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Advanced energy materials: synthesis of noble metal monatomic catalysts for oxygen reduction by sequential coordination method and their active point coordination principles

wallpapers News 2020-12-23

proton exchange membrane fuel cells (PEMFCs) are clean energy power generation devices which are suitable for electric vehicles power plants. However the large-scale application of PEMFCs is hindered by the high price of Pt based catalysts the limited reserves of Pt on the earth. At present the activity stability of non noble metal catalysts carbon based metal free catalysts are still difficult to meet the application requirements. Therefore it is an urgent need to develop other noble metal oxygen reduction catalysts improve the utilization of noble metal atoms. Single atom catalysts (SACS) have the highest atom utilization rate the electronic interaction between metal support can adjust the d-b center of metal atoms thus affecting the catalytic activity. However the combination principle of metal atoms coordination anions at active sites is not clear which leads to the blindness of catalyst design. In addition efficient sacs need high metal loading. Zif-8 is a common precursor of sacs. Non precious transition metals rare earth metals can be added into zif-8 by doping (replacing Zn with foreign metal ions) or encapsulation (fixing target metal ions with micropores) to prepare high metal loading sacs. The metal loading of zif-8-based sacs prepared by traditional methods is usually very low (< 0.2 wt%) due to the weak coordination ability of noble metal ions compared with zinc ions or the large ion radius.

Professor Shui Jianglan School of materials science engineering Beijing University of Aeronautics Astronautics developed a sequential coordination method which successfully doped a large number of noble metal atoms into zif-8 framework prepared a series of nitrogen doped carbon supported PM (PM = IR Rh Pt PD) sacs (pm1-n / C) after pyrolysis. The noble metal loading in the catalyst is as high as 1.2 ~ 4.5% which is almost an order of magnitude higher than that of the traditional method. In acidic electrolytes ir1-n / C rh1-n / C exhibited higher orr activity than IR / C Rh / C while pd1-n / C pt1-n / C exhibited significantly lower orr activity than Pd / C Pt / C. The density function calculation shows that the orr activity of PM sac is determined by the strength of OH * adsorption the electronegativity of coordination anions on the crystal surface of PM (111): the stronger the metal adsorbs OH * the higher the electronegativity of coordination anions. IR (111) Rh (111) crystal faces have poor activity for orr intermediates such as OH * because of their strong adsorption capacity. The adsorption capacity for OH * is weakened by coordination with four strong electronegative n so ir1-n / C rh1-n / C show significantly improved orr activity. On the contrary the adsorption of OH * by Pt (111) Pd (111) is relatively weak the adsorption of OH * by metal atoms is further weakened by N coordination resulting in lower orr activity of pt1-n / C pd1-n / C than Pt / C Pd / C. The results show that ir1-n / C exhibits very high performance the limiting power density of ir1-n / C is 870 mwcm-2 in the oxyhydrogen fuel cell. At the same time the mass power density of IR is much higher than that of commercial Pt / C due to its high atomic efficiency. The principle of active site coordination in this report is helpful for researchers to design / prepare more kinds of high performance orr catalysts for fuel cells.

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