Applied Catalysis B: Environmental 117–118 (2012) 317–320

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Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb

Polymer electrolyte fuel cell supplied with carbon dioxide. Can be the reductant
water instead of hydrogen?
Motoharu Morikawa a , Naveed Ahmed b , Yuta Ogura a , Yasuo Izumi b,∗ab

Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 14 November 2011
Received in revised form 6 January 2012
Accepted 22 January 2012
Available online xxx
Keywords:
Carbon dioxide
Water
MethanolFuel cell
PEFC
ReductantProton transfer

a b s t r a c t
Polymer-electrolyte fuel cell supplying CO2 was demonstrated. Using a PEFC, 95 mol% of methanol
was selectively formed by supplying H2 to Pt catalyst and CO2 to Zn-Cu-Ga layered double hydroxide catalyst accompanying protons and electrons transfer, i.e. H2 → 2H+ + 2e− at anode followed by
CO2 + 6H+ + 6e− → CH3 OH + H2 O at cathode. Using PEFC, CO2 reduction with water was also enabled tomethanol (100–86 mol%) in contrast that negligible amount of methanol was formed if mixed oxidation
and reduction catalysts were under CO2 and moisture. At anode, water was dissociated over Pt catalyst
to provide protons (H2 O → OH(adsorbed) + H+ + e− ) that transferred to Zn-Cu-Ga catalysts at cathode on
which CO2 was progressively reduced to methanol (CO2 + 6H+ + 6e− → CH3 OH + H2 O).
© 2012 Elsevier B.V. All rights reserved.

1. Introduction
Chemical/biological conversion of carbon dioxide is attracting
greater attentions to reduce one of the major green house gases[1]. Solar-driven thermal dissociation of CO2 was reported withwater utilizing cerium oxide [2]. The photocatalytic CO2 conversion
is also investigated to solve global environmental problem [1,3,4].
The choice of reductant, e.g. hydrogen or water and the selectivity
to fuels, e.g. methanol are keys for the practical application. Especially, CO2 conversion with water is a difﬁcult problem and thissituation hampers environmental application of catalysts to CO2
removal.
In this work, the CO2 -reducing catalyst was separated from a
reductant (hydrogen or water) and the oxidation catalyst by polymer electrolyte (PE) ﬁlm and thus exclusive methanol formation
was enabled. In the analogy to PE fuel cells (FC), Pt/C catalyst
was chosen for the oxidation of H2 and moisture [5]. The Zn-CuGa-based reduction catalyst was chosen based on the elemental
composition of industrial methanol synthesis catalysts startingfrom CO and/or CO2 [6–8]. When hydrogen was used, PEFC using
CO2 was enabled for the ﬁrst time. Even when water was used, partial oxidation of water and CO2 reduction to methanol successfully∗ Corresponding author. Tel.: +81 43 290 3696; fax: +81 43 290 2783.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
0926-3373/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2012.01.026

proceeded owing to the separation of the two half-reactions by thePE ﬁlm.
The catalytic reaction mechanism was also investigated bychanging the oxidation catalysts (Pt/C, C only), reduction catalysts(Zn-Cu-Ga/C, Zn-Cu-Ga only), and the reactant (H2 and/or water).
In addition, the kinetic tests using PEFC were compared to tests ina Pyrex reactor under CO2 + H2 gas or CO2 + H2 O gas.

2. Experimental
50 ␮m thick perﬂuorosulfonic acid ionomer Naﬁon (Scheme 1;
NR-212, Dupont; >95%, acid capacity >9.2 × 10−4 equivalent g−1 )was pasted with 20 wt% of Pt/C (19 mg; Vulcan XC72, Cabot)
attached on water-repellent C paper (EC-20-10-7, Electrochem)or with C powder (Vulcan XC72R, 19 mg) attached on waterrepellent C paper on one side and also layered double hydroxide[Zn1.5 Cu1.5 Ga(OH)8 ]+ 2 (CO3 )2− ·mH2 O (abbreviated as Zn-Cu-Ga
LDH, m ∼ 4; 6.9–10 mg) [8,9] and/or Vulcan XC72R (6.9 mg) on theother side. The area of both catalysts was 3.8 cm2 . After adding
0.4 mL of 3% Naﬁon dispersion solution (DE521-CS, Wako Pure
Chemical) to each side, the membrane-electrode assembly (MEA)
was pressed with 2.0 MPa at 393 K for 10 min using Model SA-302,
Tester Sangyo Co.
Catalytic CO2 reduction tests were conducted using a PEFC
based on a Model PEM-004 (Chemix Co.). To the Pt/C or C catalyst, 10 mL min−1 of H2 (101 kPa) or 50 mL of N2 (101 kPa) was318

