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近年来,淡水危机日益严重,非常规水资源制备淡水是解决淡水危机的重要途径之一[1]。目前应用较为广泛的淡水制备工艺有电渗析(electrodialysis,ED)、反渗透(reverse osmosis,RO)和纳滤(nanofiltration,NF),但这些工艺普遍存在能耗高、操作复杂且具有二次污染的风险[2]。近年来,流动电极电容去离子(flow electrode capacitive deionization,FCDI)作为一种新型的脱盐技术,具有能耗低、操作简单、绿色无污染等优点而备受关注[3]。
FCDI技术是由电容去离子技术(capacitive deionization,CDI)衍变而来,在CDI装置的基础上引入离子交换膜和流动电极,其原理是通过在电极上施加电压,将进水中的盐离子吸附在电极微孔与溶液界面的双电层(electric double layer,EDL)上,从而达到脱盐的目的[4-5]。通过在两侧电极与中间腔室间加入离子交换膜能够有效的抑制CDI装置中的同离子效应,显著提高了其电荷效率,电荷效率可由60%左右提升至85%[6]。此外,流动电极相比固体电极操作模式更加灵活,能够通过调节电极循环方式实现电极在吸附的同时完成再生,克服了固体电极吸附容量低,无法连续运行的问题[7]。
FCDI技术中所用到的流动电极由电极材料、导电剂和流体介质组成,如何提高流动电极的导电性和电极材料对离子的吸附性能一直受到研究者们的广泛研究。其中,对电极材料进行掺杂改性是提高流动电极的电化学性能的有效手段。ALSULTAN等[8]和YAN等[9]分别以经过直接蓝(direct blue,DB)分子修饰和硝酸、对氨基苯磺酸磺化改性后的炭黑(carbon black CB)和碳纳米管(carbon nanotubes CNT)为电极材料,均有效解决了其由疏水性导致的团聚和沉降问题,显著提升了系统的平均脱盐速率,其中磺化CNT流动电极对比CNT流动电极平均脱盐速率提升了74.6%。但上述研究中对改性材料的需求量较高,往往需要较高的成本,且会对环境造成二次污染[10]。有研究表明,在流动电极中添加具有强导电性的碳材料作为导电剂,能够在电极材料之间建立电荷传输通道,提高流动电极的导电性和离子吸附性能。LIANG等[11]和CHO等[12]以活性炭(activated carbon, AC)作为电极材料并分别将CB和CNT作为导电剂投加到流动电极中,均有效地提高了流动电极的脱盐性能。但CB、CNTs的高疏水性会导致其难以在流动电极中均匀分布,不仅影响导电效果,还会显著增加流动电极的粘度,增加泵送能耗[13]。有研究表明,在碳材料表面引入含氧官能团能够改善其疏水性,增加比电容,进而提升了其对离子的吸附性能[14-15]。H2O2作为一种常用的氧化剂,有易分解、无过量氧化剂残留与二次污染风险等优点,这些优势为H2O2对导电剂氧化改性的应用提供了可能。因此,探究工艺简单、绿色无污染的导电剂氧化技术对于推进FCDI在脱盐领域的发展势在必行。
本论文利用H2O2氧化CB,对氧化后的CB进行了物理和电化学特性表征,并考察了其作为导电剂添加到流动电极中,FCDI装置的脱盐性能,进一步探讨了在FCDI体系中提升流动电极脱盐性能方面的机制,并考察了CB氧化程度、运行电压、投加量对FCDI系统脱盐性能的影响,以期为后期工程化应用提供参考。
改性炭黑导电剂强化流动电容去离子脱盐性能的机制
Enhancement mechanism of desalination performance of the flow electrode capacitive deionization by modified carbon black conductive agent
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摘要: 流动电极电容去离子是一种可连续运行、低能耗的新型电化学脱盐技术。在流动电极中添加导电剂是提升其脱盐性能的常用手段,但常用的导电剂如炭黑 (carbon black CB)、碳纳米管 (carbon nanotubes CNT)等的强疏水性会影响流动电极的电子传递效率和流动性,因此,提高导电剂亲水性并充分利用其吸附能力是提高流动电极脱盐性能的关键。本研究利用H2O2对炭黑进行氧化改性,通过调节氧化时间,制备了系列具有不同亲水性、表面含氧官能团的炭黑改性材料(CBO),并对其进行了详细的物理和电化学特性表征。在此基础上进一步探究了系列炭黑材料作为导电剂对装置脱盐效果的影响规律,同时优化了系统运行参数。结果表明,由H2O2氧化12 h所制备的炭黑材料作为导电剂时,装置具有较好的脱盐性能,在CBO-12投加量为0.25%、施加电压为1.2 V时,装置的平均除盐速率和电荷效率分别为0.464 μmol∙(cm2∙min)−1和83.1%。进一步表征发现,优异的脱盐性能归因于适当的氧化改性,可有效改善CB的疏水性,并在其表面引入大量含氧官能团,在改善流动电极电荷传递能力的同时,还能充分利用改性CB的吸附性能。Abstract: Flow electrode capacitive deionization is a new electrochemical desalination technology which can run continuously and has low energy consumption. Adding a conductive agent to the flow electrode is an effective method to improve the desalination performance of the flow electrode. However, the strong hydrophobicity of commonly used conductive agents (such as carbon black CB, carbon nanotubes CNT) will affect the electron transfer efficiency and the fluidity of the flow electrode. Therefore, how to increase the hydrophilicity of the conductive agent and make full use of its adsorption capacity is the key to improve the desalination performance of the flow electrode. In this study, a series of carbon black modified materials (CBO) with different hydrophilic properties and surface oxygen-containing functional groups were prepared by adjusting the oxidation time with H2O2 oxidation, and the detailed physical and electrochemical properties were characterized. On this basis, the effect of series of carbon black materials as conductive agent on the desalination effect of the device was further explored, and the operating parameters of the system were optimized. The experimental results showed that the device had a good performance when the carbon black material prepared by H2O2 oxidation for 12 hours was used as the conductive agent. When the dosage of CBO-12 was 0.25% and the applied voltage was 1.2 V, the average desalination rate and charge efficiency of the device could reach 0.464 μmol∙(cm2∙min)−1 and 83.1%, respectively. Further characterization showed that the excellent desalination performance was due to proper oxidation modification, which effectively changed the hydrophobicity of CB, and introduced a large number of oxygen-containing functional groups on its surface, which not only improved the charge transfer ability of the flow electrode, but also made full use of the adsorption performance of modified CB.
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