Proton Conductivity of Natural Diatomite
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Proton Conductivity of Natural Diatomite
Bo Wang Research and Applications, Imerys, San Jose, CA 95134, U.S.A.
ABSTRACT Proton conductivity of the natural diatomite was studied by ac complex impedance technique. At room temperature, the highest proton conductivity was found to be 4.5 x 10-7 S·cm-1. By hydrating the diatomite, the proton conductivity was increased to two orders of magnitude higher. The room temperate proton conductivity of the hydrated diatomite (5.5 x 10-5 S·cm-1) was comparable to other hydrated solid proton conductors. The natural diatomite could be used as potential cost-effective proton conductor for electrochemical applications. INTRODUCTION Solid proton conducting materials from organic polymers to inorganic oxides have been extensively investigated for various electrochemical applications such as hydrogen and humidity sensors, water electrolyzers and fuel cells [1]. For examples, there are many studies on polymer proton exchange membranes including fluorinated membranes, hydrocarbon membranes, aromatic membranes and hybrid membranes for fuel cell and electrolyser applications [2 - 3]. Hydrated or water-containing proton conductors have also been studied for the similar applications [4 - 5]. Compared to these synthetic materials, natural minerals such as diatomite are economically more attractive. Diatomite is a sediment enriched in biogenic silica in the form of the siliceous frustules of diatoms with intricate and porous structure. Silanol (≡Si-OH) groups often occur on the natural diatomite surface. The concentration of the silanol groups can be controlled by hydrating or dehydrating the diatomite. The porous and intricate structure of silica unique to diatomite in combination surface silanol groups offers distinguishing characteristics as a solid proton conductor. Thus far, there have been no reports on the proton conductivity of diatomite material. In this work, we have investigated proton conductivity and structure of a natural diatomite material as potential cost-effective proton conductor for electrochemical applications. EXPERIMENTAL A natural diatomite crude from Tuxpan deposit in Mexico was used for this study. Chemical composition of this diatomite was determined using a Thermo ARL ADVANT’XP XRay Fluorescence (XRF) spectrometer equipped with a 60 KV rhodium target X-ray source. Specific surface area was determined using a Micromeritics Gemini III 2375 BET surface area analyzer. Pore size distribution was determined using a Micromeritics AutoPore IV 9500
mercury porosimetry analyzer. Fourier Transform Infrared (FTIR) spectrum was recorded on a Thermo Scientific Nicolet 6700 FTIR spectrophotometer on the diatomite powder in the range 400 to 4000 cm-1. Proton conductivity was determined by an AC complex impedance analysis at frequencies from 0.1 Hz to 50 KHz using a Solartron 1250 frequency response analyzer. The natural diatomite crude material was cut into small plates or milled into fine powders and then pressed into pellets. Gold contacts were deposited on the faces of the pla
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