M. Morikawa et al. / Applied Catalysis B: Environmental 117–118 (2012) 317–320

Scheme 1. The structure of perﬂuorosulfonic acid ionomer used in this study(Naﬁon).

ﬂowed via glass ﬁlter with pores of 5–10 ␮m ﬁlled in water set
at 343 K. The tube between the glass ﬁlter bubbler and the inlet to
PEFC was kept at 348 K. To the Zn-Cu-Ga LDH catalyst with/without
C, 240 mL min−1 of CO2 (2.6 kPa) [+H2 (27 kPa)] + balance He to
101 kPa or 240 mL min−1 of He (101 kPa) was circulated in a loop of
172 mL.
The PEFC was maintained at 413 K for 5 h and the products atthe cathode were analyzed using an online gas chromatograph(GC) with thermal conductivity detector (Shimadzu, Model GC8A)
equipped with 2 m of polyethylene glycol-6000 supported on Flusin
P and 3 m of Molecular Sieve 13X-S columns (GL Sciences).
The outlet gas from the anode was trapped in a ﬂask cooled inliquid N2 bath and obtained aqueous solution (∼3 mL) was mixed
with 15 ␮L of 30 wt% Ti(SO4 )2 solution (Wako Pure Chemical). The
concentration of H2 O2 was evaluated based on the UV absorption
intensity at 404 nm using a UV–visible spectrophotometer (JASCO,
Model V-650). The MEA was washed with 5 mL of water after catalysis test and the wash was analyzed on UV–visible spectrometer ina similar manner.
Alternatively, 10 mL min−1 of CO2 (101 kPa) was ﬂowed via thewater bubbler (343 K) and the tube between the bubble and PEFC
was set at 348 K. 240 mL min−1 of water (2.8 kPa) + He (99 kPa)
was circulated in a loop to the Pt/C catalyst. During the catalytic
tests, the MEA was maintained as wet. The products at the anodewere analyzed using the online MS 13X-S column. Control tests
using the physical mixture of Zn-Cu-Ga(6.9 mg) + Pt/C(19 mg) andZn-Cu-Ga(6.9 mg) alone were performed in a Pyrex reactor in
CO2 (2.7 kPa) + H2 O(2.0 kPa) and CO2 (2.7 kPa) + H2 (27 kPa), respectively.
3. Results
The result of CO2 reduction test over Zn-Cu-Ga/C catalyst separated from Pt/C catalyst under H2 + moisture is listed in Table 1a.
Methanol was the major product (95 mol%, C-basis) accompaniedwith minor CO. The average current for the reaction of 5 h was
88 ␮mol-e− h−1 gcat −1 , corresponding to 97% of the summation of
six and two electrons required to reduce CO2 into methanol and CO,
respectively (Fig. 1a and Scheme 2a). In our knowledge, this is theﬁrst report of the feasibility of PEFC supplied with CO2 and produce
methanol at the same time.
Compared to the entry a, negligible current was monitored
when moisture only was fed to Pt/C catalyst (Table 1b). Methanol
was produced with the rate of 35% compared to that in entry a. The
methanol selectivity was >86 mol% if the detection limit of CO using
GC was considered. The methanol formation was enabled even from
CO2 and water utilizing PEFC in entry b.
Two thermally reversible transitions are known below 423 K
and above 513 K for Naﬁon [10]. Below 423 K, short-range segmental motions (glass transition) within a static network occur
while above 513 K the long-range molecular mobility begins due
to destabilization of the electrostatic network. Though the reaction temperature in this study (413 K) was near the ﬁrst reversible
transition, the structure was stable until 493–553 K in aqueous

