Capillary Effect-Enabled Water Electrolysis for Enhanced

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Capillary Effect-enabled Water Electrolysis for Enhanced Electrochemical Ozone Production by Using Bulk Porous Electrode Chen Zhang, Yingfeng Xu, Ping Lu, Xiaohua Zhang, FangFang Xu, and Jianlin Shi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Journal of the American Chemical Society

Capillary Effect-enabled Water Electrolysis for Enhanced Electrochemical Ozone Production by Using Bulk Porous Electrode Chen Zhang,†,‡ Yingfeng Xu,†,‡ Ping Lu,† Xiaohua Zhang,†,‡ Fangfang Xu†,‡ and Jianlin Shi*,†,‡ †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P.R. China. KEYWORDS. Bulk porous electrode; Electrochemical ozone production; Capillary effect; Oxygen evolution reaction ABSTRACT: A significant overpotential necessary for the electrochemical oxygen evolution reaction (OER) is one of the most serious disadvantages in water electrolysis, which, on the contrary, gives the probability to electrochemically produce ozone alternative to the common corona discharge. To effectively suppress the competitive OER and improve gaseous ozone escaping, here, we present a capillary effect-enabled electrolysis strategy by employing an unusual partial-submersed mode of anode composed of a β-PbO2 cuboids-loaded bulk porous Pb, and realize a much enhanced electrocatalytic gaseous ozone production in comparison to the cases of solid Pb counterpart and/or usual submersion operation. Detailed study reveals a capillary pressure-induced “molecular oxygen-locking effect” in the electrolyte fully filled in the porous structure of the electrode area above the electrolyte pool level, which unexpectedly leads to the production of unusual •O3- intermediate. Distinctive from the traditional electrochemical ozone production (EOP) mechanism dependent on the essential reaction between the atomic oxygen and molecular oxygen, the •O3- intermediate generation favors the EOP process in the special case where the capillary action is relevant for a porous bulk anode.

1. INTRODUCTION Ozone (O3) is one of the most environmental-friendly oxidants currently available with a high oxidation potential of 2.07 V,1 and has found wide consumer and industrial applications in disinfection,2 sterilization,1 organic synthesis,3-5 and so forth, leaving no potential secondary pollutions of harmful reaction residuals. However, limited to its extreme instability and poor storability, ozone’s practical utilization depends inevitably on the on-site producing techniques, traditionally employing the ultraviolet light,6 corona discharge7-9 and cold plasma10 to excite the gaseous dioxygen into trioxygen. During the last decades, the promising electrochemical ozone production (EOP) route, using a facile electrochemical strategy to split and oxidize the water molecule into O3 on the anode, has gained great research interests due to its remarkable advantages, such as simple equipments needed, independency of air quality and no harmful by-products, etc.11,12 Unfortunately, ozone could not be produced during the typical water electrolysis, because water is more preferential to be oxidized to O2 instead of O3, which is theoretically indicated by the much lower anodic potential of oxygen than that of ozone, as respectively expressed in the total anodic reactions in Eq. 1 and Eq. 2.12 To realize the favorable O3 production (1) 2H 2 O  → O 2 + 4H + + 4e E Θ = 1.23 V

3H 2 O  → O 3 + 6H + + 6e -

E Θ = 1.51 V

(2)

under electrolysis, effective approaches should be developed to substantially elevate the overpotential of the oxygen evolution reaction (OER), thus to prohibit the competitive oxygen generation. To date, various catalysts including Pt,13 PbO2,14

SnO215,16 and boron-doped diamond (BDD),17,18 have been demonstrated for feasible EOP by taking advantage of their inherently favorable molecular oxygen adsorption. However, the traditional approach for OER overpotential elevation is intrinsically determined by the kinds of the anodic material, which is still far from being satisfactory for an efficient EOP. Therefore, other facile strategies are greatly needed for efficient EOP by further enhancing the thermodynamic barrier of the undesirable OER. On the other hand, the polar O3 molecule is much more soluble than O2 in the aqueous electrolyte,19,20 such an apparent disadvantage largely restricts the practical productivity of gaseous O3 in traditional EOP process. In fact, before escaping into the atmosphere, a part of the generated O3 molecules will be dissolved and absorbed in the electrolyte, and subsequently decompose into dioxygen.11,21 Such a persistent ozone absorption and decomposing effect during electrolysis has significantly diminished the gaseous O3 producing in comparison to the theoretical productivity.22 Although the solid polymer electrolyte (SPE) technology employing proton-exchange membrane has been developed to achieve a high EOP efficiency,12,23,24 it still suffers from these intractable issues of the relatively-low OER potential and the unfavorable gaseous ozone escaping (when O3 gas is the target product instead of the ozone-dissolved water in the common SPE-based EOP22,24), which hopefully provide a room for further improving the gaseous ozone productivity. According to the commonly-accepted EOP mechanism, O3 formation depends essentially on the intermediate reaction between the hydroxyl free radical-derived active atomic oxygen and the adsorbed oxygen molecule, whilst the later will competitively desorb into gaseous O2 as an OER process.12,14,25-28 In theory,

