Solar-driven CO2 to CO reduction utilizing H2O as an electron donor by earth-abundant Mn-bipyridine complex and Ni-modified Fe-oxyhydroxide catalysts activated in a single-compartment reactor

Article abstract of DOI:10.1039/c8cc07900e

Photoelectrochemical CO₂ to CO reduction was demonstrated with 3.4% solar-to-chemical conversion efficiency using polycrystalline silicon photovoltaic cells connected with earth-abundant catalysts: a manganese complex polymer for CO₂ reduction and iron oxyhydroxide modified with a nickel compound for water oxidation. The system operated around neutral pH in a single-compartment reactor.

Full text of DOI:10.1039/c8cc07900e

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Solar-driven CO to CO reduction utilizing
2
H O as an electron donor by earth-abundant
2
Cite this: Chem. Commun., 2019,
5, 237
Mn–bipyridine complex and Ni-modified
Fe-oxyhydroxide catalysts activated
in a single-compartment reactor†
5
Received 3rd October 2018,
Accepted 3rd December 2018
DOI: 10.1039/c8cc07900e
Takeo Arai, * Shunsuke Sato,
Takeshi Morikawa
Keita Sekizawa,
Tomiko M. Suzuki
and
rsc.li/chemcomm
Photoelectrochemical CO
.4% solar-to-chemical conversion efficiency using polycrystalline solar cell) as a light absorber. A two-compartment reactor separated
silicon photovoltaic cells connected with earth-abundant catalysts: by a bipolar membrane was used in these systems. The theoretical
a manganese complex polymer for CO reduction and iron oxyhydroxide potential difference of a perfectly permselective bipolar membrane
modified with a nickel compound for water oxidation. The system for the generation of H and OH was reported to be 0.83 V by
2
to CO reduction was demonstrated with heterojunction with intrinsic thin layer (HIT) solar cell (silicon
7
3
2
+
À
8
operated around neutral pH in a single-compartment reactor.
calculation, and this will be an additional required potential to
operate the CO electrolyzer. These systems used two types of
2
The development of artificial photosynthesis systems that can electrolyte to reduce the operating potential by the chemical bias
reduce carbon dioxide (CO ) to useful and sustainable organic accompanied with the pH difference between catholyte and anolyte.
2
chemicals using sunlight is important to mitigate worldwide However, the pH differences reported in these systems were around
climate change and fossil fuel shortages. A photoelectrochemical 6, which suggests a potential loss of 0.47 V by the membrane. On
approach is one candidate method to reduce CO
2
2
through the use of the other hand, we have reported a simple CO reduction system in
solar energy. Many reports on photoelectrochemical CO
system have been reported. We also have reported a monolithic which has the advantage of being a simplified system. Herein,
artificial photosynthesis device with molecular and metal-oxide we demonstrate an example of an earth-abundant CO electrolyzer
2
reduction a single-compartment reactor without an expensive membrane,
1–7
4
2
catalysts attached with a semiconductor photosensitizer, which in a single-compartment reactor by the combination of a manganese
can produce formate with a solar to chemical conversion efficiency complex catalyst and iron oxyhydroxide. The Mn-complex is a
4,5
of 4.6% in a simple single-compartment reactor. However, the candidate CO reduction catalyst that consists of earth-abundant
2
device requires noble metal catalysts, such as a ruthenium complex elements. Many Mn-complexes have been reported for electro-
9
–12
polymer as a CO
2
reduction catalyst, and iridium oxide as a water chemical or photocatalytic CO
2
reduction.
We have reported
oxidation catalyst. In view of future practical applications, such a that a Mn-complex polymer (hereinafter referred to as [Mn–MeCN])
system requires efficient catalysts that consist of inexpensive and on multi-walled carbon nanotubes (MWCNTs) facilitated electro-
abundant elements. There have been several reports on systems catalytic CO
2
reduction in a mixed aqueous solution of potassium
composed of earth-abundant metal catalysts. Carbon monoxide borate and sulfate (hereinafter referred to as KBB) with near neutral
CO) is a raw material of high industrial value. Solar-driven CO to pH, while the bare Mn-complex did not function as a catalyst in the
6,7
(
2
11
CO conversion using a SnO modified CuO nanowire electrode for same solution. The overpotential of electrocatalytic CO reduction
2
2
2
both CO reduction and the oxygen evolution reaction (OER) has over [Mn–MeCN] was significantly reduced to 100 mV by the
6
+
been reported by Schreier et al. 14.4% of the total solar-to-fuel synergetic effect of the MWCNT support and K cations. Iron
efficiency was observed by the combination of GaInP/GaInAs/Ge oxyhydroxide (FeOOH) has been reported as an earth-abundant
1
3–19
solar cells as a light absorber. Urbain et al. observed a solar-to- OER catalyst.
