NaCl 용액에서 티타늄 결합 활성탄소천 전극을 이용한 축전식 이온제거

류민웅 전남대학교 대학원 2002 국내박사

여러 탄소재료의 물리화학적 성질과 전기장 하에서 흡착성질을 조사하여 CDI 전극재료로 선정한 활성탄소천(ACC) 표면에 물리흡착을 억제하고 전기장흡착을 증대시키기 위해 ACC에 티타늄, 지르코늄, 실리콘, 알루미늄 원소를 결합시켰다. 티타늄이 결합되면 ACC의 극성 관능기가 줄어들고 티타늄 원자가 전기장 흡착점으로 작용하여 NaCl 용액에서 CDI 성능이 크게 향상되었다. 티타늄이 결합된 ACC에서 이온의 흡착등온선을 구하여 티타늄 결합에 따른 이온제거 성능의 향상 원인을 고찰하고 다중 CDI 셀에서 이를 검증하였다. 직조된 페놀계 활성탄소천은 전기전도도가 높고, 표면적이 넓으며, 극성 관능기가 적어, 입상 활성탄소, 활성탄소섬유, 에어로겔탄소에 비해 CDI 전극으로서 성능이 우수하였다. 특히 이온의 물리흡착량이 적어 재생 효율이 높으며, 전기장하에서 산화-환원 반응이 일어나지 않아 CDI 전극으로 적절하였다. ACC의 극성 관능기와 금속 알콕사이드의 반응으로 금속을 ACC의 표면에 결합시킬 수 있다. 금속은 산소를 다리로 산화물 상태로 고르게 결합되므로, 금속의 결합량이 많아도 ACC의 미세 세공은 그대로 유지되었다. 금속은 극성 관능기와 결합되어 표면의 물리흡착점을 줄어준다. ACC 표면에 결합된 티타늄 원자는 전기장에 따라 산화 상태가 가변적이어서, 전기장흡착량이 많아져 CDI 성능이 크게 향상되었다. 이에 비해 실리콘과 알루미늄이 결합된 ACC에서는 이들 금속의 산화 상태가 쉽게 변하지 않아 CDI 성능이 향상되지 않았다. 티타늄이 결합된 ACC에 대한 이온의 흡착은 단분자층 흡착을 가정한 Langmuir 흡착등온선으로 잘 모사할 수 있었다. 흡착현상의 정량적 해석으로부터 티타늄의 결합으로 전기장흡착점이 많아지고 흡착 세기가 커져서 전기장 하에서 이온의 흡착량이 많아진다고 제안하였다. CDI 다중셀에서도 유통식과 순환식으로 수행한 NaCl의 제거 실험에서도 티타늄 결합으로 인한 ACC의 CDI 성능향상을 검증할 수 있었다. Deionization of NaCl solution using capacitive deionization (CDI) cell composed of activated carbon cloth (ACC) electrodes was discussed with relating to their surface and electrochemical properties. ACC - with a large surface area, small number of functional groups and high electric conductivity - was selected as a CDI electrode material from various carbon materials of activated carbon power, activated carbon fibers and carbon aerogel. Titanium, zirconium, silicon and aluminium were incorporated on ACC suppress physical adsorption of ions and enhance the electric field adsorption. Among them titanium-incorporated ACC showed significantly enhanced CDI performance, resulting in introducing of oxidizable and reducible titanium atom under electric potential difference. The adsorption of various electrolytes on titanium-incorporated ACC was examined its enhanced CDI performance was confirmed using a multi CDI cell. Metal could be incorporated on ACC by the reaction of metal alkoxides react with surface polar groups of the ACC. Metal atoms were bonded with the ACC surface through oxygen atoms as oxide and dispersed highly without forming any large particles. Micropores of ACC were not blocked by metal atoms, retaining high surface area even though a considerable amount of metal incorporation. Physical adsorptions of ions were decreased by metal incorporation because of diminishing surface polar groups, but electric field adsorption of ions were increased considerably by titanium incorporation. Easily changable oxidation states of titanium atoms a significant improvement of CDI performance. No remarkable enhancement of CDI performance was observed on silicon- and aluminium-incorporated ACC, because the applied electric field was too small to charge the oxidation states of these metals. Ion adsorption on titanium-incorporated ACC can be nicely simulated using Langmuir isotherm derived under the assumption of monolayer adsorption. Quantitative analysis of ion adsorption revealed that titanium-incorporation on ACC induced significant increases in the number of electric field adsorption sites and their adsorption strength, resulting in the considerable enhancement of electric field adsorption of ions. This enhancement of CDI performance of ACC electrode by titanium-incorporation was confirmed at the ion removal tests of NaCl solution with flowing and circulating modes in CDI cell composed of multi electrodes.

