Journal of Energy Chemistry 39 (2019) 23–28

Contents lists available at ScienceDirect

Journal of Energy Chemistry

journal homepage: www.elsevier.com/locate/jechem

An effective oxygen electrode based on Ir 0.6

Sn 0.4

O 2

for PEM water

electrolyzers

Guang Jiang

a , b , Hongmei Yu

a , ∗, Jinkai Hao

a , Jun Chi a , b , Zhixuan Fan

a , b , Dewei Yao

a , b , Bowen Qin

a , b , Zhigang Shao

a

a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b University of Chinese Academy of Sciences, Beijing 10 0 039, China

a r t i c l e i n f o

Article history:

Received 12 December 2018

Revised 28 December 2018

Accepted 12 January 2019

Available online 16 January 2019

Keywords:

PEM water electrolyzer

OER electrode

Low Ir loading

a b s t r a c t

An effective oxygen evolution electrode with Ir 0.6 Sn 0.4 O 2 was designed for proton exchange membrane

(PEM) water electrolyzers. The anode catalyst layer exhibits a jagged structure with smaller particles and

pores, which provide more active sites and mass transportation channels. The prepared IrSn electrode

showed a cell voltage of 1.96 V at 2.0 A cm

−2 with Ir loading as low as 0.294 mg cm

−2 . Furthermore,

IrSn electrode with different anode catalyst loadings was investigated. The IrSn electrode indicates higher

mass current and more stable cell voltage than the commercial Ir Black electrode at low loading.

© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of

Chemical Physics, Chinese Academy of Sciences

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. Introduction

Nowadays, hydrogen energy is in highly demand as a carbon-

ree and renewable energy. Hydrogen can be produced by steam

eforming, natural gas reforming and water electrolysis [1] . Water

lectrolysis provides several advantages over traditional technolo-

ies: high energy efficiency, renewable sources and high purity [2] .

Three techniques of water electrolysis are mainly employed: al-

aline water electrolysis (AWE), solid oxide electrolysis (SOEC) and

roton exchange membrane water electrolysis (PEMWE) [3] . How-

ver, AWE is limited by poor current density and operating pres-

ure, strong caustic electrolyte and bulk mass. SOEC is still under

evelopment without commercially available product [4] . PEMWE

an not only avoid the above disadvantages but endows fast re-

ponse to volatile renewable energy source [5,6] . Currently, high

apital cost and short lifetime hinder its further practical applica-

ion for PEMWE [7,12] . Due to the high consumption and scarcity

tore of iridium in membrane electrode assembly, PEMWE-made

ydrogen possesses high cost [3,5,9] . According to DOE’s report,

nly when the investment cost is dramatically decreased from

0 0 0 −20 0 0 € kW

−1 to 30 0–60 0 € kW

−1 , can PEMWE be com-

etitive for virtually large scale applications [8,12] . Furthermore,

hort lifespan is another key limitation [10,11] . Proton Onsite Co.

eported a 60,0 0 0 h test in 2017 which is still inefficient to AWE’s

∗ Corresponding author.

E-mail address: hmyu@dicp.ac.cn (H. Yu).

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ttps://doi.org/10.1016/j.jechem.2019.01.011

095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press

0,0 0 0 h [12,13] . It is urgent to develop cost effective electrode for

EMWE [14] .

To reduce hydrogen cost, many efforts have been made by many

nvestigators to improve the electrode structure and cut down the

oble metal consumption. Noble free metals, such as Fe, Ni, Li et

l., have been realized high activity at low overpotentials [15,16] .