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an increased level of adsorbed oxygen molecule will increase the reaction barrier to Eq. 1, and simultaneously benefit the ozone formation. Additionally, aimed at preventing these asgenerated O3 from the undesirable quick decomposing in a large quantity of electrolyte, it is necessary to find an effective yet simple solution by shortening the gaseous O3 escaping distance to atmosphere. Based on these considerations, we here propose a partialsubmersed electrolysis strategy by employing a bulk porous anode for enhanced electrochemical gaseous O3 production. Through a capillary effect as illustrated in Figure 1, the 3Dinterconnected porous network will enable the electrolyte to penetrate and fill into the whole electrode, including the part elevated above the electrolyte level and exposed in atmosphere. In comparison with the submersed case, the electrolytic reaction could equally take place inside the electrolytesaturated porous structure of the electrode above the electro-

Figure 1. Schematic diagram of the capillary effect-enabled partial-submersed electrolysis strategy by using a bulk porous anode.

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lyte level, which will diminish the escaping distance of gaseous O3 in travelling through the aqueous absorber. More importantly, a unique “molecular oxygen-locking effect” induced by the capillary pressure has been identified in the nonsubmersed part of the porous electrode, which not only thermodynamically suppresses the oxygen evolution (Eq. 1), but unexpectedly change the traditional EOP mechanism into an unusual •O3- intermediate-involved pathway. 2. RESULTS AND DISCUSSION 2.1 Design, preparation and characterizations of the desirable bulk porous electrode. Here, metallic Pb was selected as a qualified matrix to make bulk porous anode due to its low melting point for easy shaping, relative inertness in acidic electrolyte and cost-effectiveness. The bulk porous Pb (BPP) was prepared by a novel alloying-dealloying strategy (Figure 2a), in which a reasonable amount of metallic Na was doped in Pb lattice to form a uniform solid solution as confirmed by the X-ray diffraction (XRD) analysis (Figures 2b and S1). Then the alloy was etched in aqueous solution to remove Na while detain Pb component, thus leaving a porous Pb framework. Notably, benefiting from the huge electrode potential contrast between Na and Pb, the Na sacrificing in the dealloying process would not affect the remaining Pb networks by potential oxidization. Optimally, Pb-Na alloy of 10 : 1 in mole ratio was found to be the best precursor to prepare the desirable BPP with robust and abundant 3D-interconnected porous network after a facile dealloying treatment (Figure S2). Excessive Na amount would lead to a serious segregation of Na-rich intermetallic compound (Pb3Na), whose violent dealloying process would destroy the bulk porous matrix of Pb (Figure S3 and Table S1). Judging from the scanning electron microscopic (SEM) images, the surface of the obtained BPP can be found

Figure 2. (a) Schematic diagram of the fabrication process of the bulk porous lead. (b) XRD patterns of the as-prepared Pb-Na alloys with varied molar ratios. (c) Low-magnification SEM image of the surface of bulk porous lead. Insert is the enlarged SEM image highlighting the porous structure of several micrometers. (d) Close-up SEM image showing the porous framework. The yellow dotted lines indicate the regions of the micron-sized pores. (e) Further close-up SEM image of the rectangle-marked region in (d), showing the nanoporous structure. The insert is the further enlarged SEM image.


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to be of well-defined hierarchical porous structure, where the nanoporous ligaments as the framework build the micronsized porous structure of metallic Pb bulk (Figure 2c-e). In addition to the surface, the SEM images of the crosssectionally fractured surface of the obtained BPP confirm the similar architecture inside the whole BPP bulk, comprising 3D-interconnected nanoporous ligaments and connective micron-sized pores (Figure S4), and the resultant porosity is higher than 25 %. To further elevate the inherent OER overpotential on the electrode, the as-synthesized BPP was then packed with copper wire and treated by a convenient anodic oxidation in sulfuric acid solution at 10 V (vs. RHE) for 5 min. During this process, the metallic Pb (0) was revealed to be initially and partially oxidized to PbSO4 (Pb (II)) crystals as confirmed by the EDS and electron back-scattered diffraction (EBSD) analysis (Figure S5), which were firmly deposited on the bulk porous Pb marix. Then, these PbSO4 were further oxidized into a higher-valance form of tetragonal β-PbO2 as the XRD pattern indicated (Figure 3a). In contrast to the orthorhombic α-phase of PbO2, the β-crystallographic phase is known to possess a much better conductivity for ozone generation.25 These in-situloaded β-PbO2 presents a well-defined and cuboid-faceted morphology, and distribute uniformly on the maintained bulk porous Pb matrix (Figure 3b), which makes the argenteous BPP to be black-brown-coated (Figure 3c). From the selected area electron diffraction (SAED) pattern (Figure 3d), the βPbO2 cuboids are revealed to be well-defined single-crystalline particles, with no undesirable amorphous layer on the surface, which is evidently confirmed by their clear crystalline edge observed in the high-resolution transmission electron microscopy (HRTEM) image (Figure 3e). However, these PbO2 crystals are imperfect with slight lattice distortions and disloca-

