Due to the severe decrease in the current efficiency for chlorine evolution reaction (CER) of DSAs in dilute chloride solutions, it was necessary to investigate the alternative electrode materials for improving the CER efficiency. Our strategy to enhance the CER efficiency was the adoption of catalysts that can suppress the competitive reaction of CER, namely oxygen evolution reaction (OER). Iron and cobalt were introduced as an OER suppressor because their relatively much higher overpotential of OER compared with that of CER was expected to enhance the CER selectivity. For reliable stability, iridium oxide was mixed with each electrode material.
First, we fabricated the iridium-iron mixed oxide electrode (0.3IrOy∙0.7FeOz) to enhance the current efficiency for CER in dilute Cl− solutions. The 0.3IrOy∙0.7FeOz
exhibited far better current efficiency for CER compared with IrO2 as a representative of DSA, not only in a dilute Cl− solution but also in a concentrated Cl− solution. The charge transfer resistance (Rct) of 0.3IrOy∙0.7FeOz for CER was as fast as that of IrO2, while Rct for OER was much higher than that of IrO2. It is attributed to the synergistic effect originating from amphoteric behavior, including FeOx-like (slow OER rate) and IrO2-like (fast CER rate) aspects. The 0.3IrOy∙0.7FeOz also showed reliable stability and safety regarding byproducts
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formation. Moreover, the feasibility of 0.3IrOy∙0.7FeOz for environmental application was successfully proved with arsenite oxidation, ammonium removal, and direct electrolysis of tap water.
Secondly, the iridium-cobalt mixed oxide electrode (ICO) was also fabricate d for improving CER current efficiency in dilute chloride solutions. The IC O showed higher CER activity and lower energy consumption than that of t he IrO2 not only in dilute aqueous solutions but also in concentrated soluti ons. It is attributed to its low CER overpotential with high OER overpotent ial and low CER charge transfer resistance. The ICO electrode exhibited re markable stability (> 200 h) in 0.5M H2SO4 solution. In addition, the ICO outperformed IrO2 for ammonium degradation in dilute aqueous solutions. C onsequently, the ICO could act as an optimal electrocatalytic anode in dilut e chloride solutions for its high CER current efficiency, low energy consu mption, low cost and high anodic stability.
The results suggest that 0.3IrOy∙0.7FeOz and ICO can be a good alternative to DSA and has the potential to expand the application of electrochlorination systems in dilute Cl− solutions.
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국문 초록
본 연구에서는 산소발생반응을 억제할 수 있는 촉매를 이용하여 저농도 Cl− 용액에서도 효율적으로 염소를 발생시킬 수 있는 전극을 개발하였다.
Dimensionally stable anode (DSA; IrO2, RuO2)는 염소발생성능과 안정성이 뛰어난 전극으로 알려져 있지만, DSA는 저농도 Cl−용액에서 염소발생효율이 급격하게 떨어지는 단점이 있다. 이는 DSA의 촉매적 특성에 기인한 것으로, 염소발생반응의 경쟁반응인 산소발생반응도 매우 빠르게 일어나기 때문이다.
이로 인해, 원수의 Cl−농도에 따라 염소발생시스템의 사용이 매우 제한되거나, 염소발생장치에 높은 Cl−농도를 지속적으로 유지시켜주기 위한 부가장치들이 필요하다. 따라서 저농도 Cl−용액에서도 효율적으로 염소를 발생시킬 수 있는 전극개발이 필요하다.
본 연구에서는 산화철과 산화코발트를 산소발생억제 촉매로 도입하여 염소 발생효율을 향상시키고자 하였다. 산화철은 주로 산소발생용 전극으로 사용되 는 물질로, 다양한 장점이 있지만 산소발생속도가 느리다는 단점이 있다. 산 화철의 단점이라고 여겨지던 느린 산소발생속도는 염소발생반응의 관점에서 보면 경쟁반응을 억제시켜 염소발생효율을 향상시킬 수 있는 장점이 될 수 있 다. 또한 산화철은 다른 전극물질에 비해 산소발생 과전압이 염소발생 과전압 보다 매우 높은 특성이 있어 염소발생 선택도를 향상시켜 줄 수 있다. 이와 비슷하게 산화코발트는 산화철보다 산소발생 과전압은 낮지만, 산소발생과전