Tianjin Key Laboratory of Composite and Functional Materials, Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education), School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China.
Nanoscale. 2017 Jun 29;9(25):8623-8630. doi: 10.1039/c7nr02385e.
Two-dimensional (2D) nanocatalysts with a large specific surface area and efficient charge conductivity are promising candidates for catalyzing the sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which are at the heart of various electrochemical energy conversion and storage technologies. Here, we report the synthesis of an ultrathin CoO nanofilm with a thickness of nearly 1.8 nm via a surfactant- and template-free facile hydrothermal route. The proposed synthesis strategy can be extended to the preparation of 2D NiCoO and FeCoO nanostructures. The synthesized CoO nanofilm exhibited bifunctional activity that was superior to that of the counterpart CoO nanoparticles, including a lower overpotential and higher reduction and evolution current densities, and demonstrated faster catalytic kinetics over the 2D nanofilm surface. In comparison with precious metal-based catalysts, to achieve an OER current density of 40 mA cm the overpotential of the nanofilm (461 mV) was lower than that of RuO (526 mV), whereas the ORR on the nanofilm proceeded via a dominant 4e transfer mechanism, which is similar to that of commercial carbon-supported Pt (Pt/C). The CoO nanofilm enabled the assembly of rechargeable Zn-air batteries with a lower overpotential (0.72 V), higher round-trip efficiency (62.7%), and a longer cycle lifetime (175 cycles). The remarkable bifunctional activity contributes to an increase in the number of electrochemically active sites, a large interfacial contact area with the electrolyte, and the enrichment of Co ions on the surface, which facilitates the adsorption and activation of oxygen-containing species. This study should shed light on the future development of new electroactive materials with optimized 2D nanostructures to enhance the overall bifunctional ORR/OER performance of rechargeable metal-air batteries.
二维（2D）纳米催化剂具有较大的比表面积和高效的电荷导电性，是催化缓慢的氧析出反应（OER）和氧还原反应（ORR）的理想候选材料，而 OER 和 ORR 是各种电化学能量转换和存储技术的核心。在这里，我们通过无表面活性剂和模板的简便水热途径报告了一种厚度接近 1.8nm 的超薄 CoO 纳米薄膜的合成。所提出的合成策略可以扩展到制备 2D NiCoO 和 FeCoO 纳米结构。合成的 CoO 纳米薄膜表现出优于对应 CoO 纳米粒子的双功能活性，包括更低的过电位和更高的还原和析出电流密度，并且在 2D 纳米薄膜表面上表现出更快的催化动力学。与贵金属基催化剂相比，为了实现 40mAcm 的 OER 电流密度，纳米薄膜（461mV）的过电位低于 RuO（526mV），而纳米薄膜上的 ORR 通过主导的 4e 转移机制进行，类似于商业碳载 Pt（Pt/C）。CoO 纳米薄膜使可充电锌空气电池的组装具有更低的过电位（0.72V）、更高的往返效率（62.7%）和更长的循环寿命（175 次循环）。显著的双功能活性有助于增加电化学活性位点的数量、与电解质的大界面接触面积以及表面上 Co 离子的富集，从而促进含氧物种的吸附和活化。这项研究应该为未来开发具有优化 2D 纳米结构的新型电活性材料提供启示，以提高可充电金属空气电池的整体双功能 ORR/OER 性能。
