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理解电极表面氧气泡演化对提升大规模水分解的效率具有重要意义. 本文提出了一种基于气泡边界的溶解氧通量的电极表面氧气泡生长的数值模型, 研究了反应区域和电流的大小对气泡生长的影响. 结果表明, 由气泡边界的氧通量计算得到气泡直径与气泡在化学反应控制阶段的生长关系吻合较好. 随着反应区域增大, 在气泡生长过程中, 由扩散控制向化学反应控制阶段过渡的时间也变长. 微电极表面的浓度峰值明显高于大电极表面的浓度峰值, 从而导致微电极表面与气泡表面之间的浓度梯度更加陡峭. 随着电流增大, 气泡的生长速率增大, 时间系数降低得越快. 电流为0.06 mA时的气泡直径与光电解水实验中电流为0.1 mA 的气泡直径能较好吻合. 这是因为生长的气泡对光的散射会导致气泡底部电流密度的降低.In order to enhance the efficiency of large-scale water decomposition, it is important to understand the oxygen bubble evolution on the electrode surface. In this work, a numerical model for the growth of oxygen bubbles on the electrode surface is proposed based on the dissolved oxygen flux at the bubble boundary, and the mechanisms of the reaction area and current during the bubble growth are investigated. The results show that the bubble diameters calculated from the oxygen flux at the bubble boundary are in good agreement with thediameters of the bubbles growing in the control phase of the chemical reaction. As the reaction region increases, the transition time from the diffusion-controlled stage to the chemical reaction-controlled stage becomes longer during the bubble growth. The concentration maximum value on the microelectrode surface is significantly higher than that on the large electrode surface, which leads to a steeper concentration gradient between the microelectrode surface and the bubble surface. As the current increases, the bubble growth rate increases and the time coefficient decreases faster. The bubble diameter at a current of 0.06 mA accords well with the bubble diameter at a current of 0.1 mA in the photoelectrochemical water splitting experiments. This is because the scattering of light by the growing bubbles leads to a decrease in the current density at the bottom of the bubble.
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Keywords:
- oxygen bubble evolution /
- time coefficient /
- numerical simulation /
- photoelectrochemical water splitting
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图 3 四组网格计算得到的出口处的流速的r方向分量
Fig. 3. The r-direction component of the flow velocity at the outlet calculated by the four sets of grids.
图 4 模拟得到的气泡生长直径和快照
Fig. 4. Bubble growth diameter and snapshot obtained through simulation.
图 5 不同反应区域时气泡的生长直径 (a)气泡的直径随时间的变化; (b) 30 s时的直径
Fig. 5. Growth diameter of the bubble at different reaction regions: (a) Diameter of the bubble as a function of time; (b) the diameter at 30 s.
图 6 不同反应区域下生长的气泡的时间系数
Fig. 6. Time coefficient of bubbles growing in different reaction regions.
图 7 不同反应区域的气泡边界的溶解氧通量
Fig. 7. Flux of dissolved oxygen at bubble boundaries in different reaction regions.
图 8 电极表面浓度随时间变化 (a)光斑直径1.8 mm; (b)光斑直径0.1 mm
Fig. 8. Electrode surface concentration varies with time: (a) Laser diameter of 1.8 mm; (b) laser diameter of 0.1 mm.
图 10 光电极表面析气反应区域的示意图 (a)小气泡; (b)大气泡
Fig. 10. Schematic diagram of the gas evolution reaction area on the photoelectrode surface: (a) Small bubble; (b) large bubble.
图 11 不同电流的气泡边界的溶解氧通量
Fig. 11. Flux of dissolved oxygen at bubble boundaries for different currents.
图 12 不同电流下气泡生长的时间系数 (a)气泡生长时的时间系数; (b)生长时间为0—3 s
Fig. 12. Time coefficients of bubble growth at different currents: (a) Time coefficient when the bubble grows; (b) growth time from 0 to 3 s.
表 1 沸腾气泡与光解气泡的异同
Table 1. Similarities and differences between boiling bubble and photolysis bubble.
沸腾气泡 光解气泡 生长过程 成核、生长和脱离 成核、生长和脱离 相间作用 异相成核 异相成核 驱动力 过饱和温度 过饱和浓度 气泡成分 水蒸气 氢气/氧气 能质传输 涉及热能与物质的传递 涉及热能、化学能、光能及物质的传递 生长规律 生长速率受热传导和蒸气压支配, 快速膨胀后脱离 生长受限于光强、反应速率及气体扩散, 可能持续缓慢生长 稳定性 上升时可能因冷却而凝结 化学性质稳定, 不易再溶解 气泡尺寸/m 10–3—10–2 10–9—10–4 生长时间/s 10–2—10–1 10–3—102 
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表 2 模拟参数
Table 2. Parameters used in simulation.
参数 值 表面张力σ/(mN·m–1) 70 电流I/mA 0.03—0.12 光斑半径rlaser/mm 0.05—0.9 转移电子数z 4 氧气摩尔质量MO2/(g·mol–1) 32 氧气扩散系数DO2/(m2·s–1) 2.1×10–9 气泡初始半径rb/μm 15 参考压力pref/kPa 101.325 浓度建立时间twait/s 1.2×10–5
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