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Dovepress Salt-free electrolysis of water v .30 mV s−1 (Figure 1), the hysteresis was not caused by the capacitive current, but may have been caused by the time required for reaching the steady-state for the redox cycling. The electrolysis time for reaching the limiting current was approximately 3 seconds for the 0.1 V potential domain at v = 30 mV s−1 in Figure 1. The thickness of the diffusion layer was 50 μm, which was comparable to the interdistances of the electrodes. Therefore, the hysteresis should be contained in the voltammograms. Conventional voltammograms in 1 M KCl + H2 aque- ous solution were made to understand the reactions in the thin layer cell. Figure 2 shows the voltammogram in the potential domain covering water decomposition. Peak (a) is the oxidation of H2, of which current was proportional to v1/2. Waves (b) and (c) are ascribed to the oxidation and the reduction of water, respectively. The potential difference of the two waves is 2.0 V. In contrast, the potential differ- ence of the redox cycling in Figure 1 (a) was approximately 0.4 V. Consequently, 1.6 V was gained by use of the thin layer cell. Figure 3 shows variation of the limiting current, Ilim, of the voltammograms in Figure 1 with the concentration of HCl. The current is approximately proportional to the H+ con- centration [H+] when [H+] ,0.2 mM. A possible reason for the deviation from the proportionality for [H+] .0.3 mM is blocking of the product (H2) at the cathode by adsorption.36–39 The other possibility is ambiguity of the determination of the limiting currents, as will be discussed later. It is predicted that the current is inversely proportional to w owing to the control by diffusion in the finite domain w.29 Figure 4 shows dependence of the limiting current on 1/w, as shown in circles. The plot at narrow distances between the electrodes deviated from the proportional line. Since the electrode surface was polished before every voltammetric run using a polishing buff, it was deformed to a rounded surface,40 as shown in the inset of Figure 4. Values of w were read from the moving distance from the closest contact point of the two electrodes by a micrometer gauge. Therefore the average distance should be larger than w. We measured w2 with the microscope as shown in the inset, and obtained the curvature, from which we evaluated the effective interelectrode distance, w1 = w + 10 μm. The plot of the limiting current with 1/w1 (triangles) fell on the proportional line. We derive here expressions for current-voltage curves of the redox cycling reaction, 2H+ + 2e− ↔ H2, when mass transport of both species is controlled only by diffusion in the x-direction of the cell under the steady state. It is Reports in Electrochemistry 2013:3 100 0 −100 −1 0 1 E versus Ag|AgCl/V Figure 2 Voltammogram of 1 M KCl + saturated h2 solution at the platinum disk electrode 1.6 mm in diameter for v = 30 mV s–1. Notes: (a) oxidation of h2, (b) oxidation of water, and (c) reduction of water. (b) (a) (c) Powered by TCPDF (www.tcpdf.org) 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 [H+]/mM Figure 3 Dependence of the limiting current of the voltammograms in Figure 1 on concentrations of hCl. Abbreviations: h+, concentration of hydrogen ion; Ilim, the limiting current. w2 w1 w 50 0 Figure 4 Variation of the limiting currents with 1/w (circles) and 1/w1 (triangles) under the conditions of Figure 1, where w1 = w + 10 μm. Note: The inset is the illustration of the rounded electrode surfaces. Abbreviations: Ilim, the limiting current; w, distance between two facing electrodes. 0 10 20 30 w−1/mm−1 submit your manuscript | www.dovepress.com Dovepress 9 Ilim/μA Ilim/μA I/μAPDF Image | Salt-free electrolysis of water facilitated by hydrogen gas
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