文献精选-20160310
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OER背景介绍：
电解水可以分为析氢反应(HER)和析氧反应(OER)两个半反应，其中，OER反应由于具备很高的过电位(overpotential)，极大地限制了电解水的效率。基于此，大量的研究工作集中于制备高效高稳定性且较为低廉的OER催化剂，目的就是要降低OER反应的过电位。目前，酸性条件下最好的催化剂是贵金属Ir, Ru氧化物;碱性条件下NiFeOx等过渡金属氧化物，氢氧化物的催化效果也非常好，目前已知最好的是NixFeyOOH。
1. 本文采用Ni基MOFs（类普鲁士蓝化合物）作为前驱体合成了C包覆的NiPx多孔纳米片，表现出了优异的OER活性。这种方法的优势主要是：1）由MOFs引入的无定型炭可以增加导电性和电子传递能力；2）材料的多孔性能增加了暴露的活性位点，有利于电解质传质；3）原位生成的Ni氧化物物种作为活性位点很好地附着在导电性良好的NiPx上，有利于电子传递；4)NiPx的存在可以OER反应中H+的传递。
Electrochemical water splitting provides an attractive way to produce hydrogen fuel. Unfortunately, the efficient and large-scale H2 production is still hindered by thesluggish kinetics of oxygen evolution reaction (OER) at the anode side of a water electrolyzer. Starting from metal-organic frameworks (MOFs), we demonstrate a template-engaged strategy to transform Ni-Ni Prussian blue analogue (PBA) nanoplates into porous carbon coated nickel phosphides nanoplates with mixedphases of Ni5P4 and Ni2P. For comparison, NiO and Ni(OH)2 porous nanoplates with the similar morphology have also been synthesized from the same precursor. Benefitting from their structural and compositional merits, the as-derived nickel phosphides manifest excellent electrocatalytic activity for OER superior to NiO and Ni(OH)2.
文章评论：本文是典型的材料合成的思维，突出的是合成方法的优异性。从电化学的角度考虑，本文所设计的催化剂并没有state-of-art的NixFeyOOH催化剂效果好。而采用无定型炭增加导电性的思路，斯坦福大学戴宏杰教授已经发表了相关工作了。不过本文提供的合成方法，对于优化OER催化剂还是有借鉴意义的。个人觉得，follow-up的工作可以从NiFeMOFs的角度入手，应该可以得到更好的催化活性。
2. 这篇Adv. Mater. 和上面那篇Energy Environ. Sci.有类似之处,都在导电性方面下了功夫，设计出了独特的结构。相对而言，本文对催化材料与OER活性的之间的关系叙述得更加充分。
We demonstrated that dual electrical behavior regulation on electrocatalysts successfully realized an enhanced electrochemical water oxidation. Benefiting from dual electrical behavior regulation which can bring about faster electron transport inside the electrocatalyst bulk component and on the surface ofcatalyst, Ni3C/C shows enhanced OER catalytic activity compared withthat of NiO, NiO/C, and pure Ni3C. Additionally, the catalytic mechanism of Ni3C/C was studied in detail by the XAFS characterization, HRTEM, and EELS spectroscopy; and the NiOx/Ni3C/C heterostructure was proved to be the actual effective species for the OER process. This work not only provides an in-depth catalytic mechanism insight for nonoxide metal-based electrocatalyst for OER but also paves a new way to the design of advanced electrocatalysts.
ORR背景介绍：
氧还原反应(ORR)是能量转化反应中非常重要的一环，在燃料电池，金属-空气电池以及电解池中都有很重要的应用。目前最好的ORR催化剂是Pt基催化剂，特别是Pt基合金催化剂，比如说PtNi。此外，研究的比较多的催化剂体系包括Fe-N-C,Ni-N-C以及掺杂型石墨烯C材料等。
3. 本文提出Fe/Fe3C Nanoparticles的存在可以显著地提高Fe-Nx的催化活性。同时存在以上两者是Fe-N-C材料高ORR活性的保证。本文对于理解Fe-N-C材料以及bottom-up来设计Fe-N-C材料具有很重要的指导意义。
Understanding the origin ofhigh activity of Fe−N−C electrocatalysts in oxygen reduction reaction (ORR) is critical but still challenging for developing efficient sustainable nonprecious metal catalysts in fuel cells and metal−air batteries. Herein, we developed a new highly active Fe−N−C ORR catalyst containing Fe−Nx coordination sites and Fe/Fe3C nanocrystals (Fe@C-FeNC), and revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance. The detailed analyses unambiguously confirmed the coexistence of Fe/Fe3C nanocrystals and Fe−Nx in the best catalyst. A series of designed experiments disclosed that (1) N-doped carbon substrate, Fe/Fe3C nanocrystals or Fe−Nx themselves did not deliver thehigh activity; (2) the catalysts with both Fe/Fe3C nanocrystals and Fe−Nxexhibited the high activity; (3) the higher content of Fe−Nx gave the higher activity; (4) the removal of Fe/Fe3C nanocrystals severely degraded the activity; (5) the blocking of Fe−Nx downgraded the activity and the recovery ofthe blocked Fe−Nx recovered the activity. These facts supported that the highORR activity of the Fe@C-FeNC electrocatalysts should be ascribed to that Fe/Fe3C nanocrystals boost the activity of Fe−Nx. The coexistence of highcontent of Fe−Nx and sufficient metallic iron nanoparticles is essential forthe high ORR activity. DFT calculation corroborated this conclusion by indicating that the interaction between metallic iron and Fe−N4 coordination structure favored the adsorption of oxygen molecule. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.
4. 本文提出另一种选择性刻蚀PtNi表面Ni的方法，这种方法得到的PtNi材料比传统的酸刻蚀得到的PtNi材料活性更高，稳定性更好。本文通过两种方法的对比研究对leaching机理以及Ni对PtNi催化剂的作用有了更加深刻的认识。
We report new insights in dissolution mechanisms of nickel in PtNi bimetallic nanoparticles (NPs) to develop active and durable oxygen reduction catalysts for fuel cells. Leaching out nickel by using acidic aqueous solution has been regarded as one of the most efficient chemical treatments to obtain a platinum-rich surface, which has shown both increased activity and stability during oxygen reduction reaction. In this work, we introduce a new approach using hydroquinone dissolved in ethanol to leach out nickel from PtNi NPs. The degree of alloying level is followed by X-ray photoelectron and absorption spectroscopies. Electrochemical measurements including potential cycling under oxygen reduction conditions allow us to investigate the dissolution behavior of nickel, depending on the chemical systems, and assess the relationship with electrochemical activity and stability. From comparative studies regarding the traditional acid treatment and the hydroquinone method introduced in this article, it is revealed that, while acid treatment preferentially removes oxidized Ni clusters, hydroquinone dissolves Ni atoms close to surface platinum. Electrochemical measurements help with the understanding of the different leaching mechanisms and highlight the influence of alloyed nickel on the activity of platinum and durability of the catalyst in the oxygen reduction reaction.
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