tions detected in the Fourier filtering HRTEM images. Moreover, an unspecified broad plasmon peak at around 50 eV can be detected in the low-loss electron energy loss spectrum (EELS) of the β-PbO2 cuboids (Figure 3f), confirming a nonextrinsic collective electron oscillation at high frequency in the defective crystals, which indicates the presence of highdensity oxygen vacancies in the as-synthesized β-PbO2. Such an oxygen vacancy-rich nonstoichiometric PbO2 obtained from the in-situ anodic oxidation will benefit the electron conduction and oxygen molecule adsorption,25 which favors the EOP with high current efficiency. 2.2 Capillary effect in the porous electrode. Metallic Pb is an extremely soft and malleable metal, and its expected low mechanical strength of the porous Pb-based working anode is a significant concern in practical EOP operation. Interestingly, in spite of a pore-rich structure, the matrix of β-PbO2 cuboidloaded BPP (PbO2@BPP) show significantly enhanced compressive strength, with a four-time enhancement in the yield stress compared to that of the solid counterpart (Figure 4a), namely the β-PbO2 cuboid-loaded solid Pb matrix obtained from the similar anodic oxidation (PbO2@Pb, Figure S6). Such an unexpected compressive strength increase of porous Pb is proposed to be due to the trace retention of Na after the alloying-dealloying treatment (Figure S2b), which induces a significant solid solution strengthening effect to the Pb matrix. In addition to the micro-scale porosity directly observed by the SEM, the N2 sorption isotherms of PbO2@BPP further confirm the maintained nano-scale porosity with abundant mesopores and macropores (Figure 4b). The 3D interconnected porosity endows the bulk electrode with a strong capillary effect to uptake the aqueous solution to fully fill the whole porous network (Figure 4c and d), generating a localized electrolytic cell inside the anode electrode under continuous

Figure 3. (a) XRD patterns of the as-synthesized BPP before (Pre-AO) and after anodic oxidation treatment (Post-AO). (b) SEM image of the PbO2 cuboids-loaded BPP. The insert is the enlarged image highlighting a PbO2 cuboid loaded on the well-maintained porous Pb matrix. (c) Digital photographs and schematic diagrams of the BPP and PbO2 cuboids-loaded BPP. (d) SAED pattern of a representative PbO2 cuboid as shown in the insert TEM image. (e) Representative HRTEM image of a PbO2 cuboid. The insert is the fast Fourier transformed image of the square-marked region. Images on the right are the corresponding inverse fast Fourier transformed images by applying circular masks shown in the insert. The arrows indicate the distortion and dislocation in lattice. (f) Representative low-loss and Pb M-edge coreloss EEL spectra of the PbO2 cuboid.



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Figure 4. (a) Compressive stress-strain curves of the PbO2@Pb and PbO2@BPP. The cycles and arrows indicate the yield points. (b) N2 sorption isotherms of the PbO2@BPP at 77.4 K and the corresponding pore size distribution obtained from the adsorption branch using the BJH algorithm. (c) Successive digital photographs recording the fast water absorbing process of the PbO2@BPP. (d) Schematic diagram of the capillary effect-induced aqueous solution penetration into the whole PbO2@BPP framework.

electrolyte supply from electrolyte pool. This special electrode fully saturated with electrolyte may partially enable the electrolytic reactions to transfer into the porous structure of the electrode above the electrolyte pool. By employing a standard three-electrode system equipped with a hand-operated microelevator in 0.5 M H2SO4 aqueous solution (Figure S7), the capillary effect-enabled and aboveelectrolyte height-dependent electrochemical performances of the PbO2@BPP working electrodes were characterized in comparison with PbO2@Pb. Under the intermittent elevations of the working electrodes out of the electrolyte pool by 1 mm per step, the current is significantly and stepwise decreased for the PbO2@Pb electrode (Figure 5a), which results from a stepwise diminished contact area of the electrode with the electrolyte. Moreover, during the elevating operation, the current was found to be initially extremely unsteady due to the inevitable fluctuation of the electrolyte level, featuring a vio-

lent drop followed by an immediate increase. In contrast, the current fluctuation for the PbO2@BPP electrode during the elevating process is negligible, and the decrease is less than 5 % even when 75 % of the electrode has been out of the electrolyte. These results suggest a desirable capillary soaking effect of the electrolyte in penetrating and filling the porous structure of the bulk PbO2@BPP electrode, leading to an almost fully-retained contact with the electrolyte during the electrode elevation. Moreover, thanks to the 3D interconnected capillary structure, though the area of the electrolytesaturated PbO2@BPP electrode above the electrolyte level is exposed in atmosphere, the continuous mass supply from the electrolyte pool is ensured, along with an unblocked electron conduction in the metallic network. 2.3 Partial-submersed electrolysis for enhanced EOP. To study the above-electrolyte height-dependent electrocatalytic activities of the two electrodes for oxygen evolution, the corresponding linear sweep voltammetry (LSV) measurements

Figure 5. (a) Time-course current change during the intermittent elevations of the PbO2@Pb and PbO2@BPP electrodes above the electrolyte level by 1 mm per step. The insert demonstrates the elevating process of the electrodes. (b) Representative LSV curves of PbO2@Pb and PbO2@BPP electrodes measured in the submersed and half-submersed cases. (c) Comparison between the Tafel plots of PbO2@BPP electrodes obtained in the submersed and half-submersed cases. S0 is the true geometric area of the active regions of the bulk porous electrode.