활성탄의 후처리에 의하여 제조된 전기이중층 커패시터용 전극재의 전기 화학적 특성

WU JINGYU 명지대학교 일반대학원 2008 국내석사

Electrochemical Performance of Activated Carbon Electrode Materials with Post Treatment for EDLC Jingyu Wu Dept.of Chemical Engineering Graduate School Myongi University Two types of commercial activated carbons of coconut shell based and coal char based were applied for EDLC electrode by removing impurities with chemical treatments, and controlling pore size distribution and content of surface functional group with heat treatment. The electrochemical properties were evaluated with coin cell using these activated carbons as electrode. The porosity and surface area of the electrode materials were analyzed by nitrogen adsorption at 77K, and the surface functional groups were studied by Boehm and TPD method. The effects of the pore structure and surface functional groups on the electrochemical performance of the activated carbon electrodes were investigated. The initial gravimetric capacitance of the commercial activated carbon electrodes with coconut shell based and coal char based were 66 and 70 F/g respectively, and after 100 cycles the values were decreased to 54 and 66 F/g respectively showing 82% and 94% of charge-discharge efficiency. Their initial volumetric capacitances were 39 and 36 F/cc, and after 100 cycles, the values were turned out to be 32 and 34 F/cc. The properties of CV graph with the commercial activated carbon electrodes showed the serious polarization as the result of additional reaction between electrolyte and impurities of the electrode materials. So, it was concluded that the commercial activated carbons were not suitable as the EDLC electrode materials. In order to remove the impurities, the commercial activated carbons were chemical treated with various acids. The initial gravimetric capacitances of coconut shell and coal char based activated carbon electrodes which were acid treated by aqua regia were 75 and 79 F/g, and after 100 cycles the values were turned out to be 63 F/g and 71 F/g, showing the highest capacitance among the various acid treatments. The commercial activated carbons were heat treated after nitric acid treatment to control the surface functional groups and were used as electrode to evaluate the electrochemical properties. The initial gravimetric and volumetric capacitances of coconut shell based activated carbon electrodes which were acid treated by HNO3 and then heat treated at 800℃ were 90 and 42 F/cc respectively with 94% of charge-discharge efficiency, showing much improved capacitance. The commercial activated carbons were alkali treated to remove the impurities, which were not soluble in acid solution. The commercial activated carbon treated by NaOH showed a little bit improved gravimetric capacitance but the