owever, durability is inferior for the practical standpoint at

igh overpotentials. Moreover, rational electrode structures with Ti

17,18] , TiO 2 [19] , Ta 2 O 5 [20] , TiC [21] or Au/C [22] substrates were

onstructed to enhance the mass activities. KŠP et al. [21] fabri-

ated nano-sized Ir film on a TiC substrate to reduce the loading

f noble metal. However, poor performance (1.93 V @2.0 A cm

−2 )

nd high facility cost hinder the cutting down of the electrode

ost. Besides, Shi et al. [23] sprayed the catalyst layers innova-

ively on a water-swollen Nafion membrane to enhance the cat-

lyst layer structure. A current density of 2.0 A cm

−2 (@1.95 V)

as achieved when Ir loading was 2.0 mg cm

−2 . In Table 1 ,

ublished data is listed for comparison of the voltage, anode elec-

rocatalyst loading and the membrane of electrode. Currently, the

tudy about practical electrode with no noble catalyst is lack, as

rO 2 or RuO 2 is the main OER catalyst [24–27] .

Based on the above considerations, electrode was fabricated

ith effective OER catalyst (Ir 0.6 Sn 0.4 O 2 ) in this work. The features

re as follows: (1) smaller particle ( ∼10 nm) offers more surface

ctive reaction sites; (2) the dropping of Sn contributes to increase

ridium mass active and reduce electrode cost. Additionally, direct

pray is adopted to prepare the electrode, as it takes advantages

f reliable and feasible operation. Finally, a non-crystalline Nafion

and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

24 G. Jiang, H. Yu and J. Hao et al. / Journal of Energy Chemistry 39 (2019) 23–28

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layer has been introduced between the membrane and catalyst

layer, which provides larger interfacial area from the catalyst layer

to the solid electrolyte. Benefiting from the smaller particle and

larger interfacial area, IrSn electrode shows promising performance

at low catalyst loading.

2. Experimental

2.1. Preparation of MEA

The anode and cathode catalysts were Ir 0.6 Sn 0.4 O 2 synthesized

in our previous work [33] and commercial Pt/C (40 wt%, Johnson

Matthey Co.), respectively.

As mentioned in the reference [33] , the preparation of

Ir 0.6 Sn 0.4 O 2 was showed as follows. 0.012 M H 2 IrCl 6 • 5H 2 O and

0.08 M SnCl 4 • 5H 2 O were added into the mixture of deionized (DI)

water and Pluronic F127 (0.0 0 0 02 M) at 60 °C. After stirring for 6 h,

12 mL NaBH 4 solution (1 M) was added by slowly dripping to react

with metal precursors. Then, the reaction mixture was stirred for

12 h and aged for 12 h. Finally, the resultant powder was calcined

at 400 °C for 2 h in air atmosphere to terminate the reaction. Anode

and cathode catalyst ink was prepared by mixing catalyst, Nafion

ionomer (5 wt%, DuPont) and isopropanol (GR, Tianjing Kermel Co.)

with a mass ratio of 1:0.2:60 (catalyst: isopropanol: Nafion) . Then,

the harmonious mixture was prepared in an ultrasonic bath for 1 h.

For the spraying, 0.6 mg cm

−2 Nafion ionomers were firstly

sprayed on both sides of a Nafion 115 (N115) membrane on a hot

plate at 80 °C. Then, catalyst inks were sprayed to produce the cat-

alyst coated membrane (CCM). Ir 0.6 Sn 0.4 O 2 and traditional Ir Black

(Johnson Matthey Co.) electrodes in low, medium and high cata-

lyst loadings (1.5 mg cm

−2 , 1.0 mg cm

−2 and 0.5 mg cm

−2 , respec-

tively) were prepared by the same process. The cathode catalyst

loading was 0.4 mg Pt cm

−2 . Besides, IrSn electrode without anode

Nafion layer was prepared to compare the jagged structure. The

above samples are tagged as IrSn electrode and IrB electrode.

After spraying, membrane electrode assemble (MEA) was pre-

pared by sealing a pair of polyester frames. In detail, the CCM and

wet-proof carbon paper (attached on the side of cathode) were hot

pressed together at 140 °C and 0.1 MPa for 1 min. Anode gas diffu-

sion layer is porous Ti disc (0.8 mm in thickness). The active area of

the MEA is 5 cm

2 . Single cell was assembled by a pair of Pt-coated

Ti end plates.

