Shape-Controlled Pd Nanocrystal-Polyaniline Heteronanostructures with Modulated Polyaniline Thickness for Efficient Electrochemical Ethanol Oxidation

Abstract

For fossil fuels currently used as a major energy source, it is expected to be replicated in 100 years due to limited reserves. In addition, the combustion of fossil fuels causes many environmental problems. Therefore, the development of energy sources that can replace fossil fuels is essential, and many studies are underway for environmentally friendly and renewable energy sources. Fuel cell is one of the promising future energy-supplying technologies; in particular, the polymer electrolyte membrane fuel cell (PEMFC) is of special interest because of its high energy density, low operation temperature, and low environmental impact. For commercialization of PEMFC, securement of high electrocatalytic activities accompanying high-stability with various fuel conditions is required. Platinum (Pt) is the most typical catalyst for PEMFC. However high cost, low stability, and poisonous intermediates such as CO are the challenges to be overcome. Accordingly, Palladium (Pd) has been investigated as an alternative because of relatively inexpensive cost compared to Pt, and similar properties via negligible mismatch of atomic structure.

There are various methods to increase the activity of metal catalysts such as shape, size and controlled compositional structure. Moreover, metal-polymer heterostructure inhances not only electrocatalytic property but also durability.

Constructing metal nanocrystal (NC)-polymer heteronanostructures (HNSs) with excellent properties in terms of ethanol adsorption and activation will lead to catalysts with significantly enhanced electrochemical ethanol oxidation reaction (EOR).

Herein, we present a facile and effective synthesis method for shape-controlled Pd NC-polyaniline (PANI) HNSs with various PANI thicknesses, which are achieved by using cubic and octahedral Pd NCs as metal NC seeds. By investigating the electrocatalytic properties of various Pd NC-PANI HNSs and their counterparts, we found that electrocatalytic property of the Pd NC-PANI HNSs highly depends on both the exposed facets of Pd NCs and thickness of the PANI coating. The cubic Pd NC-PANI (Pdcube-PANI) HNSs with cubic Pd NCs and 0.7 nm PANI coating significantly improved electrocatalytic performance for the EOR than the Pdcube-PANI HCNs with different PANI coating thicknesses, octahedral Pd NC-PANI (Pdocta-PANI) HNSs with octahedral Pd NCs, cubic Pd NCs, and commercial Pd/C because of synergistic advantages of the favorable exposed facet and optimal PANI coating thickness. |본 연구에서는 팔라듐을 나노크기의 육면체와 팔면체로 각각 합성을 하고 표면에 전도성 고분자인 폴리아닐린을 얇게 둘러 팔라듐-폴리아닐린 복합나노구조체를 합성함으로써 에탄올 산화 효율 및 촉매 반응 동안 안정성을 높였습니다.

주사 전자 현미경과 투과 전자 현미경을 통해 팔라듐-폴리아닐린 복합나노구조체의 모양과 아닐린의 농도에 따른 폴리아닐린의 두께 변화를 관찰 하였으며, HAADF-STEM-EDS을 통해 팔라듐 육면체 및 팔면체 표면에 골고루 형성된 폴리아닐린을 확인하였으며. 적외선 분광법을 통해 얇게 둘러싼 물질이 폴리아닐린임을 확인하였습니다. X선 회절 분석법을 통하여 촉매의 fcc 구조를 확인하였으며, X선 광전자 분석법을 통해 팔라듐과 팔라듐-폴리아닐린 복합나노구조체의 전자 거동 차이에 대해 확인하였습니다.

팔라듐-폴리아닐린 복합나노구조체의 대조군으로는 팔라듐 육면체, 팔라듐 팔면체, 상업용으로 판매되는 탄소에 담지된 팔라듐을 선택하였으며, 알칼리성 용액에서의 에탄올 산화 반응의 전기화학적 특성을 확인결과, 상업용 팔라듐과 팔라듐 육면체와 비교하여 크게 향상된 에탄올 산화 효율을 확인하였습니다. 이는 {100}면이 나타내는 효율적인 전기화학적 산화 효율과 더불어 폴리아닐린의 π-π 컨쥬게이션으로 인한 높은 전기 전도도에 기인합니다.

이 연구를 통해 촉매의 모양과 조성의 조절을 통해 전기화학적 에탄올 산화를 향상시켰으며, 친환경적이고 지속가능한 새로운 에너지원 개발에 대한 전략을 제공 할 수 있습니다.