system and the sulfate ligand (Scheme 1) is enough thermally stable [10–12]. Furthermore, reversible shape memory effect of Naﬁon
was demonstrated at 413 K [10].
To conﬁrm the stability, reaction tests of entries a and b at
373–413 K were repeated in total 45 h (35 h at 413 K) keeping the
MEA always in moisture. The methanol formation rate for fresh
MEA for entry b negligibly changed within 3% compared to arepeated test for the condition of entry b at the end of 45 h of use.
This fact excludes the possibility that the signiﬁcant irreversible
change of Naﬁon affected the catalysis at 413 K in the presence ofmoisture.
As for the reaction mechanism in entry b, overall reaction of
CO2 + 2H2 O → CH3 OH + 1.5O2 was assumed, however, O2 evolution
was not detected at the anode (<0.2 ␮mol h−1 gcat −1 ; Table 1b). By
assuming another reaction mechanism via H2 O2 , H2 O2 was monitored during the PEFC test. The amount was less than the detection
limit of 20 nmol at the outlet of anode and in the washing of used
MEA, excluding the possibility of water disproportionation [13].
Further, the electronic state of Pt in the MEA during entry b wasinvestigated by Pt L3 -edge X-ray absorption ﬁne structure at SPring8, Ako, Japan (white line peak intensity, EXAFS curve ﬁts). The molar
ratio of population of Pt0 and PtIV O2 sites was 0.81:0.19 before thePEFC test and essentially unchanged at 0.78:0.22, 0.83:0.17, and0.81:0.19 at 2, 5, and 20 h of reaction, respectively. Thus, the possibility that sacriﬁcial oxidation of Pt surface produced H species
from water was also denied.
Therefore, partial oxidation of water should proceed following
the equation (onset, in SHE [14])
H2 O → OH(adsorbed) + H+ + e− ,E (273–323 K) = +0.55 V

on Pt/C catalyst and subsequent reaction at cathode (in SHE [15,16])
CO2 + 6H+ + 6e− → CH3 OH + H2 O,
E ◦ (298 K) = +0.016 or + 0.046 V
occurred. Based on the standard redox potential values for theanodic and cathodic reactions, this cell reduction did not proceed spontaneously, but required a catalyst for CO2 reduction.
The available protons in Naﬁon (∼5.0 × 10−5 equivalent) were bymore than one order greater than the amount of H needed toform methanol and CO in the entries a–g. Thus, consumed protons of Naﬁon in proximity to cathode catalyst (Zn-Cu-Ga/C) would
be compensated from ones generated from water in proximity toanode catalyst (Pt/C; Scheme 2b). The protons apparently moved
from anode to cathode side in Naﬁon, and the negative charge
consumed for the CO2 reduction at cathode could be also compensated as the apparent move of hydroxy ions in the same
direction (Scheme 2b). Due to this situation, negligible current
was monitored in entry b. The overall reaction was expressed as
CO2 + 5H2 O → CH3 OH + 6OH(ads).
When H2 was added to CO2 at the cathode (entry c), the test
result was essentially identical with that of entry a. The direct
reaction from CO2 + H2 was predominant.
Next, the selective methanol formation using Pt/C catalyst for
activating water and Zn-Cu-Ga/C catalyst for activating CO2 (entryb) was compared to entry d where the Pt content was depleted. The
methanol formation rate (0.7 ␮mol h−1 gcat −1 ) was only 13% of thatin the presence of Pt and the current was also negligible (Table 1b
and d). Thus, Pt was found to be essential for the activation of water.
The protons in Naﬁon could be consumed to form methanol at cathode, but protons were not compensated from water at anode in theabsence of Pt.
In comparison to the entry a where 97% of FC current wasmonitored during the CO2 conversion to methanol and CO, pure ZnCu-Ga LDH was set in the cathode, not mixed with C (Table 1e). In
the entry e, PEFC current was negligible. Thus, mixed C at cathodeM. Morikawa et al. / Applied Catalysis B: Environmental 117–118 (2012) 317–320

319

Table 1Results of CO2 reduction tests for 5 h using the Zn-Cu-Ga catalyst separated from Pt/C or C catalyst by Naﬁon and using Zn-Cu-Ga and/or Pt/C catalyst(s) in a Pyrex reactor.
Anode

