testing was performed at 80 °C for 500 h at 0.3 A/cm² using 100% humidification and a 100%/40% humidification cycle that stresses the membrane and enhances release of the fluoride ion and promotes a more corrosive environment for the bipolar plates. Periodic high frequency resistance potential-current scans during the 500 h fuel cell test and post-test analysis of the membrane indicated no resistance increase of the plates and only trace levels of metal ion contamination.     

The effect of coal syngas containing AsH3 on the performance of SOFCs: Investigations into the effect of operational temperature,current density and AsH3 concentration

The performance of solid oxide fuel cells (SOFCs) using simulated coal-derived syngas, with and without arsine (AsH₃), was studied. Anode-supported SOFCs were tested galvanostatically at 0.25 and 0.5 A cm⁻² at 750 and 800 °C with simulated coal syngas containing 0.1, 1, and 2 ppm AsH₃. The tests with simulated coal syngas containing 1 ppm AsH₃ show little degradation over 100 h of operation. The tests with simulated coal syngas containing 2 ppm AsH₃ show some signs of degradation, however no secondary arsenide phases were found. Extended trial testing with 0.1 ppm AsH₃ showed degradation as well as the formation of a secondary nickel arsenide phase in the anode of the SOFC.     

Natural degradation and stimulated recovery of a proton exchange membrane fuel cell

In this paper, the stimulated recovery of a proton exchange membrane (PEM) fuel cells after natural degradation has been investigated. The performance degradation of a 63-cell PEM fuel cell stack over a storage interval of 40,000 h at temperature 24 °C and relative humidity 65% was analyzed by static and dynamical tests. The average cell voltage degradation rate was 309 μV h⁻¹, averaged over a range of currents. The performance was then partially recovered by application of a high frequency pulsing procedure after which the effective average degradation rate (from the commencement of storage to after the recovery) was approximately 170 μV h⁻¹. This indicates the existence of both recoverable and irrecoverable degradations in the fuel cell. Furthermore, the equivalent circuit model and membrane resistance were used to investigate the degradation mechanisms, suggesting that the natural degradation of the fuel cell is mainly caused by the increase of the resistance, which is most likely caused by membrane dehydration.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

Preparation of supported SrCeO3-based membrane by spin coating method

SrCe_0.9Y_0.1O_3−δ (SCY10) powder with a pure perovskite phase is prepared by solid-state reaction method. NiO is dispersed uniformly in SCY10 powder to fabricate NiO-SCY10 anode substrate. The starting powder, the mixture of SrCO₃, CeO₂ and Y₂O₃, is deposited directly on the green substrate instead of SCY10 powder by spin coating. After co-firing at 1300 °C for 3 h, the starting powder reacts to form SCY10 top layer on the substrate. SEM micrographs show that the top layer is defect-free and adheres well with the anode substrate without any delamination. A single fuel cell is assembled with anode-supported SCY10 membrane as electrolyte membrane and Ag as cathode. The electrochemical property of the fuel cell is tested with hydrogen as fuel in the temperature range of 600-800 °C. The open circuit voltage (OCV) reaches 1.05 V at 800 °C, and the maximum power density is 50 mW cm⁻², 155 mW cm⁻², 200 mW cm⁻² at 600 °C, 700 °C, 800 °C, respectively.     

Fabrication and characterization of a PTFE-reinforced integral composite membrane for self-humidifying PEMFC

A novel PTFE-reinforced self-humidifying membrane based on low-cost sulfonated poly (ether ether ketone) (SPEEK) resin was fabricated. In the membrane a base layer and a thin protective layer were bonded by porous polytetrafluoroethylene (PTFE) film. The base layer, which is composed of silicon oxide supported platinum catalyst (abbreviated as Pt-SiO₂) dispersed in SPEEK resin, can suppress reactant crossover and achieve good membrane hydration due to the imbedded hygroscopic Pt-SiO₂ catalysts. The thin protective layer, which constitutes of H₂O₂ decomposition catalyst Pt-SiO₂ and high H₂O₂-tolerant Nafion resin, aims to prevent the SPEEK resin degradation by H₂O₂ produced at the cathode side by incomplete reduction of oxygen. The porous PTFE film tightly bonds with the SPEEK and the Nafion resins to form an integral membrane and accordingly to avoid delamination of the two different resins. The self-humidifying membrane was characterized by TEM, SEM and EDS, etc. The self-humidifying membrane exhibits higher open circuit voltage (OCV) of 0.98 V and maximum power density value of 0.8 W cm⁻² than 0.94 V, 0.33 W cm⁻² of SPEEK/PTFE membrane under dry condition, respectively. The primary 250 h fuel cell durability experiment was conducted and suggested that this low-cost self-humidifying membrane was durable both on fuel cell performance and the membrane structure under fuel cell operation condition with dry H₂/O₂.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

