Comprehensive Stochastic Modeling of Nanoscale Structure-Transport Characteristics and Catalytic Performance of Polymer Electrolyte Fuel Cell Catalyst Layers

Abstract

본 연구에서는 연료전지 촉매층 내 전기화학반응에 대한 성능 결정 인자를 밝히기 위해, 촉매층 구조의 무작위적 특성에 기초한 통계적 접근법을 사용하여, 고성능 고분자 전해질 연료전지 (PEFC) 내 복합 촉매층 구조가 모델링 되었다. 촉매 성능 개선을 위한 연료전지 촉매층 내 질량 수송 특성 및 촉매 이용을 평가하기 위해 준 무작위적 나노 구조 모델(QRNM)에 기초한 3차원 격차 볼츠만 모델 (LBM)이 제안되었다. 95% 신뢰도의 정확한 통계적 분석을 위해, 각각의 데이터 점은 총 25개의 촉매층 전산 표본으로부터 추출되었다. 촉매층 내부의 물질 전달 현상은 LBM 모델을 통해 시뮬레이션 되었으며, 유효 물질 전달 특성 및 상응하는 촉매활용도에 영향을 미치는 촉매층 구조, 조성, 상응하는 형태학적 구조는 통계적으로 분석되었다. 일반적 촉매층 및 수직 배열 탄소나노튜브(VACNT) 촉매층 내 기체투과도, 유효확산계수와 같은 구조-수송 특성은 촉매층 성능 향상을 위해 일련의 수치 실험 데이터로부터 통계적으로 예측되었다. 통계학적 형상 분석은 크누센(Knudsen) 확산이 촉매층 내 물질전달현상에 지배적인 영향을 미침을 보여준다. 반응 기체의 평균 유효 확산 계수는 앞서 추정된 크누센 확산 계수와 유선의 비틀림도(Tortuosity) 로부터 예측되었으며, 앞선 선행연구 그룹의 실험적으로 측정된 유효 확산 계수를 성공적으로 예측하였다. 크누센 확산 계수가 상대적으로 낮음에도 불구하고, VACNT 촉매층의 유효 확산 계수는 일반적 촉매층과 비교할 때 더 높은 것으로 계산되었는데, 이는 VACNT 구조에서 기인한 개선된 기공 구조 및 낮은 비틀림도에 의한 것으로 확인되었다. 이 결과는 VACNT 촉매 지지체를 포함한 PEFC 촉매층이 보다 효과적인 물질전달현상을 제공함으로써 전기화학반응을 위한 촉매 이용을 향상시킬 수 있음을 분명히 나타낸다. 고분자 전해질 연료전지 촉매층의 전기화학적 성능은, 앞서 도출된 유효 물질전달 특성으로부터 성공적으로 시뮬레이션 되었다. 촉매층 단위 부피당 전기화학적 활성 표면적 및 등가의 표면적 비율 또한 통계적으로 평가되었다. 추정된 분극 곡선 (Polarization curve)의 결과는 VACNT 촉매층이 높은 전기화학적 활성 표면적 및 제한전류밀도로 인해, 기존의 촉매층과 비교할 때 개선된 전기화학 성능을 나타냄을 분명히 보여준다. 일련의 수치 실험으로부터 얻은 이러한 통계적 결과는 VACNT를 촉매 지지체로 함유하는 PEFC 촉매층이 보다 효율적인 반응물 수송 및 보다 넓은 전기화학적 활성 표면적을 제공함으로써, 종래의 촉매층과 비교할 때 개선된 전기화학적 성능을 달성할 수 있음을 보여준다. |In the present study, the advanced polymer electrolyte fuel cell (PEFC) composite catalyst layers were modeled using a statistical approach based on the inherent random nature of the structures of the catalyst layers to elucidate the performance determining factors for electrochemical reactions in fuel cell catalyst layers. A three-dimensional lattice Boltzmann model (LBM) based on the quasi-random nanostructural model is proposed to evaluate the mass transport properties and catalyst utilization of fuel cell catalyst layers in pursuance of catalyst performance improvement. For a reliable statistical analysis, each of the data points was extracted from a total of minimum 25 computational catalyst layer specimens to achieve a 95% confidence level. The reactant mass transport phenomena inside the catalyst layers were simulated by the LBM model, and the structure, composition and corresponding morphological structure influences of the catalyst layers on the effective mass transport properties and catalyst utilization were statistically analyzed. The structure–transport characteristics of both conventional and vertically aligned carbon nanotube (VACNT) catalyst layers, such as the tortuosity, permeability and effective diffusion coefficients, were statistically estimated from a series of numerical experimental data derived by the LBM in pursuit of performance improvement. The statistical morphology analyses revealed that Knudsen diffusion had a dominant effect on the reactant mass transport phenomena in the catalyst layers. The average effective diffusion coefficient of reactant gas was estimated from the pre-estimated Knudsen diffusion coefficient and tortuosity and successfully predicted the experimental effective diffusion coefficient data of other research groups. Despite the relatively lower Knudsen diffusion coefficient, the effective diffusion coefficient of the VACNT catalyst layers was computed to be higher than that of conventional catalyst layers, mainly owing to the improved pore structures and low tortuosity. This results clearly indicates that the PEFC catalyst layers containing the VACNT catalyst supports can provide more efficient reactant transport, resulting in enhanced catalyst utilization for electrochemical reactions. The catalytic performance of polymer electrolyte fuel cell catalyst layers was successfully simulated based on the pre-estimated effective transport characteristic of the conventional and VACNT catalyst layers. The electrochemically active