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were then conducted. As expected, owing to the oxygen vacancy-rich β-PbO2 crystals, both kinds of the electrodes show a poor catalytic activity toward the OER with a high onset OER potential of 1.84 ± 0.10 V (vs. RHE, correlated to an OER overpotential of 0.61 ± 0.10 V, determined at a given faradaic current of 1 mA) in the submersed case without any air-exposure (Figure S8). However, the differences in the onset OER potentials between the two kinds of electrodes could be clearly observed when these electrodes were partially elevated above the electrolyte level. In this partial-submersed case, it is exciting to find that the OER overpotential is significantly enhanced by about 500 mV for the PbO2@BPP electrode, in contrast to the non-statistical changes observed for the PbO2@Pb electrode. Subsequently, the two typical conditions of submersed and half-submersed cases for the PbO2@Pb and PbO2@BPP electrodes were selected to further reveal the underling electrochemical processes and the corresponding EOP performances. The representative LSV curves are shown in Figure 5b. A marked elevation on the OER onset potential can be visually observed after the half-submersed operation on the submersed PbO2@BPP electrode, but the elevation is negligible for the solid PbO2@Pb electrode. Though it is extremely difficult to obtain the true geometric area of the active regions of the bulk porous electrode, due to the ignorable change in electrolyte contact after elevating the porous PbO2@BPP electrode (Figure 5a), its active area in both cases could be regarded as a constant of S0. Thus, the relative values of the Tafel slopes based on the logarithmic current density could be employed and analyzed to further understand such an unusual electroly-

sis (Figure 5c). The curves typically exhibit two intersecting linear regions, corresponding to two parallel anodic processes. The low-current-density region is dominated by the oxygen evolution reaction.29 In this region, the largely increased slope for the half-submersed case reveals a much diminished OER catalytic kinetics, suggesting a potential suppression effect on the mass transport of oxygen species. At higher current density, the secondary decreased Tafel slope is co-determined by the synchronous OER and EOP processes. The more dramatic change of the Tafel slope in half-submersed case indicates a much more efficient EOP catalytic kinetics than the submersed one. It should be noted that the half-submersed operation of the solid PbO2@Pb electrode is unstable, with a serious fluctuation observed in the polarization curve, which disables further Tafel analysis. All these results verify an efficient and desirable suppression on the OER in PbO2@BPP electrode by using the proposed half-submersed strategy. Furthermore, to give a qualitative comparison on the practical EOP efficiency between the submersed and halfsubmersed cases, we suspended a moistened starch potassium iodide test paper above the working electrode in the anode, which will undergo a sensitive chromogenic reaction from white to blue in the presence of gaseous O3. After a sufficient purge with N2 gas to get rid of the dissolved oxygen in the electrolyte of 0.5 M H2SO4 aqueous solution, the subsequent electrolysis was then performed at 5 V (vs. RHE). For the PbO2@Pb electrode in both submersed and half-submersed cases, although numerous fine bubbles were quickly produced on the submersed electrode once powered on, the test paper kept almost unchanged in white in 30 min (Figure S9). Despite

Figure 6. (a) Moistened starch potassium iodide paper giving a qualitative comparison on the PbO2@BPP-mediated EOP under the submersed and half-submersed conditions. (b) Schematic diagram of the real-time quantitative measurement on the gaseous ozone production by continuous-flow oxidization of nitric monoxide. (c) Time-course gaseous ozone productivity under varied electrolytic modes as shown in the insert. (d) Chronoamperometry measurement (time-course relative current, black curve) of the half-submersed PbO2@BPP electrode at a constant potential of 5 V (vs. RHE) in 0.5 M H2SO4 solution, with time-course analyses on the Faraday efficiency (blue curve) of the gaseous ozone production and the free Pb2+ releasing (red bars) in the electrolyte.