Performance of a miniaturized silicon reformer-PrOx-fuel cell system

A fuel cell made with silicon is operated with hydrogen supplied by a reformer and a preferential oxidation (PrOx) reactor those are also made with silicon. The performance and durability of the fuel cell is analyzed and tested, then compared with the results obtained with pure hydrogen. Three components of the system are made using silicon technologies and micro electro-mechanical system (MEMS) technology. The commercial Cu-ZnO-Al₂O₃ catalyst for the reformer and the Pt-Al₂O₃ catalyst for the PrOx reactor are coated by means of a fill-and-dry method. A conventional membrane electrode assembly composed of a 0.375 mg cm⁻² PtRu/C catalyst for the anode, a 0.4 mg cm⁻² Pt/C catalyst for the cathode, and a Nafion™ 112 membrane is introduced to the fuel cell. The reformer gives a 27 cm³ min⁻¹ gas production rate with 3177 ppm CO concentration at a 1 cm³ h⁻¹ methanol feed rate and the PrOx reactor shows almost 100% CO conversion under the experimental conditions. Fuel cells operated with this fuel-processing system produce 230 mW cm⁻² at 0.6 V, which is similar to that obtained with pure hydrogen.     

Measurement of effective gas diffusion coefficients of catalyst layers of PEM fuel cells with a Loschmidt diffusion cell

In this work, using an in-house made Loschmidt diffusion cell, we measure the effective coefficient of dry gas (O₂-N₂) diffusion in cathode catalyst layers of PEM fuel cells at 25 °C and 1 atmosphere. The thicknesses of the catalyst layers under investigation are from 6 to 29 μm. Each catalyst layer is deposited on an Al₂O₃ membrane substrate by an automated spray coater. Diffusion signal processing procedure is developed to deduce the effective diffusion coefficient, which is found to be (1.47 ± 0.05) × 10⁻⁷ m² s⁻¹ for the catalyst layers. Porosity and pore size distribution of the catalyst layers are also measured using Hg porosimetry. The diffusion resistance of the interface between the catalyst layer and the substrate is found to be negligible. The experimental results show that the O₂-N₂ diffusion in the catalyst layers is dominated by the Knudsen effect.     

Diagnosis of MEA degradation under accelerated relative humidity cycling

The objective of this work is to identify the failure mode diagnosis protocols for polymer electrolyte membrane (PEM) fuel cells with the use of accelerated testing conditions. The single cells used in this work were constructed using commercial Ion Power^® membrane electrode assemblies (MEAs) and the performance degradation was studied under accelerated dynamic reactant relative humidity (RH) conditions. The influence of RH on cell performance was investigated, and a strong dependence of degradation with respect to relative humidity was found. RH cycling was seen to not only cause the gradual decrease in performance at the beginning of cell operation, but it was also related to the rapid decline in performance observed after 330 h operation. This change in degradation rate is seen a change in the material degradation failure mechanism at this point in the operational history of the cell. The increase in cell resistance, membrane crossover, fluoride release rate and decrease in electrochemical surface area (ESA) were also observed with time, and these results were correlated to change in degradation rate. Infra-Red (IR) imaging of an aged MEA was utilized to show varying temperature profiles and outline the possibility of cracks, tears or pinholes in the membrane.     

Ni/YSZ anode - Effect of pre-treatments on cell degradation and microstructures

Anode supported (Ni/YSZ-YSZ-LSM/YSZ) solid oxide fuel cells were tested and the degradation over hundreds of hours was monitored and analyzed by impedance spectroscopy. Test conditions were chosen to focus on the Ni/YSZ anode degradation and all tests were operated at 750 °C, a current density of 0.75 A cm⁻². Oxygen was supplied to the cathode and the anode inlet gas mixture had a high p(H₂O)/p(H₂) ratio of 0.4/0.6. Commercially available gasses were applied. The effect of different types of pre-treatments on the Ni/YSZ electrode degradation during subsequent fuel cell testing was investigated. Pre-treatments included operating at OCV (4% and 40% H₂O in H₂) prior to fuel cell testing, cleaning of the inlet H₂ gas at 700 °C and processing the anode half cell via multilayer tape casting. Analyses of impedance spectra showed that the increase in the charge transfer reaction resistance in the Ni/YSZ (R_Ni,TPB) was decreased to ¼ or less for the pre-treated and fuel cell tested cells when compared with a non-pre-treated reference tested cell; all operated at the same fuel cell test conditions. Scanning electron microscopy and image analyses for the non-pre-treated reference tested cell and selected pre-treated cells showed significant differences in the area fractions of percolating nickel both in the active anode and support layer.