CathodeAverage formation rate(␮mol h−1 gcat −1 )Ent.aCatalyst

Reactant

Catalyst

Reactant

Methanolb

COb

O2 c

a
b
b
c
c
d
e
f
g

Pt/C
Pt/C
–
Pt/C
–
C
Pt/C
Pt/C
Pt/C

H2 + H2 O
H2 O
–
H2 O
–
H2 O
H2 + H2 O
H2 O
H2 O

Zn-Cu-Ga/C
Zn-Cu-Ga/C
Zn-Cu-Ga/C + Pt/C
Zn-Cu-Ga/C
Zn-Cu-Ga
Zn-Cu-Ga/C
Zn-Cu-Ga
Zn-Cu-Ga
Zn-Cu-Ga/C

CO2CO2
CO2 + H2 O
CO2 + H2
CO2 + H2
CO2
CO2
CO2
(He)

14.7
5.1
0.7
15.3
14.6e
0.7
4.5
5.6
0.9

0.7
<0.8
<0.04
<0.8
<0.04e
<0.8
<0.8
<0.8
<1.0

–
<0.2
–
–
–
–
–
–
–

Methanol selectivity(mol%, C-base)

Current(␮mol-e− h−1 gcat −1 )/6b , d

95
>86
>95
>95
>99
>46
>85
>88
>47

14.7
−0.10
–
1.1
–
0.17
0.18
0.12
−0.10

Entries a–g were done in PEFC. Entries b and c were tested in Pyrex glass system.
Values per 1 g of Zn-Cu-Ga catalyst.
cValues per 1 g of Pt/C catalyst.
dDivided by 6 to compare to six electron reduction rates from CO2 to methanol. The current sign is positive if electrons moved from anode to cathode.
e
Average rates in 2 h.
ab

was essential for PEFC supplied with CO2 (Scheme 2a). Interestingly, the methanol formation rate (4.5 ␮mol h−1 gcat −1 ) was in thesame order as that for the entry b (5.1 ␮mol h−1 gcat −1 ) where C was
mixed with Zn-Cu-Ga LDH catalyst but only moisture was supplied
to the Pt/C catalyst.
Compared to entry e, only moisture was fed to Pt/C catalyst in
entry f. The current value and methanol formation rate were essentially identical to those in the entry e (Table 1f). Thus, the protons
generated on Pt surface derived either from H2 O or H2 should havetransferred to the Zn-Cu-Ga catalyst, suggesting similar reaction

mechanism including apparent protons/hydroxy species transferfrom anode to cathode side in Naﬁon in entries b, e, and f.
In the comparison of entries a and e supplying H2 for anode,
mixed C in cathode was critical for external current and fastermethanol formations. In contrast, entries b and f supplying moisture for anode, mixed C exhibited essentially no catalytic difference.
In fact, in entries b and f, effective electron ﬂow was not initiated at the Pt/C electrode (Scheme 2b) on which the half reaction
H2 O → OH(adsorbed) + H+ + e− occurred but the electrons would
be unfortunately trapped at electronegative OH(adsorbed) species.

Fig. 1. Time course of CO2 reduction tests using PEFC. (a) H2 + moisture fed to Pt/C and CO2 fed to Zn-Cu-Ga LDH/C, (b) moisture to Pt/C and CO2 to Zn-Cu-Ga/C, (c) moisture
to Pt/C and CO2 + H2 to Zn-Cu-Ga/C, (d) moisture to C and CO2 to Zn-Cu-Ga/C, (e) H2 + moisture to Pt/C and CO2 to Zn-Cu-Ga, (f) moisture to Pt/C and CO2 to Zn-Cu-Ga, and(g) moisture to Pt/C and He to Zn-Cu-Ga/C. CO2 pressure: 2.6 kPa. Moisture pressure: 31 kPa. H2 pressure: 101 kPa except for (c) (27 kPa). Control reaction tests in a Pyrex
reactor in CO2 (2.7 kPa) + H2 O (2.0 kPa) (b ) and in CO2 (2.7 kPa) + H2 (27 kPa) (c ).