surface area per unit volume of catalyst layer and equivalent surface ratio for various catalyst layers were also statistically evaluated and plotted. The estimated polarization curve clearly showed that the VACNT catalyst layer exhibits an improved catalytic performance compared to the conventional catalyst layers owing to its high electrochemically active surface area and limiting current density. These statistical results obtained from a series of numerical experiments confirm that the PEFC catalyst layers containing the VACNT catalyst supports can yield an improved catalytic performance compared to the conventional catalyst layers by providing more efficient reactant transport and enlarged electrochemically active surface area.; In the present study, the advanced polymer electrolyte fuel cell (PEFC) composite catalyst layers were modeled using a statistical approach based on the inherent random nature of the structures of the catalyst layers to elucidate the performance determining factors for electrochemical reactions in fuel cell catalyst layers. A three-dimensional lattice Boltzmann model (LBM) based on the quasi-random nanostructural model is proposed to evaluate the mass transport properties and catalyst utilization of fuel cell catalyst layers in pursuance of catalyst performance improvement. For a reliable statistical analysis, each of the data points was extracted from a total of minimum 25 computational catalyst layer specimens to achieve a 95% confidence level. The reactant mass transport phenomena inside the catalyst layers were simulated by the LBM model, and the structure, composition and corresponding morphological structure influences of the catalyst layers on the effective mass transport properties and catalyst utilization were statistically analyzed. The structure–transport characteristics of both conventional and vertically aligned carbon nanotube (VACNT) catalyst layers, such as the tortuosity, permeability and effective diffusion coefficients, were statistically estimated from a series of numerical experimental data derived by the LBM in pursuit of performance improvement. The statistical morphology analyses revealed that Knudsen diffusion had a dominant effect on the reactant mass transport phenomena in the catalyst layers. The average effective diffusion coefficient of reactant gas was estimated from the pre-estimated Knudsen diffusion coefficient and tortuosity and successfully predicted the experimental effective diffusion coefficient data of other research groups. Despite the relatively lower Knudsen diffusion coefficient, the effective diffusion coefficient of the VACNT catalyst layers was computed to be higher than that of conventional catalyst layers, mainly owing to the improved pore structures and low tortuosity. This results clearly indicates that the PEFC catalyst layers containing the VACNT catalyst supports can provide more efficient reactant transport, resulting in enhanced catalyst utilization for electrochemical reactions. The catalytic performance of polymer electrolyte fuel cell catalyst layers was successfully simulated based on the pre-estimated effective transport characteristic of the conventional and VACNT catalyst layers. The electrochemically active surface area per unit volume of catalyst layer and equivalent surface ratio for various catalyst layers were also statistically evaluated and plotted. The estimated polarization curve clearly showed that the VACNT catalyst layer exhibits an improved catalytic performance compared to the conventional catalyst layers owing to its high electrochemically active surface area and limiting current density. These statistical results obtained from a series of numerical experiments confirm that the PEFC catalyst layers containing the VACNT catalyst supports can yield an improved catalytic performance compared to the conventional catalyst layers by providing more efficient reactant transport and enlarged electrochemically active surface area.