(Invited) Hybrid Polyxometallate and Metal Oxide Based Materials of Defined Structure, Electrocatalytic Activity and Charge Storage Properties

Abstract

Our research interests aim at establishing structure/property relations leading to rational designs of functionalized materials for efficient electrocatalysis and electrochemical energy conversion and storage. Graphene is, in principle, a promising material for consideration as component (support, active site) of electrocatalytic materials, particularly with respect to reduction of oxygen, an electrode reaction of importance to low-temperature fuel cell technology. Different concepts of utilization, including nanostructuring, doping, admixing, preconditioning, modification or functionalization of various graphene-based systems for catalytic electroreduction of oxygen are elucidated, as well as important strategies to enhance the systems’ overall activity and stability are discussed. There has been growing interest in the electrochemical reduction of carbon dioxide, a potent greenhouse gas and a contributor to global climate change. Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large overpotentials. To optimize the hydrogenation-type electrocatalytic approach, we have proposed to utilize nanostructured metallic centers (e.g. Cu, Pd or Ru) in a form of highly dispersed and reactive nanoparticles generated within supramolecular network of various polytungstate systems. Among important issues are the mutual completion between hydrogen evolution and carbon dioxide reduction and specific interactions between coordinating centers and metallic sites. Upon incorporation or modification of various noble metal nanostructures with ultra-thin polyoxometallate films, highly reactive and selective systems not only toward reductions of carbon dioxide and oxygen but also oxidations of simple organic fuels (formic acid, methanol or ethanol) have been obtained. Another possibility to enhance electrocatlytic reactions is to explore direct transformation of solar energy to chemical energy using transition metal oxide semiconductor (WO3, Cu2O) materials (e.g. toward water splitting or photoelectrochemical reduction of carbon dioxide mostly to methanol). The potential materials for solid-state electrochemical applications including charge storage are expected to contain three-dimensionally distributed highly concentrated redox centers between which fast electron self-exchange (hopping) is feasible. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile. The applicable materials also must host mobile counter-ions that are capable of providing charge balance during electron transfers, thereby serving the same purpose as the supporting electrolytes in conventional electrochemistry. The population of these ions must be sufficient to support diffusive mass transport of electrons and to minimize ohmic effects. The emphasis is on the elements of dynamics for the efficient delivery of charge and on reactivity of the ‘redox conducting’ (certain polyoxometallates or metal oxide) materials. The effective (apparent) diffusional mechanism is critical to the success of effective charging-discharging in solid or semi-solid state.