ELSEVIER Journal of Electroanalytical Chemistry 433 (It.~7) 187-193 f-d (Photo! anodic decomposition of 3-methyl oxazolidin-2-one An in-situ FTIR study Petr Krtil, Ladislav Kavan *, Kate i ina Macounovfi ,I, H~,yrorxk~ Inxtitlae ¢~f Phyxiod Chemist~3', Aoulemy ¢~[ S¢ietwes ~l"the C=erh Rel~uhlic. l)oh~j,~kora 3, Prague CZo 18223, Czech Republic' Received 18 March I~J97;revised 21 May 11J97 Al~tract Oxidative bl*eakdown of electrolyte solutions in 3omethyloxatolidino2o~me (NMO) on phot(~xcited 'l'iO~ and platinum ele¢lrt~les was studied by (photo~lectnvdlemistry, in-situ H'IR and GC-MS. The solvent oxidation at the platinum electr{~le starts at about 4.5 V w. LilLi ". It pn~ceeds by a stepwise mechanism, while an intermediate product is detectable within a time scale of It) l to i(): s. Carbon dioxide is one of the fired products. Another product is an electrochemicallyactive species, showing irreversible reduction at 3.7 to 3.9 V, and pseudo-reversible reduction/oxidation at 2.9 to 3. I V. This product was tentatively identified as a nitrogenocontaining heter~ycle. © 1997 Elsevier Science S.A. I. Introduction The optimisation of photoelectrochemical solar cells and 4 V-lithium batteries requires stable solvent/electro- lyte systems to be lbund, while the stability at extreme positive potentials is a crucial parameter. Cyclic carbon- ates, such as ethylene carbonate and propylene carbonate, are popular solvents, both for lithium batteries and solar cells. The (photo)anodic and cathodic stability of propyo lene carbonate was studied in detail [I-10], but relatively little is known about ethylene carbonate [8]. The electrolyte solutions in ethylene carbonate + acetoo nitrile [I I] and ethylene carbonate + propylene carbonate [12] were used for highly efficient regenerative !;11- cells with a sensitised TiO~ photoanode (the so-called GPdtzel cell [13,14]). Further optimisation of the Gfiitzel cell pointed at 3-methyl oxazolidin-2-one (NMO) as a promis- ing solvent: a cell with NMO + acetonitrile (90:!0, v:v) displayed a conversion efficiency of 10% upon AM 1.5 solar irradiation [15]. NMO was also tested in a mixture with methyl-hexyl-imidazolium iodide for the same ap- plications [ 16]. NMO is a suitable electrochemical solvent, based on its high dielectric constant (77.5), high boiling point (87 to 90°C at I Tort), low density (!. 1702 g/cm 3) and viscosity (2.45 g m-I s-l) [17]. The accessible potential ranges of * Corresponding author. E-math kavan@jh-inst.cas.cz. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All right~ reserved. Pll S0022-07 28(97)00296-9 various electrolyte + NMO solutions on Pt, Au, Hg and glass-like carbon were summarised by Kelly and Heine° man [18], but no details about the breakdown product~ were given. In the presence of tetraalkylammonium salts, the positive limit appeared at 1.3 V vs. an aqueous AglAgCIIsat. KCI reference electrode, regardless of the electrode material (Pt, Au. C). Dry NMO + NBuaBF4 (18 to 22 ppm H20) showed a positive limit of 1,6 V vs. aglagCI [16]. Our previous in-situ FTIR investigation of the photoexo cited Tie: electrode in contact with acetonitrileo propylene carbonate [7] and dimethyl suifoxide [19] solutions have demonstrated a possibility to monitor some products of the solvent photo-oxidation. These studies have complemented the investigation of dark anodic oxidation of acetonitrile [20]. propylene carbonate [5-8,21] or dimethyl sulfoxide [ 19] solutions on platinum or glass-like carbon electrodes, Usually, the IR-detectable products of photoanodic and dark-anodic oxidation were qualitatively identical [7019], Nevertheless, the mechanism of (photo)anodic breakdown of these solutions is quite complex, and far from being understood, For instance, acetonitrile solutions of perchlo~ rates or tetrafluomboratel~ decompose oxid~tive!y to ~,ive CO~, CH~OH. acetamJde, HCN, N~, NH:~, NO,, CH,~, O~,~H~Oz, succinonitrile, pyrrole, polyacetonitrile, as well as some solute-specific products: HCIO~, CIO~, CI ~, and CH 3CNBF3 [20], A trial to rationali~ some contradictions about the acetonitrile oxidation was given in Ref, [20],