Alternatives for Pt-based catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells (PEMFCs), especially carbon-based catalysts were synthesized and characterized. In the acidic media, carbon-based catalysts show relatively much lower ORR activity compared to Pt-based cata-lysts; therefore, improving their performance is one of the major objectives for commercializing PEMFCs. In this study, heteroatom (boron, nitrogen, phosphorus, and sulfur) doped carbon-based catalysts were synthe-sized to improve the ORR performance.
P and/or S heteroatoms are additionally doped into NDC to improve its ORR activity in acidic media. The binary and ternary doping of P and/or S in the NDC produces many edge sites and increases the portion of pyridinic-N sites in the carbon materials, which is favourable for the ORRs. Moreover, the addition of P and/or S enhances the asymmetry of the atomic charge density in C atoms and strengthens the adsorption of oxygen molecules on the carbon atoms. The ORR activity of the carbon materials is significantly modified through the additional doping of P and/or S in the NDC. In particular, S-doping in NDC reveals 3.1 times higher mass activity (mA mgnon-metal-1) compared to that of NDC at 0.75 V (vs. RHE), and derives the four-electron pathway in ORRs with high stability.
As a catalyst for ORRs in acidic media, graphene was modified by doping of N and additional do-pants, B and P. In the modification steps, DCDA, boric acid, and phosphoric acid were used as doping sources for N, B, and P, respectively, and it was confirmed that the heteroatoms were doped evenly on the graphene surface. Regardless of the dopant types, the graphene-based catalysts revealed high onset potentials, ~0.86 ± 0.01 V (vs. RHE). However, additional doping of B and P in NGr significantly improved catalytic activity in ORRs. The NGr showed 0.45 mA mg-1 of mass activity at 0.75 V, but BNGr and PNGr indicated 0.53 and 0.80 mA mg-1 of mass activities, respectively. These values were 1.2 and 1.8 times higher than that of the NGr, and much higher activities compared with those of other graphene-based catalysts reported elsewhere. Moreover, the presence of B and P also increased the catalytic stability against a severe acidic environment. Additional doping of B and P in NGr derived electrophysical modifications of graphene, resulted in enhancement of asymmetry of spin density, facile electron transfer on the graphene basal plane, and decrement of energy gap between HOMO and LUMO. The electrophysical modifications result in the production of reactive complex in graphene with an increment of conductivity through the basal plane, and it was suggested that these modifications induced improvement of catalytic activity for graphene materials for ORRs. Therefore, it is expected that this study will provide a new guideline for the catalyst design of the graphene-derived catalysts that have significantly higher ORR activities than those of novel graphite- or CNT-derived catalysts.
고분자 전해질 막 연료전지용 양극촉매로서 탄소를 기반으로 한 양극 촉매를 합성하고 그 특성을 평가하였다. 산 분위기에서, 탄소기반 촉매는 백금 촉매에 비하여 비교적 낮은 산소환원 활성을 보인다. 그러므로, 이를 개선하는 것이 고분자 전해질 막 연료전지의 상용화를 위하여 수행되어야 할 주 목적 중 하나이다. 본 연구에서는, 이종원소 (붕소, 질소, 인, 그리고 황)이 도핑된 탄소기반 촉매를 합성하였으며 이를 통해 탄소기반 촉매의 산소환원활성을 증가시켰다.
인과 황이 질소가 도핑된 탄소에 추가적으로 도핑되었으며, 이는 산소환원활성에 긍정적인 영향을 미치는 edge site를 증가시킬 뿐만 아니라 pyridinic-N site 역시 증가시키는 것으로 나타났다. 또한 탄소 원자의 atomic charge density를 변화시켜 산소분자의 흡착을 강화시켰다. 합성된 촉매들은 질소만 도핑된 탄소에 비하여 약 3배 이상 높은 산소환원활성을 보였으며, 4전자반응 및 좋은 안정성역시 확인되었다.
산 분위기에서의 산소환원반응 촉매로서, 질소 뿐만 아니라 붕소와 인이 도핑된 그래핀을 합성하였다. 합성된 촉매들은 모두 약 0.86 V (vs. RHE) 보였으며, 붕소와 인이 도핑될 경우 질소만 도핑되었을 경우 보다 훨씬 향상된 산소환원 활성을 보임을 확인하였다. 또한, 강한 산 분위기에서도 높은 안정성을 보였다. 이종원소의 도핑이 asymmetry of spin and charge density 를 강화시켰으며, HOMO와 LUMO사이의 에너지 gap을 감소시킴으로서 산소환원 활성을 증가시켰다.
