【作者】 李佳；

【导师】 陈四国；

【作者基本信息】 重庆大学 ， 化学工程与技术， 2020， 博士

【摘要】 质子交换膜燃料电池（PEMFC）具有高效和环境友好等突出优点,是最有发展前途的一种电池,可广泛用于移动电源和便携式电源。但是PEMFC在大规模商业化之前还需要解决催化剂价格昂贵、稳定性差和利用率低等一系列问题。因此开发低价、高活性、高稳定性的非Pt或低Pt载量的催化剂替代Pt基催化剂,已成为PEMFC大规模商业化的关键。本论文从合成方法的角度开展了如下几个方面的研究工作:首先,针对传统Fe-N-C催化剂在高温碳化过程中存在的前驱体快速烧失、催化剂产率低、氮掺杂效率低等问题,开发了一种以低共熔盐为模板的多孔Fe-N-C催化剂合成方法。该方法以具有三维立体大孔结构的ZnCl₂/KCl低共熔盐为模板,邻苯二胺为前驱体,过硫酸铵为氧化剂,铁盐为助催化剂,采用固相聚合法在ZnCl₂/KCl低共熔盐表面引发邻苯二胺氧化聚合。在高温热解过程中,低共熔盐不仅可以作为模板剂和造孔剂,而且其低温熔融状态提供的限域微环境还可以有效避免金属有机聚合物前驱体在高温碳化过程中的热解损失、表面烧结和孔结构坍塌,提高催化剂产率和氮掺杂效率。此外,ZnCl₂高温气化后形成的大量微孔可以提供大量催化活性中心,提高催化剂活性位密度,而除去KCl后形成的大孔可以作为氧气和水的传输通道,提升反应物和产物的传质能力。采用该方法制备的Fe/N/C催化剂在酸性和碱性介质中都表现出了较高的催化活性,在酸性介质中的氧还原半波电位达0.809 V,以其为阴极组装的单电池最大输出功率达到了580 m W cm^-2。在碱性介质中的氧还原半波电位达到了0.921 V,比商业化Pt/C催化剂正移了41 m V,以其为阴极组装的锌空电池最大输出功率206 m W cm^-2,远高于以商业化Pt/C为阴极组装的锌空电池(173 m W cm^-2)。其次,通过有效调控高温热解升温速率,制备了Zn含量高达9.33 wt%的Zn单原子催化剂（Zn-N-C-1）。采用循环伏安加速寿命实验,结合HADDF-STEM、XAFS、LSV和XPS对所制备催化剂的催化活性和稳定性进行了研究。HADDF-STEM结果显示,经过酸刻蚀和二次碳化处理后,Zn-N-C-1催化剂的Zn单原子密度高达9.31 atoms/nm²,为目前所报道的单原子催化剂最高值。EXAFS分析证明,Zn原子与四个N原子结合配位形成稳定的Zn-N₄结构。电化学测试显示,Zn-N-C-1催化剂在酸性和碱性介质中的活性可以与相应的Fe-N-C-1催化剂媲美。更重要的是,不论是酸性还是碱性介质中,Zn-N-C-1催化剂均表现出比Fe-N-C-1催化剂更加优异的电化学稳定性。通过XPS和DFT计算进一步揭示了Zn-N-C-1更稳定的原因。通常,氧气存在时,吡啶氮的质子化会导致C-N键变长,最终导致其断裂。然而,与Fe-N-C-1中的吡啶氮相比,Zn-N-C-1中的吡啶氮更难于质子化,因而C-N键更为稳定。此外,与Fe-N₄相比,Zn-N₄结构中*Zn形成*Zn（OH）_x的自由能更高,更难于被氧化。最后,针对传统单原子催化剂缺少相邻的金属中心,而许多反应需要多原子配位效果的问题,本论文首次发明了一种新型多原子催化剂（MAC-Pt/Zn Fe-N-C）,该催化剂具有紧密相连的Pt-Pt原子并100%暴露于催化剂表面,通过Pt-N键紧密结合在原子级分散的Zn Fe-N-C载体上。X射线吸收精细结构分析表明,多原子Pt催化剂中每个Pt原子与～2.6个N原子和～4.3个Pt原子配位。电化学性能测试表明,MAC-Pt/Zn Fe-N-C催化剂的电化学活性表面积高达177.62 m² g^-1,约为商用Pt/C催化剂(71.76 m² g^-1)的2.48倍。此外,由于Pt-N键的强相互作用,可以抑制Pt原子的溶出、迁移、团聚、长大,导致催化剂具有很强的电化学稳定性。同时,单电池测试证实了MAC-Pt/Zn Fe-N-C催化剂具有超高的电化学活性表面积,在Pt载量为0.035 mg cm^-2时,H₂-O₂燃料电池最大输出功率高达1.02 W cm^-2。最后,结合密度泛函理论计算,研究了多原子催化剂具有高稳定性的机理,发现Pt-N键的存在对于Pt的溶出、迁移、团聚、长大具有很强的抑制作用。更多还原

