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. E-mail: jlshi@mail.sic.ac.cn. University of Chinese Academy of Sciences, Beijing 100049, P.R. China. S1
1. Supplementary Figures and Tables Figure S1. Comparison of the normalized (111) XRD peak of Pb/Na alloys with varied mole ratios of Pb to Na. At Pb : Na 10:1, no Pb-Na intermetallic compound could be detected, and the (111) peak of Pb matrix is broadened with a notable shift towards higher angle along with the increase of Na content. Such a result indicates that only small amounts of Na atoms (mole ratio of Pb : Na 10:1) could well dissolve into the Pb lattice, forming uniform solid solution without any detectable segregation. S2
Figure S2. Elemental analysis of the as-synthesized Na/Pb alloy (Pb : Na = 10: 1) before (a) and after (b) dealloying treatments. (a 1, b 1 ) Typical SEM images, EDS element content analyses (a 2, b 2 ) and corresponding element mappings of Pb (a 3, b 3 ) and Na (a 4, b 4 ). The Pb and Na elements are distributed highly homogeneously with each other without clearly detectable Pb- or Na-rich regions, indicating the formation of a uniform solid solution of Pb and Na at the mole ratio of 10 : 1. Note that the little oxygen observed in (a 2 ) is from the fast surface oxidation of Na in moist air, which is unavoidable in SEM sample preparation. After the dealloying treatment, the Pb framework was formed in porous morphology with negligible Na retained. There is no detectable surface oxidation of the bulk porous Pb, which benefits from the much lower electrode potential of Na in water preventing the Pb framework from oxidation. S3
Figure S3. SEM image (a) and corresponding element mappings of Pb (b) and Na (c) of the Pb/Na alloy of 5 : 1 in mole ratio. Notable Na-rich segregation could be observed, which exhibits an inhomogeneous distribution in the Pb/Na solid solution. Judging from the XRD result, the Na-rich phase is confirmed to be NaPb 3 intermetallic compound. S4
Table S1. Typical digital photographs of the Pb/Na alloys with varied mole ratios of Pb to Na before and after dealloying treatments, and corresponding pore parameters of the obtained bulk porous Pb. a Porosity calculated by (1- the measured density/ the theoretical density) 100 %. b Surface area determined by BET equation. c BJH adsorption cumulative volume of pores between 1.7 and 300 nm. Note that the Pb framework will collapse spontaneously during dealloying treatment when the mole ratio of Pb to Na in Pb/Na alloy is lower than 10 : 1. NA, not available. S5
Figure S4. Step-by-step zoom-in SEM images of the fresh fracture of the bulk porous Pb obtained from the dealloying treatment on the Pb/Na alloy of 10 : 1 in mole ratio. Besides the surface, these images evidently confirm the existence of hierarchical porous structure inside the as-prepared bulk porous Pb. S6
Figure S5. (a) SEM image of the BPP after anodic oxidation treatment in 5 M H 2 SO 4 aqueous solution at 10 V (vs. RHE) for 15 s. A number of non-cuboid particles can be observed on the Pb matrix at this stage, which is the precursors for the final β-pbo 2 cuboids. (b) EDS element analysis of a representative non-cuboid particle marked in (a). The mole ratio of Pb : S : O is approximately 1:1:4, indicating PbSO 4 species. (c) Further EBSD pattern of the marked substance shows the well-matched Kikuchi bands of the orthorhombic PbSO 4 phase, confirming that the non-cuboid particles on the Pb matrix at this stage of anodic oxidation is PbSO 4. (d) 3D phase view of the marked PbSO 4 nanocrystal. S7
Figure S6. (a) Digital photographs of the solid Pb electrode before and after anodic oxidation treatment in 5 M H 2 SO 4 aqueous solution at 10 V (vs. RHE) for 5 min. (b) SEM image of the surface of the β-pbo 2 cuboid-loaded solid Pb. (c) XRD pattern of the solid Pb electrode after anodic oxidation treatment, confirming the β-pbo 2 loading. S8
Figure S7. Experimental setup of the electrochemical measurement. The as-prepared working anode and cathode (Pt counter electrode and Ag/AgCl reference electrode) were separated in the U-pipe containing 50 ml of 0.5 M H 2 SO 4 solution. The U-pipe 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, which enables accurate control on the height of Pb anode above the level of the electrolyte pool. S9
