Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium
Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium
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DOI：
10.1021/acscatal.6b00643
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发表时间：
2016-05-01
期刊：
影响因子：
12.9
通讯作者：
Jaouen, Frederic
中科院分区：
文献类型：
--
作者：
Choi, Chang Hyuck;Baldizzone, Claudio;Jaouen, Frederic
为了成功地替代Pt，迄今为止已经进行了巨大的努力来开发用于聚合物电解质膜燃料电池（PEMFC）中的氧还原反应（ORR）的高性能Fe-N-C催化剂。然而，与活性的显著进步相比，Fe-N-C催化剂的稳定性仍然是关键的。假设酸性介质中的Fe脱金属是影响总寿命的一个关键因素。与一般的看法相反，我们在此使用操作光谱分析证明，由于相对高的开路电位（约1000 V），暴露于酸性电解质的催化非活性Fe颗粒不能通过酸洗完全去除。0.9 V-RHE）导致形成不溶性铁物质，而这些颗粒在PEMFC操作条件（E-阴极<0. 7V-RHE）下溶解，这是由于操作还原成可溶性亚铁阳离子。为了克服这个问题，我们展示了两种方法：（i）合成不含Fe颗粒的Fe-N-C催化剂和（ii）通过使用外部恒电位仪或内部还原剂（即，SnCl 2）。操作光谱分析证实，在一个给定的伏安协议期间，Fe脱金属显着降低合成和合成后改性的Fe-N-C催化剂，而初始ORR活性没有显着变化。然而，所有这些催化剂在短期PEMFC耐久性测试中表现出类似的性能衰减，表明缺乏从非活性Fe颗粒中浸出的亚铁阳离子对催化剂失活所起的作用。这支持了活性主要由FeNxCy部分赋予的观点。尽管如此，所提出的指导方针是普遍适用于整个类的Fe-N-C催化剂，以尽量减少铁脱金属的PEMFC，这提供了重要的进展，为未来设计的稳定的电催化系统的长期运行。
For a successful replacement of Pt, tremendous efforts have hitherto been made to develop high-performing Fe-N-C catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In comparison to the remarkable progress in activity, the stability of Fe-N-C catalysts still remains critical, however. Fe demetallation in acidic medium is hypothesized to be one critical factor for the overall lifetime. In contrast to the general belief, we herein demonstrate using an operando spectroscopic analysis that catalytically inactive Fe particles exposed to acid electrolytes cannot be fully removed by acid washing due to a relatively high open circuit potential (ca. 0.9 V-RHE) leading to the formation of insoluble ferric species, whereas these particles dissolve under PEMFC operating conditions (E-cathode < 0.7 V-RHE) due to operando reduction to soluble ferrous cations. To overcome this issue, we demonstrate two approaches: (i) synthesis of Fe-N-C catalysts free of Fe particles and (ii) postsynthesis removal of exposed Fe particles through the control of potential using an external potentiostat or an internal reducing agent (i.e., SnCl2). Operando spectroscopic analyses verified that Fe demetallation during a given voltammetric protocol was dramatically decreased for both synthetically and postsynthetically modified Fe-N-C catalysts, while the initial ORR activity did not significantly change. However, all of these catalysts showed similar performance decay over short-term PEMFC durability tests, demonstrating the lack of a role played by ferrous cations leached from inactive Fe particles on catalyst deactivation. This supports the view that the activity is mainly imparted by FeNxCy moieties. Nevertheless, the presented guidelines are generally applicable to the whole class of Fe-N-C catalysts in order to minimize Fe demetallation in PEMFCs, which provides important advances for the future design of stable electrocatalytic systems for long-term operation.