High Capacity SiO[sub 2]-Graphite Composite Electrodes with Chemically Incorporated Metal MHCFs for Electrochemically Switched Alkaline Cation Exchange

Abstract

Bulk metal hexacyanoferrate ͑MHCF͒ has been synthesized within the porous microstructure of conductive composite materials using a simple chemical procedure. The composite materials consist of micrometer-size graphite particles encapsulated in a SiO 2 solid matrix using a sol-gel technique. A strong electrochemical signal from the chemically treated composites is associated with electrochemically switched alkaline-cation exchange. The exchange capacity of these composite electrodes exceeds by more than two orders of magnitude that of thin-film MHCF electrodes of comparable dimensions. The effects of some fabrication parameters on the cation-exchange capacity and on the distribution of the hexacyanoferrate within the composite electrodes are presented.Metal hexacyanoferrates ͑MHCFs͒ are inorganic compounds capable of selectively exchanging alkaline cations in aqueous media. 1,2 This particular property makes MHCFs attractive for the selective removal of radioactive Cs ϩ from high Na ϩ and K ϩ nuclear wastes as well as in the water deionization processes in general. 2 The cation-exchange capability of MHCF compounds arises from an inherent charge imbalance; alkaline cations must intercalate into ͑and deintercalate out of͒ the MHCF matrix in order to maintain electroneutrality. Equation 1 is a simplified representation of this reversible process for a MHCF whose stoichiometry is analogous to soluble prussian blue 3,4where M ϭ Ni II , Co II , Zn II , Fe II , or Cu II , A is an intercalated alkaline cation, and A ϩ is an alkaline cation in solution. Note that Eq. 1 assumes completely oxidized and completely reduced iron states. If the iron centers are accessed electrochemically, the life expectancy of MHCF can reach thousands of cation-exchange cycles with minimal capacity losses. 5-9 This process, termed electrochemically switched ion exchange ͑ESIX͒, 2,10,11 controls the cation loading and unloading processes via an applied electrode potential. The separation media used in ESIX processes must first be selective for a target ion; MHCFs are selective for Cs ϩ . Electroactive nickel hexacyanoferrate ͑NiHCF͒, in fact, intercalates ca. 10 4 more Cs ϩ into its matrix than competing alkaline cations present in solution. 4 The separation material must also be able to empty all intercalated alkaline cations as the matrix approaches a fully oxidized state. Recent work using Raman and energy dispersive spectroscopies 12,13 molecular dynamics simulations, 14 and X-ray diffraction ͑XRD͒ and extended X-ray absorption fine structure ͑EXAFS͒ 12 has probed the oxidation state, stoichiometry, cation content, and structure of electroactive NiHCF to address this issue. The third and final characteristic important for an ESIX separation material is high cation-exchange capacity.Thin-film, electroactive NiHCF electrodes can be easily prepared by anodic dissolution of a nickel substrate in a ferricyanide solution 5,11 or by cathodic deposition onto platinum and gold substrates from dilute solutions of ferricyanide and Ni͑II͒. 3,4 Nominal ...

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