High-temperature Polymer Electrolyte Fuel Cells Research Articles

Design of Supported PtCo Electrocatalysts for Pemfcs

Pt-alloy electrocatalysts have been investigated for operation in both hydrogen and methanol-fed polymer electrolyte membrane (PEM) fuel cells with the aim of evaluating their performance and resistance to degradation. Nanosized PtCo catalysts with particle size of about 3 nm were prepared by using a sulphite complex route [1-4]. To achieve a high degree of alloying a carbothermal reduction was used. This caused the occurrence of a primitive cubic ordered structure. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He+ at 1kV). The formation of a Pt skin layer on the surface of the alloy electro-catalyst was studied. Oxide supports were evaluated as alternative to carbon black materials with the aim to increase the electrochemical stability. The structural and surface characteristics of the samples as well as the type of catalyst support appear to strongly influence the PtCo catalyst performance and degradation.

Poly(vinylpyrrolidone)–wrapped carbon nanotube-based fuel cell electrocatalyst shows high durability and performance under non-humidified operation

Low durability and performance have limited the wide commercialization of high-temperature polymer electrolyte fuel cells (HT-PEFCs).

Dynamic Operation of HT-PEFC: In-Operando Imaging of Phosphoric Acid Profiles and (Re)distribution

Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples.

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

Accelerated stress tests are commonly applied in order to obtain a prediction of fuel cell life time within a short testing period. The stress test in this work considers a fuel cell which is constantly under load with frequent periods of very high current. Four different cells were operated each with a specific load profile. As a result severe performance degradation was observed in the region of high current densities. A short overview over the phenomena which may contribute to this specific fuel cell degradation is compiled from literature. Based on this overview major degradation modes are identified and combined with a simple polarization curve model. The modeling results allow for two different interpretations. Carbon corrosion reactions can explain the observed effects if cell voltage is assumed as the driving force for degradation. A better fit was obtained by using the overall heat flux as degradation criterion. In this case an increase in local temperature could lead to redistribution and loss of phosphoric acid from the MEA. The results are supported by the fact that the model yields consistent time dependend results for four different and non-periodic load cycles.

Observing proton motion on the nanoscale in polymeric electrolyte membranes with quasielastic neutron scattering

The mechanism of proton conductivity in high temperature polymer electrolyte fuel cells (HT-PEFCs) has been investigated with macroscopic conductivity measurements and on a microscopic scale with quasielastic neutron scattering techniques. Polybenzimidazole membranes, which are used in HT-PEFCs, are doped with phosphoric acid (PA) to achieve the desired proton conductivity. Neutron spin echo experiments showed that the polymer matrix is very rigid incoherent scattering experiments, but incoherent scattering revealed rather fast diffusion processes, compatible with macroscopic conductivity measurements. The measured diffusion is faster than anticipated from the conductivity of a phosphoric acid doped PBI membrane, but slower than that expected for pure phosphoric acid. Over larger distances the fractal polymer membrane network slows down the locally fast diffusion to the macroscopic values. With elastic fixed window scans on a backscattering spectrometer an activation energy of 7.6 kJ/mol is obtained at typical conditions in an HT-PEFC.

Phosphate-Tolerant Oxygen Reduction Catalysts

Increased oxygen reduction reaction (ORR) kinetics, improved CO tolerance, and more efficient water and heat management represent significant advantages that high-temperature polymer electrolyte fuel cells (HT-PEFCs) operating with a phosphoric acid-doped polybenzimidazole (PBI) membrane offer over traditional Nafion-based, low-temperature PEFCs. However, before such HT-PEFCs become viable, the detrimental effect of phosphate chemisorption on the performance of state-of-the-art Pt-based cathode catalysts needs to be addressed. In this study, we propose a solution to the severe poisoning of Pt-based PEFC cathode catalysts with phosphates (H2PO4– and HPO42–) by replacing standard Pt/C catalysts with phosphate-tolerant, nonprecious metal catalyst (NPMC) formulations. Catalysts with a very high surface area (845 m2 g–1) were synthesized in this work from polyaniline (PANI), iron, and carbon using a high-temperature approach. The effects of metal precursors and metal loading on the morphology, structure, and O...