volumetric capacitance was similar to the raw materials of commercial activated carbon electrode. The removal of impurities from the electrode materials was more efficient with acid treatment than with alkali treatment, showing higher capacitance. In order to remove impurities more efficiently the commercial activated carbons were alkali and acid treatment, and then were heat treated to control the surface functional groups. The surface functional groups decreased with the increased heat temperature and the specific capacitance increased with the decreased surface functional groups. The initial volumetric capacitance of coconut shell based activated carbon electrode which was alkali and nitric acid treated, and then heat treated at 800℃ was 44 F/cc. After 100th cycles the value turned out to be 42 F/cc, showing 95% of charge-discharge efficiency. Such a good electrochemical performance can be possibly applied to the medium capacitance of EDLC. 국문요약 Coconut shell 및 coal char 계 상용활성탄을 전기이중층 커패시터의 전극재로 적용하기 위하여 화학적 처리에 의하여 전극재의 불순물 성분을 제거하였으며 열처리에 의하여 표면 관능기를 제어하고자 하였다. 제조된 활성탄을 전극재로 사용하여 coin cell형태의 전기이중층 커패시터를 제작하여 전기화학적 특성을 평가하였다. 질소등온흡착에 의하여 비표면적과 세공특성을 분석하였고 Boehm 법 및 TPD 방법에 의하여 전극재의 표면의 산성관능기의 종류와 양을 측정하여 전극재의 물리화학적 특성과의 상관관계를 조사하였다. Coconut shell 및 coal char 계 활성탄을 별도의 처리 없이 전기이중층 커패시터의 전극재로 사용하였을 때 초기무게용량은 각각 66과 70 F/g, 100사이클 후의 용량은 54와 66 F/g으로 각각 82%와 94%의 효율을 나타내었고, 초기부피용량은 각각 39와 36 F/cc, 100사이클 이후 32와 34 F/cc로 감소되었다. CV 특성도 사이클 진행에 따라 불순물과 전해질의 부반응에 의한 분극현상이 크게 발생하여 미처리한 상용활성탄은 전극재로서의 사용에 적합하지 않다는 것을 알 수 있었다. 원료의 불순물을 제거하여 전해질과의 전기화학적 부반응을 감소시키기 위하여 상용활성탄을 여러 가지 산에 의하여 화학적으로 처리하였다. 산 처리를 한 활성탄의 무게용량은 원료에 비하여 높은 용량을 나타내어 왕수 처리를 한 coconut shell 및 coal char계 활성탄을 전극 물질로 사용했을 때 초기무게용량은 각각 75와 79 F/g을 나타냈으며 100사이클 후에 63과 71 F/g을 보임으로서 가장 높은 용량을 나타내었다. 열처리에 의하여 산 처리된 활성탄의 관능기를 제어하고 이를 전극재로 사용하여 전기화학적 특성을 평가하였다. 질산 처리된 coconut shell 및 coal char계 활성탄은 열처리 이후 용량이 모두 증가되었으며, 사이클 특성도 우수하였다. 질산 처리 된 coconut shell 계 활성탄을 800℃에서 열처리하여 전극재로 사용한 경우 초기 무게용량은 90 F/g, 100사이클 충•방전 반복 후 무게용량은 85 F/g으로 가장 높은 용량을 나타냈으며, 94%의 충•방전 효율을 나타내었다. 화학적 처리 및 열처리에 의하여 용량에 기여 가능한 크기의 micropore가 증가함에 따라 무게용량과 부피용량이 모두 증가하였으며 mesopore 와 macropore가 증가함에 따라 무게용량과 부피용량이 모두 감소하였다. 활성탄을 알칼리에 용해 가능한 불순물을 제거하기 위하여 여러 가지 알칼리 용액으로 처리하여 전극재로서의 특성을 평가한 결과 수산화나트륨용액으로 처리한 활성탄이 상대적으로 높은 용량을 나타내었으며 원료에 비하여 무게용량은 다소 증가되었으나 부피용량은 원료와 비슷하였다. 알칼리 처리에 의한 전극재의 불순물 제거효율은 산 처리에 의하여 제조된 전극재에 비하여 상대적으로 낮으므로 산 처리에 의하여 제조된 전극재의 용량의 향상효과가 더 높은 것을 알 수 있었다. 불순물을 보다 효과적으로 제거하기 위하여 활성탄을 알칼리와 산으로 차례로 처리하였으며 관능기 제어를 위하여 열처리를 하여 전극물질로 사용하였다. Boehm 법과 TPD 방법을 이용하여 분석한 결과 열처리에 따라 관능기의 양이 감소하는 것을 확인할 수 있었다. 이에 따라 무게용량과 부피용량이 모두 원료에 비하여 증가되었으며 사이클 특성도 우수하여 coconut shell 계 활성탄을 알칼리 및 질산 처리 후 800℃에서 열처리한 전극재의 경우 초기 부피용량 44 F/cc, 100사이클 후 42 F/cc로서 실용화 가능한 수준의 높은 부피용량 및 95% 이상의 높은 충•방전효율을 나타내었다.

