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                                                                                                      【電催化】高穩定氧還原催化劑——具有拉伸應變效應的N摻雜Pt納米催化劑
                                                                                                      2020-03-19



                                                                                                      鉑基納米材料仍然是當前氧還原反應(ORR)*可行的催化劑,也是質子交換膜燃料電池(PEMFCs)中*為關鍵的材料。然而,對于鉑基催化劑而言,由于其儲量有限、價格昂貴、穩定性較差以及易被毒化等諸多因素的存在,嚴重制約了燃料電池發展。因此,如何提高催化劑的活性和耐久性、減少貴金屬用量,有效降低燃料電池的制造成本成為近些年來的研究熱點。


                                                                                                      目前常用的提升Pt基催化劑電催化活性和耐久性的策略主要包括:(1)組分調控;(2)晶面調控;(3)形貌調控。值得注意的是,目前高活性和高耐久性的Pt基電催化劑的報道絕大多數集中在過渡金屬摻雜以及形成Pt合金。目前,關于非金屬元素摻雜的Pt基催化劑的報道還非常少,尤其是關于N摻雜的Pt基材料的研究更是****。


                                                                                                      *近,中國科學院上海高等研究院聯合寧波中科科創新能源科技有限公司開發了一種N摻雜的具有極高氧還原穩定性的Pt/C催化劑。通過均相沉淀法,在水溶液中,制備得到尺寸均一、粒徑小、分散度高的N摻雜Pt黑和Pt/C催化劑。通過此方法制備的催化劑展現出能與商業化Pt催化劑相媲美的活性以及極其優異的穩定性。經過20,000次加速循環耐久性測試(ADT),質量比活性衰減僅為3.7%,遠低于商用鉑碳催化劑。特別值得注意的是,該方法步驟簡單,沒有使用表面活性劑,并且已經實現催化劑單批次百克級制備。因此,這種具有前景的制備策略為經濟高效地生產Pt基催化劑提供了巨大潛力,相關論文發表在Journal of Catalysis 雜志上。


                                                                                                      經過課題組多年基礎的積累以及反復地實驗,經過酸洗、干燥,*后制備得到尺寸均一、粒徑小、分散度高的Pt/C催化劑(Pt的質量百分數為60%)。

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                                                                                                      Fig. 1.?Schematic illustration of the synthetic strategy of N-doped?Pt/C catalysts.

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                                                                                                      通過XPS和EXAFS表征發現,通過這種方法合成得到Pt/C催化劑存在著Pt-N鍵的信號。此外,XRD結果表明該催化劑中Pt的(1 1 1)晶面與商用鉑碳催化劑相比有0.23o的負移。通過球差校正電鏡結果的進一步分析,發現合成的Pt/C催化劑中Pt的晶格發生明顯扭曲,EELS能譜在Pt顆粒中檢測出了明顯的N信號。通過計算發現Pt(1 1 1)晶面的晶面間距為0.232 nm,較商業化Pt/C的0.229 nm有約1.3%的增加。作者認為,N原子摻雜進了Pt原子的晶格中,并且N的摻雜使Pt晶格的晶面間距增大,產生晶格拉伸應變效應。

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                                                                                                      Fig. 2.?(a)?XPS spectra for N 1s in N-doped Pt NPs/C.?(b) XPS spectra for Pt 4f in N-doped Pt NPs/C and N-doped Pt NPs/C-H2. (c)?XRD diffraction patterns of different catalysts. (d) Enlarged region of the (111) diffraction peaks of Fig. 2c. (e) Pt L3-edge XANES for all?the samples. (f) The k3-weighted R-space Fourier-transform EXAFS spectra?of different catalysts and reference samples.

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                                                                                                      Fig. 3. (a) High resolution HAADF-STEM image of N-doped Pt NPs.?(b) HAADF-STEM?image of one N-doped Pt NP.?(c)?N element K-edge?EELS spectrum of the N-doped Pt NP in Fig. 3b. (d) The integrated pixel?intensity taken along the Pt (111) spacing direction marked by purple square in Fig. 3b.?Inset of Fig. 3d is FFT pattern from the purple square at the Pt NP shown in Fig. 3b.