2.2. Physicochemical characterization

Electrode surface and cross-section morphology were observed

with a scanning electron microscopy (SEM, JEOL, JSM-7800F).

Cross-section was obtained by quenching the sample in liquid ni-

trogen. The sample was obtained by immersing the electrode in al-

cohol and the structure was observed by transmission electron mi-

croscopy (TEM, JEM-20 0 0EX) at 120 kV. The crystal and electronic

structure of the electrode were investigated by X-ray diffraction

(XRD, PANalytical X’pert PRO) with a Cu- K α tube and X-ray pho-

toelectron spectroscopy (XPS, ESCALABXi) with an Mg anode as a

X-ray source, respectively. The XPS result was corrected with the

Table 1. Performance of the electrode in references.

Ref. Anode loading (mg cm

−2 ) Anode catalyst Membrane Ce

[28] 3.0 RuO 2 /SnO 2 Nafion 115 1.7

[29] 3.0 Ru x Pd 1- x O 2 Nafion 115 2.0

[30] 1.5 Ru 0.9 Ir 0.1 O 2 Nafion 115 1.8

[23] 2.0 Ir Nafion 117 1.7

[31] 1.91 IrO 2 Nafion 212 1.5

[2] 2.5 IrO 2 /Ti x Ta 1- x O 2 Nafion 117 1.8

[25] 1.0 Ir x Ru 1- x O 2 Nafion 212 1.6

[32] 1.94 IrO 2 Nafion 117 1.7

eak of adventitious carbon (C 1 s ) (284.4 eV). Half-cell test was car-

ied out by Gamry Interface 10 0 0E in a typical three electrodes

ystem, which contained a glassy carbon electrode coated with cat-

lyst layer as working electrode, a Pt foil counter electrode and a

aturated calomel reference electrode (SCE). Catalyst ink was the

ixture of 1 mL ethanol, 5.0 mg catalyst and 50 μL Nafion solution

5 wt%). 25 μL ink was coated on the surface of glassy carbon elec-

rode.

DI water was circulated at the anode side and heated to 80 °Cy heating rods in the end plates. Steady I −V curves were tested

y MATRIX MPS-3020 with step speed of 40 mA cm

−2 /25 s in the

urrent range of 0–300 mA cm

−2 and 100 mA cm

−2 /25 s in the cur-

ent range of 30 0–20 0 0 mA cm

−2 . Anode cyclic voltammograms

CV) were recorded with cathode at H 2 atmosphere as the refer-

nce electrode (RHE: H 2 /Pt, 80 °C) and the counter electrode at

he scan rate of 50 mV/s from 0 to 1.2 V. CV curve was measured

y Gamry 50 0 0E. Voltammetric charge Q was calculated as follow:

=

∫ E2

E1

| i | ν

d E (1)

here i is the current density obtained in CV, ν is the scan rate of

0 mV/s.

Electrochemical impedance spectroscopy (EIS) is an in-situ di-

gnostic characterization incorporated in single cell [25,34–36] .

t was carried out by using Solartron 1287 + 1260 at 1.45 V

nd 80 °C. The alternated signal is 10 mA with frequency from

0 kHz to 0.1 Hz. Nyquist impedance diagram was simulated by

View program of Solartron. Modeling circuit was R ohm

( R ct1 C dl1 )

R ct2 C dl2 ) as showed in Fig. S1 [31] .

. Results and discussion

.1. Physical characterization

Physical characterizations of IrSn electrode were carried out by

RD, XPS and TEM to confirm the structure of IrSn catalyst layer.

The IrSn catalyst layer was analyzed by XRD in Fig. 1 (a). As

afion ionomer is contained, the peaks at 17.77 ° and 26.24 ° can be

ndexed to the Nafion patterns. Typical IrO 2 lattice pattern is pre-

ented. Compared with IrO 2 JCPDS(03-065-2987), typical peaks at

5.16 °, 40.06 ° and 54.11 ° correspond to the (101), (200), and (211)

attices planes of IrO 2 , respectively. However, there is no clear fea-

ure of SnO 2 lattice pattern. This result indicates that the introduc-

ng of Sn did not change the crystalline phase of IrO 2 .