We have recently reported akaganeite-type
syngas conversion efficiency of 4.3% using a system consisting of iron oxyhydroxide hyperfine nanorod doped with 1 at% nickel
a copper-zinc cathode for CO reduction to CO and a photoanode (b-FeOOH:Ni) surface-modified with 21 at% amorphous nickel
2
comprised of nickel foam catalyst for H O oxidation and a hydride (a-Ni(OH) ) in which Ni amounts are defined as Ni/Fe
2
2
19
ratio (hereinafter referred to as b-FeOOH:Ni/a-Ni(OH)2). b-FeOOH:
Ni/a-Ni(OH) exhibited higher performance than other Fe-based
oxide and oxyhydroxide materials, and an overpotential of 430 mV
2
Toyota Central Research and Development Laboratories, Inc., 41-1 Yokomichi,
Nagakute, Aichi 480-1192, Japan. E-mail: takeo–arai@mosk.tytlabs.co.jp
À2
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc07900e
(1.66 V vs. RHE) at 10 mA cm was observed during the OER under
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strong alkaline conditions. However, Fe-based materials generally +1.40 V (vs. RHE) in 1 M and 0.1 M KOH solutions, respectively
have low activity at near neutral pH. Therefore, the utilization of (Fig. 1(b)). The b-FeOOH:Ni/a-Ni(OH) electrode after use in 1 M
2
b-FeOOH:Ni/a-Ni(OH) as an OER catalyst in near neutral pH is KOH solution also showed a cathodic peak at +1.49 V (vs. RHE)
2
a challenging and important approach. To conduct CO reduction in 0.1 M KBB solution, while a broad and ambiguous peak was
2
in a single-compartment reactor, the b-FeOOH:Ni/a-Ni(OH) catalyst observed for the bare b-FeOOH:Ni/a-Ni(OH) electrode in 0.1 M
2
2
was used in KBB aqueous solution and the pre-treatment of KBB solution. Fig. 1(c) shows the results of current–potential
b-FeOOH:Ni/a-Ni(OH) in strong alkaline condition was found to measurements repeated in 0.1 M KBB solution for the bare
be effective to maintain the OER activity in near neutral solution. b-FeOOH:Ni/a-Ni(OH) electrode. The anodic current for the OER
Photoelectrochemical CO reduction was subsequently demon- was decreased by repeating the measurement. The overpotential
strated by combining the earth-abundant CO
photovoltaic cells.
2
2
2
2
electrolyzer with was increased during the three times potential sweep. The
b-FeOOH:Ni/a-Ni(OH)2 electrode after used in the 1 M KOH
The current–potential characteristics of b-FeOOH:Ni/a-Ni(OH)2 solution showed stable current–potential characteristics, even
see ESI† for detail) on carbon paper were evaluated in 1 M KOH in 0.1 M KBB solution (Fig. 1(d)). The pH dependence of the
pH 13.6), 0.1 M KOH (pH 12.9) and 0.1 M KBB (pH 6.9) aqueous Ni–Fe oxyhydroxide catalyst supported on Vulcan XC-72r (NiFeO /C)
solutions (Fig. 1(a)). The apparent surface area (ASA) of the has been investigated by G o¨ rlin et al. They reported that a 0.1 M
(
(
x
16
2
b-FeOOH:Ni/a-Ni(OH)
during the OER increased significantly with a decrease in the pH catalytic OER activity in 0.1 M phosphate buffer (pH 7) solution
of the electrolyte from alkaline to neutral. The same pH depen- during cyclic voltammetry measurements. Ni(OH) /NiOOH redox
dence has been reported for the activity of a Fe-free NiOOH peaks were observed around +1.4 V and +1.5 V (vs. RHE) at pH 13
2 x
electrode was 2 cm . The overpotential KOH (pH 13) pre-cycled NiFeO /C electrode showed a rapid loss of
2
2
0
catalyst. Diaz-Morales et al. identified the active species as a and pH 7, respectively. The potentials of cathodic peaks shown in
deprotonated g-NiOOH surface phase that was generated by the Fig. 1(b) were almost correspond to the results of G ¨o rlin et al. at
electrochemical oxidation of Ni(OH)
mechanism for the OER on NiOOH that involves deprotonation Ni(OH)
of the polymeric hydrous nickel oxyhydroxide by hydroxide ion strongly related to the OER activity in several reports.
to form a superoxo-type intermediate (NiOO ) that acts as According to the Pourbaix diagrams reported by Beverskog et al.,
2
. They have proposed a each pH, which suggests these are Ni(OH)
2
/NiOOH redox peaks.
