Calum R. I. Chisholm

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

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 (p[H2O] = 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.

Superprotonic behavior of Cs2(HSO4)(H2PO4) – a new solid acid in the CsHSO4–CsH2PO4 system

Investigations into the CsHSO₄–CsH₂PO₄ system have yielded a new solid acid, Cs₂(HSO₄)(H₂PO₄), with a superprotonic phase transition that occurs over the temperature range 61–105°C. In the room temperature structure, the SO₄ and PO₄ groups are randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs₂(HSO₄)(H₂PO₄) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, ΔH=44±2 J/g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with a₀=4.926(5)A, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO₄ groups (X=P or S) at the center. The conductivity in the high temperature phase at 110°C is 3×10⁻³ Ω⁻¹ cm⁻¹, and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO₄ group reorientations in the cubic phase of Cs₂(HSO₄)(H₂PO₄), as is known to occur in the high temperature, tetragonal phase of CsHSO₄.

Polymer solid acid composite membranes for fuel-cell applications

A systematic study of the conductivity of polyvinylidene fluoride (PVDF) and CsHSO4 composites, containing 0 to 100% CsHSO4, has been carried out. The polymer, with its good mechanical properties, served as a supporting matrix for the high proton conductivity inorganic phase. The conductivity of composites exhibited a sharp increase with temperature at 142°C, characteristic of the superprotonic phase transition of CsHSO4. At high temperature (160°C), the dependence of conductivity on vol % CsHSO4 was monotonic and revealed a percolation threshold of ~10 vol %. At low temperature (100°C), a maximum in the conductivity at ~80 vol % CsHSO4 was observed. Results of preliminary fuel cell measurements are presented.

Alcohol Fuel Cells at Optimal Temperatures

High-power-density alcohol fuel cells can relieve many of the daunting challenges facing a hydrogen energy economy. Here, such fuel cells are achieved using CsH2PO4 as the electrolyte and integrating into the anode chamber a Cu-ZnO/Al2O3 methanol steam-reforming catalyst. The temperature of operation, ~250°C, is matched both to the optimal value for fuel cell power output and for reforming. Peak power densities using methanol and ethanol were 226 and 100 mW/cm^2, respectively. The high power output (305 mW/cm^2) obtained from reformate fuel containing 1% CO demonstrates the potential of this approach with optimized reforming catalysts and also the tolerance to CO poisoning at these elevated temperatures.

Structure and thermal behavior of the new superprotonic conductor Cs2(HSO4)(H2PO4)

Ongoing studies of the CsHSO(4)-CsH(2)PO(4) system, aimed at developing novel proton conducting solids, resulted in the new compound Cs(2)(HSO(4))(H(2)PO(4)) (dicesium hydrogensulfate dihydrogenphosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Cs(2)(HSO(4))(H(2)PO(4)) to crystallize in space group P2(1)/n with lattice parameters a = 7.856 (8), b = 7.732 (7), c = 7.827 (7) Å, and beta = 99.92 (4) degrees. The compound has a unit-cell volume of 468.3 (8) Å(3) and two formula units per cell, giving a calculated density of 3.261 Mg m(-3). Six non-H atoms and two H atoms were located in the asymmetric unit, with SO(4) and PO(4) groups randomly arranged on the single tetrahedral anion site. Refinement using all observed reflections yielded weighted residuals of 0.0890 and 0.0399 based on F(2) and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and fixed isotropic temperature factors for the two H atoms. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron in a neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (1;01). Thermal analysis of the superprotonic transition in Cs(2)(HSO(4))(H(2)PO(4)) showed that the transformation to the high-temperature phase occurs by a two-step process. The first is a sharp transition at 334 K and the second a gradual transition from 342 to 378 K. The heat of transformation for the entire process ( approximately 330-382 K) is 44 +/- 2 J g(-1). Thermal decomposition of Cs(2)(HSO(4))(H(2)PO(4)) takes place at much higher temperatures, with an onset of approximately 460 K.

High-temperature phase transitions in K3H(SO4)2

Two phase transitions in K 3 H(SO 4 ) 2 were discovered in the temperature range 25-300°C. The transition temperatures for the mid and high-temperature phases are T c = 190°C (ΔH = 17.0 kJ/mol) and T c = 227°C (ΔH = 7.4 kJ/mol), respectively, for freshly heated samples. A slow decomposition process begins above 270°C. The conductivity of K 3 H(SO 4 ) 2 in these phases (σ = 1.68 × 10 -3 at 198°C and 2.19 × 10 -2 Ω -1 cm -1 at 251°C) is comparable to that of other M 3 H(XO 4 ) 2 compounds (M = Cs, NH 4 , Rb and X = S, Se) in their superprotonic phases. The activation energy for proton transport in the highest temperature phase of K 3 H(SO 4 ) 2 -, is 0.45 eV, a value slightly higher than in the related compounds. Despite the similarity between the electrical properties of K 3 H(SO 4 ) 2 and other M 3 H(XO 4 ) 2 compounds, the structural properties are quite distinct. Specifically, high-temperature X-ray powder diffraction measurements show that neither of the high-temperature phases of K 3 H(SO 4 ) 2 , is trigonal, indeed, the symmetry of the structure decreases at the first transition, in contrast to the superprotonic phases in other M 3 H(XO 4 ) 2 compounds.

Proton (deuteron) conductivity in Cs1.5Li1.5H(SO4)2 and Cs1.5Li1.5D(SO4)2 single crystals

Abstract Calorimetric (DSC), NMR and electrical studies of Cs 1.5 Li 1.5 X(SO 4 ) 2 (X=H, D) single crystals have been performed in the temperature range from 300 to 533 K. No phase transitions are observed upon heating and the conductivity follows an Arrhenius temperature dependence to the point of decomposition at ∼470 K. Despite the high protonic conductivity of Cs 1.5 Li 1.5 X(SO 4 ) 2 ( σ ∼10 −3 S cm −1 ) at high temperature, these compounds cannot be classified as superprotonic because of the large value of the activation energy ( E a ∼1 eV). The proton NMR studies confirmed that two crystallographic proton sites exist in Cs 1.5 Li 1.5 H(SO 4 ) 2 , presumably corresponding to two minima in the single, crystallographically distinct (and asymmetric) hydrogen bond. A measurable isotope effect in the conductivity data confirmed that protons/deuterons, as opposed to lithium ions, are the mobile species. The high activation energy for proton transport in Cs 1.5 Li 1.5 X(SO 4 ) 2 most probably results from the asymmetry of the hydrogen bonds, and from electrostatic repulsion between the protons and the Li + ions which likely hinders reorientation of XSO 4 − groups.