【Abstract】 Proton exchange membrane fuel cell（PEMFC）is a promising technology which can be used in transportation and other fields,due to its high zero emission,high power density and high efficiency.Oxygen reduction reaction（ORR）,the kinetically sluggish cathode reaction,is a more important research topic in PEMFC that requires more attention and effort.So far,Pt and Pt-based alloys are the most active catalysts for ORR.However,the high cost and scarcity of Pt limit the application of PEMFC.To overcome this,it is of great significance to develop highly active Platinum group metal-free（Pt-free）or low-Pt catalysts for ORR.Based on these,the preparation of Pt-free catalysts with higher catalytic activity and development of low-Pt catalysts with high utilization of active sites are the focus and main direction of current research on PEMFC electrocatalysts.Firstly,we demonstrate a“eutectic salt-assisted semi-closed carbonization”technique for fabricating high-density active-sites hierarchically porous Fe/N/C catalyst by using ZnCl₂/KCl eutectic salt as template.Our technique allows for the pyrolysis of Fe/N/C precursor under the protection of the molten ZnCl₂/KCl eutectic salt,which can not only provide an ion liquid-confined space to suppress the large weight loss and N evaporation of precursor in a very wide temperature range from 390°C to 923°C,but also play a key role in modulating the porous structure,specific area and graphitization degree of Fe/N/C catalyst.Accordingly,the as-prepared Fe/N/C catalyst exhibits excellent ORR activity and stability in both acidic（half-wave potential of 0.803V versus reversible hydrogen electrode）and alkaline（half-wave potential of 0.918V versus reversible hydrogen electrode）media.More importantly,real cathodes made from the Fe/N/C catalysts further demonstrated superior performance in H₂-O₂ fuel cells and Zinc–O₂batteries,respectively.This strategy provides a new avenue for the design and development of advanced porous carbon materials for different applications.Secondly,atomically dispersed Zn-N-C nanomaterials are promising Pt-free catalysts for oxygen reduction reaction（ORR）.However,the fabrication of high Zn loading Zn-N-C catalyst remains a formidable challenge due to the high volatility of Zn precursor during high-temperature annealing.Here,we report that the atomically dispersed Zn-N-C catalyst with an ultrahigh Zn loading of 9.33 wt%can be successfully prepared by simply adopting a very low annealing rate of 1°/min.The Zn-N-C catalyst exhibits comparable ORR activity with Fe-N-C catalyst,and significantly better ORR stability than Fe-N-C catalyst in both acidic and alkaline media.Further experiments and DFT calculations demonstrate that Zn-N-C catalyst is less susceptible to protonate than Fe-N-C catalyst in acidic medium.DFT calculations reveal that the Zn-N₄ structure is more electrochemically stable than the Fe-N₄ during ORR process.Finally,while single-atom catalysts（SACs）are drawing wide attention because they offer properties that differ from those of conventional nanoparticle（NP）-based catalysts,the lack of neighboring metal centers to cooperate in catalysis limits their real application in many important chemical processes.Here,we report the synthesis of a multi-atom platinum（Pt）catalyst that consists of cross-linked Pt-Pt metal centers stabilized by atomically dispersed Zn Fe-N-C support through Pt-N bonds.X-ray absorption fine structure analysis reveals that each Pt atom in the multi-atom Pt catalyst coordinates with～2.6 N atoms and～4.3 Pt atoms.This novel multi-atom Pt catalyst combines the merits of SACs and NP,resulting in extremely high Pt efficiency,excellent stability and high activity for the oxygen reduction reaction（ORR）in both acidic and alkaline media.With an ultralow Pt loading of 0.035 mg cm^-2 at the cathode,the fuel cell assembled by this catalyst delivers a 1.02 W cm^-2maximal power output.Density functional theory（DFT）calculations revealed that the strongly coupled Pt-N bond is critical for stabilizing the cross-linked Pt.This cross-linked multi-atom catalyst provides a new direction to reduce metal usage and enhance the activity and stability of supported catalysts.更多还原