The chemical oxidation state of Ir in the IrSn electrode was

btained by XPS. For SnO 2 and IrO 2 crystallize in similar tetrag-

nal structure [37] , Sn is a desired dropping element to improve

he dispersion of Ir. In Fig. 1 (b), Ir 4 f 7/2 peak at 62.13 eV shifts to

igher binding energy in comparison with which of the reagent

rade IrO 2 (61.9 eV) [11] . As Ir obtains weak binding force, a higher

inding energy is beneficial to the catalytic activity. Besides, the

urface Ir content is 58.23% in Table.S1, which is in good agree-

ent with the mass percentage of Ir Sn O .

0.6 0.4 2

ll voltage (V @10 0 0 mA cm

−2 ) Cell voltage (V @20 0 0 mA cm

−2 )

23

3 2.45

7

5 1.942

3 1.62

7

7

1.9

G. Jiang, H. Yu and J. Hao et al. / Journal of Energy Chemistry 39 (2019) 23–28 25

Table 2. Electrochemical properties of the IrB and IrSn electrodes.

Loading (mg Ir cm

−2 ) Charge Q (mC cm

−2 ) R ct ( Ω cm

2 ) Cell voltage (V)

@0.3 A cm

−2 @1.5 A cm

−2 @2.0 A cm

−2

IrB electrode 0.5 72.4 1.8 1.572 1.97 2.144

IrSn electrode 0.‘ 109.8 0.6 1.543 1.85 1.963

Fig. 1. The physical characterization of IrSn electrode. (a) XRD patterns and (b) XPS spectra.

Fig. 2. SEM images of (a) the surface of IrSn electrode, (b) IrB electrode with anode

loading at 0.5 mg cm

−2 .

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Fig. S2 shows the nano-sized catalyst particles in a diameter

f about ∼10 nm. Nafion ionomer shows dendrite structure, which

hows the morphology of Nafion and catalyst.

.2. IrSn catalyst layer

The surface morphology was scanned by SEM. In Fig. 2 (a), IrSn

lectrode shows a uniform surface with small particles ( < 50 nm

n diameter) and pores. The smaller particle and pores attributes

o the nano-sized raw catalyst (Ir Sn O ). However, for IrB elec-

0.6 0.4 2

Fig. 3. Electrochemical activity at 80 °C of IrSn and IrB electrode.

rode in Fig. 2 (b), particle diameter is 20 0–50 0 nm. Lots of particles

re covered by ionomer, which will hinder the electrochemical ac-

ive site and mass transport.

Single cell CV was adopted to measure the voltammetric charge

[37] . In Fig. 3 (a), two peaks at 0.42 and 0.82 V vs. RHE corre-

pond well to the redox couples Ir III /Ir IV and Ir IV /Ir VI , respectively.

t is obvious that the peak currents of IrSn electrode are higher

han that of IrB electrode. A uniform structure provides more elec-

rochemical active sites. Generally, electrochemical active area was

escribed by voltammetric charge Q in the reaction of OER in

n Fig. 3 (b). IrSn electrode provides higher voltammetric charge

Table 2 ).

SnO 2 , which is similar to IrO 2 in lattice parameters, can not

nly promote the dispersion of the nanoparticle but also efficiently

emove absorbed hydroxyl spices to release more active sites of

ridium oxide [37] . Catalysts with different tin contents were in-

estigated in a half cell and a single cell. In Fig. S3(a), linear sweep

oltammetry (LSV) curves in a rotating disk electrode (RDE) was

hown. The over potentials of Ir 0.6 Sn 0.4 O 2 and Ir 0.8 Sn 0.2 O 2 are 270

nd 370 mV at 10 mA cm

−2 . Ir 0.4 Sn 0.6 O 2 obtains low OER activity.

hen, Ir 0.6 Sn 0.4 O 2 and Ir 0.8 Sn 0.2 O 2 electrode with the same Ir load-

ng (0.294 mg cm

−2 ) were tested in a single cell. In Fig. S3(b), the

ncreasing of cell voltage (82 mV) appears for the Ir 0.8 Sn 0.2 O 2 elec-

rode at 20 0 0 mA cm

−2 . Tin plays a positive role in a certain ratio.