Effect of Nanostructure of the Electrocatalyst Based on Carbon Nanotube for the Activation Overpotential for PEFC

Polymer electrolyte fuel cells (PEFC) are considered as one of the promising power sources for automobile and house in near future due to their high energy conversion efficiency and compactness. Recently high temperature PEFC using the polymer membrane doped with liquid phosphoric acid (PA) is attracting strong interest since it shows higher power density, CO poisoning on platinum (Pt) can be avoided and no cooling and humidifying devices are required. However, it was pointed the high temperature PEFC has poor durability due to leaching of PA from the membrane, which causes the deterioration of PEFC performance during long-term operation[1]. To overcome this issue, poly(vinylphosphonic acid) (PVPA) was used as the electrolyte instead of PA. We found the membrane doped with PVPA was free from the leaching of the electrolyte since PVPA is polymer and solid acid. Although this PEFC showed high durability, this catalyst had large overpotential in the area of low current density, probably because the poisoning of Pt by PVPA coating[2] or the lack of electrolyte in catalyst layers[3]. In this study, first we explored the new preparation method to avoid the Pt poisoning and investigated the catalytic performance by electrochemical techniques. And then, we increased the amount of acid in the electrolyte and compared the overpotential.[1] M. Hori et al. J. Power Sources 2012, 210, 366. [2] C. Roth et al. J. Phys. Chem. C 2013, 117, 6210. [3] J.H. Jang et al. Int. J. Hyd. E. 2010, 35, 2119.

Evaluation of structural changes of HT-PEFC electrodes from in-situ synchrotron X-ray radiographs

One of the main goals for improvement of high-temperature polymer electrolyte fuel cells (HT-PEFCs) is the increase of the fuel cell performance under different operating conditions. We investigated the correlation between operating conditions and structural changes in the electrodes by means of in-situ through-plane synchrotron X-ray radiography. From the radiographs it is possible to clearly distinguish between the electrode crack structure beneath the ribs and beneath the channels of the flow field. We present a statistical method to analyze these crack structures. For this purpose a ‘radar’ method was developed in order to obtain the width of the cracks at many different locations and the distribution of crack widths. We found a different behavior of cracks located beneath the ribs and beneath the channels and an influence of the operating conditions on local regions of the crack structure.

On the Positive Effect of CO during Start/Stop in High-Temperature Polymer Electrolyte Fuel Cells

The influence of CO in the fuel gas on cathode carbon corrosion during start/stop cycles in high temperature polymer electrolyte fuel cells (HT-PEFC) was investigated. The fuel cell underwent simulated start/stop cycles with a constant time interval, reactant gas flow rate and temperature by switching between H2/CO mixtures and O2 at the fuel electrode. The results reveal that increasing amounts of CO in the fuel gas reduce the amount of carbon corrosion at the air electrode. Therefore, HT-PEFC operation with CO containing fuel has the benefit of mitigating start/stop induced degradation effects.

Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates for HT-PEFCs

This work investigates various commercially available austenitic stainless steels and Ni-based alloys as possible metallic bipolar plates for high-temperature polymer electrolyte fuel cells (HT-PEFCs). The dynamic formation and depletion of passivation layers that depends strongly on applied potential was analyzed in 85 wt% phosphoric acid at RT and 130°C by means of cyclic voltammetry and steady-state polarization. All materials showed a beneficial passivation in the potential window 0.3–1.2 V, which is mainly based on a stable inner Cr2O3 layer, and a reduction of the passive layers at cathodic polarization. Alloy 2.4869 (Cronix 80) with 80 wt% Ni and 20 wt% Cr reveals the lowest corrosion rates of 16.1 μA cm−2 at 130°C in the passive region at 0.6 V and a free corrosion potential Ecor of 235 mV. The improvement of passivity was achieved by the dominant superficial Ni3(PO4)2 layer. Alloying additions Mo and Ti revealed a characteristic increase of passive current densities due to instability of these passivation components at 130°C. Passivation based on Fe oxides and phosphates in stainless steels shows to be beneficial at RT, but less efficient at 130°C.

Evaluation of Neutron Imaging for Measuring Phosphoric Acid Distribution in High Temperature PEFCs

The possibility of using neutron imaging for non-invasive investigation of the phosphoric acid distribution in high temperature polymer electrolyte fuel cells (HT-PEFC) was explored with a small scale test cell. In particular, the issue of providing a suitable reference – necessary for distinguishing the neutron attenuation due to the acid from the attenuation due to the structural components – was solved by using in situ deuteration/protonation of the phosphoric acid, a fully reversible process. Experiments with a non-operating cell have shown that this isotope exchange can be performed in less than 20 minutes. The possibility of imaging the acid distribution either over the cell area (through-plane imaging) or across the cell structure (in-plane imaging) was demonstrated. Although some discrepancies between the two modes remain, quantitative analysis resulted in a good agreement with the amount of acid used in the cell.