Figure S8. Onset OER potentials of PbO 2 @Pb and PbO 2 @BPP working electrodes measured at varied heights (h) above the level of 0.5 M H 2 SO 4 aqueous electrolyte. Four electrodes were individually measured for each group and the data are mean ± s.d. The value of the OER potential is determined at a given faradaic current of 1 ma. In contrast to the negligible influence of h on the onset OER potential observed in PbO 2 @Pb electrodes, the onset OER potential is significantly increased by about 500 mv for the PbO 2 @BPP electrode in the partial-submersed case compared to that of submersed case. Please note that no statistical differences can be detected between the onset OER potential measured at varied h values for PbO 2 @BPP. S10
Figure S9. Moistened starch potassium iodide paper giving a qualitative comparison on the EOPs from the PbO 2 @Pb electrode under the submersed and half-submersed conditions. S11
Figure S10. Relationship between the O 3 concentration (n = 4, data are mean ± s.d.) and the detected NO removal amount. The gaseous ozone was produced by a commercial corona-discharge generator, whose varied concentration was obtained by using nitrogen dilution. The actual O 3 concentration was determined by the conventional method based on the characteristic absorbance of ozone at 258 nm. The result shows a good linearity, firmly confirming the accuracy of the developed NO-O 3 system to quantitatively monitor the O 3 production. All the repeatedly-measured points are within short error bars, indicating the good stability of this developed measurement method. S12
Figure S11. Time-course gaseous ozone productivity after the initiation of the PbO 2 @Pb-mediated EOP under the submersed and half-submersed conditions, which is shown in the insert. Despite numerous bubbles (oxygen) generated in the anode after powered on, the concentration of the gaseous nitric monoxide in the half-submersed case experiences negligible changes, which indicates the ineffectiveness of gaseous oxygen to simultaneously oxidize the low-concentration nitric monoxide into nitric dioxide. Comparatively, little gaseous ozone (please note that the concentration is too low to visually distain the test paper as evidenced in Figure S9.) could be detected in the submersed case, which is due to the doubled amount of the catalytic active site compared to that in the half-submersed case. S13
Figure S12. Nyquist diagrams of the PbO 2 @Pb electrode in the submersed and half-submersed cases. The insert is the corresponding equivalent circuit. Note that the measurement in the half-submersed case is instable due to the inevitable fluctuation of the electrolyte level on the PbO 2 @Pb electrode. S14
Table S2. Parameters in the simulated equivalent circuit diagrams of the PbO 2 @Pb and PbO 2 @BPP electrodes in submersed and half-submersed cases. S15
Figure S13. Bode plots of the PbO 2 @BPP electrode in the submersed and half-submersed cases measured at a bias of 100mV overpotential. S16
2. Supplementary Discussion 2.1 Calculation of the Faraday efficiency of the gaseous ozone production The Faraday efficiency of the gaseous ozone production (Φ O3 ) in the EOP system are calculated according to the following equation: Q t Q Φ O3 = = I t I O3 O3 (1) where I is the working current (A), Q O3 is the quantity of effective electric charge in gaseous O 3 production per unit time (C s -1 ), which can be calculated as follows: Q = F M = 6F M (2) O3 e O3 where F is the Faraday s constant of 96485 C mol -1, M e is the mole number of the transferred electron in the produced O 3 per unit time (mol s -1 ), which is six times the mole number of the produced O 3 per unit time (M O3, mol s -1 ). Furthermore, M O3 can be estimated from the oxidation reaction of nitric monoxide as follows: M (c c ) OFF ON O3 = M NO = V (3) mno where M O3 is the oxidized consumption of nitric monoxide by gaseous ozone per unit time (mol s -1 ), c OFF and c ON are the stable concentration of the nitric monoxide before and after the power on (g L -1 ), m NO is the molar mass of nitric monoxide molecule (30 g mol -1 ), V is the velocity of the carrier air flow (set as 4 ml s -1 here). In combination of the Equation (1), (2) and (3), the Faraday efficiency of the gaseous ozone production can be obtained by the following equation: 7719 (c c ) I OFF ON Φ O3 = 100 % (4) By substituting the measured time-course I and (c OFF - c ON ) in Eq. 4, the real-time gaseous ozone production and corresponding Faraday efficiency can be obtained. 2.2 Estimation of the capillary pressure-enhanced OER overpotential When numerous oxygen bubbles have been formed inside the capillary tube in the porous network, each convex liquid-gas interface will induce an additional Laplace pressure (P Lap ), which can be determined by the Young-Laplace equation: 1 S17
