工业温度下的重构化学及碱性电解水性能评估学术资讯

来源：X一MOL资讯

氢能具有来源广泛（包括化石燃料制取、化工副产物中提取、利用风能/太阳能等制备）、燃烧热值高（约为汽油的3倍）、清洁高效（产物为H2O）等特点，被业界专家称为“终极能源”。其制备、存储、运输及其商业应用已成为21世纪备受关注的热点；其中，制取氢气是大规模使用氢能源的首要环节。在电解制氢方面，目前碱性电解水技术最为成熟，是一种制备高纯氢的能源转化技术。考虑到我国电价处于中低水平，因此发展廉价、高效、稳定的非贵金属基催化剂是解决其能耗高问题和提高电解效率的主要策略。
在碱性电解水中，动力学缓慢的析氧反应（OER）是制约电解水反应整体效率的瓶颈。尽管当前在高效稳定的OER催化剂方面取得了一定的进展，其催化机制/本征活性物种的研究、OER/电解水的性能评估等内容，大多数是在常温（20-25 ℃）下开展的，与工业碱性电解水（一般在50-80 ℃）的测试参数存在差异；实际上，这些差异还包括电解液浓度、压力等。因此，研究工业参数下催化剂的重构化学及性能评估，对催化剂的合理设计、本征催化机制及应用具有推进作用。
近日，武汉理工大学麦立强教授、吴劲松教授和中国科学技术大学倪堃博士，以温度参数为例，研究了钼酸盐析氧前催化剂在常温和工业温度下的重构差异性和OER/电解水性能评估。重要发现点包括：
（1）温度调控的重构工程（Figure 1）：发现钼酸镍前催化剂在较高温度（>30℃）下测试后，恢复到常温时的OER过电位会显著降低；

Figure 1 a) Chronopotentiometric measurements of NiMoO4 pre-catalyst and nickel foam (NF) at 10 mA cm-2 in 1 M KOH with solution temperatures of T. In detail, the initial 0-12 h stage was tested at 25.0 ℃, and the next 12-48 h stage was tested at T ℃ (T = 25.0, 32.4, 39.6, 51.9), and the final 48-60 h stage was tested at 25.0 ℃. The finally obtained catalysts are denoted as cat.-T for subsequent performance evaluation. For NF-T sample, T is 51.9. b) Schematic diagram for the temperature-dependent potential curve, which associates with the thermal-induced reconstruction results. The reduced potential ΔV is based on the difference value of potentials at 60 and 12 h in (a). c-e) LSV curves, the corresponding overpotentials, and Tafel values of cat.-T tested at 25.0 ℃, respectively.
（2）非原位物相/微结构表征及原位高低温拉曼（Figure 2）：发现在常温下仅发生表面重构的钼酸镍前催化剂，在工业高温下可以发生全部相转变（全重构）并形成羟基氧化物催化活性物种，实现了催化组分的高效利用；高温促使催化剂活性提升归因于更多的高活性催化物种的产生；

Figure 2 a-d) HRTEM and e-h) HAADF-STEM images and the corresponding elemental mappings of cat.-T (T = 25.0, 32.4, 39.6, 51.9). The white dotted lines in (a-c) represent the obviously observed reconstruction boundary. i) In situ low-temperature Raman spectra and the corresponding chronopotentiometric curve of NiMoO4 pre-catalyst with the interval time of 200 s in 1 M KOH at 25.0 ℃. j) In situ high-temperature Raman spectra and the corresponding chronopotentiometric curve of NiMoO4 pre-catalyst with the interval time of 80 s in 1 M KOH at 52.0 ℃.
（3）重构程度（Figure 3）：当前涉及有限重构机理的报道很少，部分报道认为有限的电子/离子传输或金属浸出引诱的晶格氧氧化等电子结构的调制，是导致有限重构的主要原因。本工作发现，致密重构层是导致钼酸镍在常温测试时仅发生表面重构的主要原因；而高温环境加速了Mo物种的浸出和使得重构层变得疏松，因而促进了电解液的深度浸润和加深了重构程度；

