voltage (OCV) of 1.07 V (Fig. 3B), with good
reproducibility (fig. S7). Such enhancement of
the power output is attributed to the synergic
effects of the LEF at the interface and the
metallic state of the NCO surface, which
accelerated proton transport through the in-
terface of NCO and CeO 2. The NCO/CeO 2 de-
vice can even run at 370°C and maintains a
power density output above 100 mW cm−^2
(Fig. 3B). The high OCV (e.g., 1.07 V at 520°C)
is in agreement with the Nernst theoretical
potential, indicating that the ionic transfer-
ence number is near unity and the electron
conduction has been suppressed ( 18 , 19 ). We
have also investigated fuel cells that consist
of different NaxCoO 2 composite samples with
varied Na content (x= 0.47, 0.55, and 0.71) that
support the existence of the metallic state (fig.
S8). These cells demonstrated a power density
output of >550 mW cm−^2 at 520°C (fig. S9), thus
further supporting the electrolytic function of
the NCO/CeO 2 heterostructure.
The Nyquist plot of the Na0.6CoO 2 /CeO 2
(2/8) cell under an OCV condition measuredat 490°C is shown in Fig. 3C. The total ohmic
area-specific resistance of this cell is particu-
larly low (0.26 ohm·cm^2 ) for a NCO/CeO 2 thick-
ness of 400mm, which is comparable to the
target value (0.15 ohm·cm^2 )fora15-mm-thick
electrolyte used for high-performance PCFCs.
The Nyquist plot is modeled by an equivalent
circuit with an ohm resistor (Ro) and serial ele-
ments, each consisting of a resistor (Ri,i=1,2),
and a constant phase element (CPEi,i= 1, 2),
as summarized in table S2. The two semi-
circles (Fig. 3C) originate from the electrode
polarization resistances. The corresponding
ionic conductivity of NCO/CeO 2 measured in the
fuel cell condition is presented in Fig. 3D, in
comparison with several other oxygen ion–and
proton-conducting electrolyte materials ( 20 – 22 ).
NCO/CeO 2 exhibits the highest ionic conduc-
tivity (0.30 S cm−^1 at 520°C and 0.24 S cm−^1 at
500°C), as well as the lowest activation energy
(0.27 eV).
Proton conduction in NCO/CeO 2 was further
verified with two experimental approaches.
In the first, we constructed a fuel cell device
with a proton-conducting BaZr0.8Y0.2O 3 −d(BZY)
filter (fig. S10), where the NCO/CeO 2 was sand-
wiched between two BZY ion-filter layers, allow-
ing only protons to pass through and enabling
determination of proton transport property
( 23 ). The device with BZY filters achieved a
power output of 890 mW cm−^2 at 520°C (Fig.
3E), which is very close to the power output
(1000 mW cm−^2 ) of the NCO/CeO 2 fuel cell.
The difference can be attributed to the ohmic
losses caused by two additional BZY layers
with limited proton conductivity, which was
substantially lower than that of the NCO/
CeO 2 (Fig. 3D). Additional contact interfaces
between BZY and NCO/CeO 2 also contrib-
uted to polarization loss. This result verifies
proton-dominating conduction in the NCO/
CeO 2 heterostructure. In another experiment,
we measured the proton conduction isotopic
effect, in which conductivities of NCO/CeO 2
were measured in a 5% H 2 /95% Ar mixture
and a 5% D 2 /95% Ar mixture at various tem-
peratures. The associated conductivities are
substantially increased for 5% H 2 relative to
5% D 2 , exhibiting a clear H/D isotope effect
(Fig. 3F). This finding provides further evi-
dence of proton conduction in the designed
material ( 24 ).
Whereas the best-performing proton-
conducting BaZr0.4Ce0.4Y0.1Yb0.1O 3 thin-film
electrolyte fuel cell has a power output of
500 mW cm−^2 at 500°C, the NCO/CeO 2 device
has outputs of 800 mW cm−^2 at 500°C and
1000 mW cm−^2 at 520°C (Fig. 3B). In addition,
NCO/CeO 2 as an electrolyte in different molar
ratios also generates substantial outputs (fig.
S11). The NCO/CeO 2 heterostructure has dem-
onstrated promising advantages for PCFCs
( 25 , 26 ). The durability of the NCO/CeO 2 de-
vice was tested under 100 mA cm−^2 during a186 10 JULY 2020•VOL 369 ISSUE 6500 sciencemag.org SCIENCE
Fig. 3. Performance and proton transport measurement of NCO/CeO 2 cell.(A) Power output of
NCO, CeO 2 , and NCO/CeO 2 devices operated at 520°C. (B)I-VandI-Pcharacteristics of the NCO/CeO 2
device operated at various temperatures. (C) Nyquist plot obtained under an OCV condition at 490°C
for a Ni/NCAL/NCO/CeO 2 /NCAL/Ni cell. Z, the complex impedance measured from the fuel cell;
L, inductance; Ro,R 1 , and R 2 , resistors; CPE 1 and CPE 2 , constant phase elements. (D) Ionic conductivity
of NCO/CeO 2 compared with that of other oxygen ion–conducting electrolytes (dotted lines) and
proton conductors (solid lines). H-SNO, hydrogenated samarium nickelate; BCY, barium-cerium/yttrium
oxide; GDC, gadolinium-doped ceria; LSGM, lanthanum strontium gallate magnesite; BZY, yttrium-doped
barium zirconate; YSZ, yttrium-stabilized zirconia. (E)I-VandI-Pcharacteristics of the NCO/CeO 2
(2/8) device operated at various temperatures with proton filters. (F) Temperature dependence of
conductivities in the NCO/CeO 2 (2/8) device operated in a 5% H 2 /95% Ar mixture and a 5% D 2 /95%
Ar mixture at various temperatures.
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