A Comprehensive Corrosion Study on Metallic Materials for HT-PEFC Application

This work investigates various commercially available austenitic stainless steels and Ni-based alloys as possible metallic bipolar plates for high-temperature polymer electrolyte fuel cells (HT-PEFCs). The dynamic formation and depletion of passivation layers that depend strongly on applied potential were analyzed in 85 wt. % phosphoric acid at RT and 130 °C by means of steady-state polarization. Alloy 2.4869 (Cronix 80) reveals the lowest corrosion rates of 3 – 26 uA∙cm-2 at 130 °C in the passive region under anodic polarization from 0 V to 1.0 V. Negative potentials lead to reduction of the passive layer and to susceptibility to corrosion in the ongoing polarization. Interestingly, the promising alloy 2.4856 with additions of molybdenum and niobium shows a lower corrosion resistance. The release of metal ions was measured using inductively coupled plasma – optical emission spectrometry (ICP-OES). This study clearly demonstrates that the use of bare stainless steels and Ni-based alloys as bipolar plates is not realizable due to meager corrosion resistance.

Novel composite Zr/PBI-O-PhT membranes for HT-PEFC applications

Novel composite membranes for high temperature polymer-electrolyte fuel cells (HT-PEFC) based on a poly[oxy-3,3-bis(4′-benzimidazol-2″-ylphenyl)phtalide-5″(6″)-diyl] (PBI-O-PhT) polymer with small amounts of added Zr were prepared. It was shown in a model reaction between zirconium acetylacetonate (Zr(acac)4) and benzimidazole (BI) that Zr-atoms are capable to form chemical bonds with BI. Thus, Zr may be used as a crosslinking agent for PBI membranes. The obtained Zr/PBI-O-PhT composite membranes were examined by means of SAXS, thermomechanical analysis (TMA), and were tested in operating fuel cells by means of stationary voltammetry and impedance spectroscopy. The new membranes showed excellent stability in a 2000-hour fuel cell (FC) durability test. The modification of the PBI-O-PhT films with Zr facilitated an increase of the phosphoric acid (PA) uptake by the membranes, which resulted in an up to 2.5 times increased proton conductivity. The existence of an optimal amount of Zr content in the modified PBI-O-PhT film was shown. Larger amounts of Zr lead to a lower PA doping level and a reduced conductivity due to an excessively high degree of crosslinking.

Synchrotron X-ray radioscopic in situ study of high-temperature polymer electrolyte fuel cells - Effect of operation conditions on structure of membrane

We present the first high-resolution synchrotron X-ray study on high temperature polymer electrolyte fuel cells (HT-PEFCs) in through-plane mode. Distribution and evolution of the phosphoric acid in the membrane electrode assembly was monitored in situ/in-operando under different operating conditions at steady states. At current densities of 350 mA cm−2 and 600 mA cm−2 significant changes in local media distribution were detected mainly beneath the channels of the flow field. We assign this effect to a varying degree of acid doping over the active region. Furthermore it was found that parts of the electrode structure were moved and partly irreversibly displaced after cell operation at load conditions. This effect might contribute to structural aging of the electrodes.

The Effect of Microporous Layer in Phosphoric Acid Doped Polybenzimidazole Polymer Electrolyte Membrane Fuel Cell

A polybenzimidazole (PBI) based polymer electrolyte fuel cells, which called high temperature polymer electrolyte fuel cells (HT-PEMS), operate at higher temperatures (120-200°C) than conventional PEM fuel cells. Although it is known that HT-PEMS have some of the significant advantages as non-humidification requirements for membrane and the lack of liquid water at high temperature in the fuel cell, the generated water as a result of oxygen reduction reaction causes in the degradation of these systems. The generated water absorbed into membrane side interacts with the hydrophilic PBI matrix and it can cause swelling of membrane, so water transport mechanism in a Membrane Electrode Assembly (MEA) needs to be well understood and water balance must be calculated in MEA. Therefore, the water diffusion transport across the electrolyte should be determined. In this study, it is investigated firstly the water content in a MEA in the case of with/without Microporous Layer (MPL). Secondly, in the case of with MPL the effect of microporous layer’s thickness on the water management in fuel cell is investigated. For this aim, two-dimensional fuel cell with interdigitated flow-field is modeled using Comsol Multiphysics 4.2a software. The operating temperature and doping level is selected as 180°C and 6.75 RPU H3PO4/PBI, respectively. The results of this work brought out that MPL significantly effects to water content in MEA and reduces H2O concentration in MEA. Thus it can be prevented to flooding in MEA and so durability of the cell is increased.

