Characterization of redox active materials to enable electrochemical separations through faradaic deionization
Faradaic deionization (FDI) is an emerging electrochemical separations technique which has received tremendous research interest in the past decade. Established electrochemical separations techniques such as capacitive deionization are limited by low charge storage capacity due to electric double layer charge storage, high energy consumption, and inability to selectively remove specific ions. FDI employs redox active materials with higher charge storage capacity, high selectivity, and offers low energy consumption for the separation process by operating within a moderate potential window. This work is focused on fundamental characterization of materials for FDI to enable a broad range of applications ranging from saltwater desalination, selective ion removal from water streams in agriculture and industry, and carbon dioxide capture from the atmosphere. Nickel hexacyanoferrate (NiHCFe) is chosen as an intercalation host compound for removal of sodium ions from saltwater. To enable high-rate capability, microscopic mechanisms that limit cation intercalation within porous electrodes comprised of NiHCFe nanoparticles, conductive carbon additives and polymer binder are investigated. A combination of experimental characterization, particle-scale transport modeling, and porous electrode theory show that slow electron conduction through nanoparticle agglomerates is linked to rate limitations during galvanostatic cycling, while naïve predictions based on crystal-scale diffusion alone suggest rate 4-5 orders of magnitude higher than in experiment. These insights are used to target improvement in electronic conductivity of porous electrodes of NiHCFe, resulting in significantly improved desalination performance. Investigation of cation intercalation is carried further from Na+ to insertion of Mg2+ and Ca2+ into NiHCFe in aqueous electrolytes. Capacity fades of up to 75% are observed within 200 cycles during intercalation of Mg2+(aq) and Ca2+(aq). A combination of experimental characterization, first-principles electronic structure calculations, statistical mechanics, and lattice-percolation modeling of electron transfer are used to elucidate the mechanisms responsible for the degradation of NiHCFe and its partial retention of capacity. It is quantitatively demonstrated for the first time that Mg2+ can displace Ni2+ sites within the NiHCFe crystal lattice, leading to structural degradation and removal of ferricyanide and Ni2+ ions during electrochemical cycling. Since NiHCFe is shown to be limited in its application to electrolytes with alkali ions, and has a modest volumetric charge storage capacity, another class of materials – NASICONs (sodium superionic conductors) are investigated. Among NASICONs, sodium vanadium phosphate (NVP) offers three times higher volumetric charge storage density than NiHCFe and potentially minimal degradation in divalent electrolytes. However, during galvanostatic cycling in aqueous electrolyte, complete capacity loss is observed in the first charging cycle due to vanadium dissolution. Titanium alloying is shown to substantially improve stability of NVP, and hence sodium titanium vanadium phosphate (NTVP) is synthesized and characterized. Using electrochemical characterization and first principles calculations, insertion mechanism of Na+ into NTVP is investigated. Given the high affinity of Na+ towards NASICON compounds, intercalation of various other alkali, alkaline earth, and ammonium ions is characterized to understand selectivity. NTVP is found to exhibit ion recognition towards Na+ with a selectivity sequence as Na+ >> Li+ > NH4+ >> K+, Mg2+, Ca2+. This selectivity makes NTVP attractive for excess Na+ removal for agricultural feed and waste waters in saline regions to prevent micronutrient poisoning of plants. Operation of NTVP is demonstrated in a custom faradaic deionization cell with mixed K+, Na+ electrolytes. Extending the application of FDI from cation capture, CO2 capture from the atmosphere and industrial emissions is explored. Most industrial techniques rely on thermally driven CO2 separations, which can require up to 10 times the thermodynamic minimum energy requirement. Recent efforts with electrochemical pH swing CO2 capture aim to lower this energy consumption. By utilizing the pH-dependent solubility of CO2 in aqueous media, capacitive and pseudocapacitive electrodes have been used to modulate the pH of the feed stream consisting of CO2 dissolved as bicarbonates in water and desorb pure CO2 from a low pH stream obtained by reversibly capturing protons. Building upon these concepts, new materials for FDI based CO2 capture are explored in this work, aimed at significantly improving the rate of CO2 capture and lowering the overall energy consumption.
Faradaic deionization, electrochemical separations, aqueous batteries, Prussian blue analogues, Nickel hexacyanoferrate, porous electrodes, diffusion, NASICON, Sodium titanium vanadium phosphate, selective capture, CO2 capture
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign