Although they hold the promise of clean energy, state-of-the-art fuel cells based on polymer electrolyte membrane fuel cells are inoperable above$100\textdegree C$, require cumbersome humidification ...systems, and suffer from fuel permeation. These difficulties all arise from the hydrated nature of the electrolyte. In contrast, "solid acids" exhibit anhydrous proton transport and high-temperature stability. We demonstrate continuous, stable power generation for both$H_{2}/O_2$and direct methanol fuel cells operated at$\sim 250\textdegree C$using a humidity-stabilized solid acid$CsH_{2}PO_{4}$electrolyte.
We report the spontaneous formation of nanoparticles on smooth nanofibres in a single-step electrospinning process, as an inexpensive and scalable method for producing high-surface-area composites. ...Layers of nanofibres, containing the proton conducting electrolyte, caesium dihydrogen phosphate, are deposited uniformly over large area substrates from clear solutions of the electrolyte mixed with polymers. Under certain conditions, the normally smooth nanofibres develop caesium dihydrogen phosphate nanoparticles in large numbers on their external surface. The nanoparticles appear to originate from the electrolyte within the fibres, which is transported to the outer surface after the fibres are deposited, as evidenced by cross-sectional imaging of the electrospun fibres. The presence of nanoparticles on the fibre surface yields composites with increased surface area of exposed electrolyte, which ultimately enhances electrocatalytic performance. Indeed, solid acid fuel cells fabricated with electrodes from processed nanofibre-nanoparticle composites, produced higher cell voltage as compared to fuel cells fabricated with state-of-the-art electrodes.
Solid acids as fuel cell electrolytes Haile, Sossina M; Boysen, Dane A; Chisholm, Calum R. I ...
Nature (London),
04/2001, Letnik:
410, Številka:
6831
Journal Article
Recenzirano
Fuel cells are attractive alternatives to combustion engines for electrical power generation because of their very high efficiencies and low pollution levels. Polymer electrolyte membrane fuel cells ...are generally considered to be the most viable approach for mobile applications. However, these membranes require humid operating conditions, which limit the temperature of operation to less than 100 °C; they are also permeable to methanol and hydrogen, which lowers fuel efficiency. Solid, inorganic, acid compounds (or simply, solid acids) such as CsHSO4 and Rb3H(SeO4)2 have been widely studied because of their high proton conductivities and phase-transition behaviour. For fuel-cell applications they offer the advantages of anhydrous proton transport and high-temperature stability (up to 250 °C). Until now, however, solid acids have not been considered viable fuel-cell electrolyte alternatives owing to their solubility in water and extreme ductility at raised temperatures (above approximately 125 °C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) operating at 150-160 °C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit. Moreover, the solid-acid properties were not affected by exposure to humid atmospheres. Although these initial results show promise for applications, the use of solid acids in fuel cells will require the development of fabrication techniques to reduce electrolyte thickness, and an assessment of possible sulphur reduction following prolonged exposure to hydrogen.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200-300 degrees C) fuel cells. In order to settle the question of the high temperature behavior of this ...material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (pH2O = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 degrees C, with the conductivity rising to a value of 2.2 x 10(-2) S cm(-1) at 240 degrees C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH2PO4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH2PO4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm(-2). Thus, for fuel cells in which the supported electrolyte membrane was only 25 microm in thickness and in which a peak power density of 415 mW cm(-2) was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH2PO4, a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.
Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based ...electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 molH2/gcath.
