could provide a highly conductive path in
termsof out-of-plane doping control of the
in-plane electronic charge ( 11 ). The diffusion
barriers are 3.17 eV (e 1 ) for proton migration in
CeO 2 , 3.89 eV (e 2 )inNa0.6CoO 2 , and 0.15 eV
(e 3 ) at the interface. The proton barrier energy
at the interface is reduced by a factor of >20
compared with those of bulk CeO 2 and NCO.
The interface between NCO and CeO 2 can
provide a weak bonding interaction, thus build-
ing a proton transport highway with much
lower migration energy (Fig. 1C).
The heterostructure exhibits a spherical
CeO 2 cluster epitaxially grown at NCO, form-
ing a uniform sphere-cluster–by–layer mor-
phology with a top CeO 2 cluster and an NCO
nanosheet (Fig. 2A). The high-resolution im-
age in Fig. 2B demonstrates that the (111) plane
of cubic CeO 2 has good contact with the (001)
planes of the NCO nanosheets. The crystal struc-
ture and component have also been charac-
terized (fig. S3). The oxygen K-edge spectra
(Fig. 2C) in different local chemical environ-
ments of NCO/CeO 2 display an additional
peak localized at 539 eV and associated with
the interface region. This additional peak
results from the distinctive electronic struc-
ture caused by the aggregation of oxygen in
the interface region ( 12 ). Kelvin probe force
microscopy (KPFM) is used to characterize the
interface properties of CeO 2 and NCO ( 13 – 15 ).
The topographic spatial map and the corre-
sponding surface potentials of NCO/CeO 2
(Fig. 2, D and E) reveal notable changes be-
tween interfaces. In an illustrative line scan
crossing the interface of NCO and CeO 2 (Fig.
2F), the surface potential difference between
NCO and CeO 2 is ~15 mV, demonstrating the
existence of a LEF at the NCO/CeO 2 interface,
which is also verified by the NCO/CeO 2 hetero-
structure film device (Fig. 2G) with a clear
rectifying effect.
We then performed chemical and structural
characterizations of the NCO/CeO 2 hydrogen-ation process. Hydrogenation occurs only in
the NCO structure, with H···OCo bond forma-
tion (peak at 1071 cm−^1 ) confirmed by in situ
Raman measurement (Fig. 2H and fig. S4). In
contrast, no change was observed in the CeO 2
structure (Fig. 2I). Ex situ x-ray absorption
near-edge structure (XANES) measurements
oftheCoK-edgeandCeL 3 -edge from pristine
and tested NCO/CeO 2 samples are shown in
fig. S5, confirming that hydrogenation takes
place only during the in situ process.
The results of the designed NCO/CeO 2 het-
erostructure demonstrated in a PCFC device
are presented in Fig. 3. The pristine NCO is
an electron conductor ( 16 , 17 ) and does not
show any performance in a fuel cell device
(Fig. 3A). Owing to the existence of oxygen
vacancies (fig. S6), pure CeO 2 electrolyte ex-
hibits 300 mW cm−^2 of power output at 520°C.
After formation of the NCO/CeO 2 heterostruc-
ture interfaces, the device reaches 1000 mW cm−^2
at 520°C, corresponding to an open-circuitSCIENCEsciencemag.org 10 JULY 2020•VOL 369 ISSUE 6500 185
Fig. 2. Characterizations of NCO/CeO 2 .(A) Transmission electron microscopy
(TEM) image. (B) High-resolution TEM image. (C)OxygenK-edgespectrain
different local chemical environments. a.u., arbitrary units. (D) Atomic force
microscopy image and (E) scanning KPFM image of NCO/CeO 2. The scanning
area is 2.5mmby2.5mm. (F) Contact potential difference along the white line in (E).
(G)I-Vcurve of the NCO/CeO 2 heterostructure deposited on an ITO (indium tin
oxide)/glass substrate with Au electrodes. (HandI)InsituandexsituRaman
spectra of NCO (H) and CeO 2 (I) under air and H 2 atmospheres.RESEARCH | REPORTS