유기전해질 전기이중층캐패시터에서 마이크로파로 표면개질한 활성탄소 전극의 전기화학적 특성

선진규 檀國大學校 大學院 2001 국내석사

마이크로파 처리를 통하여 활성탄소 표면 개질에 따른 유기전해질 전기이중층캐패시터의 전기화학적 특성에 미치는 영향을 확인하기 위하여 제조된 petroleum cokes 활성탄소와 pitch cokes 활성탄소 그리고 시판되는 BP25 활성탄소에 시간을 변화하여 각각 1.5분, 5분, 15분 동안 마이크로파 처리하였으며, 표면 개질된 활성탄소를 전극물질로 사용하고 도전제 KJ-black과 바인더로 PTFE를 혼합하여 제조된 전극과 유기전해질로 구성한 전기이중층캐패시터의 Impedance특성, 충방전특성, CV특성등 전기화학적 특성을 조사하였다. 마이크로파는 최대출력 700W, 주파수 2450 MHz의 microwave oven으로 활성탄소를 처리하였으며, 전기이중층캐패시터의 유기전해질로는 IM LiCIO_(4)/PC + DME(1:1) 전해질과 1M LiPF_(6)/EC+EMC+DMC(1:1:1) 전해질을 사용하였다. 1. 마이크로파로 표면개질된 활성탄소의 물리적 특성 Petroleum cokes를 출발물질로 하여 KOH를 사용한 약품활성법으로 제조된 petroleum cokes 활성탄소, Pitch cokes를 출발물질로 하여 KOH를 사용한 약품활성법으로 제조된 pitch cokes 활성탄소 그리고 시판되는 BP25 활성탄소를 시간을 늘리면서 마이크로파로 처리하였을 때 시간에 비례하여 비표면적과 세공부피는 감소하고, 평균세공직경은 증가하였다.활성탄소 표면의 관능기를 확인하기 위하여 FT-IR 시험을 하였으며, 그 결과 활성탄소 표면에 산소결합을 나타내는 peak들이 마이크로파로 처리하면서 감소하는 경향이 나타났다. 2. 마이크로파로 표면개질된 활성탄소의 전기화학적 특성 각각의 활성탄소와 마이크로파로 표면 개질한 활성탄소들로 제조된 전극을 사용한 유기전해질 전기이중층캐패시터의 전기화학적 특성은 1MLiO_(4)/PC+DME(1:1) 전해질을 사용한 경우와 1 M LiPF6/EC+EMC+DMC(1:1:1) 전해질의 경우 모두에서 표면개질된 활성탄소로 제조한 전기이중층캐패시터의 계면저항이 개질하지 않은 활성탄소에 비해 크게 감소하였으며, 방전용량은 증가하였다. 이때의 방전용량은 petroleum cokes 활성탄소에서 개질하지 않은 경우가 96.4 F/g 이였고, 15분 동안 마이크로파로 처리한 경우가 131.8 F/g 이였다. Pitch cokes 활성탄소에서는 개질하지 않은 경우가 77.0 F/g 이였고, 15분 동안 마이크로파로 처리한 경우가 96.0 F/g 이였다. BP25 활성탄소의 방전용량은 개질하지 않은 경우가 78.80 F/g 이였고, 5분 동안 마이크로파 처리한 경우에서 105.3 F/g을 나타내었다. BP25 활성탄소에서는 15분 동안 마이크로파로 처리하였을 경우 방전용량의 감소가 나타났는데, 이것은 마이크로파 처리에 의한 비표면적의 감소가 큰 영향을 준 것으로 나타났다. 또한 CV시험에서도 활성탄소를 표면 개질하였을 때가 더 좋은 방전특성이 나타났다. We modified surface of activated carbons with microwave. The raw materials of activated carbon were petroleum cokes and pitch cokes, and they were prepared by chemical activation with KOH in 800℃. Also they were modified at the condition of microwave power of 700W and frequency of 2450MHz. So we investigated the characteristics of modified activated carbon according to a various microwave radiation time in above the conditions. We investigated physical characteristics of modified activated carbon with FT-IR, BET surface analysis. And we investigated the electrochemical characteristics of electric double layer capacitor electrode prepared from modified activated carbons using organic electrolyte. The organic electrolytes using in this study were 1 M LiClO_(4)/PC+ DME(1:l) and 1 M LiPF_(6)/ EC+EMC+DMC(l:l:l). Also we compared BP25 activated carbon for sale with the prepared activated carbons. 1. Physical characteristics of modified activated carbon Specific surface area and pore volume were decreased according to increasement of the microwave radiation time for petroleum cokes activated carbon, pitch cokes activated carbon, and BP25 activated carbon. But average pore diameter was increased. For all activated carbons, the FT-IR spectrum showed that the peaks associated with oxygen contained functional groups were decreased as the microwave radiation time increased. 2. Electrochemical characteristics of modified activated carbon For the using of 1 M LiClO_(4)/PC+ DME and 1 M LiPF_(6)/ EC+EMC+DMC electrolyte, interface resistance of modified activated carbons was largely reduced than it of non-modified activated carbon, also discharge capacitance was increased in proportion to microwave radiation time. Discharge capacitance of the respective activated carbons follows, 1. For petroleum cokes activated carbon, discharge capacitance of non- modified activated carbon was 96.4 F/g, and it of activated carbon radiated with microwave for 15 minutes was 131.8 F/g. 2. For pitch cokes activated carbon, discharge capacitance of non-modified activated carbon was 77.0 F/g, and it of activated carbon radiated with microwave for 15 minutes was 96.0 F/g. 3. For BP25 activated carbon, discharge capacitance of non-modified activated carbon was 78.8 F/g, and it of activated carbon radiated with microwave for 5 minutes was 105.3 F/g. For BP25 activated carbon radiated with microwave for 15 minutes, discharge capacitance was decreased, because specific surface area largely are reduced. From the result of CV test, discharge characteristics of modified activated carbons was better than its of non-modified activated carbon.

