The widespread application of intermediate-temperature solid oxide fuel cells is mainly being hurdled by the cathode's low efficiency on oxygen reduction reaction and poor resistance to carbon dioxide impurity. Here we report the fabrication of a hierarchical shell-covered porous cathode through infiltration followed by microwave plasma treatment. The hierarchical shell consists of a dense thin-film substrate with cones on the top of the substrate, leading to a three-dimensional (3D) heterostructured electrode. The shell allows the cathode working stably in CO2-containing air, and significantly improving the cathode's oxygen reduction reactivity with an area specific resistance of ∼0.13 Ωcm2 at 575°C. The method is also suitable for fabricating functional shell on the irregularly shaped substrate in various applications.
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Figures at a glance
Figure 1: Schamatic of preparation of the hierarchical LN shell protected BSCF cathode. 
(a) A two-step infiltration process is empolyed to introduce porous LN precursor shell onto the surface of BSCF scaffold and followed by microwave plasma treatment to obtain hierarcical LN shell. (b) In the 1st infiltration of the two-step infiltration process, La(NO3)3 and Ni(NO3)2 aqueous solution is infiltrated into BSCF scaffold. The LN substrate shell is obtained after heating at 850°C for 5 h. In order to obtained hierarcical LN precursor shell, ctrate added La(NO3)3 and Ni(NO3)2 aqueous is inflitrated and fired at 850°C for 5 h. Here the ctrate is used as the template to obtain bigger pores in the second LN shell. Finally, the microwave-plasma is used to heat the precurosr shell to make hierarchical LN shell.
Figure 2: SEM images of the LN shell on BSCF scaffold prepared in different conditions. 
The LN substrate shell with 6wt% LN loading before (a) and after (b) microwave plasma treatment. The hierarchical LN shell with 12 wt% LN loading before (c) and after (d) microwave plasma treatment. The hierarchical LN shell with 26 wt% LN loading (the second infiltration) before (e) and after (f) microwave plasma treatment.
Figure 3: CO2 resistvity of the BSCF powders and the BSCF-LN powders with 26wt% LN loading before or after MP. 
The three types of powders were treated in air+10vol% CO2 at 600°C for 1 h. (a) FT-IR spectra of the treated powders. The blue arrows indicates the vibrating peaks for carbonates. (b) CO2-TPD of the treated powders. The peak area related to the amount of the carbonates.
Figure 4: Electrochemical analysis of the electrodes. 
(a) Arehnius plots of ASRs of the various electrodes based on the symmetric cells (BSCF-6%LN and BSCF-26%LN are the BSCF-LN cathodes before MP treatement). (b–d) The impedance spectroscopy of the electrode tested at 600°C in air, air+10vol% CO2, and after removal of CO2 from air. (b) 6wt% LN loading before microwave plasma treatment; (c) 26wt% LN loading before microwave plasma treatment; (d) 26wt% LN loading after microwave plasma treatment. The time in the bracket indicates the time after the introdtuction of CO2 into air or after the remavol of the CO2.
Figure 5: Stability of the hierarchical LN protected BSCF cathode under working conditions. 
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