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a higher onset potential than the standard potential of ozone generation, this result suggests a significant oxygen evolution due to the insufficient suppressions on the oxygen evolution reaction in Eq. 1, or/and undesirable dissolution of O3 in the electrolyte if produced, by employing the solid PbO2@Pb electrode. For the PbO2@BPP electrode in the submersed case, the gaseous O3 was detectable from the gradually blue-colored test paper in about 12 min (Figure 6a), which visually confirms the generation of gaseous ozone. Excitingly, the EOP efficiency was found to be significantly enhanced when the electrode was half-elevated out of the electrolyte, as evidenced by the much faster blue-chromogenic test paper in 2 min, which quickly became fully blue in 15 min. Due to the rapid-decomposition nature, it is of great difficulties to give a quantitative and real-time measurement on the productivity of the moist gaseous ozone directly. Nevertheless, as shown in Figure 6b, we here set up a device allowing for the sensitive oxidation reaction of nitric monoxide in a continuous-flow mode to monitor the gaseous ozone production, where the low-concentration nitric monoxide will be immediately oxidized into nitric dioxide once contacting O3 molecules.30,31 After complete drying to remove the water vapor, the detected decrease in the nitric monoxide concentration could support a real-time quantification of the gaseous ozone production and the corresponding Faraday efficiency (Supplementary Discussion). This method has confirmed to be of reasonably high enough sensitivity, accuracy and stability (Figure S10). In accordance with the qualitative findings, the ozone production is negligible for the PbO2@Pb electrode in the half-submersed electrolysis after powered on (Figure S11). In the submersed case, a very low amount of gaseous O3 (less than 2 µmol min-1 after reaching to the reaction equilibrium) could be detected, whose concentration is too low to visually distain the test paper as evidenced in Figure S9. This result suggests a remarkable sensitivity of such a novel quantitative measurement. In contrast, for the porous PbO2@BPP electrode, the half-submersion operation results in an immediate generation of gaseous O3 once powered on, whose productivity is quickly elevated to be around 14 µmol min-1 after the reaction equilibrium, achieving more than three times of EOP productivity that of the submersed case (Figure 6c). Such an unreported phenomenon of half-submersionpromoted EOP by employing bulk porous electrode may originate from the increased current density in the immersed part, instead of the role of the non-immersed part as aboveenvisioned. To check this plausible cause, we then cut off the electrode part above the level of the electrolyte solution to exclude any influences from the above electrode part in the atmosphere. As a result, the remained immersed electrode will receive a much intensified current density. However, the result turns out to induce no promotion effect, actually a slight reduction, on the gaseous ozone production (Figure 6c). This experiment soundly confirms the key role of the electrode part located above the level of the electrolyte in enhancing EOP efficiency. Preliminary assessment on the stability of the PbO2@BPP electrode in the half-submersed operation was carried out by the chronoamperometry measurement at a constant potential of 5 V (vs. RHE) for a prolonged period of 10 h, accompanied by the time-course analyses on the Faraday efficiency of the gaseous ozone production and the lead releasing in the electrolyte. The PbO2@BPP-mediated half-submersed electrolysis

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was preliminarily found to be of fair technical stability, featuring a well-maintained relative current with less than 5 % attenuation in 10 h (Figure 6d). In addition, the oxidized quantity of the nitric monoxide kept steady during this period, corresponding to a stable Faraday efficiency of the gaseous ozone production at ca. 21 % with minimal fluctuation (Supplementary Discussion). Besides the technical stability aforementioned, the material stability is another important concern in the practical application. Though the PbO2@BPP electrode is not absolutely stable, it was found to be relatively corrosionresistant in acidic solution under such a drastic polarization condition. Less than 1.2 ppm Pb ions can be detected in the electrolyte in 10 h electrolysis, suggesting a low anodic digesting rate of free lead ions. Despite a preliminary stability of the electrode demonstrated in this pilot study, more efforts should be made to further mitigate the undesirable lead releasing, such as integrating the promising SPE technology.12,23,29,32 Taken together, all these results evidently confirm that the proposed partial anode submersed strategy could be an efficient and durable strategy in enhancing EOP by employing the porous PbO2@BPP electrode. 2.4 Investigations on the anodic reaction mechanism. Despite the remarkable improvement on the EOP efficiency, it is still unclear the detailed origin of the above-revealed suppression on the OER catalytic activity by the partial submersion strategy. To unveil the potential kinetic differences on the conductivity and diffusion, electrochemical impedance spectroscopic studies of the electrodes were then carried out at a bias of 100 mV overpotential. The electrodes in both cases show a similar Nyquist diagram, featuring a charge transferassociated semicircle in a relatively high frequency range and a mass transfer-controlled linear trailing at lowered frequency (Figure 7a and S12). Based on the further quantitative analyses on the electrical analogue (Supplementary Table S2), the circuit parameters of the elevated PbO2@BPP electrode show slight increases in intrinsic resistance (Rs) and charge transfer resistance (Rct), which can be directly reflected in the negligible change in the frequency dependence of phase angle shown in the Bode plots along with a slight enhanced impedance (Figure S13). In addition, the corresponding Warburg impedance (element W in the equivalent circuit) was also found to be slightly higher than that in the submersed case, representing an enhanced mass transfer resistance at the electrode/electrolyte interface inside the nanoporous network after non-submersed operation. Nevertheless, it is still puzzled why such a slightly increased inhibition in electron and mass transport could lead to the substantial OER suppression and EOP promotion as revealed, indicating a potentially different reaction pathways of the ozone generation between the submersed and the half-submersed cases. To further investigate the anodic reaction pathway, electron spin resonance (ESR) spectroscopy was used to detect the radical intermediates generated during the electrolysis with 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trap. In the submersed case, a well-defined quartet lines with peak height ratio of 1: 2: 2: 1 can be observed in the ESR spectra (Figure 7b), which is assigned to the typical spin adduct of DMPO and •OH (DMPO-OH, Figure 7c). Unexpectedly, the ESR spectra are substantially different in the half-submersed case. Several unidentified peaks were detected in addition to the quartet DMPO-OH lines in 1 min electrolysis, and these signals were well-maintained in the long-time electrolysis for 15 min. These seven unusual lines (four of which are well


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Figure 7. (a) Nyquist diagrams of the PbO2@BPP electrode in the submersed and half-submersed cases. The inserts are the corresponding equivalent circuit and the enlarged image of the low-frequency region. (b) Representative ESR signals during electrolysis at a potential of 5 V (vs. RHE) in the submersed and half-submersed cases using DMPO as the spin trap. (c) Structure diagram of the DMPO-spin adducts with the electrolysis-generated radicals by the PbO2@BPP electrode. (d) Time-course records of the dissolved oxygen concentrations in different regions (as the insert shown) of the PbO2@BPP and PbO2@Pb electrodes.