活性炭素纖維 電極에 의한 중금속 이온의 電氣的 吸·脫着 特性

이상문 충남대학교 대학원 2000 국내박사

The study on the electrosorption of cobalt ions oil an activated carbon fiber(ACF) felt electrode was performed to remove the cobalt ions from the radioactive liquid wastes resulting from chemical or electrochemical decontamination and to concentrate the cobalt ions adsorbed in the fresh solution. Cyclic voltammetry was investigated on a rotating-disk electrode(RDE) the determine the region of potentials within which only double-layer charging should occur. The application of an electric potental to the ACF felt effects its adsorption capacity for cobalt ions in an aqueous solution. The adsorption capacity decreases in anodic polarization while the amount adsorbed apparently increases cathrolic polarization. The ACF felt electrode regenerated by electrodesorption resulting from reversing the current and potential shows virgin capacity. Therefore the electrosorption on the ACF felt electrode can be used to remove the cobalt ions. The high concentration of electrolytes increased the adsorbed and deserted amount of cobalt ions because the high concentration of electrolyte increases the conductivity of the solution and lowers the ohmic voltage drop. The adsorption percentage of cobalt ions removed from the solution was 100%, and desorption percentage from the electrode was 95% in a 0.1N NaCl electrolyte solution. However, in a 0.01N NaCl electrolyte solution the adsorption percentage was 75% and the desorption percentage was only 57% at maximum. The amount of cobalt ions desorbed was increased by using the fresh solution, because the electrolyte concentration was higher than that of the non-fresh solution, A current of -5mA/+5mA is more effective than -0.5mA/+0.5mA in adsorption and desorption, and a potential of +0.7V is more effective than a potential of +0.5V in desorption. Low intial pH increase desorption percentage, because excess H^+ ions were easily exchanged with adsorbed Co^2+ ions on the ACF felt electrode by producing a sufficient concentration of H^+ ions. Thus, the condition of lower pH was very effective in dissolving cobalt ions from the ACF felt electrode. In singular systems, Co(II) could be easily removed at all negative potentials, but adsorption of Cs(I) from the solution exhibited an adsorption percentage under 15% within the applied potential range, which adsorption of Sr(II) exhibited a significant potential dependence. In binary systems, a Cs-Co system could be easily separated at all applied negative potentiasl, while a Co-Sr system could be separated at less negative potential. A Sr-Cs system could be separated at more negative potential. In ternary system, a Co-Sr-Cs system could only be separated at 0V, in which removal differences of individual components were very high. The electrosorption process using an ACF felt electrode was confirmed to be effective in the removal of Co(II), and an ACF felt electrode could be continuously used in the purification of liquid waste. The selective separation of Co(II), Sr(II), Cs(II) in binary, ternary systems could be obtained. The electrosorption process as a new technology without producing secondary wastes, appears to be a promising separation technique in liquid waste treatment.

