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Previous work by the some of us has demonstrated enhanced electrochemical performance of polybenzimidazole (PBI) membranes with high phosphoric acid content prepared by the so-called PPA process.1 Methods of PBI synthesis that involve casting from an organic solvent cannot incorporate the more rigid PBIs characteristic of the PPA process due to their limited solubility. Recently, a carefully controlled drying method has been discovered in which a gel PBI membrane, made in the PPA process, is transformed into a dense PBI film. This method provides a synthetic route to PBI films without the use of any organic solvents. The dense PBI films with repeat unit para-PBI (Fig.1), were originally developed for flow battery applications. However they also can be re-doped in phosphoric acid to be used as a membrane in high-temperature fuel cells. The re-doped PBI films have displayed unprecedented ionic conductivity at elevated temperatures, similar to the starting PBI gel membranes prepared by the PPA process1, while also exhibiting enhanced mechanical properties. Temperature-dependent conductivity data for two of these films are displayed below. Sample A is the dried, densified, and re-acidified version of sample B, with a much lower acid content, ~18 mol PA/PBI repeat unit compared to 30 mol PA/PBI RU, respectively, but exhibits comparable conductivity (Fig. 2). Thus, in high-temperature fuel cell operation the electrochemical performance is similar, but performance degradation due to mechanical creep is reduced, leading to greater durability of the Version A PBI. In attempting to understand the enhanced proton conductivity of the modified PBI membrane despite its lower acid content, we performed both 1H and 31P nuclear magnetic resonance (NMR) measurements. Proton self-diffusion coefficients for four membranes two (A,C) prepared by the modified process and two (B,D) by the original PPA process, were measured by pulsed gradient NMR and plotted in Fig. 3. The results show significantly enhanced proton diffusivity, both in magnitude and in lower activation energy for sample A, which is consistent with its high conductivity despite lower acid content compared to the original PPA-processed membranes (B, D). We have also performed solid state 1D 31P and 2D 31P{1H} HETCOR NMR measurements, which yield additional information on structural differences between the membranes and how these differences may affect transport properties. Multinuclear NMR Study of the Effect of Acid Concentration on Ion Transport in Phosphoric Acid Doped Polybenzimidazole Membranes, N. Suarez, N.K.A.C. Kodiweera, P. Stallworth, S.G. Greenbaun, S. Yu, and B. Benicewicz, Journal of Physical Chemistry B, 116, 12545-51(2012). Figure 1

New and novel systems for the reversible electrochemical storage of energy are required to sustain the increasing global energy demands. Indeed the high cost, the intrinsic energy limits and the strategic aspects of lithium and lithium technology-related metals (e.g., Co) are prompting the research towards the development of battery systems based on novel chemistries. In this regard, light multivalent metals are considered the holy grail. Magnesium and aluminum, for example, can be considered one of the best choices due to their low reduction potential (-1.66 and -2.37 V vs. SHE for Al3+/Al and Mg2+/Mg, respectively), high volumetric and gravimetric theoretical capacities (2980 Ah∙kg-1 and 8046 Ah∙dm-3 for Al, and 2205 Ah∙kg-1 and 3832 Ah∙dm-3 for Mg), high abundance in the Earth’s crust and low cost (1’925 and 2’700 $∙ton-1 for Al and Mg, respectively) [1, 2]. The main roadblock to the development of these technologies is the discovery of efficient, stable and high-performing electrolytes. Ionic liquids have been demonstrated to be a good choice to be used as solvents in multivalent metal electrolytes, thanks to the low volatility and high solvating powers [3, 4]. Despite large efforts, there are no examples in the literature that demonstrate the advantage of the synergic effect of the coupling of Mg2+/Al3+ in the deposition and stripping processes. Moreover, the fundamental understanding of ion coordination in this novel class of electrolytes would be a cornerstone information necessary to reveal the conduction mechanism. In this work we study the properties of a family of hybrid Al/Mg electrolytes with formula [Pyr14Cl/(AlCl3)1.5]/(δ-MgCl2)x (x = 0, 0.056, 0.091 and 0.146), to foster the synergic effect of these two metals for application in multivalent metal secondary batteries. In this study we combine thermal analyses, vibrational, electric and nuclear magnetic spectroscopies to achieve a clear picture of the ionic coordination occurring in the proposed electrolytes and how ions are able to rearrange and form ionic coordination nanodomains. It is demonstrated that AlCl4 - and Al2Cl7 - species are formed, which act as ligands for δ-MgCl2 units. Thus, the Mg2+/Al3+ conduction occurs through the exchange of anionic species between the different ionic clusters. These phenomena are significantly correlated to the host medium dynamics of the IL matrix. The rise of the temperature results in an increased degree of disorder in the nanodomains, which modifies the long-range charge migration process. Finally, the electrochemical activity in the synergic Al/Mg alloy deposition and stripping is demonstrated. Acknowledgements This project has received funding from the program “Budget Integrato per la Ricerca Interdipartimentale - BIRD 2018” of the University of Padova (protocol BIRD187913). References [1] R. Dominko, J. Bitenc, R. Berthelot, M. Gauthier, G. Pagot, V. Di Noto, Magnesium batteries: Current picture and missing pieces of the puzzle, Journal of Power Sources, 478 (2020) 229027. [2] D.J. Kim, D.-J. Yoo, M.T. Otley, A. Prokofjevs, C. Pezzato, M. Owczarek, S.J. Lee, J.W. Choi, J.F. Stoddart, Rechargeable aluminium organic batteries, Nature Energy, 4 (2019) 51-59. [3] F. Bertasi, C. Hettige, F. Sepehr, X. Bogle, G. Pagot, K. Vezzù, E. Negro, S.J. Paddison, S.G. Greenbaum, M. Vittadello, V. Di Noto, A Key concept in Magnesium Secondary Battery Electrolytes, ChemSusChem, 8 (2015) 3069-3076. [4] F. Bertasi, F. Sepehr, G. Pagot, S.J. Paddison, V. Di Noto, Toward a Magnesium-Iodine Battery, Adv. Funct. Mater., 26 (2016) 4860-4865.