overlapped with the quartet DMPO-OH lines) indicate the generation of ozone radical (•O3-), whose electronegativity is much higher than that of hydroxyl radical and superoxide anion. Different from the spin adduct of DMPO with the hydroxyl radical or superoxide anion,33-35 the β-site-added •O3- in DMPO will activate or stimulate the insensitive γ-hydrogen atom to anticipate into the isotropic hyperfine splitting along with the nitrogen and β-hydrogen (Figure 7c, hyperfine splitting constants AN = Aβ-H = Aγ-H = 7.5 G). In addition to the traditional hydroxyl radical-dominated EOP, this result confirms the coexistence of an unusual O3 evolution pathway associated with the •O3- immediate generation in the partialsubmersed case, which is essentially different from that in submersed case for the bulk porous electrode. In addition to the radical immediate, the quantification of the involved molecular oxygen species in electrolyte will be helpful to understand the whole anodic reaction. By employing an oxygen microelectrode, the dissolved oxygen levels in different regions of the PbO2@BPP electrode and that in the electrolyte close to the solid PbO2@Pb electrode during the half-submersed electrolysis operations were then monitored (Figure 7d). Along with the electrochemical oxygen evolution after powered on, the dissolved oxygen concentration in the electrolyte inside the submersed part of porous PbO2@BPP electrode was found to increase rather slowly at a rate slightly higher than that in the electrolyte close to the solid PbO2@Pb electrode, suggesting a fast oxygen dilution effect in this case due to a direct contact with the massive electrolyte. In con-

trast, the dissolved oxygen concentration in the electrodecontained electrolyte above the electrolyte pool shows a sharp rise within several seconds and keeps at a highlysupersaturated level after powered on.36 According to the classic Henry’s law, i.e., the amount of dissolved gas molecule is proportional to its partial pressure in the gas phase,37,38 and the experimentally measured the highlysupersaturated dissolved oxygen in Figure 7d suggests an extraordinarily high partial pressure of gaseous O2 entrapped within the non-submersed porous network. Understandably, as schematically illustrated in Figure 8a, the electrochemically generated gaseous O2 inside the bulk porous electrode will lead to a gas embolism effect in these interconnected capillary tubes,39,40 forming numerous oxygen bubbles to block the electrolyte. These convex liquid-gas interfaces will cause an additional Laplace pressure to cumulatively increase the partial pressure of O2 inside the bubble chamber,39,41 thus leading to a significant enhancement in the molecular oxygen dissolution as detected. Different from the submersed part, these oxygen bubbles blocking in the electrolyte inside the non-submersed part fail to keep a direct contact with the bottom massive electrolyte, thus resisting a fast depletion through a successive dissolution and dilution process. Accordingly, the elevated partial pressure of O2 could result in largely increased thermodynamic resistance of oxygen evolution half-reaction of Eq. 1, causing an additional potential (φadd) over the standard potential need for OER. By further applying the Nernst equation based on a simplified capillary system (Supplementary Discussion), the additional potential



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has been found to be essentially determined by the capillary radius (r) and number of bubble chambers (n) inside the nonsubmersed porous network (Figure 8b). The decreased r and increased n weight the contribution of the Laplace pressure, prominently boosting the additional potential. It is highly exciting to find that the calculated φadd values range from 0.44 to 0.55 under reasonable values of r and n, which quantitatively agrees with the experimental OER overpotential obtained in the partial-submersed electrolysis. For the bulk porous electrode, these results firmly support the essential role of the capillary pressure-derived “molecular oxygen-locking effect” in elevating the OER overpotential in partial-submersed case.

nal ozone generation depends on the essential reaction between the •OH immediate-derived atomic oxygen and the OER-generated molecular oxygen adsorbed on anode (Eqs. 46).12,14,26,27 However, these adsorbed oxygen molecules are generally of a low concentration due to their free desorption into gaseous or dissolved counterparts. This intrinsic disadvantage significantly impedes the ozone generation reaction kinetically. Substantially different from the conventional EOP process observed in the submersed electrolysis, the capillary pressure-promoted oxygen dissolving in the non-submersed electrode part could lead to significant suppression on the adsorbed oxygen desorption due to the equilibrium of Eq. 7. Following the generation process of dioxygen species in OER (Eqs. 3-5), the remarkably-enhanced adsorbed oxygen level as revealed is believed to create the possibility of •O3- generation through a plausible electrochemical reaction of Eq. 8, thus fundamentally giving rise to an unusual •O3- intermediateinvolved EOP pathway for the capillary-enabled electrolysis as proposed in Figure 9b. Superior to the traditional EOP mechanism based on the chemical combination of adsorbed molecular oxygen and atomic oxygen, the •O3- intermediate generated in the special capillary-dominated case favors the generation of ozone molecules through a simple singleelectron electrochemical oxidization (Eq. 9). Such a unique EOP pathway essentially supported by the bulk porous electrode, along with the suppressed oxygen evolution reaction and the shortened path of gaseous O3 escaping from the electrolyte adsorption, synergistically account for the enhanced EOP performance of PbO2@BPP electrode in the capillary effect-enabled partial-submersed electrolysis.

