Synthesis and studies of polybenzimidazoles for high-temperature fuel cells

A number of polybenzimidazoles (PBIs) were synthesized and tested in real fuel cells. The possibility of introducing phosphoric groups in PBIs was studied. The phosphorylated and fluorine-containing PBIs obtained by click reactions were investigated.     

Fluorinated polyoxadiazole for high-temperature polymer electrolyte membrane fuel cells

For the first time a fluorinated polyoxadiazole doped with phosphoric acid as a proton-conducting membrane for operation at temperatures above 100 °C and low humidities for fuel cells has been reported. Fluorinated polyoxadiazole with remarkable chemical stability was synthesized. No changes in the molecular weight (about 200,000 g mol⁻¹) can be observed when the polymer is exposed for 19 days to mixtures of sulfuric acid and oleum. Protonated membranes with low doping level (0.34 mol of phosphoric acid per polyoxadiazole unit, 11.6 wt.% H₃PO₄) had proton conductivity at 120 °C and RH = 100% in the order of magnitude of 10⁻² S cm⁻¹. When experiments are conducted at lower external humidity, proton conductivity values drop an order of magnitude. However still a high value of proton conductivity (6 × 10⁻³ S cm⁻¹) was obtained at 150 °C and with relative humidity of 1%. In an effort to increase polymer doping, nanocomposite with sulfonated silica containing oligomeric fluorinated-based oxadiazole segments has also been prepared. With the addition of functionalized silica not only doping level but also water uptake increased. For the nanocomposite membranes prepared with the functionalized silica higher proton conductivity in all range of temperature up to 120 °C and RH = 100% (in the order of magnitude of 10⁻³ S cm⁻¹) was observed when compared to the plain membrane (in the order of magnitude of 10⁻⁵ S cm⁻¹).     

Silica-facilitated proton transfer for high-temperature proton-exchange membrane fuel cells

High-temperature proton-exchange membrane fuel cells (HT-PEMFCs) have shown a broad prospect of applications due to the enhanced reaction kinetics and simplifie...     

Recent advances in phosphoric acid-based membranes for high-temperature proton exchange membrane fuel cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are pursued worldwide as effi-cient energy conversion devices.Great efforts have been made in t...     

Synthesis and characteristics of hyperbranched polymer with phosphonic acid groups for high-temperature fuel cells

Two different molecular weight hyperbranched polymers (HBP(L)-(PA)₂ and HBP(H)-(PA)₂) with two phosphonic acid groups as a functional group at the periphery and a low molecular weight hyperbranched polymer (HBP(L)-(PA)₂-Ac) with both two phosphonic acid groups and an acryloyl group as a cross-linker at the periphery were successfully synthesized as thermally stable proton-conducting electrolytes. A cross-linked electrolyte membrane (CL-HBP(L)-(PA)₂) was prepared by thermal polymerization of the HBP(L)-(PA)₂-Ac using benzoyl peroxide. Ionic conductivities of the HBP(L)-(PA)₂, the HBP(H)-(PA)₂, and the CL-HBP(L)-(PA)₂ under dry condition and their thermal properties were investigated, and also, the effect of the phosphonic acid group number on them was discussed. Ionic conductivities of the HBP(L)-(PA)₂ and the HBP(H)-(PA)₂ were found to be 1.5?×?10^?5 S cm^?1 at 150 °C and 3.6?×?10^?6 S cm^?1 at 143 °C, respectively, under dry condition, and showed the Vogel–Tamman–Fulcher type temperature dependence. The hyperbranched polymers and the cross-linked electrolyte membrane were thermally stable up to 300 °C, and the cross-linked electrolyte membrane (CL-HBP-(PA)₂) had suitable thermal stability as an electrolyte membrane for the high-temperature fuel cells under dry condition. Fuel cell measurement using a single membrane electrode assembly cell with the cross-linked membrane was performed.     

