Science - USA (2020-07-10)

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diffusion, which is different from the well-

known Grotthuss and vehicle mechanisms

( 2 , 3 ), was confined to the NaxCoO 2 /CeO 2

heterostructure interface (see the figure).

This was driven by an induced local elec-

tric field (LEF) within the interface region

because of the unbalanced charge distribu-

tion between the Ce–O and Co–O layers.

The authors demonstrated

a cell with 400-mm electro-

lyte thickness that achieved

peak power densities of

1000 and 830 mW cm–2 at

furnace temperatures of

520° and 490°C. These val-

ues are substantially higher

than the benchmark PCFC

with a well-known thin-

film BaCe0.7Zr0.1Y0.1Yb0.1O3-d

electrolyte (455 mW cm–2

at 500°C) ( 4 ) and the OCFC

(500 mW cm–2 at 550°C) ( 5 ).

Protonic conductors with high conductiv-

ity are also in demand for hydrogen produc-

tion, electrochemical synthesis of ammonia,

dehydrogenation or hydrogenation, hydro-

gen separation, and other applications. In

the reversed operation, the PCFCs can func-

tion as protonic ceramic electrolyzer cells

(PCECs) for hydrogen production through

steam electrolysis ( 6 ). Compared with low-

temperature (below 100°C) polymer-based

electrolyzer cells that require a noble metal

catalyst such as platinum and high electri-

cal energy input, a nonprecious catalyst

can be used for PCECs, with the required

electrical energy input partially replaced by

thermal energy. Thus, the reversible opera-

tion of PCFCs makes them a good candidate

for storing excess renew-

able solar or wind power

and effective for utilization

of industrial waste heat.

Ammonia synthesis is

a key industrial chemical

process because ammonia

is widely used in fertilizer

and many other industries.

Unlike the dominating

Haber-Bosch process at a

high pressure (~30 MPa) and

with a low conversion (15%),

protonic conductor–based

electrochemical cells can synthesize am-

monia at atmospheric pressure with a high

hydrogen-to-ammonia conversion of 78% ( 7 ).

Successful implementation of this technol-

ogy requires protonic conductors with high

conductivity and suitable catalysts with high

selectivity toward ammonia synthesis.

The fuel cell properties reported by Wu

et al. are inspiring because they offer a

different strategy for the development of

proton-conducting electrolyte materials.

It is notable how the LEF can accelerate

the hopping of the proton but not the elec-

tron. If such a mechanism is reliable, it

may also be applied for the development

of electrolytes with high oxygen-ion con-

ductivity. Much more work is still needed

to realize the practical application of the

authors’ device. The composite electrolytes

were developed by means of dry pressing

without high-temperature sintering, which

may make the electrolytes porous. Direct

oxidation of fuel by oxygen gas through

the pores will reduce the overall fuel ef-

ficiency ( 8 ). The metastability of NaxCoO 2

under a hydrogen atmosphere is another

concern. The reduction of this compound

will lead to the formation of metallic co-

balt and NaO. The NaO could further react

with water to form NaOH. This compound

itself has a low melting point of 318°C,

and molten NaOH can conduct OH– at

the fuel cell’s operating temperature of

370° to 520°C.

The electrode that the authors devel-

oped is rich in cobalt and nickel oxides, and

both of these compounds are widely used

as electrodes in conventional alkaline fuel

cells. Additional comprehensive studies will

help illuminate the processes involved in

the fuel cell operation. An evaluation of the

long-term stability of the cell is important

for determining whether there is a viable

pathway to commercialization.

Different cathode and anode materials

are needed for improving highly efficient

PCFCs that operate in the 300° to 500°C

range. For example, sulfur deposits and

coke can build up on anode materials, so

developing materials that are resistant to

these processes will improve performance

and durability. Water produced at the cath-

ode side is especially harmful because it

may impede the oxygen reduction over the

cathode of PCFCs. Solving these sorts of

problems should lead to the successful ap-

plication of protonic conductors with high

conductivities in PCFCs and other relevant

electrochemical systems. j

REFERENCES AND NOTES

1. Y. Wu et al., Science 369 , 184 (2020).


2. H. Wang et al., J. Ind. Eng. Chem. 60 , 297 (2018).


3. C. Zhou et al., J. Mater. Chem. A 7 , 13265 (2019).


4. C. Duan et al., Science 349 , 1321 (2015).


5. T. Suzuki et al., Science 325 , 852 (2009).


6. L. Lei et al., Adv. Funct. Mater. 29 , 1903805 (2019).


7. G. Marnellos, M. Stoukides, Science 282 , 98 (1998).


8. H. Xu et al., J. Power Sources 440 , 227102 (2019).

ACKNOWLEDGMENTS

M.N. is thankful for the grants (project nos. PolyU 152214/17E

and PolyU 152064/18E) from the Research Grant Council,

University Grants Committee, Hong Kong SAR.

10.1126/science.abc9136

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The Grotthuss mechanism Super proton transportation

by means of metallic state–induced LEF

The vehicle mechanism

Zr O Zr

O O O O

OH

H+

H+

H+

Zr OO Zr O

Zr O Zr

O Ba Ba Ba

Ba Ba Ba

O O O

Zr O Zr

O O O O

O Zr OO Zr O

O O O O

O Zr OO Zr O

H+ H+

Zr Zr

O O O O

OH- O

OH-

OH-

H+

Zr OO Y O

Zr O Zr

Vo

Vo

Vo

Vo Vo

Vo

Na VNa

Na Na

Na

Na Na

Na

Na

Na

Na

Na

Na

VNa VNa

VNa VNa VNa

Vo Vo

Ba Ba Ba

Ba Ba Ba

O

Y Zr

O O O

O Zr OO Zr O

O O O O

O Zr O Zr O

H+

CeO 2

NaxCoO 2

Interface

“In the reversed

operation, the PCFCs

can function

as protonic ceramic

electrolyzer

cells...for hydrogen

production...”

10 JULY 2020 • VOL 369 ISSUE 6500 139

Three different proton diffusion mechanisms

Proton (H+) transport through electrolytes is attractive for lower-temperature applications. The Grotthuss

mechanism allows either an “excess” H+ or a defect to diffuse through the hydrogen bond network of water

molecules through the formation and cleavage of bonds. The vehicle mechanism allows H+ to migrate bound

to another element. An example is the formation of OH– and an oxygen vacancy (Vo) that occurs in perovskite

structured oxides. Wu et al. propose transport by a local electric field (LEF)–promoted diffusion mechanism

in which H+ migrates along the oxide interface. This requires an unbalanced charge distribution generated from

oxygen and sodium vacancies (VNa).