Based on the fast consumption and depletion of OH^- that generate considerable local pH variations from reference bulk values during anodic processes on platinum, rotating ring-disk electrode (RRDE) studies are expected to contribute in elucidating mechanisms and pH implications on kinetics. As more recently demonstrated by Feliu and co-workers[1] , significant pH variations of up to 1.5 pH units during the voltammetric electro-oxidation of ethanol in alkaline media over platinum, even under hydrodynamic conditions, were detected. Using Pt_disk-Pt_ring electrodes we investigated glycerol electro-oxidation in alkaline media under both conventional (Figure 1) and oscillatory conditions. All ring currents were corrected by the collection efficiency of the electrode, N, of 25.6%. Importantly, the intrinsic ohmic drop of the system might bring about distortions on the ring currents that must be taken into account in order to rule out the cross-talk of the disk and ring electrodes. Parameters such as ionic strength, distance between working and reference electrodes, and, current magnitudes in the disk require special attention in order to avoid misleading conclusions. The detection of pH variation under voltammetric conditions was done by polarizing the ring electrode at 100mV vs. RHE. The local generation of H⁺ is seen by the cathodic currents registered at the ring (Fig.1,bottom) whose potential is shifted to lower values due to the well-known pH dependence on the equilibrium potential (Nernstian potential). In oscillatory conditions[2] the situation is rather complicated as many soluble intermediate species are produced at the disk, introducing relevant mass-transport limitations to the oscillatory dynamics being recorded. For example, galvanostatic oxidation of glycerol is only observable in stationary conditions: rotation of the electrode modifies importantly the population of intermediates necessary to sustain oscillations. A systematic study of the effects of rotation speed and of different ring polarizations was conducted. Additionally, the interaction of platinum oxides and glycerol in alkaline media, at open circuit potential, was also followed by collection experiments with the ring polarized at different potentials. The  results so far demonstrate particular features for the glycerol system unseen for other model systems, yet to be understood.

Acknowledgments: A.A.Z., P.B.P. E.G.M. and H.V. acknowledge São Paulo Research Foundation (FAPESP) and Sao Paulo University Research Office for the scholarships (grants #2013-06205-3 and #2012/07313-1) and financial support (grants #2009/07629-6, #2012/21204-0, and #2013/16930-7).

 

References:

[1] M.C. Figueiredo, R.M. Arán-Ais, V. Climent, T. Kallio, J.M. Feliu, ChemElectroChem 2 (2015) 1254.

[2] C.P. Oliveira, N. V. Lussari, E. Sitta, H. Varela, Electrochim. Acta 85 (2012) 674.

Figure 1 In black: Voltammetric profile at 25mV/s of the system 0.3 M Gly + 0.1 M KOH. Ring currents upon 100mV polarization, in red.