RESUMO
Carbon dioxide (CO2) chemical absorption and regeneration was investigated in two diamino carboxylate protic ionic liquids (PILs), dimethylethylenediamine formate (DMEDAH formate) and dimethylpropylenediamine acetate (DMPDAH acetate), using novel calorimetric techniques. The PILs under study have previously been shown to possess a CO2 absorption capacity similar to the industrial standard, 30% aqueous MEA, while requiring much lower temperatures to release the captured CO2. We show that this is in part due to the fact that the PILs exhibit enthalpies of CO2 desorption as low as 40 kJ mol(-1), significantly lower than the 85 kJ mol(-1) required for 30% aqueous MEA. Computational and spectroscopic analyses were used to probe the mechanism of CO2 capture, which was found to proceed via the formation of carbamate moieties on the primary amine of both DMEDAH and DMPDAH. Evidence was also found that weakly acidic counter-ions such as formate and acetate provide, unexpectedly, an additional proton acceptor site in the traditional carbamate mechanism, revealing opportunities to increase CO2 uptake capacity in the future through careful design of the anion and cation used in the PIL capture agent.
RESUMO
Metal-air batteries are a well-established technology that can offer high energy densities, low cost and environmental responsibility. Despite these favourable characteristics and utilisation of oxygen as the cathode reactant, these devices have been limited to primary applications, due to a number of problems that occur when the cell is recharged, including electrolyte loss and poor efficiency. Overcoming these obstacles is essential to creating a rechargeable metal-air battery that can be utilised for efficiently capturing renewable energy. Despite the first metal-air battery being created over 100 years ago, the emergence of reactive metals such as lithium has reinvigorated interest in this field. However the reactivity of some of these metals has generated a number of different philosophies regarding the electrolyte of the metal-air battery. Whilst much is already known about the anode and cathode processes in aqueous and organic electrolytes, the shortcomings of these electrolytes (i.e. volatility, instability, flammability etc.) have led some of the metal-air battery community to study room temperature ionic liquids (RTILs) as non-volatile, highly stable electrolytes that have the potential to support rechargeable metal-air battery processes. In this perspective, we discuss how some of these initial studies have demonstrated the capabilities of RTILs as metal-air battery electrolytes. We will also show that much of the long-held mechanistic knowledge of the oxygen electrode processes might not be applicable in RTIL based electrolytes, allowing for creative new solutions to the traditional irreversibility of the oxygen reduction reaction. Our understanding of key factors such as the effect of catalyst chemistry and surface structure, proton activity and interfacial reactions is still in its infancy in these novel electrolytes. In this perspective we highlight the key areas that need the attention of electrochemists and battery engineers, in order to progress the understanding of the physical and electrochemical processes in RTILs as electrolytes for the various forms of rechargeable metal-air batteries.