Correlation of Synchrotron X-ray Radiography and Electrochemical Impedance Spectroscopy for the Investigation of HT-PEFCs

Differently conditioned high temperature polymer electrolyte fuel cells (HT-PEFCs) were investigated by electrochemical impedance spectroscopy to study the influence of the break-in procedure on the overall fuel cell performance during the heating-up procedure as well as during different operating conditions. It is shown that a classical 70 h break-in leads to better fuel cell performance compared to a fuel cell which is used without any break-in procedure. A combination of synchrotron X-ray radiographic measurements with in situ time-dependent impedance spectroscopy was used to investigate the de-/hydration processes of the phosphoric acid type electrolyte within the components of the membrane electrode assembly (MEA) of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions.

Investigation of HT-PEFCs by Means of Synchrotron X-ray Radiography and Electrochemical Impedance Spectroscopy

Differently conditioned high temperature polymer electrolyte fuel cells (HT-PEFCs) were investigated by electrochemical impedance spectroscopy to study the influence of the break-in procedure on the overall fuel cell performance during the heating-up procedure as well as during different operating conditions. It is shown that a classical 70 h break-in procedure leads to the best fuel cell performance compared to fuel cells which are used after a rapid break-in or rather without any break-in procedure. A combination of synchrotron X ray radiographic measurements with in-situ performed time-dependent impedance spectroscopy was used to investigate the de-/hydration processes within the electrolyte of the membrane electrode assembly (MEA) components of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions.

Electrode Compositions for High-Temperature Polymer Electrolyte Fuel Cell (HT-PEFC)

One theoretical advantage of High Temperature Polymer-Electrolyte Fuel Cells (HT-PEFC) is the higher kinetic of the anode and cathode reaction. Whereas on the anode side the higher kinetic can be achieved practically, the kinetic on the cathode side is in practice lower than in low temperature PEFCs. In this paper different kind of electrode compositions for anode and cathode are prepared to improve the interface between an acid-base blend membrane (Polybenzimidazol-sulfonated Polysulfon) in respect to the cathode kinetic and the ohmic resistance of the Membrane-Electrode-Assembly (MEA).

Current Density Distribution Measurement in HT-PEFC Stacks Operated with Reformate Gas from Middle Distillates

A promising application for high temperature polymer electrolyte fuel cells (HT-PEFC) is the use in auxiliary power units (APU). In combination with a fuel gas processor for middle distillates like kerosene or diesel it can efficiently provide on board electric energy for mobile applications. Current R&D work focuses on robust, long-term stable and efficient operation of HT-PEFC stacks. In order to gain insight into internal electrical and thermal behavior current density and temperature distribution measurements in a five cell short stack have been performed. With this method it is shown, how reactant supply and temperature management impacts on local polarization characteristics and temperature distribution.

Demonstration of a 20 W class high-temperature polymer electrolyte fuel cell stack with novel fabrication of a membrane electrode assembly

Acid-doped polybenzimidazole (PBI) membrane and polytetrafluoroethylene (PTFE)-based electrodes are used for the membrane electrode assembly (MEA) in high-temperature polymer electrolyte fuel cells (HTPEFCs). To find the optimum PTFE content for the catalyst layer, the PTFE ratio in the electrodes is varied from 25 to 50 wt%. To improve the performance of the electrodes, PBI is added to the catalyst layer. With a weight ratio of PTFE to Pt/C of 45:55 (45 wt% PTFE in the catalyst layer), the fuel cell shows good performance at 150 °C under non-humidified conditions. When 5 wt% PBI is added to the electrodes, performance is further improved (250 mA cm −2 at 0.6 V). Our 20 W class HTPEFC stack is fabricated with a novel MEA. This MEA consists of 8 layers (1 phosphoric acid-doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-diffusion media, 2 gas-sealing gaskets). The sub-gasket mitigates the destruction of a highly acid-doped PBI membrane and provides long-term durability to the fuel cell stack. The stack operates for 1200 h without noticeable cell degradation.