P Lap 2γcosθ = (5) r where r is the radius of a certain capillary tube. To simplified the calculation model, all these capillary tube inside the porous electrode are supposed to possess a uniform radius. γ is the surface tension, a constant of 72 mn m -1 at 25 C. 2 θ is the native contact angle on the surface which equals to 0 (cos θ = 1) when the convex liquid-gas interface presents a perfect hemispherical shape in the critical condition. Thus, based on the hypothesis that the number of the interconnected and interacted liquid-gas interface is n (Figure 8a), the maximum partial pressure of O 2 inside the porous electrode during electrolysis can be expressed as: P = P + np (6) O2 0 Lap where P 0 is the atmospheric pressure. As to the following half-reaction: + - Θ 2H 2O O 2 + 4H + 4e E = 1.23 V (7) the practical potential (E) of electrochemical oxygen evolution can be calculated according to the Nernst equation: Θ E = E + ϕ (8) add ϕ RT = ln[( P / P ) c ] zf Θ 4 add O + 2 H (9) Obviously, the elevated partial pressure of O 2 will induce an increase in thermodynamic resistance, and cause an additional potential (φ add ) deviated from the standard OER potential (E Θ ), which is regarded as the origin of the enhanced OER overpotential. In Eq.(9), R is the universal gas constant of 8.314 J K -1 mol -1. T is the temperature in kelvins. z is the number of moles of electrons transferred in the OER half-reaction, namely z = 4. F is the Faraday constant of 9.6485 10 4 C mol -1. P Θ is the standard pressure of 10 5 Pa. c H+ is the concentration of hydrogen ion in the electrolyte inside the non-submersed network. The persistently-produced hydrogen ion will be significantly accumulated in the bubble-blocking electrolyte drops isolated from the electrolyte pool. In a simplified model, all the produced hydrogen ion uniformly dissolves in these bubble-blocking electrolyte drops inside the non-submersed porous network. Under an ideal condition, the increased amount of the hydrogen ion (M H+ ) is four times that of the produced S18
oxygen (M O2 ) according to Eq. (7). Neglecting the negligible part of the dissolved oxygen, the amount of the gaseous oxygen can be calculated by: M O2 = V V Θ O2 m (10) where V Θ O2 is the total volume of the gaseous oxygen under the standard pressure P Θ. V m is a constant of the molar volume of gas, which equals to 24.5 L mol -1 at 25 C. 3 According to the Clapeyron equation, 4 the standard-pressure volume (V Θ i ) of the non-standard-pressure oxygen bubble can be calculated as follows: Pi V Θ i = V0 (11) Θ P where P i is the partial pressure of O 2 in ith oxygen bubble determined by Eq. (6). V 0 is the volume of the bubble, which can be hypothetically regarded as a sphere with r in radius. In combination with Eqs. (6), (10) and (11), the increased amount of the hydrogen ion (M H+ ) can be calculated as follows: 4 4 2γ cosθ 4 M + = = (1+ i) π H V V P 3 m n n Θ 3 Vi r (12) Θ i= 1 m i= 1 r Moreover, the total volume of the bubble-blocking electrolyte drops (V liq ) can be roughly estimated due to the difference between the total pore volume inside the half-submersed part of the bulk porous electrode (V pore ) and the total volume occupied by gas bubble (V gas ). These two parameters can be respectively calculated as follows: 1 Vpore = p Velectrode (13) 2 4 3 Vgas = π r n (14) 3 where p is the porosity of the bulk porous electrode, which possesses an average of 25 % from five independent measurements. V electrode is the geometric volume of the bulk porous electrode (8 mm*8 mm *4 mm). Generally, c H+ can be expressed as follows: c + H = 1 + M + + H = 1 + M H V V - V liq pore gas (15) In combination with Eqs. (6), (9) and (12-15), the capillary radius (r, nm) and number of the interconnected and interacted liquid-gas interface (n) dependences of additional overpotential (φ add, S19
V) can be simply expressed as follows: ϕ 28 3 25 2 1440n+ r 6.84 10 nr + 4.92 10 nn ( + 1) r = 0.0148lg +0.0592lg[1 + ] r 1.6 10 4.19 10 nr add 8 27 3 (16) Please note the boundary condition of V pore - V gas > 0, namely deriving the following relationship between n and r: 3 18 nr < 3.8 10 (17) S20
3. Supplementary References (1) Joung, Y. S.; Buie, C. R. Nat. Commun. 2015, 6. (2) Vargaftik, N.; Volkov, B.; Voljak, L. J. Phys. Chem. Refer. Dat. 1983, 12, 817-820. (3) Mohr, P. J.; Taylor, B. N.; Newell, D. B. J. Phys. Chem. Refer. Dat. 2012, 41, 043109. (4) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723-1732. S21