3 years ago

On the Feasibility of Sodium Metal as Pseudo-Reference Electrode in Solid State Electrochemical Cells

On the Feasibility of Sodium Metal as Pseudo-Reference Electrode in Solid State Electrochemical Cells
Muhammad E. Abdelhamid, Andrea Boschin, Patrik Johansson
A set-up of a sodium metal anode vs. a solid polymer electrolyte (SPE) comprising poly(ethylene oxide) (PEO) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) has been evaluated in detail for the feasibility to use sodium metal as a pseudo-reference electrode (pseudo-RE). To evaluate the stability and reproducibility, we monitored the half-wave potential (E1/2) of added decamethylferrocene (Me10Fc) and the stability of the interface by electrochemical impedance spectroscopy (EIS). The sodium/SPE interface resistance (RNa/SPE) increases with time, up to 2.8 kΩ cm−2, and causes the E1/2 of the Me10Fc+/0 reference redox couple to drift up to 15 mV during 88 hours. Moreover, the sodium potential is very irreproducible, even initially after cell assembling the values can differ by 60 mV, likely due to extreme sensitivity of the metal surface even to an “inert and dry” glove box environment. Indeed, freshly cut sodium readily reacts with water, forming NaOH, and adsorbs impurities that can be present even in a glove box atmosphere. The oxidation layer and the amount of adsorbed impurities increase with the exposure to the glove box atmosphere, as revealed by ATR-FTIR spectroscopy. Altogether, this calls for attention when evaluating any battery materials in half-cell configurations using sodium metal as the pseudo-RE. A reference electrode assessment. Sodium metal in contact with NaTFSI (PEO)9 solid polymer electrolyte (SPE) is characterized electrochemically. The potential of the redox couple Me10Fc+/0 vs. sodium metal drifts with time and it is not reproducible, and hence sodium is not suitable as a pseudo-RE – even when an SPE is used.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/celc.201700273

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