— 前言—

　　固体氧化物燃料电池的发展依赖于高效的阴极氧还原催化剂。目前，钙钛矿型氧化物，比如La1?xSrxCoO3?δ(LSC)，其催化活性十分优越，但是性能衰减严重。本文通过原位电化学阻抗谱揭示了LSC表面只有一些位点是高活性的。当表面沉积4%单层的SrO时，活性会严重衰退；而Co的则会重新活化催化剂。

　　— 图文快解 —

　　通常认为钙钛矿型氧化物的ORR动力学衰减，与表面Sr的富集有关。另外，也有可能与反应气体中的污染物引起的沉积有关。但是，不论是Sr的富集，还是污染物的沉积，其详细的机理都不清楚。比如，以Sr富集为例，可能形成 致密绝缘的SrO (or Sr(OH)2/SrCO3) ，也有可能堵塞ORR的活性位点。

　　在本文的结果表明，LSC表面的ORR活性位的非均匀性，只有少量的高活性位点存在。表面组成的轻微变化，都可能降低活性位数量，进而严重降低活性。

　　本文的创新点：（1）通过沉积组成成分的量，可以精细调控LSC表面（2）原位阻抗谱表征沉积SrO, Co3O4and La2O3过程的变化和电化学行为。

图1 LSC表面上的ORR活性位点的不均匀分布

　　Figure 1: Sketch of an LSCelectrode with an inhomogeneously active surface for the oxygen reductionreaction (ORR).The LSC surface is largely but notcompletely SrO-terminated10; a few highly active sites, either Co itself or inthe vicinity of Co, catalytically enhance the ORR.

图2 PLD的原位阻抗装置

　　Figure 2 a, Setupduring the ablation of target material with the sample positioned below thetarget and plasma plume. b,Sample with the macroscopic 5 × 5mm2 denseLSC working electrode contacted by a Pt tip from above and the porous LSCcounter electrode connected to a Pt sheet. c,Sketch of the entire setup (WE, working electrode; CE, counter electrode).

图3 通过表面修饰得到的阻抗图谱

　　Figure 3 RepresentativeNyquist plots normalized to the surface area of the LSC working electrode ofas-prepared thin films decorated with deposits from ‘Sr’ (a) and ‘Co’ (b)target measured in situ at 450°Cand 5 × 10?1 mbar p(O2). The legends specify the total number of laser pulsesafter which the impedance spectra were recorded.

图4 用不同材料对LSC表面修饰后，Rsurf exch的变化

Figure 4 a, Dependence of the surface exchange resistance and surfaceexchange coefficient kq on the number of laser pulses used to decorate the LSC working electrodesurface of 16 samples at 450°C in 5 × 10?1 mbar p(O2). b, Surface exchange resistanceplotted versus the amounts of cations deposited on the LSC working electrodesurface (log?log plot, lines are guides to the eye); the surface exchangecoefficientkqplotted on the rightordinate is not an independent quantity but can be simply calculated from themeasuredRsurfexch by37kq = RT/(4F2Rsurf exchcO) with R being the gas constant, T the absolute temperature, F Faraday’s constant, and cO the oxygen lattice site concentration in LSC. The nominal amount ofcations needed to form a (100) perovskite monolayer is indicated as well. c, Increase of the surfaceexchange resistance relative to its initial value with ongoing deposition of‘Sr’ and ‘Sr (sc)’. Effects of ‘Sr’ and ‘Sr (sc)’ decoration are the samewithin the error bars(coloured area). Error bars show the standard deviation of the mean from threesamples including two to three measurements for each sample.

图5 用两种氧化物修饰后表面阻抗的变化

　　Figure 5 Decrease of the surface exchange resistance ofsamples already decorated with 88 laser pulses of the ‘Sr’, ‘Sr (sc)’ and ‘La’target after applying 10, 30 and 80 laser pulses to the ‘Co’ target at 450°C in 5 × 10?1 mbarp(O2).

图6 LSC非均匀活性表面的模型

　　Figure 6 a, Sites ofas-prepared thin films with Co termination (violet) show a higher exchange ratethan regions with Sr termination (black). b,SrO causes an activity decrease (resistance increase). c, Co-oxide decoration leads toan increase of the ORR activity. d,The surface of a SrO decorated LSC electrode can be reactivated to its initialvalue by adding Co-oxides, provided the SrO layer is not too thick yet.