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•Superprotonic CsH2PO4 enables electrochemical cell operation at 250°C•Ammonia decomposition catalyst integrated with hydrogen electrooxidation catalyst•Ammonia converted to hydrogen with 100% faradic efficiency•Hydrogen production rate of 1.5 mol per gram catalyst per hour at 0.4 V bias
Ammonia has received increasing attention in recent years as a possible energy carrier, in particular, as a carrier of hydrogen for use in fuel cells. The traditional approach of thermal decomposition suffers from high concentrations of residual ammonia, which poison the catalysts in polymer electrolyte membrane fuel cells, whereas newer strategies based on electrochemical decomposition in aqueous solution operate at high overpotentials, implying low efficiency. Our approach integrates a thermal decomposition catalyst (Cs-promoted Ru on carbon nanotubes) with an all-solid-state electrochemical conversion cell (based on the proton-conducting electrolyte, CsH2PO4) in a device that is operable at 250°C. The resulting polarization curves indicate high current density at a modest voltage (far beyond what can be attained from alkali electrolyte cells), as well as catalyst utilization efficiency that far exceeds traditional thermal decomposition.
Ammonia has received increasing attention in recent years as an enabler of a sustainable energy future, in particular, as a carrier of hydrogen for use in fuel cells. Using superprotonic CsH2PO4 and a bilayer cathode structure, we show ammonia-to-hydrogen conversion with 100% faradic efficiency. Cs-promoted Ru serves as the ammonia decomposition catalyst, whereas Pt serves as the hydrogen electrooxidation catalyst and also as the hydrogen evolution catalyst at the anode. Zero ammonia crossover and zero side reactions result in an ultrahigh-purity product.
We demonstrate cathodes for solid acid fuel cells fabricated by vapor deposition of platinum from the metalorganic precursor Pt(acac)2 on the solid acid CsH2PO4 at 210 °C. A network of platinum ...nanoparticles with diameters of 2−4 nm serves as both the oxygen reduction catalyst and the electronic conductor in the electrode. Electrodes with a platinum content of 1.75 mg/cm2 are more active for oxygen reduction than previously reported electrodes with a platinum content of 7.5 mg/cm2. Electrodes containing <1.75 mg/cm2 of platinum show significantly reduced catalytic activity and increased ohmic resistance indicative of a highly discontinuous catalytic-electronic platinum network.
A low-cost Ru-based chalcogenide catalyst has been used as a cathode catalyst in solid acid fuel cells (SAFC). With sequential addition of Se and Mo on Ru/C in a controlled manner, the resulting ...physical and electrochemical properties have been discussed in detail. The oxygen reduction reaction (ORR) in the presence of phosphoric acid has been performed to appraise the tolerance of the catalyst in the presence of phosphate anions. Considering the phosphate-rich environment during cell operation, this study is especially relevant for designing catalysts for s. In order to estimate the coverage of phosphate anions on active sites, a semiquantitative analysis of the corresponding Tafel plots has been done. Electrochemical, thermogravimetric, and in situ X-ray absorption spectroscopic experiments have been performed to get a deeper perception of the catalyst–electrolyte interface and account for the high stability of the chalcogenide catalyst at room temperature as well as at elevated temperature. Steady-state polarization curves in SAFC have been collected for over 120 h using the chalcogenide catalyst operating at 250 °C.
Open up any textbook on fuel cells and the reader finds a list of five classic types of fuel cells, each differentiated by the nature of the electrolyte: polymer electrolyte membrane (or sometimes ...proton exchange membrane), phosphoric acid, alkali, molten carbonate, and solid oxide.
The performance of hydrogen fuel cells based on the crystalline solid proton conductor CsH2PO4 is circumscribed by the mass activity of platinum oxygen reduction catalysts in the cathode. Here we ...report on the first application of an alloy catalyst in a solid acid fuel cell, and demonstrate a mass activity 4.5 times greater than Pt at 0.8 V. This activity enhancement was obtained with platinum-palladium alloys that were vapor-deposited directly on CsH2PO4 at 210 °C. Catalyst mass activity peaks at a composition of 84 at% Pd, though smaller activity enhancements are observed for catalyst compositions exceeding 50 at% Pd. Prior to fuel cell testing, Pd-rich catalysts display lattice parameter expansions of up to 2% due to the presence of interstitial carbon. After fuel cell testing, a Pt-Pd solid solution absent of lattice dilatation and depleted in carbon is recovered. The structural evolution of the catalysts is correlated with catalyst de-activation.
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