Instability of sulfate and selenate solid acids in fuel cell environments
TL;DR: In this paper, the chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods, and the overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H 2O + SO_3 with Cs 2S_2O_7 appearing as an intermediate byproduct at slow heating rates.
Abstract: The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO_4, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at ∼175°C under inert conditions. The overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H_2O + SO_3 with Cs_2S_2O_7 appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO_4 + 4H_2 → Cs_2SO_4 + 4H_2O + H_2S. In the absence of fuel cell catalysts, this reduction reaction is slow; however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H_2S. In the case of M_3H(XO_4)_2 compounds, where M = Cs, NH_4, or Rb and X = S or Se, a similar reduction reaction occurs: 2M_3H(XO_4)_2 + 4H_2 → 3M_2XO_4 + 4H_2O + H_2X. In an operational fuel cell based on CsHSO_4, performance degraded with time, presumably as a result of H_2S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160 °C.
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TL;DR: This work demonstrates continuous, stable power generation for both H2/O2 and direct methanol fuel cells operated at ∼250°C using a humidity-stabilized solid acid CsH2PO4 electrolyte.
Abstract: 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°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 ∼250°C using a humidity-stabilized solid acid CsH_2PO_4 electrolyte.
425 citations
TL;DR: In this paper, a review summarises and discusses the key areas of research in recent years for non-polymer based high temperature membranes or so-called solid acid membranes, including the reasons for operating at high temperatures, the proton transport mechanisms, the limitations of current polymer membranes and their modification and on the future of solid acid membrane elaborating on future pathways which may bring about tangible enhancements in this technology.
274 citations
TL;DR: An overview of the different materials currently thought to be potential proton exchange membrane materials for fuel cells operated at medium temperatures (100-200°C) is given in this article.
184 citations
01 Sep 2006
TL;DR: In this paper, composites are classified in terms of four main classes: inorganic proton conductors suspended in inert polymers, inorganic particles added to extend polymeric ionomers, polymeric acid complexes between basic polymers and acidic inorganic particle.
Abstract: Composite membranes consisting primarily of a polymer and an inorganic proton conducting particle or a proton conducting polymer containing inorganic particles for use as proton exchange membranes in low and intermediate temperature fuel cells are reviewed. The chemistry of major inorganic additives that have been used is described in terms of their structure and intrinsic ability to conduct protons. Composites are classified in terms of four main classes: inorganic proton conductors suspended in inert polymers; inorganic particles added to extend polymeric ionomers; inorganic proton conductors blended with polymeric ionomers; and polymer acid complexes between basic polymers and acidic inorganic particles.
159 citations
TL;DR: In addition, fuel cells have been shown to have high efficiency and low emissions, and are attractive for their modular and distributed nature, and zero noise pollution as discussed by the authors, and they will also play an essential role in any future hydrogen fuel economy.
141 citations
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TL;DR: A status report on development of the direct methanol fuel cell (DMFC) and relevant fundamental and applied electrochemistry is the objective of as discussed by the authors, where emphasis is put on strategies and approaches rather than on individual results.
1,217 citations
TL;DR: It is shown that a cell made of a CsHSO4 electrolyte membrane 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.
Abstract: 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 degrees 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 degrees 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 degrees C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) operating at 150-160 degrees 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.
809 citations
TL;DR: The self-assembly of conjugated polymer/silica nanocomposite films with hexagonal, cubic or lamellar mesoscopic order using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers is reported.
Abstract: Nature abounds with intricate composite architectures composed of hard and soft materials synergistically intertwined to provide both useful functionality and mechanical integrity. Recent synthetic efforts to mimic such natural designs have focused on nanocomposites, prepared mainly by slow procedures like monomer or polymer infiltration of inorganic nanostructures or sequential deposition. Here we report the self-assembly of conjugated polymer/silica nanocomposite films with hexagonal, cubic or lamellar mesoscopic order using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic monomers uniformly within a highly ordered, inorganic environment. Polymerization results in polydiacetylene/silica nanocomposites that are optically transparent and mechanically robust. Compared to ordered diacetylene-containing films prepared as Langmuir monolayers or by Langmuir-Blodgett deposition, the nanostructured inorganic host alters the diacetylene polymerization behaviour, and the resulting nanocomposite exhibits unusual chromatic changes in response to thermal, mechanical and chemical stimuli. The inorganic framework serves to protect, stabilize, and orient the polymer, and to mediate its function. The nanocomposite architecture also provides sufficient mechanical integrity to enable integration into devices and microsystems.
520 citations
TL;DR: In this paper, the conductivity measurements were made by means of impedence spectroscopy for crystals of the Me 3 H(SeO 4 ) 2 (Me: NH 4, Cs, Rb) group.
137 citations
TL;DR: In this article, it is suggested that the nomenclature high-temperature phase transition (HTPT) around T p should be replaced by that onset of partial polymerization at reaction sites distributed on the surface of solids.
131 citations