High-Performance Oxygen Reduction Electrocatalysts based on Cheap Carbon Black, Nitrogen, and Trace Iron
High-Performance Oxygen Reduction Electrocatalysts based on Cheap Carbon Black, Nitrogen, and Trace Iron
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基于廉价炭黑、氮气和微量铁的高性能氧还原电催化剂
DOI：
10.1002/adma.201302786
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发表时间：
2013-12-01
影响因子：
29.4
通讯作者：
Xu, Weilin
中科院分区：
文献类型：
--
作者：
Liu, Jing;Sun, Xiujuan;Xu, Weilin
由于能源危机和贵金属的日益稀缺，可持续的非贵金属或无金属氧还原反应（ORR）电催化剂在燃料电池系统中越来越受到关注。[1-6]已经发现杂原子（N、B、S、P、Fe、Co或Mn）掺杂的碳材料，例如碳纳米管（CNT）、[7-9]石墨烯、[10]石墨阵列[11]和无定形碳[12-14]，对ORR表现出优异的电催化性能。与传统的铂基催化剂相比，用于ORR的非贵金属基催化剂具有多种优势，例如更高的活性、长期操作稳定性、耐毒物性，最重要的是可持续性。在所有这些碳基非铂催化剂中，很少有与铂处于竞争水平的。[1，3，6，9，13，15]这些最有希望的Pt替代物中的一个家族主要是具有约3-10重量%的相对高的最佳金属含量的铁或钴掺杂的碳材料。[1-3在此，通过在0.02 - 20重量%的宽范围内调节Fe含量和在廉价炭黑（CB）上与氮共掺杂，我们发现，除了先前报道的在高浓度范围内的最佳Fe含量之外，在极低浓度范围内存在第二或真实的最佳值。新发现的最佳催化剂（CB-NFe）的微量铁含量低至0.05重量%，显示出上级的高性能相比，其他非铂电催化剂的ORR。与通过碱性直接甲醇燃料电池测试明确验证的商业Pt/C相比，其在酸性介质中相当[3，6，15]并且在碱性介质中更好[1，2，9]。这些CB-NFe催化剂是用于ORR的最有效的电催化剂之一。由于CB的成本仅为Pt的万分之一，因此这些CB-NFe电催化剂是迄今为止燃料电池中用于ORR的Pt的最有前途的替代品。CB-NFe的合成是基于一个简单的过程，使用廉价的化学品BP 2000（BP），三聚氰胺（C3 h6 N6）和FeCl 3（支持信息）。为了比较，也以类似的方式获得BP、BP-N和BP-Fe。为了表征不同ORR电催化剂的性能，通常使用在旋转圆盘电极（RDE）上O2饱和的0.1MKOH中线性扫描伏安法（LSV）的起始电位（Eonset）和半波电位（E1/2）两个参数进行比较。[1]图1a显示了热解温度对催化剂性能的影响。可以看出，催化剂在热解温度为约900 ℃时表现出最佳性能。我们进一步改变初始三聚氰胺/碳（M/C）重量比以调节催化剂中的最终氮含量。如图1 B和c所示，催化剂活性显示出对N含量或初始M/C比的火山形依赖性。当初始M/C质量比为10时，最佳N含量为2.4wt%。值得注意的是，当初始M/C重量比高于5时，催化剂中的最终N含量几乎饱和，但活性在很宽的范围内变化，表明高ORR电催化活性的关键不是与表观N含量直接相关，而是与碳中N和Fe的掺杂模式有关。在此基础上，通过改变铁含量，考察了铁含量对催化剂性能的影响
Due to the energy crisis and increasing scarcity of noble metals, sustainable non-precious metal or metal-free electrocatalysts for oxygen reduction reactions (ORR) are attracting more and more attention for fuel cell systems.[1–6] Heteroatom (N, B, S, P, Fe, Co, or Mn)-doped carbon materials, such as carbon nanotubes (CNTs),[7–9] graphene,[10] graphitic arrays,[11] and amorphous carbon,[12–14] have been found to exhibit excellent electrocatalytic performance for ORR. Compared with traditional Pt-based catalysts, non-precious metal-based catalysts for ORR possess several advantages, such as higher activity, long-term operation stability, tolerance to poisons, and, most importantly, sustainability. Among all these carbon-based non-Pt catalysts, very few are on a competitive level with platinum.[1, 3, 6, 9, 13, 15] One family of these most promising alternatives to Pt are mainly the iron or cobalt-doped carbon materials with a relatively high optimal metal content of about 3–10 wt%.[1–3, 5, 15–19] Here, by tuning the Fe content and co-doping with nitrogen on cheap carbon black (CB) over a wide range from 0.02 to 20 wt%, we found, in addition to the previously reported optimal Fe content in a high concentration range, that there is a second or real optimal value in an extremely low concentration range. The new-found optimal catalyst (CB-NFe) with a trace Fe content down to 0.05 wt% showed a superior high performance compared with the other non-Pt electrocatalysts for ORR. It was comparable in acidic medium [3, 6, 15] and better in alkaline medium [1, 2, 9] than commercial Pt/C validated unambiguously by the alkaline direct methanol fuel cell tests. These CB-NFe catalysts are among the most efficient electrocatalysts for ORR. Since the cost of CB is only one in ten thousandth the cost of Pt, these CB-NFe electrocatalysts are the most promising alternatives to Pt for ORR in fuel cells to date. The synthesis of CB-NFe was based on a simple procedure with cheap chemicals BP2000 (BP), melamine (C3h 6N 6), andFeCl 3 (Supporting Information). For comparison, BP, BP-N, and BP-Fe were also obtained in a similar way. In order to characterize the performance of different ORR electrocatalysts, generally two parameters of onset potential (Eonset) and half-wave potential (E1/2) from linear sweep voltammetry (LSV) in O 2-saturated 0.1 M KOH on a rotating disk electrode (RDE) are used for comparison.[1] Figure 1a shows the effect of pyrolyzing temperature on the performance of the catalyst. It can be seen that the catalyst shows best performance with a pyrolyzing temperature of about 900 C. We further varied the initial melamine/carbon (M/C) weight ratio to tune the final nitrogen content in the catalyst. As shown in Figure 1 b and c, the catalyst activity shows a volcano-shaped dependence on N content or the initial M/C ratio. The optimal N content was found to be about 2.4 wt% with an initial M/C mass ratio of 10. It is worth noting that when the initial M/C weight ratio is higher than 5, the final N contents in the catalysts are almost saturated, but the activities vary over a wide range, indicating the key to high ORR electrocatalytic activity is not directly related to the apparent N content, but to the doping pattern of N and Fe in the carbon. Based on the above optimal conditions, we studied the effect of Fe content on the catalyst performance by varying Fe