320

M. Morikawa et al. / Applied Catalysis B: Environmental 117–118 (2012) 317–320

Scheme 2. Proposed reaction mechanism using PEFC starting from H2 and CO2 (a) and from H2 O and CO2 (b).

Thus, mixed C in cathode in entry b did not have a chance to conduct
current.
As a blank test to entry b, He was fed to Zn-Cu-Ga/C catalyst instead of CO2 (Table 1 g). The current was negligible, andthe methanol formation rate (0.9 ␮mol h−1 gcat −1 ) was 17% of thatfor entry b. The C source was carbonates those were intercalated
between Zn-Cu-Ga cation layers. Two thirds of carbonates of LDH
catalyst remain at the reaction temperature 413 K, corresponding
to 4.9–7.2 ␮mol per 6.9–10 mg of Zn-Cu-Ga LDH [8]. The amount
was greater than the methanol obtained in the entry g.
In a Pyrex reactor ﬁlled with Zn-Cu-Ga LDH catalyst in CO2 + H2
(entry c ), the methanol formation rate was essentially equivalent
to that in entry a. The selective catalysis to methanol was also
possible in Pyrex reactor, but FC current was obtained in entry a(Scheme 2a). In the Pyrex reactor ﬁlled with the physical mixtureof Zn-Cu-Ga/C and Pt/C in CO2 + H2 O (entry b ), negligible reaction
proceeded.
4. Discussion
In the comparison of Table 1a and c, the efﬁcient catalysis proceeded in similar rates if the H2 and CO2 supply was separated byNaﬁon ﬁlm to generate current in entry a. The PEFC supplied with
H2 + CO2 at 413 K in this work will be further developed to photoPEFC supplied with CO2 or air at 290 K because the cathode catalyst
in this work worked at 290 K under UV–visible light to convert CO2
into methanol [8,9].
The feasibility of PEFC supplied with H2 O + CO2 at 413 K could
not be demonstrated as current, but methanol was selectively
produced. Partial decomposition H2 O → OH(adsorbed) + H+ + e−
should proceed, but the rate was slower than proton transfer
rate in Naﬁon and CO2 reduction reaction over Zn-Cu-Ga catalyst.
The rate balance of reaction steps in PEFC disabled stoichiometric current based on Scheme 2a, and charges transferred in
Naﬁon. Nevertheless, photo-PEFC supplied with H2 + CO2 at 290 K
is possible in principle [17], if excellent water-splitting photocatalyst consisting of deeper bandgap energy is used to form O2[18].
The partial oxidation of water at anode in entry b can be supported by previous reports. The O H bond dissociation over Pt
surface was reported to be relatively easy (72 kJ mol−1 on Pt(1 1 1)face) compared to over Cu, Au, Ni, or Pd surface [19]. The dissociative adsorption of water was reported to be less stable only by
6.3 kJ mol−1 compared to non-dissociative adsorption on Pt(1 1 1)
face [20]. In the comparison of kinetic tests in entries a–g, O H
bond of water was dissociated over Pt surface and the protons
transferred to cathode Zn-Cu-Ga LDH catalyst to reduce CO2 intomethanol.

5. Conclusions
Stoichiometric current was obtained when H2 and CO2 were
fed to Pt/C and Zn-Cu-Ga LDH catalysts at anode and cathode,respectively, in PEFC. The PEFC supplied with CO2 simultaneously
produced methanol. When moisture and CO2 were fed, owing theseparation of oxidation and reduction catalysts by the PE ﬁlm,
methanol synthesis was enabled. If the PE ﬁlm was not used, negligible reaction proceeded. In PEFC, water partially decomposed toOH(adsorbed) + H+ + e− and transferred protons in Naﬁon reduced
CO2 to methanol in cathode.
Acknowledgments
The authors are thankful for ﬁnancial support from a Grantin-Aid for Scientiﬁc Research C (2255 0117) from MEXT and the
Iwatani Naoji Foundation’s Research Grant (2011). The X-ray
absorption experiments were performed under the approval of the
Photon Factory Proposal Review Committee (no. 2011G033) andthe grant of the Priority Program for Disaster-Affected Quantum
Beam Facilities (2011A1977) supported by both SPring-8 and
KEK-PF.
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