A systematic investigation of BH 4 electrooxidation in 2 M NaOH on Pt was carried out using the electrochemical quartz crystal microbalance technique (EQCM). Four sets of experiments were conducted: (i) Pt in NaOH, (ii) BH 4-(between 10 and 60 mM) on Pt, (iii) thiourea (TU) on Pt, and (iv) BH 4in the presence of TU (between 0.01 and 2.1 mM) on Pt. The BH 4 electro-oxidation mechanism was developed by correlating the EQCM results with density functional theory (DFT) calculations from the literature revealing the energetically most favorable adsorbates. The surface coverage by key intermediates was estimated using the van der Waals molecular areas. On the anodic scan, the BH 4 electrosorption was followed by dissociation of BH 4,ad generating BH y,ad and (4y)H ad , with y between 1 and 3, depending on the available surface sites. BH y,ad is further oxidized as a function of electrode potential in Eley-Rideal and Langmuir-Hinshelwood type mechanisms with the participation of OHand OH ad , respectively. The oxidative desorption on the cathodic scan at potentials between 0.1 and 0.5 V of strongly adsorbed intermediates such as BOH ad and BH 2 OH ad is essential for recovering the Pt electrocatalytic activity. TU adsorption on Pt produces a characteristic potential dependent adsorption-desorption hysteresis. Furthermore, at TU concentrations above 0.045 mM for 30 mM BH 4 a bilayer is formed on the surface, which is stabilized by Lewis acid-base interactions between TU and BH 4 -. As a result, the BH 4 oxidation overpotential is increased leading to incomplete oxidation, whereas the BH 4 thermocatalytic hydrolysis is inhibited.

The early stages of the process of anodic zinc oxidation in CO32-, SO42-, and C1-solutions around pH 11.5, were studied by cyclic voltammetry, as in part I. Unusual peak current dependence on sweep rate is observed at the first anodic peak (As). This behavior is explained in terms of a mixed process involving coupled diffusion and film formation. By correlating the shapes of peaks and peak potentials for different Zn surface preparations, and the relation of the peak current to sweep rate and to electrode rotation rate, r, the roles of film formation and diffusion of solution species are distinguished. A new method of plotting out cyclic voltammetry peak currents, i,, as a function of sweep rate, s, is proposed that treats the anodic currents in terms of parallel components for dissolution into solution, with diffusion (proportional to sl/2), and for film formation or reduction (proportional to s). This allows a separation of diffusion-controlled and film formation currents to be made. The RDE experiments yield i, = A + Br '/2 and sweep rate experiments showed iD = k,s + k2s '/2 relations apply to the i, for the A, peak. Two processes therefore take place: diffusion-controlled dissolution and direct film formation. The importance of studies at single-crystal surfaces is stressed where resolution of the various processes involved is much clearer. Thus, it is shown that only the direct oxide film formation process is predominant at the single-crystal Zn (0001) face. This compact film is responsible for passivation. The results enable a model for dissolution and coupled-film formation, associated with passivation, at the Zn electrode, to be suggested. The participation 6f these two types of process depends on pH and the types of ions present. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.69.4.4 Downloaded on 2015-06-21 to IP Vol. 134, No. 4 ZINC OXIDATION AND REDEPOSITION PROCESSES II ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.69.4.4 Downloaded on 2015-06-21 to IP

The effects of carbonate ions on the electrolytic oxidation and reduction processes that take place at zinc electrodes in aqueous alkaline solutions have been studied by cyclic voltammetry; the behavior is compared with that in sulfate and chloride solutions at the same pH. A reaction which corresponds in the first anodic peak false(A1false) of the cyclic voltammograms for zinc in carbonate solutions is found to be first order with respect to hydroxide ions in the pH range 12.0–13.5. It has a diffusion‐controlled current component. A second anodic peak false(A2false) is only observed in carbonate solutions, but the diffusing species associated with the processes at peaks A1 and A2 is found to be the same, as indicated by the identity of slopes of inormalp vs. r1/2 plots in rotating disk electrode experiments. It is suggested that this behavior can be explained in terms of a residual current passing through a porous oxide/hydroxide film that is already formed in the A1 process. The breakdown of the passive films in Cl− or SO42− solutions, and the resistivity of the passive film formed in CO32− solutions, is examined and discussed.

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The quartz crystal microbalance technique coupled with cyclic voltammetry (referred to as electrochemical quartz crystal microbalance (EQCM)) was employed to study the oxidation of BH 4 − on Au in alkaline media (0.1 M and 2 M NaOH, respectively) using a newly designed vertical cell to minimize the interference by evolved H 2 . The electrode potential was scanned between −0.7 and 0.7 V vs. SHE with scan rates in the range of 0.01 V s −1 and 1 V s −1 . The relative mass changes of the Au resonator-electrode were interpreted in combination with density functional theory (DFT) modeling and pertinent experimental results from the literature. The potential scan rate dependent mass change profiles revealed: i) weak adsorption of BH 4 − below −0.2 V leading to high anodic overpotential, ii) accumulation of reaction intermediates on the surface between approximately −0.2 V to 0.3 V (scan rate dependent), and iii) Au surface hydroxide and oxide assisted oxidation of BH 4 − and possibly of other active species present such as BH 3 OH − formed by hydrolysis.Direct borohydride fuel cells have received increased attention over the past decade as potential power source candidates for electronic devices and transportation. 1-4 It is convenient and common to conceptualize the BH 4 − electro-oxidation using Eq. 1. However, the complexity of the reaction mechanism and the number of possible faradaic (i.e., involving electron-transfer with the electrode) and nonfaradaic (thermocatalytic) pathways have a significant influence on the anode performance at both open-circuit and under polarization conditions.Most electrode materials that are electrocatalytic toward the borohydride oxidation reaction (such as Pt 5,6 or Ni 7,8 ) possess also thermocatalytic activity for hydrolysis of BH 4 − , generating H 2 and BH 3 OH − (in the first stages) according to: 9The generation of BH 3 OH − could lower the borohydride utilization efficiency with respect to Eq. 1, by up to about 60%. 10 It was widely accepted in the literature in the past, that Au does not promote the BH 4 − hydrolysis, hence, it is a faradaically efficient eight-electron borohydride oxidation reaction (BOR) catalyst. 5,11 Cheng and Scott have studied the BH 4 − electrooxidation kinetics on Au rotating disk electrode (RDE) and showed that the number of electrons exchanged per BH 4 − anion increases from four at 0.09 V vs. SHE to eight at 0.54 V vs. SHE. 12 Chatenet et al. estimated 7.5 electrons at BH 4 − concentrations lower than 50 mM and proved also the formation of BH 3 OH − at higher concentrations (≥ 50 mM), as the hydrolysis is first order with respect to BH 4 − concentration. 13 Krishnan et al. carried out a detailed analysis using Au rotating ring-disk electrode (RRDE) voltammetry and detected BH 3 OH − on the ring and its further oxidation to BH 2 (OH) 2 − between −0.8 and −0.6 V. They concluded that both the formation and stability of BH 3 OH − is pH dependent and were the first to discuss in depth the BH 4 − hydrolysis on Au. 10 Chatenet et al. in their more recent w...