(a) CV curves at 50 mV s −1 and (b) voltammetric charge Q .

26 G. Jiang, H. Yu and J. Hao et al. / Journal of Energy Chemistry 39 (2019) 23–28

Scheme 1. The illustration of the CCM with anode Nafion layer, and without Nafion layer.

Fig. 4. SEM pictures of the cross-section of the electrodes (a) IrSn anode without

sprayed Nafion layer, (b) IrSn anode with sprayed Nafion layer.

Fig. 5. Steady I–V curves of IrSn electrode with jagged structure.

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When tin content is over 40%, the electrochemical activities will

decrease sharply.

3.3. The jagged structure

As showed in Scheme 1 , a reference IrSn electrode without an-

ode Nafion layer was introduced to demonstrate the function of

jagged structure. On the cathode side of N115, double Nafion load-

ing (1.2 mg cm

−2 ) was sprayed to maintian the same total ohmic

resistance.

In Fig. 4 (a), a flat interface between the catalyst layer and

electrolyte is observed, while the electrode with Nafion layer ex-

hibits an embedded interface (jagged) structure in Fig. 4 (b). Fur-

thermore, the forming of jagged structure can be deduced. For

the Nafion ionomer and catalyst ink were sprayed in the shapes

of grain or dripping, it is a tendency for Nafion layer to form a

rough surface. The catalyst dripping will fill the rough surface of

Nafion layer, by which the jagged structure is fabricated success-

fully. However, for the IrB electrode, enormous particle and severe

agglomeration hinder the filling in defects. Pointed by the red line

in Scheme 1 , it is obvious that the electrode with Nafion layer pro-

vides larger interfacial area between electrolyte and catalyst layer.

Triple phase boundaries (TPB), as the significant sites of electrode

without anode Nafion layer at 80 °C electrochemical reaction and

mass transfer, is built by the unit of catalyst and ionomer. The

larger interfacial area will contribute to increase TPB.

Single cell test was adopted to intuitively certify the improve-

ment of TPB. In Fig. 5 , steady I–V curve shows slow cell volt-

age (about 44 mV @ 20 0 0 mA cm

−2 ) for the electrode with an-

ode Nafion layer. The result of EIS in Fig. S4 presents a smaller R ct

with Nafion layer. Only one difference is the anode Nafion layer

between the two electrodes. That means that low cell voltage and

low charge resistant benefiting from the larger the interfacial area

provided by the jagged catalyst layer.

3.4. Electrocatalyst loading effect

Electrodes with different electrocatalyst loadings were prepared

to investigate the relationship between the anode catalysts loading

with the cell performance. The morphology and thickness of the

anode catalyst layer were observed by SEM, in Fig. S5.It is obvious

that IrSn catalyst layer contains a jagged structure, particularly for

the low IrSn loading ones.

Steady I-V curves of IrSn and IrB electrodes are showed in

ig. 6 (a, b). For IrSn electrode, cell voltages only reach to 1.963,

.919 and 1.890 V (@2.0 A cm

−2 ) with low, medium and high

atalyst loading. However, cell voltages for IrB electrodes rise to

.144, 2.007 and 1.948 V (@2.0 A cm

−2 ), respectively. There is no

oubt that the IrSn electrode obtains lower cell voltage under the

ame catalyst loading. The IrSn electrode with low catalyst loading

hows the cell voltage is 1.963 V, less than 120 mV, compared with

he IrB under the same condition.

