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Dovepress Salt-free electrolysis of water (a) (b) (c) (d) (c) 1 0.5 0 20 0 (b) (a) 0 0.1 0.2 0.3 0 0.2 ∆E/V 0.4 ∆E/V Figure 7 Voltammograms of water including hydrogen gas without deliberately adding h+ for w = (a) 30, (b) 40, (c) 50 and (d) 70 μm at v = 30 mV s−1. Abbreviations: ∆E the applied voltage; I, the current. features of the voltammograms are: (1) Steady-state voltammograms; (2) A sigmoidal wave rising up at 0.2 V; (3) Values of the limiting current being 100 times larger than the theoretical value; (4) The limiting currents which increase with a decrease in w. Features 1 and 4 suggest the presence of some kind of redox cycling, whereas features 2 and 3 cannot be simply explained in terms of the redox cycling of H2 ↔ 2H+ + 2e−. The appearance of the inactive potential domain (from 0 V to 0.2 V) suggests participation in the kinetics other than diffusion. A possible kinetics is a supply of H+ from the dissociation of water rather than from the solution bulk, because the dissociation is the only source of H+ in the present experiment. We considered the problem of mass transport including the dissociation when the redox cycling by 2H+ + 2e− → H2 occurs in the thin layer cell; the redox cycling is composed of the dissociation through the CE reaction, H2O ↔ H+ + OH− and 2H+ + 2e− → H2. It is assumed that the diffusion coef- ficients of H+ and OH− have a common value, D. Feature 3 implies that the consumed amount of hydrogen gas is much smaller than the bulk concentration of hydrogen gas. Therefore cH2 can be regarded as the bulk value. It is [H+] and [OH−] that vary in the thin layer cell. Expressions for [H+] and [OH−] will be derived in Supplementary materials, and they will yield current-voltage curves. Dimensionless current (g) versus voltage curves can be cal- culated from Equation S24 for some values of λ, and are plotted in Figure 8. Sigmoidal curves were obtained and are similar to the experimental ones (Figure 7). However, the potential shift of the experimental curves is larger than the theoretical ones. A possible reason is ohmic drop of water. The resistivity of water 30 minutes after use was 1.7 MΩ ⋅ cm, which yields Reports in Electrochemistry 2013:3 Figure 8 Theoretical voltammograms calculated from Equation S24 for λ = (a) 10, (b) 45 and (c) 100. Notes: Conditions of (b) corresponds, for example, to w = 0.05 mm at kr = 1.4 × 1011 M−1 s−1 andD=5×10−5 cm2 s−1. Abbreviations: ∆E, the applied voltage; jw/Fc*D, the dimensionless current. 0.14 V potential shift at 0.5 μA. This value explains the poten- tial difference. We have assumed that values of the diffusion coefficients of H+ and OH− were common, although the ratio of the former to the latter is approximately 1.7. It provides 1.3 times difference in λ through Equation S20. This ratio is negligibly small in the variations in Figure 8. The condition of taking the limiting current is 2λ = g3/2 from Equation S24. Then the limiting current is expressed by 1/3 jlim = Fc*D(2λ)2/3/w = Fc*Dw−1/3 (2krc*/D ) (2) Equation 2 indicates that the limiting current has the −1/3 power of w. However, the experimental results show −0.8 power of w, as exhibited in Figure 9, probably because Equation 2 has been derived on the assumption that the reaction 0 −0.1 −0.2 Slope-1 Slope-1/3 1.8 1.9 2 −0.3 1.5 1.6 1.7 Log (w/mm) Figure 9 Plot of the limiting currents in Figure 6 against w in the logarithmic scale, where w was added as 10 μm to the geometrical interelectrode distance. Abbreviations: w, distance between two closest electrodes; Ilim, the limiting current. Powered by TCPDF (www.tcpdf.org) submit your manuscript | www.dovepress.com Dovepress 11 Log (Ilim/μA) jw/Fc*D I/μAPDF Image | Salt-free electrolysis of water facilitated by hydrogen gas
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