II
III
2
/NiOOH (or Ni /Ni ) redox transition is considered to be
1
6,17,20
À
2
1
2
preferential oxygen precursor at pH 4 11. NiOOH was described the formation of Ni(OH) seems difficult around neutral pH.
as an unsuitable electrocatalyst for applications in neutral Therefore, it is inferred that the pre-formation of the Ni(OH)2/
or moderately alkaline pH (in the range 7–11). In the case of NiOOH redox couple using b-FeOOH:Ni/a-Ni(OH) under strong
2
1
9
b-FeOOH:Ni/a-Ni(OH)2, it is considered that the amorphous alkaline conditions is important to maintain the OER activity
1
6
Ni(OH)
OER activity. Cathodic peaks were observed at +1.36 V and current–potential characteristics of the 0.1 M KOH pre-cycled
NiFeO /C electrode reveals a Ni(OH) /NiOOH redox peak in
.1 M phosphate buffer. However, the OER activity was gradually
2
on b-FeOOH:Ni influences the pH dependence of the under neutral conditions. According to G ¨o rlin et al., the
x
2
0
decreased with repeated cyclic voltammetry measurements and
the Ni(OH) /NiOOH redox peak diminished due to a loss of Ni
2
3À
atoms. Lee et al. reported that PO
tends to destroy the
structure of Ni(OH)2. It has also been reported that a 0.1 M
KOH pre-cycled NiFeO /C electrode showed stable OER activity
4
2
2
x
1
6
in 0.1 M borate buffer (pH 9.2). According to these reports, the
important factor to maintain OER activity under neutral conditions
2
is not only the formation of the Ni(OH) /NiOOH redox couple, but
also the presence of borate and the absence of phosphate.
The OER activity of the b-FeOOH:Ni/a-Ni(OH) electrode was
2
slightly improved by modification with single-walled carbon
nanotubes on carbon paper (see the results for Fig. S5, ESI†).
The 0.1 M KOH pre-cycled b-FeOOH:Ni/a-Ni(OH)
2
electrode
2
(
ASA: 2 cm ) was combined with a [Mn–MeCN] electrode (ASA:
2
3.24 cm , see ESI† for detail) to compose a single-compartment
CO
2
electrolyzer (Fig. S3, ESI†). A series of six polycrystalline
silicon photovoltaic cells (hereinafter referred to as poly-Si cells)
were selected as a light absorber because it was cheap and
commercially available. The current–potential characteristics
Fig. 1 Current–potential characteristics of the b-FeOOH:Ni/a-Ni(OH)
electrode in various electrolytes. (a) Comparison of the current–potential
characteristics in 1 M KOH (pH 13.6), 0.1 M KOH (pH 12.9) and 0.1 M KBB
2
of the CO electrolyzer and poly-Si cells are shown in Fig. 2(a).
2
(
(
pH 6.9) solutions. (b) Enlargement of the yellow dotted line area in (a).
c) Current–potential characteristics of the bare b-FeOOH:Ni/a-Ni(OH)
The cross-point that indicates the operation point of the CO
electrolyzer and poly-Si cells was estimated to be 2.19 V and 5.1 mA.
2
2
electrode in 0.1 M KBB solution. (d) Current–potential characteristics of the
b-FeOOH:Ni/a-Ni(OH) electrode in 0.1 M KBB after use in 1 M KOH solution. The CO
2
2
electrolyzer was connected with poly-Si cells to convert
2
38 | Chem. Commun., 2019, 55, 237--240
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shown in Fig. 2(c). The main products were CO and H
2
for the
reduction reaction and O for the oxidation reaction. The amount
2
of other CO reduction products was negligible, and the ratio of
2
reduction products CO and H were 82% and 18%, respectively
2
(
see ESI† for a margin error). The amount of O was approxi-
2
mately stoichiometric with the CO and H
2
production. It is worth
noting that CO production selectivity of over 80% was observed
while oxygen was generated from water in a single-compartment
2
reactor. CO reduction is generally disturbed by the reduction of
O
2
as a competitive reaction. Carbon materials are reported to act
4
as catalyst supports that prevent O reduction and promote CO
reduction in a combination with metal complex catalysts.