H 2 O (ads) - e −  → .OH (ads) + H +

. OH + .O + .O

(ads)

.O

(ads)

O 2(ads) Figure 8. (a) Schematic diagram of the additional Laplace pressure caused by the convex liquid-gas interfaces of bubble chamber inside a capillary tube in the bulk porous electrode. (b) Calculated result showing the capillary radius (r) and number of bubble chambers (n) dependences of OER overpotential (φadd).

As illustrated in Figure 9a, the PbO2@BPP electrodesubmersed EOP process follows a common anodic mechanism after a primary water discharge step (Eq. 3), where the termi-

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- e−  → .O (ads) + H +

(ads)

(ads)

 → O 2(ads)  → O 2(g)  → O3(ads)  → O3(g)

(3) (4) (5) (6)

O 2(ads) ← → O 2(aq.) ← → O 2(g)

(7)

− O 2(ads) + H 2 O (ads) - e −  → .O3(ads) + 2H +

(8)

.O

− 3(ads)

- e−  → O3(ads)  → O3(g)

(9)

Figure 9. Schematic comparison on the anodic reaction mechanisms on the β-PbO2@BPP electrode administrated between submersed14 (a) and half-submersed EOP process (b). The chemicals written in circle represent the adsorbed species on the active surface of the electrode.


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3. CONCLUSION In summary, we have developed a Na-mediated alloyingdealloying method to prepare bulk porous Pb matrix, which is further partially oxidized to achieve electrochemistry-active βPbO2 cuboids-loaded bulk porous electrode. Benefiting from the 3D-interconnected porous structure of the electrode, its strong capillary effect enables an unusual partial-submersed electrolysis strategy by continuous transporting electrolyte into the atmosphere-exposed electrode area above the level of electrolyte pool, which leads to a largely suppressed OER but much enhanced electrochemical gaseous ozone production. Detailed study on the partial-submersed anodic process identifies a capillary pressure-induced “molecular oxygen-locking effect”, giving rise to a brand new and highly efficient •O3intermediate-involved EOP pathway in the special capillarydominated electrolysis supported by the bulk porous electrode. In addition to providing a promising methodology for gaseous ozone production, this study also demonstrates the possibility of tuning the intrinsic electrochemical behavior of the electrode simply by extrinsic structure design and electrolytic mode adjustment. 4. EXPERIMENTAL SECTION 4.1 Materials and Reagents. Lead shot (99.9 %), sodium cube (99.9 %) and 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, 97 %) were purchased from Sigma-Aldrich. All chemical agents used are of analytical grade and without further purification. The ultrapure water was prepared using ELGA water purification system (PURELAB Classic). 4.2 Characterizations. X-ray diffraction (XRD) data was collected on a Rigaku D/MAX-2250 V at Cu Kα radiation (λ = 1.54056 Å) with a scanning rate of 4 ° min-1. Transmission electron microscope (TEM) images and corresponding results of electron energy loss spectroscopy (EELS) were acquired on a JEM-2100F equipped with a Gatan GIF 963 operated at 200 kV. Scanning electron microscope (SEM) images, analyses of energy dispersive X-ray spectroscopy (EDS) and electron back-scattered diffraction (EBSD) were obtained on a FEI Magellen 400. Nitrogen isotherms corresponding to the Barrett-Joyner-Halenda (BJH) pore distribution and BrunauerEmmett-Teller (BET) surface area were collected on a quadrasorb SI at 77 K after degassing the samples at 150 °C for 5 h. Compression test was carried out on an Instron-5566 singlecolumn mechanical testing system with a loading speed of 0.1 mm min-1.Electrochemical characterizations were performed on a CHI 660 electrochemical workstation. 4.3 Synthesis of bulk porous Pb. All the weighing and blending processes were performed in an argon atmosphere glove box to prevent any oxidation of sodium. In brief, fresh lead and sodium were mixed and hermetically put into a casting steel mould with the dimension of 8 mm*8 mm *4 mm. Then, with the protection of argon atmosphere, the Pb/Na-containing steel mould was kept at 450 °C for 1 h with a ramping rate of 5 °C min-1 in a tubular furnace (OTF-1200X-L, Hefei Ke Jin Materials Technology Co., Ltd., China). After a natural cooling process to the room temperature, the obtained Pb-Na alloy was polished into 2 mm in thickness, dealloying treated in 500 ml of ultrapure water at room temperature (25 °C) for 12 h, and then dried in a vacuum dryer after being extensively washed with ethanol.

