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Li et al layer was much thinner than w. Therefore, the experimental results fell between w−1/3 and w−1 (Figure 9). The current value calculated from Equation 2 for D = 8.6 × 10−5 cm2 s−1, c*=10−7 M,k =1.4×1011 M−1 s−1 andw=0.05mmis r 0.08 μA. This is ten times smaller than the experimental value. The small value is ascribed to the assumption of the thin reaction layer. The value of (2λ)2/3 in Equation 2 is 14, which is a gain of H+ supplied from the dissociation. Dovepress current on w is closer to the dependence for the simple redox cycling than the theory. Acknowledgment This work was financially supported by Grants-in-Aid for Scientific Research (Grant 25420920) from the Ministry of Education in Japan. Disclosure The authors report no conflicts of interest in this work. References 1. Loewe H, Ehrfeld W. State-of-the-art in microreaction technology: concepts, manufacturing and applications. Electrochim Acta. 1999; 44(21–22):3679–3689. 2. WattsP,HaswellSJ,Pombo-VillarE.Electrochemicaleffectsrelatedto synthesis in micro reactors operating under electrokinetic flow. Chem Eng J. 2004; 101(1–3):237–240. 3. Bouzek K, Jiricny V, Kodym R, Kristal J, Bystron T. Microstruc- tured reactor for electroorganic synthesis. Electrochim Acta. 2010; 55(27):8172–8181. 4. Scialdone O, Galia A, Guarisco C, Mantia SL. Abatement of 1,1,2,2- tetrachloroethane in water by reduction at silver cathode and oxidation at boron doped diamond anode in micro reactors. Chem Eng J. 2012; 189–190:229–236. 5. Paddon CA, Pritchard GJ, Thiemann T, Marken F. Paired electrosyn- thesis: micro-flow cell processes with and without added electrolyte. Electrochem Commun. 2002;4(10):825–831. 6. Attour A, Rode S, Ziogas A, Matlosz M, Lapicque F. A thin-gap cell for selective oxidation of 4-methylanisole to 4-methoxy-benzaldehyde- dimethylacetal. J Appl Electrochem. 2008;38(3):339–347. 7. AttourA,RodeS,LapicqueF,ZiogasA,MatloszM.Thin-GapSingle- Pass High-Conversion Reactor for Organic Electrosynthesis II. Applica- tion to the Anodic Methoxylation of 4-Methoxytoluene. J Electrochem Soc. 2008;155(12):E201–E206. 8. Scialdone O, Guarisco C, Galia A, et al. Anodic abatement of organic pollutants in water in micro reactors. J Electroanal Chem. 2010; 638(2):293–296. 9. Belmont C, Girault HH. Coplanar interdigitated band electrodes for electrosynthesis, Part 3: Epoxidation of propylene. Electrochim Acta. 1995;40(15):2505–2510. 10. Rode S, Altmeyer S, Matlosz M. Segmented thin-gap flow cells for process intensification in electrosynthesis. J Appl Electrochem. 2004;34(7):671–680. 11. Sakairi M, Yamada M, Kikuchi T, Takahashi H. Development of three-electrode type micro-electrochemical reactor on anodized alu- minum with photon rupture and electrochemistry. Electrochim Acta. 2007;52(21):6268–6274. 12. Aoki KJ, Li C, Chen J, Nishiumi T. Electrolysis of pure water in a thin layer cell. J Electroanal Chem. 2013; 695:24–29. 13. Baker DR, Verbrugge MW, Newman J. A transformation for the treat- ment of diffusion and migration. Application to stationary disk and hemisphere electrodes. J Electroanal Chem. 1991;314(1–2):23–44. 14. Oldham KB. Theory of steady-state voltammetry without supporting electrolyte. J Electroanal Chem. 1992;337(1–2):91–126. 15. Amatore A, Fosset B, Bartelt J, Deakin MR, Wightman RM. Electro- chemical kinetics at microelectrodes: Part V. Migrational effects on steady or quasi-steady-state voltammograms. J Electroanal Chem. 1988;256(2):255–268. 16. Oldham KB, Cardwell TJ, Santos JH, Bond AM. Effect of ion pairing on steady-state voltammetric limiting currents at microelectrodes Part I. Theoretical principles. J Electroanal Chem. 1997;430(1–2):25–37. Reports in Electrochemistry 2013:3 We can understand the four features of the absence of hydrogen ion through the theoretical voltammograms. The steady-state (1) of the voltammograms is ascribed to the redox cycling of H2 and H+, where hydrogen ion is supplied from the dissociation of water. The sigmoidal form (2) is attributed to the potential shift by ln(λ) in Equation S24. Non-zero values of the slope of the current at ∆E = 0 in Figure 1 are caused by the reversible redox reaction when both H and H+ are present, whereas the zero values in 2 Figure 7 are obtained when either species is absent in solu- tion. The latter is in accord with the experimental results in the absence of hydrogen ion. One hundred times smaller values of the limiting current than the redox cycling currents (3) can be explained in terms of the dissociation kinetics. The inconsistency (Figure 9) between the theory and the experiment (4) regarding the dependence of the limiting cur- rents on w arises from the oversimplification of the theory by neglecting diffusion in the kinetic dissociation layer. Discussion The redox cycling of H2 ↔ 2H+ + 2e− in the thin layer cell including both H and H+ provides the steady state 2 voltammograms approximately according to the theoretical prediction. The steady state is established for v ,10 mV s−1 and w ,0.05 mm to yield the limiting currents. The limiting current is proportional to the concentration of hydrogen ion in the low concentration domain, partly because of adsorp- tion of produced hydrogen and partly because of the vague waveform. The limiting current is inversely proportional to w if the curvature effect (10 μm) of the electrode surface is taken into account. Attention must be paid to accuracy as the thickness of the thin layer cells may be less than 10 μm. Even when hydrogen ion is not added deliberately to the solution, the redox cycling occurs. The voltammogram has a sigmoidal shape with a positive potential shift. The limiting current is much smaller than that of the normal redox cycling. These variations can be explained in terms of the control by the dissociation kinetics of water. The approximation of the reaction layer includes oversimplification for the dissociation kinetics. As a result, observed dependence of the limiting Powered by TCPDF (www.tcpdf.org) 12 submit your manuscript | www.dovepress.com DovepressPDF Image | Salt-free electrolysis of water facilitated by hydrogen gas
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