2
2
11,23
The
carbon source for the generation of CO over [Mn–MeCN] was
1
1
identified as CO
Therefore, the present CO was generated by the reduction of CO
using H O as an electron donor. The turnover number for CO
2
by isotope tracer analysis in our previous study.
2
2
production was estimated to be 120 during 4 hours reaction. The
solar to chemical energy conversion efficiency for CO production
was calculated to be 3.4% (see ESI† for details) and the efficiency for
CO and H production was 4.1%. The efficiency was slightly low
2
compared with the solar-to-syngas efficiency 4.3% when using a
6,7
silicon HIT cell. However, this value is strongly dependent on the
solar-to-electricity conversion efficiency of the photovoltaic cell; the
efficiency of the poly-Si cell was 7.1%, while that of typical HIT solar
cells is 16–19%. Therefore, 4.1% efficiency achieved in the single-
compartment reactor without membrane by the combination of
these earth-abundant Mn and Fe catalysts and the inexpensive
poly-Si cells is technically advantageous.
Solar-driven CO2 reduction was also conducted with the
combination of [Mn–MeCN], Ni(OH)
2
-deposited b-FeOOH:Ni/
a-Ni(OH) and a triple-junction amorphous silicon photovoltaic
2
cell (3jn-a-Si cell) as a light absorber (Fig. S4, ESI†). Tandem
photovoltaic cells such as 3jn-a-Si provide high voltages (Fig. S6,
ESI†) that exceed 1.33 V of the theoretical potential to reduce CO
2
to CO using water as an electron donor. The tandem photovoltaic
light absorber is advantageous because CO2 can be reduced
without series connection and it is suitable for future fabrication
4
of a monolithic artificial photosynthesis system. Conventional Si
2 2
Fig. 2 Results of solar driven CO reduction using a CO electrolyzer and
cells generally have an open circuit potential of 0.7 V and a series
poly-Si cells. The CO
2
electrolyzer consisted of a [Mn–MeCN] electrode
reduction and a b-FeOOH:Ni/a-Ni(OH) electrode
O oxidation in a single-compartment reactor. The electrolyte
saturated 0.1 M KBB solution. Poly-Si cells are the series of 6 Series connection of photovoltaic cells generally leads to a loss
2
of over 3 photovoltaic cells are thus required to conduct CO
reduction to CO including the overpotential of the catalysts.
2
(
ca. 3.24 cm ) for CO
2
2
2
(
ca. 2 cm ) for H
was CO
polycrystalline silicon photovoltaic cells. (a) Current–potential characteristics of
the CO electrolyzer and poly-Si cells under simulated solar light (1 sun, AM1.5,
2
2
of voltage and current due to the resistance caused by the non-
uniformity of solar irradiation to each cell, and a conditioner is
thus required to control the voltage. The solar-to-chemical energy
conversion efficiency using the 3jn-a-Si cell as a light absorber
was estimated to be 1.0% for CO generation. The current
2
2
1
.6 cm ). Time courses of (b) the current under potentiostatic conditions (applied
bias 0 V) and (c) amount of gaseous products observed during solar driven
CO reduction using the combination of CO electrolyzer and poly-Si cells.
2
2
2 2
efficiencies for CO, H and O were 84.7%, 18.4% and 97.8%,
light energy to chemical energy. The current–time measurement respectively (Fig. S7, ESI†). The product ratios were almost the
was conducted under solar simulated light irradiation. The same as those observed for the poly-Si cells. Photoelectrochemical
applied potentials of the cathode and anode by poly-Si cells were CO reduction to CO using the same light absorber has been
2
2
4
À1.03 V and +1.12 V (vs. Ag/AgCl), respectively. The observed reported by Sugano et al., where the system was composed of
current was ca. 5 mA, which corresponds to the operation current an Au nanoparticle cathode and a photoanode that combined a
x
estimated from the current–potential characteristics, and it was 3jn-a-Si cell and a CoO catalyst. A two-compartment reactor
stable during 4 hours of measurement (Fig. 2(b)). Time courses separated by an anion exchange membrane was used for the wired
for the amount of gaseous products during CO
2
reduction are photovoltaic-photoelectrochemical cell system. The solar-to-CO
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40 | Chem. Commun., 2019, 55, 237--240
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