Enhancing proton conduction in metal organic frameworks by post synthetic modification

Abstract

The human need for energy is increasing in response to the increasing population, rapid urbanization and industrialization, especially in the developing countries. U.S. Energy Information Administration’s (EIA) recently released International Energy Outlook 2013 (IEO2013) projects claim that the global energy consumption will increase by 56% between 2010 and 2040. In response to this demand, various energy alternatives to fossil fuels are being looked at, out of which, fuel cells have proven to be a promising candidate due to their high conversion efficiency, low pollution, and high fuel flexibility. They are electrochemical conversion devices that produce electrical energy from the chemical energy of a fuel and oxygen at high thermodynamic efficiencies. They mainly have three components; the cathode, the anode, and the electrolyte. At one of the electrodes hydrogen is oxidized to protons. The electrolyte is necessary to transfer these protons to the other electrode, where they combine with the oxygen to form water. At present, Nafion is the commercially used electrolyte membrane which has a conductivity of 10-2 to 10-1 S/cm in the presence of humidity, but it fails to conduct ions at a higher temperature under anhydrous conditions. Regardless, the Nafion is still the commercially used proton conducting electrolyte because of its robust structure, excellent thermal and mechanical stability, high proton conductivity and reusability without any significant loss of activity. Various materials are being studied to be used as electrolytes in the fuel cells. Developing Metal Organic Framework (MOF) - based proton-conducting electrolytes for fuel-cell applications is an important target that has drawn a lot of attention. They are coordination framework compounds formed by linking metal ions/clusters via organic linkers and have emerged as a new class of functional materials. The modular and crystalline nature of the MOF permits controlled introduction of functional species and characterization of their composition with high precision, a task which is very difficult to achieve in amorphous polymers. This makes MOFs excellent for hosting proton bearing guests and the chemistry around their framework can be tuned to function cooperatively with the guests to obtain desirable proton conductivity values. In this context, we have developed a few dense MOFs which display very interesting proton conducting properties. Via an experimental-modeling approach, we show how the extra-framework species can be site-specifically modulated to enhance the proton conduction by constructing highly ordered hydrogen bond pathways within the MOF. From a comparison of three structurally related metal terephthalate frameworks, we realize that we need an amphoteric guest in the MOF which can form a hydrogen-bonded network and thus help in the transfer of protons. Post-synthetic loading of ethylene glycol, a strong hydrogen bonding and coordinating guest, in the MOF help us achieve a 1000-fold enhancement in the humidity-dependent proton conductivity. The post-synthetic approach we adopt makes it very generic. Similarly, we demonstrate how highly hydrating sites can be embedded into MOF to achieve up to 10000-fold enhancement in hydrous conductivity. Loading Cs+ ions to bring in more coordinated and acidic water in the same MOF helps attain conductivity values comparable to that of Nafion. Unfortunately, none of these show any high-temperature proton conductivity. In another system, wide-temperature range proton conductivity, i.e., both humidity dependent as well as anhydrous conductivity (30° to 150˚C). A 3-dimensional, dense MOF having azoles in the pores as proton-conducting guests conducts at 10-3 S/cm at 90˚C and 90% RH but fails to show any conductivity at higher temperatures under anhydrous conditions. Using a post-synthetic strategy, the conductivity value has been pushed to 10-2 S/cm at 90˚C, and 90% RH and conductivity of the order of 10-4 S/cm at 150˚C has been attained. Considering, relatively very few MOFs showing anhydrous conductivity are reported, this, we believe, is a worthy achievement. The investigations in this thesis include (i) synthesis of dense MOFs as proton conducting electrolytes; (ii) understanding the proton-transfer pathways and proton conducting characteristics of the MOFs through a combined experimental-modeling approach; (iii) post-synthetic modifications of the MOFs to enhance the proton conductivities and to achieve anhydrous proton conductivity.