Cu/C Hybrid Nanomaterials for Value-added Chemical Production via Electrochemical CO2 and CO Reduction

Abstract

Electrochemical CO2 reduction reaction (CO2RR) is receiving great attention for carbon neutrality, which can convert CO2 into value-added chemicals using renewable electricity. However, there still remains challenges for CO2RR product selectivity, production rate and energy efficiency for practical application. Therefore, we designed we designed Cu/C hybrid material-based heterogeneous electrocatalysts, and studied the effect of C supporters on enhancing product selectivity toward C2+ products and controlling CO2 mass transport for high production rate. First, we developed Cu-confined carbon nanofiber (cCu-CNF) harnessing difference of metal/C redox tendency and Boudouard reaction during calcination. In CO2RR, among the lots of production pathways, acetate is the only product that the intermediate reacts in electrolyte, not the catalysts surface. So, we expected controlling adsorption and desorption of *CH-CO can increase the acetate selectivity. To be advantageous for desorption of the *CH-CO intermediate, fabrication of catalysts proceeded for less-exposed Cu active site. Oxygen partial pressure(pO2)-controlled calcination in a closed system enabled selective C combustion – Cu reduction due to difference in temperature-dependence stability between Cu and C. As the amount of CO increased, the reverse reaction of Boudouard reaction, the thermodynamical equilibrium between C, CO and CO2, enabled inner-diffused Cu formation due to graphitic C shell formation outside of CNF. The electrochemical CORR performance proved the morphology-dependence production selectivity we expected. The thermodynamically fabricated cCu-CNF showed acetate-dominant performance while Cu-embbeded hollow carbon nanofiber (Cu/H-CNF), the high vacuum calcined, showed hydrogen evolution dominant reaction. Seconds, we utilized graphene quantum dots (GQDs) for nano-confinement effect, the proton circulation of vitamin C, which is co-catalyst of CO2RR on Cu surface. The main obstacles of CO2RR is not only selectivity but low current density is hindering the improvement of energy efficiency. Disrupting the high current density reaction is the low mass transfer of CO2 reactants, which previous studies have overcome through hydrophobic ionomer coating such as nafion or sustainion. However, such ionomers do not participate in reaction directly, but increase the local CO2 concentration or are involved in the movement of ions. Here, we modified ascorbic acid (AA, vitamin C), the ubiquitous material, on Cu nanowire (CuNW). We suggested that presence of AA promotes CO2 to CO2 – by donating proton and electron, which is rate determining step (RDS) for CO2RR, so that it can lower the energy barrier of reaction and maintain high local CO2 concentration even in high current density. However, the amount of AA on CuNW is limited, so a supporting material was needed to reversibly circulate the AA and oxidized AA. GQD, the nanoscale graphene material, enables redoxtransition of AA by nanoconfinment effect. The CO2RR performance of nanoconfined AA modified CuNW exhibited outstanding ethylene selectivity and partial current density which are almost reached industrial level. This thesis suggested effective methodology for improved CO(2)RR performances by utilizing carbon supports. Existing studies used carbon to increase stability by wrapping the catalysts, but our investigation proposed various potentials of carbon such as controlling catalytic morphology or redox reversibility of co- catalyst. Also, outstanding performances of our study suggested possibility of high-energy efficiency catalyst fabrication for the sustainable future.

전기화학적 CO(2) 환원반응(CO(2)RR)은 CO2 또는 CO 를 전기화학적으로 고부가가치의 연료 혹은 화합물로 변환할 수 있는 미래의 탄소 중립을 위해 필요한 유망한 기술이다. 그러나 선택성 제어, 전극 안정성 및 대량 반응물 전달의 문제와 같은 많은 장애물이 CO(2)RR 의 에너지 효율에 대한 과제로 여전히 남아 있다. 본 논문에서는 기존 연구와는 다른 방식으로 탄소 지지체를 활용하여 Cu/C 기반 이종 전기 촉매를 설계하였다. 먼저 금속/C 산화환원 경향의 차이와 하소 시 부두아 반응을 활용한 구리를 가둔 카본 나노 섬유(cCu-CNF)를 개발했다. 이는 구리 활성 부위의 노출을 제한함으로써 중간생성체의 반응 경로를 제어할 수 있다. cCu-CNF 를 통한 전기화학적 일산화탄소 환원 반응에서는 37.8% 페러데이 효율에 달하는 아세테이트 형성 성능을 보였다. 둘째, Cu 표면에서 CO2RR 의 조촉매인 비타민 C 의 양성자 순환을 유도할 수 있는 나노결합 효과(nanoconfinement effect)를 위해 그래핀 양자점(Grapehene quantum dots, GQDs)을 활용했다. 이러한 시너지 효과는 CO2RR 의 에너지 장벽을 낮추고 활성 부위에 다량의 반응물을 공급할 수 있게 했다. 나노결합 효과가 적용된 비타민 C 가 결합된 구리 나노와이어(AA confined by GQD-CuNW, cAA-CuNW)를 통한 이산화탄소환원 반응에서는 -1.55 V(vs RHE)에서 에틸렌 페러데이효율 60.7 %, 부분 전류밀도 539 mA/cm2 의 뛰어난 성능을 보였다. 본 논문에서는 촉매 안정성 증가를 위해 사용되었던 기존 탄소 활용과 달리, 촉매 자체의 형상 변화와 조촉매의 산화화원 가역성 제어를 통해 CO(2)RR 의 성능을 향상시킬 수 있는 효과적인 방법론을 제시한다.