Figure 3 a) HAADF-STEM image of NiMoO4 pre-catalyst after chronopotentiometric measurement at 10 mA cm-2 at 25.0 ℃ for 12 h. b) HAADF-STEM image of NiMoO4 pre-catalyst after chronopotentiometric measurement at 10 mA cm-2 at 25.0 ℃ for 12 h and then rising to 51.9 ℃. c) HAADF-STEM image and the corresponding elemental mappings of sample in (b), showing the dissolution of Mo species on the surface. d) Schematic diagram for the two reconstruction results of NiMoO4 pre-catalyst under low-/high-temperature electro-oxidation conditions.
（4）重构顺序（Figure 4）：发现高温碱刻蚀-重构的同步进行，是形成小颗粒和丰富晶界的原因。如果在电氧化重构前先进行碱刻蚀反应，则得到的催化剂是具有较高结晶性和较少晶界的片状结构，进而导致较低的催化活性。基于理论分析，也进一步证明了晶界-氧空位在高OER动力学方面的重要角色；

Figure 4 a) Chronopotentiometric measurements at 10 mA cm-2 in 1 M KOH. For in situ reconstruction, the NiMoO4 pre-catalyst was firstly measured at 51.9 ℃ and then at 25.0 ℃, and the obtained catalyst is called as in situ catalyst. For ex situ reconstruction, the NiMoO4 was firstly soaked in 1 M KOH at 51.9 ℃ and then measured at 25.0 ℃, and the obtained catalyst is called as ex situ catalyst. b-e) HRTEM images and the corresponding fast Fourier transform patterns of (b,c) in situ catalyst and (d,e) ex situ catalyst. f) O 1s XPS spectra of in/ex situ catalysts. g) Model of NiOOH (011)-NiOOH (011) twin boundary structure with O vacancy, showing the O vacancy site in the boundary region. h-j) DFT-calculated models with the optimal structures for intermediate products. k) Free energy of each reaction step on six different models, considering the effects of boundary, O/OH vacancies on OER. l) PDOS calculation of d band of Ni-2 atom near the O vacancy site in the boundary region in boundary with VO model. Inset: the partial charge density drawn from -7 to -6.8 eV with iso-surface value of 0.025 e- bohr-3, corresponding to the PDOS peak at -6.9 eV.
（5）全重构催化剂的优势（Figure 5）：发现热致全重构催化剂在常温和高温均具有非常高的催化稳定性（>250 h），归因于重构得到的催化/热稳定物相及独特的颗粒内连接结构。为评估工业温度下的碱性电解水性能，HER电极采用了我们前期报道的类Pt活性的MoO2-Ni异质纳米线（ACS Catal., 2019, 9, 2275），该材料可以在51.9 ℃至少工作115 h。在高温碱性电解水性能评估中（两电极系统），可以实现~1.49 V@10 mA cm-2并稳定工作220 h。

Figure 5 a,b) Chronopotentiometric measurement of cat.-51.9 and NiFe-LDH catalysts tested at 10 mA cm-2 in 1 M KOH at (a) 25.0 and (b) 51.9 ℃. c) Schematic diagram of NPs-interconnected structure with open 3D pores. d-g) Representative HAADF-STEM images via electron tomography of single cat.-51.9 nanowire after long-term stability measurement in (b). h) The conventional condition (almost at room temperature) has a certain gap with the industrial one (at 50-80 ℃). i) Schematic diagram of MoO2-Ni arrays on the nickel foam. j) Chronopotentiometric measurement of MoO2-Ni heterostructured nanowire arrays at 10 mA cm-2 in 1 M KOH at 51.9 ℃. k) Durability evaluation of high-temperature water electrolysis based on cat.-51.9 (anode) and MoO2-Ni (cathode) array system at 10 mA cm-2 in 1 M KOH at 51.9 ℃.
展望
（1）本工作以温度为例，探究了前催化剂在工业温度与常规测试温度下的重构差异性，尽管如此，考虑到工业电解水条件参数的复杂性，其它参数对重构及催化机制/性能评估的影响还有待进一步探究；（2）当前很多前催化剂的重构程度较浅（一般表面重构层厚度<10 nm），这种有限重构机制还有待深入研究；（3）本工作得到了钼酸盐前催化剂的重构程度越深、催化性能越好的结果，那么重构程度-催化性能之间的关系还有待进一步探索，即是否存在特例。（4）其它催化反应，如HER、CO2还原等，也可以探究实际条件下的重构化学和性能评估；（5）本工作提供了一种基于催化重构工程的新结构材料的制备新策略。
这一研究成果发表在Advanced Materials，论文第一作者是武汉理工大学博士刘熊，武汉理工大学麦立强教授、吴劲松教授和中国科学技术大学倪堃博士是通讯作者。

来源：X-molNews X一MOL资讯

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