Exploring the Oxygen Reduction Reaction in Ionic Liquids to Synthesize Silver Oxides with Unusual Oxidation State Peter Schmitz, Daniel Schröder, Marcus Rohnke, Jürgen Janek Institute of Physical Chemistry, Justus-Liebig-University Giessen Heinrich-Buff-Ring 17, 35392 Giessen, Germany
[email protected] Oxidizing metal species electrochemically is an alternative and efficient way to tailor-make metal oxides, being possibly even preferred over chemical oxidation for certain practical applications. The main goal of our work is to achieve higher/unusual oxidation states of silver with oxygen (Ag xOy) by electrochemical synthesis. The use of ionic liquids (ILs) as electrolyte offers thereby a great opportunity, since their unique electrochemical properties (wide stability window, intrinsic ionic conductivity) stand out compared to conventional organic and aqueous electrolytes. Our approach is to use an electrochemical cell, similar as reported for Na/O 2 cells [1,2], with a silver metal anode, oxygen supplied at the porous carbon cathode and either [emim][TFSI] or [Pyr13][TFSI] as electrolyte with silver salt (either 0.5 M Ag[TFSI] or Ag[OTf]) for sufficient ion transport. The intended reaction mechanism is as follows: At the anode, silver ions are stripped from the metal into the IL (Ag → Ag+ + e) and migrate to the cathode. Here, two reactions can take place, namely the backwards reaction of the anode reaction to plate silver and the reduction of the dissolved oxygen molecule to the superoxide ion, i.e. the oxygen reduction reaction (ORR: O 2 + e → O2• ). The latter cathode reaction is crucial for the formation of a silver oxide product. In literature, this reaction is expected to take place at approximately 0.98 V vs. Ag/Ag+ in [emim][TFSI] [3]. For this work, cyclic voltammograms are used to characterize the occurring electrochemical reactions within the aforementioned set-up. SEM images were taken to investigate whether the intended product was deposited in the cathode (see Fig 1, left). Besides, the existence of the superoxide anion will be investigated by ESR measurements. Exemplary results are shown in Figure 1: In a neat IL, the ORR is evident at around 0.9 V vs. Ag-wire. In presence of silver ions, the reduction of silver ions seems to be favored and thus no silver oxide is formed (Fig. 1). Based on these results, we will give an outlook on how to enforce the ORR by changing the influence of O2 pressure/concentration and temperature of the electrochemical synthesis. All in all, we want to show the advantages using electrochemistry in ILs for the synthesis of compounds not accessible through aqueous solutions.
Figure 1: left) SEM image of deposited silver on the porous carbon cathode; right) CV of neat IL ([Pyr13][TFSI]; dotted line), neat IL saturated with O 2 (solid blue line) and IL with O 2 and silver salt (0.5 M Ag[OTf]; dashed black line) at the reduction potential; silver wire as reference, silver sheet anode, glassy carbon cathode, scan rate: 20 mV/s. [1] Hartmann et al., Nat. Mater. 2012, 12, 228–232; [2] Bender et al., Angew. Chem. 2016, 128, 4716– 4726; [3] Buzzeo et al., J. Phys. Chem. A 2003, 107, 8872–8878