Herein, loading effect has been introduced to describe the re-

ationship between the catalyst loading and the single cell per-

ormance. In Fig. 6 (c), the cell voltage is 1.89 V at 2.0 A cm

−2

or IrSn electrode at 1.5 mg cm

−2 . It increased about 73 mV when

he reducing of catalyst loading to 0.5 mg cm

−2 . Moreover, the

ell voltage is 1.948 V at 2.0 A cm

−2 for IrB electrode under

.5 mg cm

−2 , it increased about 196 mV when the reducing of

atalyst loading to 0.5 mg cm

−2 . It suggests that the IrSn elec-

rode shows more stable cell voltage than IrB electrode. It is

ore possible to develop a low Ir loading electrode with IrSn

atalyst.

Current density normalized by Ir mass has been introduced to

valuate Ir utilization in Fig. 6 (d). As Ir 0.6 Sn 0.4 O 2 was used as an-

de catalysts, Ir mass fraction is about 59.8%. It means that the Ir

oadings are 0.294, 0.588 and 0.882 mg cm

−2 , respectively for the

rSn electrode with low, medium and high catalyst loading. Then,

r mass normalized current (Mass Current in Fig. 6 d) is obtained

rom current density normalized by Ir loading. At the cell voltage

f 1.95 V, mass currents are 6720 mA mg Ir −1 and 2950 mA mg Ir

−1

or 0.5 mg cm

−2 IrS and IrB electrode, respectively. The mass cur-

ent for IrSn electrode is two times higher than that of IrB elec-

rode. There is no doubt that the IrSn electrode is beneficial for

educing Ir loading and cutting down cost.

In Fig. S6, when the catalyst loading decreases, charge resis-

ance of IrSn electrode changes from 0.33 to 0.58 Ω cm

2 , while

rom 0.50 to 1.80 Ω cm

2 of IrB electrode. The growth of charge

esistance also fits “loading effect”. The lower and more stable R ct

f IrSn electrode may attribute to the uniform structure.

G. Jiang, H. Yu and J. Hao et al. / Journal of Energy Chemistry 39 (2019) 23–28 27

Fig. 6. Steady I–V curves at 80 °C of (a) IrSn electrodes, (b) IrB electrodes. (c) Comparison of cell voltages at 2.0 A cm

−2 . (d) Ir mass normalized current.

Fig. 7. Stability test for 1.5 mg cm

−2 IrSn electrode at 10 0 0 mA cm

−2 and 80 °C.

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Stability test for the IrSn electrode (1.5 mg cm

−2 ) was carried

ut at a constant current density of 10 0 0 mA cm

−2 and 80 °C. In

ig. 7 , the cell voltage increases from 1.70 to 1.748 V after 100 h

peration. The degradation rate is about 480 μV h

−1 .

. Conclusions

Nano-sized IrSn catalyst makes it possible to decrease the

hemical polarization and improves the advanced catalyst struc-

ure. Sn specie can not only increase Ir utilization, but also improve

lectrochemical activity by changing the electronic structure. Sec-

ndly, the catalyst layer with uniform surface and smaller particle

akes contribution to enlarge the voltammetric charge Q . Finally,

he jagged structure provides more electrochemical active sites by

he larger interfacial area between the catalyst layer and the elec-

rolyte.

With the above advantages, two aspects have been achieved by

rSn electrode. One is the lower cell voltage, which is only 1.963 V

t 2.0 A cm

−2 with Ir loading of 0.294 mg cm

−2 . Indicated by CV

nd EIS test, IrSn electrode shows lager voltammetric charge and

maller charge resistance. The other one is the stable loading ef-

ect. When catalyst loading decreases, IrSn electrode obtains more

table cell voltage. Besides, double mass current is achieved by

rSn electrode, compared with IrB electrode. The IrSn electrode is

romising for practical PEM electrolyzers.

cknowledgments

This work was financially supported by the National Natural

cience Foundation of China ( U1664259 ) and State Grid Corpora-

ion of China (No. SGTYHT/15-JS-191 , PEMWE MEA Preparation and

egradation mechanism).

upplementary materials

Supplementary material associated with this article can be

ound, in the online version, at doi: 10.1016/j.jechem.2019.01.011 .

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