Preparation of pitch-based CF/ACF by electrospinning and its electrode performance for EDLC

박상희 全南大學校 2003 국내박사

전기 이중층 축전기용 활성탄소 전극의 특성평가에 관한 연구

박성철 아주대학교 대학원 2000 국내석사

활성탄소/폴리아닐린/루테늄 산화물로 구성된 복합전극의 초고용량 캐폐시터 특성 연구

송락영 한밭大學校 産業大學院 2006 국내석사

초고용량 캐폐시터의 대표적인 전극 활물질인 활성탄소(MSC-30), 전기전도성고분자인 폴리아닐린(polyaniline, PA), 그리고 금속산화물인 RuO_(2)의 각각의 단점을 개선하고 장점을 향상시키기 위하여 단일전극 및 복합전극을 제조하여 cyclic voltammetry와 impedance spectroscopy에 의해 capacitance 특성을 연구하였다. ACP/PA 복합전극은 화학적 합성 방법에 의하여 활성탄소 분말 표면위에 아닐린 단량체를 흡착시켜서 제조 하였다. Capacitance 특성은 IMH_(2)SO_(4)의 수용액에서 cyclic voltammetry와 impedance spectroscopy에 의해서 조사 되었다. Cyclic voltammetry에 의하여 측정된 결과는 ACP/PA의 전극의 비 용량값이 ACP의 193F/g 보다 큰 273F/g으로 증가하였다. 그러나 impedance spectroscopy의 결과는 ACP전극이 ACP/PA전극보다 더 좋은 이온 응답시간과 높은 frequency capacitance 특성을 보여준다. 이 결과는 PA의 redox 반응의 속도 율속단계 이어서 1Hz이상의 높은 주파수 영역에서 전하저장에 기여할 수 없다는 것을 의미한다. Carbon/Nafion/RuO_(2)복합전극은 cyclic voltammetry 방법으로 제조 되었다. Carbon/Nafion/RuO_(2)복합전극의 비용량은 100 mv/s에서 513 F/g이고 루데늄 산화물 전극의 비용량은 418 F/g을 보였다. 1000 mV/s의 주사속도에서 복합전극은 456 F/g 루데늄 산화물의 단일 전극은 207 F/g 이었다. 복합전극의 주사속도에 따른 비 용량 값은 Carbon/Nafion/RuO_(2)>RuO_(2)의 순으로 관찰 되었다. PA/Nafion/RuO_(2)의 복합전극은 초고용량 캐페시터의 응용을 위하여 제조되었다. 나피온의 설포네이트 음이온은 반복적인 redox 반응을 위하여 redox site를 보호하는 복합체를 형성하는 것으로 폴리아닐린의 라디칼 cation을 안정화 시킬 수 있다. 복합전극의 수명은 PA/Nafion/RuO_(2)>PA/Nafion>PA의 순으로 관찰된다. 주사속도에 따른 비용량 값은 PA/Nafion/RuO_(2)>PA/nafion>PA의 순으로 관찰된다. ACP/PA hybrid electrodes for surpercapacitors were fabricated by adsorption of aniline monomers on the surface of activated carbon powders(ACP) and their polymerization by using a chemical method. Their capacitance performances were investigated in the 1 M H_(2)SO_(4) aqueous electrolyte solution by means of cyclic voltammetry and impedance spectroscopy. The cyclic voltammetric results showed that the ACP/PA electrodes gave an increased specific capacitance value of 273 F/g, which was larger than the capacitance of the ACP, approximately 193 F/g. However, impedance spectroscopic data showed better response time and higher frequency capacitance performance in ACP than ACP/PAn electrodes. This result means that the redox reaction of PA is rate-determined step and can not contribute for charge storage at high frequency range over 1 Hz. Carbon/Nafion/RuO_(2) composite electrode were prepared by cyclic voltammetry method. The carbon/Nafion/RuO_(2) electrodes showed a specific capacitance value of 513 F/g in the 1 M H_(2)SO_(4) electrolyte in the scan rate 100 mV/s, which value was larger than the capacitances of 418 F/g for RuO_(2) electrode. The carbon/Nafion/ RuO_(2) electrodes showed a specific capacitance value of 456 F/g in the 1 M H_(2)SO_(4) electrolyte in the scan rate 2000 mV/s, which value was larger than the capacitances of 207 F/g for RuO_(2) electrode. specific capacitance values according to scan rates were observed in the order of carbon/Nafion/RuO_(2)>RuO_(2) PA/Nafion/RuO_(2) composite electrode were prepared for supercapacitor application. sulfonate anions in Nafion can stablize radical cations of polyaniline by forming complex to protect the redox sites for repetitive redox process. RuO_(2) improved capacity performance in the composite electrode. cycle life of the composite electrode was observed in the order of PA/Nafion/RuO_(2)>PA/Nafion>PA. specific capacitance values according to scan rates were observed in the order of PA/Nafion/RuO_(2)>PA/Nafion>PA