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                                                                                                      隨后,作者對比了該N摻雜的Pt/C催化劑與商用鉑碳催化劑的ORR電催化活性和穩定性。該N摻雜的Pt納米粒子展現出能與商用鉑碳催化劑相媲美的ORR活性以及極其優異的耐久性。其ECSA和商用鉑碳催化劑相近(圖4a);0.9V(vs. RHE)電位下ORR質量比活性較商業化Pt/C提高了5%(圖4b)。經過20000次加速耐久性循環測試,發現該N摻雜Pt納米粒子的ECSA僅減小11.5%,明顯低于商用JM鉑碳催化劑的降低量(圖4e);且在0.9V/RHE的ORR質量比活性僅衰減3.7 %,顯著低于商用JM鉑碳催化劑(30.9%)(圖4f),成為已有報道中穩定性*好的Pt/C 電催化劑之一。在氫空燃料電池單電池測試中,電池電流密度為1.4A·cm-2時,電壓為0.65 V(如圖4g)。


                                                                                                      DFT理論計算表明,N原子的摻雜誘導了晶格拉伸應變效應,導致Pt上電子轉移到N上,Pt原子間因為電子斥力而產生減弱的相互作用。與純Pt相比,N摻雜的Pt納米粒子中Pt原子間的結合能以及原子脫附能更高,因此ORR過程中表層Pt原子更難被溶解、催化劑穩定性也更好(如圖5)。


                                                                                                      值得關注的是,目前所報道的Pt基催化劑,絕大多數仍以合成克級甚至毫克級為單位。而本文介紹的制備方法步驟簡單,沒有使用表面活性劑,已在寧波中科科創新能源科技有限公司實際生產中實現單批次百克級制備(如圖6)。

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                                                                                                      Fig. 4.?(a)?CVs of different catalysts in 0.1 M HClO4?solution at a scan rate of 50 mV?s-1.?(b) ORR polarization curves on?different catalysts?in O2-saturated 0.1M HClO4?with a scan rate of 10 mV?s-1?and rotation speed of 1,600 rpm. (c-d)?ORR polarization curves on different catalysts before and after 20,000 ADT cycles between 0.6 and 1.1 V/ RHE. (e) The changes in ECSAs of the different catalysts before and after 20,000 cycles. (f) The changes in mass activities of the different catalysts before and after 20,000 cycles. (g) Single cell performance of H2-air fuel cells prepared with N-doped Pt/C and commercial JM Pt/C. (Pt loading: anode-0.1 mg·cm-2; cathode-0.3 mg·cm-2. Testing condition: 80 oC, 100 RH%, back-pressure=1 atm.)

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                                                                                                      Fig. 5. (a) Pure Pt NP and (b) N-doped Pt NP. The grey and red?balls stand for the Pt?and N atoms, respectively. (c) The tensile strain of Pt NP as a function of the number of N atoms embedded into the NP. (d) Atom removal?energy of pure Pt NP and N-doped Pt NP at different atom positions (a, b, c and d represent the Pt atom position sites shown in Fig. S25). (e) Reaction free energy diagram of the ORR for two different reactive sites on N-doped Pt NPs.

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                                                                                                      Fig. 5. The scene photograph of synthesizing the N-doped Pt NPs in a large-scale.

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                                                                                                      原文:

                                                                                                      N-doping induced tensile-strained Pt nanoparticles ensuring an excellent durability of the oxygen reduction reaction


                                                                                                      Yunjie Xiong, Yunan Ma, Liangliang Zou, Shaobo Han, Hong Chen, Shuai Wang, Meng Gu,?Yang Shen, Lipeng Zhang, Zhenhai Xia,?Jun Li?and Hui Yang


                                                                                                      Journal of Catalysis, 2020, 382: 247-255 ??DOI:?10.1016/j.jcat.2019.12.025


                                                                                                      文章鏈接:

                                                                                                      https://www.sciencedirect.com/science/article/pii/S0021951719306220


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