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氧还原反应(ORR)是燃料电池和金属空气电池等洁净发电装置中阴极的主要反应,该反应动力学过程慢,电化学极化严重.Pt基电催化剂具有较好的ORR活性,然而Pt资源有限、价格昂贵,研制高活性、低成本的代Pt电催化剂意义重大.经过几十年的探索,研究者发现将含有C,N和Fe等元素的前体进行高温热处理得到的Fe-N-C电催化剂对ORR具有良好的活性,然而在高温热解过程中Fe容易发生聚集而形成大块颗粒,导致Fe的利用率不高,影响了电催化剂的ORR活性.本文分别以聚吡咯和乙二胺四乙酸二钠(EDTA-2Na)为C和N的前驱体,利用高温热解形成的富含微孔的碳材料对铁前体的吸附及锚定作用,获得了一种Fe高度分散的Fe-N-C电催化剂.采用物理吸脱附技术、高分辨透射电镜(HRTEM)和扫描电镜对Fe-N-C及其制备过程中相关电催化剂的孔结构及表面形貌进行了表征.结果表明,在第一步热解过程中,EDTA-2Na的Na对碳材料起到了活化作用,形成富含微孔的N掺杂碳材料(N-C-1),其BET比表面积达到1227 m~2/g,孔径约1.1 nm.在第二步热解过程中,N-C-1有效地抑制了Fe的聚集,产物Fe-N-C中的Fe元素均匀地分布在碳材料中,其比表面积高达1501 m~2/g.电化学测试结果表明,在碱性介质(0.1 mol/L NaOH)中,Fe-N-C电催化剂对ORR具有良好的催化活性,ORR起始电位(E_o)为1.08 V(vs.RHE),半波电位(E_(1/2))0.88 V,电子转移数n接近4,H_2O_2产率<3%,与商品20%Pt/C(Johnson Matthey)接近.电化学加速老化测试结果表明,Fe-N-C的E_(1/2)未发生明显变化,而Pt的负移45 mV,表明Fe-N-C具有很好的稳定性;在酸性介质(0.1 mol/L HClO_4)中,Fe-N-C的E_o为0.85 V,E_(1/2)为0.75 V,其E_(1/2)比Pt/C负移约0.15 V,表明在酸性介质中Fe-N-C对ORR的催化活性还有待提高.采用TEM、X射线衍射、X射线光电子能谱以及穆斯堡尔谱等方法研究了电催化剂构效关系.结果表明,Fe-N-C较好的ORR活性主要来自于高分散的Fe-N_4结构,此外,N(吡啶N和石墨N)掺杂的C也对反应具有一定的催化活性.与Pt/C相比,Fe-N-C电催化剂具有很好的耐甲醇性能.本文对比了Fe-N-C和Pt/C作为阴极催化剂的直接醇类燃料电池(DMFC)性能,采用质子交换膜的DMFC最大功率密度分别为47(Fe-N-C)和79 mW/cm~2(Pt/C),而采用碱性电解质膜的则分别为33(Fe-N-C)和8 mW/cm~2(Pt/C).结合半电池结果表明,Fe-N-C电催化剂在碱性介质中具有比Pt更为优秀的催化活性和稳定性,有望用作DMFC阴极代Pt催化剂.
Oxygen reduction reaction (ORR) is the main reaction of cathodes in clean power plants such as fuel cells and metal-air batteries, and the reaction kinetics is slow and electrochemical polarization is severe. Pt-based electrocatalysts have good ORR activity, however, Pt resources Limited and expensive, the development of high activity, low cost generation Pt electrocatalysts is of great significance.After several decades of exploration, the researchers found that the precursor containing C, N and Fe and other elements of high temperature heat treatment of Fe-NC power The catalyst has good activity on ORR, however, Fe easily aggregates to form large particles during high temperature pyrolysis, resulting in low utilization rate of Fe and affecting the ORR activity of the electrocatalyst.In this paper, polypyrrole and ethylenediamine Disodium tetracetate (EDTA-2Na) is a precursor of C and N. The adsorption and anchoring effect of iron-rich microporous carbon material formed by high-temperature pyrolysis on iron precursor obtained a Fe highly dispersed Fe -NC electrocatalyst.The pore structure and surface morphology of the electrocatalyst in Fe-NC and its preparation process were characterized by means of physical adsorption desorption, high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) During the pyrolysis process, The Na-EDTA-2Na activated the carbon material to form a microporous N-doped carbon material (NC-1) with a BET specific surface area of 1227 m 2 / g and a pore diameter of about 1.1 nm. NC-1 effectively inhibits the aggregation of Fe during the step pyrolysis, and the Fe element in the Fe-NC product is uniformly distributed in the carbonaceous material with a specific surface area of up to 1501 m 2 / g. The results of electrochemical tests show that, The catalytic activity of Fe-NC electrocatalysts was good for ORR in basic medium (0.1 mol / L NaOH). The initial OR potential of ORR was 1.08 V (vs. RHE) and the half-wave potential (E 1 / 2)) 0.88 V, the electron transfer number n was close to 4 and the yield of H 2 O 2 was less than 3%, which was close to that of commercial 20% Pt / C (Johnson Matthey) .The electrochemical aging test showed that the E 1/2 ) Showed no obvious change, while the negative shift of Pt was 45 mV, indicating that Fe-NC had good stability. The E_o of Fe-NC was 0.85 V in acidic medium (0.1 mol / L HClO_4) 2) is 0.75 V, and the E 1/2 of Pt / C negative shift about 0.15 V, indicating that the catalytic activity of Fe-NC on ORR in acidic medium needs to be improved.Using TEM, X-ray diffraction, X-ray photoelectron Energy spectrum and Mossbauer spectroscopy and other methods to study the structure-activity relationship of electrocatalysts.The results show that Fe-NC The better ORR activity is mainly from the highly dispersed Fe-N_4 structure. In addition, C doped with N (pyridine N and graphite N) also has a certain catalytic activity for the reaction.Compared with Pt / C, Fe-NC (DMFC) with Fe-NC and Pt / C as cathode catalyst. The maximum power density of DMFC using proton exchange membrane is 47 (Fe-NC) And 79 mW / cm 2 (Pt / C), respectively, compared to 33 (Fe-NC) and 8 mW / cm 2 Pt / C with alkaline electrolyte membranes, respectively. NC electrocatalyst has more excellent catalytic activity and stability than Pt in alkaline medium, and is expected to be used as a cathode catalyst for Pt catalyst in DMFC.