Electrocatalytic CO2 reduction reaction on copper : designs toward product selectivity optimization

김주연 Graduate School, Yonsei University 2023 국내박사

Encountering an environmental crisis from an atmospheric CO2 surge, the research demand on the electrochemical carbon dioxide reduction reaction (CO2RR) has increased. Foremost, research on carbon dioxide reduction reaction (CO2RR) at Cu has been considered especially promising for its capability of CO2 conversion into valuable chemical fuels without emitting pollutants. However, two hurdles are still unsolved in the selective production of desired chemical fuels, even for a promising copper-based carbon dioxide reduction reaction (CO2RR). One is a competitive hydrogen evolution reaction (HER) that proceeds at a potential range-reducing carbon dioxide, and another is a unveiled reaction mechanism stemming from byproducts accompanying complex reaction steps. To overcome these hurdles and improve the selectivity of desired products, I’ve developed three research projects to control electrode surface-adsorbed carbon monoxide (COads) and electrode surface-adsorbed hydrogen (Hads). Firstly, I found that electrodeposited-Cu10Ag14Hg0.6 multimetallic films displayed an improved selectivity of C2 chemicals production in carbon dioxide reduction reaction (CO2RR), with highly suppressed hydrogen evolution reaction (HER). To elucidate the improved reaction mechanism at electrodeposited-Cu10Ag14Hg0.6 multimetallic films, surface-interrogation scanning electrochemical microscope (SI-SECM) and in-situ IR spectroscopy was used. These surface analysis methods revealed that electrode surface-adsorbed hydrogen (Hads) was suppressed and electrode surface-adsorbed carbon monoxide (COads) was increased at the developed catalyst. These results supported that Ag and Hg metal incorporation led to C2 selectivity enhancement by surface dynamics changes. Secondly, I designed a Cu-Ag Bipotentiostatic reaction system and measured CO efficiency in CO2-CO-C2 production made between two working electrodes (Cu and Ag). Through the unique capability of this system, I applied CO efficiency calculation that had been limited in the precedent literature. By applying the potential insufficient to produce C2 from CO2 reduction on Cu, I tried to minimize interference of C2 from Cu sole operation (CO2 to C2). Through the optimization process of various reaction factors (distance between Cu and Ag, applied potential at Ag (EAg), applied potential at Cu (ECu), and cation composition in the electrolyte), the selectivity of C2+ could be vastly improved (67% including 6.5% 1-PrOH). For last, I developed the reactor to modulate electrode surface temperature and investigated the reactivity of carbon dioxide reduction reaction (CO2RR). The COMSOL-based simulation revealed that electrode surface temperature could be depressed up to -4.0℃ when the thermoelectric module was set at -40℃ temperature. Combined with the potential-pulse program, surface chilling at subzero (-40℃ thermoelectric module temperature and -4.0℃ copper foil) improved the reactivity of CH4 at large amounts (80%). Compared to the bath cooling reactor (5℃), the selectivity of CH4 in surface chilling was 9% higher even without additional overpotential consumption (cathodic potential at surface chilling and bath cooling was -1.2 V and -1.5 V vs. RHE). By in-situ spectroscopic analysis and COMSOL-based simulation result, this reactivity improvement is presumably due to increased residence time of electrode surface-adsorbed carbon monoxide (COads) and increased CO2 concentration at electrode peripheral. Toward the dream of the carbon-neutral strategy to complete, many problems were still left unsolved in the carbon dioxide reduction reaction (CO2RR). However, I believe these pieces of novel research keep going will bring the dream come to reality finally.