4.4 Preparation of β-PbO2 cuboids-loaded working electrode. Enamelled copper wire was attached to the side face of the as-synthesized bulk porous Pb or the control bulk Pb with soldering indium, and the soldering joint was subsequently covered with epoxy. Then, the prepared electrodes were anodized in 50 ml of 5 M H2SO4 aqueous solution at 10 V (vs. RHE) for 5 min with a standard three-electrode system. A platinum sheet (99.9995%) and an Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. 4.5 Electrochemical setup and characterizations. The electrochemical measurements were performed on a standard three-electrode system in a U-pipe containing 50 ml of 0.5 M H2SO4 aqueous solution (Figure S7). Apart from the platinum sheet (99.9995%, counter electrode) and the Ag/AgCl electrode (reference electrode), the as-prepared working electrode was separately put in one side of the U-pipe, which was submersed in the thermostabilized water bath to maintain the test temperature at 25 °C. The copper wire of the working electrode was fixed in a hand-operated microelevator to control the height of working electrode out of the surface of the electrolyte. Before all the electrochemical measurements, the electrolyte was fully purged with N2 gas to get rid of the dissolved O2. In a time-course potentiostatic measurement at 1 V (vs. RHE), the completely submerged working electrode was elevated out of the level of the electrolyte by 1 mm per step with an interval time of about 70 s. At varied height out of the electrolyte, the linear sweep voltammetry (LSV) was measured from -0.2 to 5 V vs. Ag/AgCl with a scan rate of 1 mV s-1. Tafel plot was obtained by plotting the curve of the potential (E) against the logarithm of current density (log j). Electrochemical impedance spectra (EIS) was measured at the frequency ranged from 10-2 to 105 Hz, with an overpotential bias of 100 mV and a.c. amplitude of 10 mV. The fit of the EIS for the equivalent circuit was performed on the Zview software. Electrochemical durability was assessed by a time-course potentiostatic measurement at a fixed potential of 5 V (vs. RHE). At given time, 200 µl of the electrolyte was collected to determine the dissolved concentration of Pb ion via the inductively coupled plasma optical emission spectrometry (ICPOES, Agilent 700 Series). The time-course concentrations of the dissolved oxygen were recorded by using a Unisense oxygen microelectrode. All the electrochemical measurements were performed without ohmic-drop correction, and the applied potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: E(RHE) = E(Ag/AgCl) + 0.059pH + 0.197 V. Four electrodes were individually measured for each group, representative curves are presented, and the statistical OER potential are mean ± s.d. 4.6 Qualitative and quantitative assessments on the productivity of gaseous ozone. For the qualitative case, a moistened starch potassium iodide test paper was suspended above the working electrode in the anode. The height from the bottom of the test paper to the electrolyte level was fixed at 1cm. After the power on with a fixed potential of 5 V (vs. RHE), the time-course color change of the test paper was recorded by the digital photographs in 30 min. A continuous-flow oxidization reaction of gaseous nitric monoxide was employed to give an indirect quantitative measurement of the gaseous ozone production. As illustrated briefly in Figure 6b, the anodic chamber of the U-pipe was exposed to a continuouslyflowing inlet gas containing 8 ppm NO with the air flow as the



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carrier gas, and the total flow rate was maintained at 240 ml min-1 at 25 °C. After a full drying to remove the water vapor through flowing a dry column containing the allochroic silicagel, the concentrations of NO in the outlet gas were realtime detected by a NOx analyzer (Thermo Fisher 42i-LS). The potential was fixed at 5 V (vs. RHE) during the electrolysis, and the power was initiated after a 30-min equilibrium of the gas flow. The gaseous dry ozone used for the test of the linearity between ozone concentration and the detected NO removal amount was produced by a commercial YEK-7000 coronadischarge generator (Shenzhen YEK High-Tech Co., LTD, China). The varied concentration of gaseous ozone was obtained by pure nitrogen dilution, and the corresponding actual ozone concentration was determined by a GENESYS 10S UVVis spectrophotometer (Thermo Scientific) based on the characteristic absorbance of ozone at 258 nm.42 4.7 Detection of the radical intermediates generated in anode. In the dark condition, an aforementioned threeelectrode system was equipped in a micro-U-pipe, which contained 4 ml of 0.5 M H2SO4 aqueous solution including 50 mM DMPO as the spin trap. The electrolyte was fully pretreated with N2 gas purge to get rid of the dissolved O2. Respectively, in the “completely-submersed” or “half-submersed” administration for the working electrode, after a electrochemical reaction at a fixed potential of 5 V (vs. RHE) for 1 min and 15 min, 2 µl of the electrolyte collected from the anode region was quickly injected into a quartz capillary for X band electron spin resonance (ESR) measurement. The roomtemperature ESR spectra were collected on a Bruker EMX8/2.7 spectrometer with the following settings: microwave frequency = 9.820 GHz, microwave power = 4.743 mW, modulation amplitude = 1.00 G and modulation frequency = 100.00 kHz.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional XRD patterns, SEM images, EDS spectura and electrochemical date. Digital photograph of the electrochemical setups. Detailed discussion on the calculation of the capillary radius and number of bubble chambers dependences of OER overpotential. (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Chen Zhang: 0000-0002-6514-7244 Yingfeng Xu: 0000-0002-6855-0848 Ping Lu: 0000-0001-7262-6174 Xiaohua Zhang: 0000-0001-6690-6911 Fangfang Xu: 0000-0002-5570-4289 Jianlin Shi: 0000-0001-8790-195X

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work has been financially supported by the National Key Basic Research Program of China (2013CB933200), National Natural Science Foundation of China (Grant No. 51372260) and

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Shanghai technical platform for testing and characterization on inorganic materials (14DZ2292900). Thanks to Jingwei Feng, Heliang Yao and Linlin Zhang from Shanghai Institute of Ceramics, Chinese Academy of Sciences for useful discussions.

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Capillary Effect-Enabled Water Electrolysis for Enhanced

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