Dhavale Vishal M, Singh Santosh K, Nadeema Ayasha, Gaikwad Sachin S, Kurungot Sreekumar
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411 008, India.
Nanoscale. 2015 Dec 21;7(47):20117-25. doi: 10.1039/c5nr04929f. Epub 2015 Nov 16.
The size-controlled growth of nanocrystalline Fe-Fe2O3 particles (2-3 nm) and their concomitant dispersion on N-doped graphene (Fe-Fe2O3/NGr) could be attained when the mutually assisted redox reaction between NGr and Fe(3+) ions could be controlled within the aqueous droplets of a water-in-oil emulsion. The synergistic interaction existing between Fe-Fe2O3 and NGr helped the system to narrow down the overpotential for the oxygen reduction reaction (ORR) by bringing a significant positive shift to the reduction onset potential, which is just 15 mV higher than its Pt-counterpart. In addition, the half-wave potential (E1/2) of Fe-Fe2O3/NGr is found to be improved by a considerable amount of 135 mV in comparison to the system formed by dispersing Fe-Fe2O3 nanoparticles on reduced graphene oxide (Fe-Fe2O3/RGO), which indicates the presence of a higher number of active sites in Fe-Fe2O3/NGr. Despite this, the ORR kinetics of Fe-Fe2O3/NGr are found to be shifted significantly to the preferred 4-electron-transfer pathway compared to NGr and Fe-Fe2O3/RGO. Consequently, the H2O2% was found to be reduced by 78.3% for Fe-Fe2O3/NGr (13.0%) in comparison to Fe-Fe2O3/RGO (51.2%) and NGr (41.0%) at -0.30 V (vs. Hg/HgO). This difference in the yield of H2O2 formed between the systems along with the improvements observed in terms of the oxygen reduction onset and E1/2 in the case of Fe-Fe2O3/NGr reveals the activity modulation achieved for the latter is due to the coexistence of factors such as the presence of the mixed valancies of iron nanoparticles, small size and homogeneous distribution of Fe-Fe2O3 nanoparticles and the electronic modifications induced by the doped nitrogen in NGr. A controlled interplay of these factors looks like worked favorably in the case of Fe-Fe2O3/NGr. As a realistic system level validation, Fe-Fe2O3/NGr was employed as the cathode electrode of a single cell in a solid alkaline electrolyte membrane fuel cell (AEMFC). The system could display an open circuit voltage (OCV) of 0.73 V and maximum power and current densities of 54.40 mW cm(-2) and 200 mA cm(-2), respectively, which are comparable to the performance characteristics of a similar system derived by using 40 wt% Pt/C as the cathode electrode.
当油包水乳液的水滴内可控制氮掺杂石墨烯（NGr）与Fe(3+)离子之间的相互辅助氧化还原反应时，可实现纳米晶Fe-Fe2O3颗粒（2-3纳米）的尺寸控制生长及其在氮掺杂石墨烯上的伴随分散（Fe-Fe2O3/NGr）。Fe-Fe2O3与NGr之间存在的协同相互作用通过使还原起始电位产生显著正移，帮助系统缩小了氧还原反应（ORR）的过电位，该起始电位仅比其对应的铂高15毫伏。此外，与将Fe-Fe2O3纳米颗粒分散在还原氧化石墨烯上形成的系统（Fe-Fe2O3/RGO）相比，Fe-Fe2O3/NGr的半波电位（E1/2）提高了相当可观的135毫伏，这表明Fe-Fe2O3/NGr中存在更多的活性位点。尽管如此，与NGr和Fe-Fe2O3/RGO相比，Fe-Fe2O3/NGr的ORR动力学明显向更优的4电子转移途径偏移。因此，在-0.30 V（相对于Hg/HgO）时，Fe-Fe2O3/NGr（13.0%）的H2O2%相比于Fe-Fe2O3/RGO（51.2%）和NGr（41.0%）降低了78.3%。系统之间H2O2生成产率的这种差异以及在Fe-Fe2O3/NGr情况下氧还原起始和E1/2方面观察到的改善表明，后者实现的活性调节归因于多种因素的共存，如铁纳米颗粒的混合价态的存在、Fe-Fe2O3纳米颗粒的小尺寸和均匀分布以及NGr中掺杂氮引起的电子修饰。在Fe-Fe2O3/NGr的情况下，这些因素的可控相互作用似乎起到了有利作用。作为实际系统层面的验证，Fe-Fe2O3/NGr被用作固体碱性电解质膜燃料电池（AEMFC）单电池的阴极电极。该系统可显示开路电压（OCV）为0.73 V，最大功率和电流密度分别为54.40 mW cm(-2)和200 mA cm(-2)，这与使用40 wt% Pt/C作为阴极电极的类似系统的性能特征相当。