High-performance polymer electrolyte membranes incorporated with 2D silica nanosheets in high-temperature proton exchange membrane fuel cells

Silica nanosheets(SN)derived from natural vermiculite(Verm)were successfully incorporated into polyethersulfone-polyvinylpyrrolidone(PES-PVP)polymer to fabricate high-temperature proton exchange membranes(HT-PEMs).The content of SN filler was varied(0.1-0.75 wt%)to study its influence on proton conductivity,power density and durability.Benefiting from the hydroxyl groups of SN that enable the formation of additional proton-transferring pathways,the inorganic-organic membrane displayed enhanced proton conductivity of 48.2 mS/cm and power density of 495 mW/cm² at 150℃ without humidification when the content of SN is 0.25 wt%.Furthermore,exfoliated SN(E-SN)and sulfonated SN(S-SN),which were fabricated by a liquid-phase exfoliation method and silane condensation,respectively,were embedded in PES-PVP polymer matrix by a simple blending method.Due to the significant contribution from sulfonic groups in S-SN,the membrane with 0.25 wt%S-SN reached the highest proton conductivity of51.5 mS/cm and peak power density of 546 mW/cm² at150℃,48%higher than the pristine PES-PVP membranes.Compared to unaltered PES-PVP membrane,SN added hybrid composite membrane demonstrated excellent durability for the fuel cell at 150℃.Using a facile method to prepare 2D SN from natural clay minerals,the strategy of exfoliation and functionalization of SN can be potentially used in the production of HT-PEMs.     

Regularities of high-temperature oxidation of current collectors of solid oxide fuel cells due to diffusion processes in subsurface regions

A complex study of regularities of oxidative reactions is carried out in subsurface layers of current collectors of solid oxide fuel cells manufactured of Crofer 22APU or Crofer 22H stainless steel. The methods of scanning electron microscopy, energy dispersive X-ray analysis, and Raman spectroscopy are used to study the distribution of the main steel elements in subsurface layers of current collectors as dependent on the operation time under the conditions of a cathodic chamber of solid oxide fuel cells. A mechanism of the process is suggested and contact resistance between the current collector and LSM cathode is calculated using the model of a Schottky barrier for a metal–semiconductor junction.     

Performance of planar electrode-supported high-temperature solid oxide fuel cell using syngas

In China, coal is a dominant energy source. In order to ensure China’s energy security, coal should be used efficiently and cleanly. Integrated gasification fuel cell hybrid power generation system is a promising system for coal utilization. It combines clean coal gasification technology with high efficient fuel cell technology. In this work, the performance of solid oxide fuel cell using syngas as fuel was investigated, based on the commercial computational fluid dynamic software and the developed program used to analyze chemical, electrochemical, heat/mass transfer, current, and electric potential. The results show that the temperature difference is about 300 K in the cell under all calculation conditions. Along the cell length, hydrogen concentration rapidly reduces, and its decrement is larger than that of carbon monoxide. The variation of current density in electrolyte layer is relatively small along the direction of gas flow, but it is obvious along the direction vertical to gas flow.     

Towards biochemical fuel cells

A biochemical fuel cell is a device which converts chemical energy into electrical power. The catalysts used in this process can be either inorganic or organic type giving rise to ‘inorganic fuel cells’ or ‘biochemical fuel cells’, respectively. Biochemical fuel cells use either micro-organism or enzymes as active components to carry out electrochemical reactions. The efficiency of such a device theoretically can be as high as 90%. The difficulty in attaining these values arises due to sluggishness of electron transfer from active site to conducting electrode. This can be overcome by using mediators or by immobilizing active components on conducting electrode. We have immobilizedfad-glucose oxidase on a graphite electrode using a semiconducting chain as a bridge. At the present stage of development, such a device tacks high current densities, which is essential for commercial power generation but can be used in applications such as pacemakers and glucose sensors.     

Solid oxide fuel cells

Despite being first demonstrated over 160 years ago, and offering significant environmental benefits and high electrical efficiency, it is only in the last two decades that fuel cells have offered a realistic prospect of being commercially viable. The solid oxide fuel cell (SOFC) offers great promise and is presently the subject of intense research activity. Unlike other fuel cells the SOFC is a solid-state device which operates at elevated temperatures. This review discusses the particular issues facing the development of a high temperature solid-state fuel cell and the inorganic materials currently used and under investigation for such cells, together with the problems associated with operating SOFCs on practical hydrocarbon fuels.     

Nafion perfluorinated membranes in fuel cells

Increasing global energy requirements, localized power issues and the need for less environmental impact are now providing even more incentive to make fuel cells a reality. A number of technologies have been demonstrated to be feasible for generation of power from fuel cells over the last several years. Proton exchange membranes (PEM) have emerged as an essential factor in the technology race. DuPont has supplied Nafion^® perfluorinated membranes in fuel cells for space travel for more than 35 years and they have played an integral part in the success of recent work in portable, stationary and transportation applications. The basis for PEM fuel cell emergence and DuPont technology utilization will be discussed.     

X-ray absorption spectroscopic studies on solid oxide fuel cells and proton-conducting ceramic fuel cells

X-ray absorption spectroscopy (XAS) is one of the best techniques to obtain the information on the electronic and local structures of materials. In the last few decades, XAS becomes a common analytical technique for the investigation of solid oxide fuel cells and proton-conducting ceramic fuel cells. In particular, operando and/or advanced XAS measurements can be recently available with the increased accessibility of synchrotron radiation. In this article, recent trends of solid oxide fuel cell and proton-conducting ceramic fuel cell researches using XAS are overviewed.     

Pervaporation membranes in direct methanol fuel cells

The membranes in direct methanol fuel cells must both conduct protons and serve as a barrier for methanol. Nafion, the most common fuel cell membrane, is an excellent conductor but a poor barrier. Polyvinyl alcohol pervaporation membranes are good methanol barriers but poor conductors. These and most other pervaporation membranes offer no significant advantages over Nafion in methanol fuel cell applications. However, polybenzimidazole membranes have demonstrated characteristics that suggest up to a 15-fold improvement in direct methanol fuel cells. This improvement may be due to an alternate form of proton conduction in which protons travel via a Grotthus or “hopping” mechanism.     

Implications of the HCN → HNC process to high-temperature nitrogen-containing fuel chemistry

We have found in our recent kinetic study of the oxidation of HCN by NO₂ in the temperature range 623–773 K that HNCO and CO₂ are very important early products. The measured kinetic data cannot be accounted for by a “conventional” mechanism involving HCN reactions with NO₂, O, and OH. However, the introduction of the isomerization reaction HCN → HNC, followed by the rapid oxidation of HNC by NO₂, O, and OH, can quantitatively simulate all measured kinetic data. A similar study of the NO₂ + HCN reaction in shock waves at temperatures between 1500 and 2400 K also required the inclusion of HNC reactions in order to quantitatively account for measured product distributions. The effects of the HNC molecule on the high temperature HCN chemistry are discussed in terms of the predicted rate constants for HNC reactions with O and OH employing the BAC-MP4 method. © John Wiley & Sons, Inc.     

Intermediate temperature solid oxide fuel cells

High temperature solid oxide fuel cells (SOFCs), typified by developers such as Siemens Westinghouse and Rolls-Royce, operate in the temperature region of 850-1000 degrees C. For such systems, very high efficiencies can be achieved from integration with gas turbines for large-scale stationary applications. However, high temperature operation means that the components of the stack need to be predominantly ceramic and high temperature metal alloys are needed for many balance-of-plant components. For smaller scale applications, where integration with a heat engine is not appropriate, there is a trend to move to lower temperatures of operation, into the so-called intermediate temperature (IT) range of 500-750 degrees C. This expands the choice of materials and stack geometries that can be used, offering reduced system cost and, in principle, reducing the corrosion rate of stack and system components.This review introduces the IT-SOFC and explains the advantages of operation in this temperature regime. The main advances made in materials chemistry that have made IT operation possible are described and some of the engineering issues and the new opportunities that reduced temperature operation affords are discussed.This tutorial review examines the advances being made in materials and engineering that are allowing solid oxide fuel cells to operate at lower temperature. The challenges and advantages of operating in the so-called 'intermediate temperature' range of 500-750 degrees C are discussed and the opportunities for applications not traditionally associated with solid oxide fuel cells are highlighted. This article serves as an introduction for scientists and engineers interested in intermediate temperature solid oxide fuel cells and the challenges and opportunities of reduced temperature operation.