Photoelectrochemical reduction of CO2 to HCOOH on silicon photocathodes with reduced SnO2 porous nanowire catalysts

Article abstract of DOI:10.1039/c7ta09672k

Electrochemical reduction of CO₂ to liquid products offers a route for the energy-dense storage of intermittent renewable electricity while simultaneously helping to mitigate greenhouse gas emissions. In this work, high-quality Si photocathodes decorated with an earth-abundant Sn porous nanowire catalyst utilized the energy from visible light absorption to provide a photovoltage-assisted conversion of CO₂ to liquid HCOOH. The Sn porous nanowire catalysts were selected for their high density of grain boundaries which was previously shown to enhance activity for formic acid formation. A faradaic efficiency of ～60% with a partial current density of 10 mA cm^-2 for HCOOH was achieved at -0.4 V vs. RHE under illumination, which reflected a positive potential shift of ～400 mV compared to the dark electrocatalytic behavior. The photo-assisted electrolysis efficiency for formic acid was calculated to be 11.0%. The results represent a promising photocathode for a narrow bandgap subcell for a tandem photoelectrode system for unbiased light-driven CO₂ electroreduction.

Full text of DOI:10.1039/c7ta09672k

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Pishgar, J. Strain, B. KUMAR, V. Atla, S. Kumari and J. Spurgeon, J. Mater. Chem. A, 2018, DOI:
1
0.1039/C7TA09672K.
Volume
4
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1
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January 2016 Pages 1–330
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ISSN 2050-7488
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C7TA09672K
Journal of Materials Chemistry A
ARTICLE
Photoelectrochemical Reduction of CO to HCOOH on Silicon
2
Photocathodes with Reduced SnO Porous Nanowire Catalysts
2
a,b
b
b
b,c
b
K. Ramachandra Rao, Sahar Pishgar, Jacob Strain, Bijandra Kumar, Veerendra Atla, Sudesh
Received 00th January 20xx,
Accepted 00th January 20xx
b
b,*
Kumari, and Joshua M. Spurgeon
DOI: 10.1039/x0xx00000x
Electrochemical reduction of CO to liquid products offers a route for the energy-dense storage of intermittent renewable
2
www.rsc.org/
electricity while simultaneously helping to mitigate greenhouse gas emissions. In this work, high-quality Si photocathodes
decorated with an earth-abundant Sn porous nanowire catalyst utilized the energy from visible light absorption to provide
a photovoltage-assisted conversion of CO to liquid HCOOH. The Sn porous nanowire catalysts were selected for their high
2
density of grain boundaries which was previously shown to enhance activity for formic acid formation. A faradaic efficiency
of ~ 60% with a partial current density of 10 mA cm-2 for HCOOH was achieved at -0.4 V vs. RHE under illumination, which
reflected a positive potential shift of ~ 400 mV compared to the dark electrocatalytic behavior. The photo-assisted
electrolysis efficiency for formic acid was calculated to be 11.0%. The results represent a promising photocathode for a
narrow bandgap subcell for a tandem photoelectrode system for unbiased light-driven CO electroreduction.
2
economically practical products for the initial establishment of
an industrial electrochemical CO
reduction process.15
Introduction
2
In recent years, an increasing effort has been directed
towards the realization of solar fuels through
Harnessing the energy of sunlight to drive chemical reactions is
one of the most promising routes for the storage of intermittent
16,
17
photoelectrochemical
CO
2
reduction.
For
1
, 2
solar energy. The formation of chemical bonds in fuels is a
particularly energy-dense storage method, and the liquid
photoelectrochemical HCOOH formation in particular,
numerous studies have used photoanode materials to drive a
products of CO
volumetric energy density than H
electrochemical CO conversion thus contributes to
2
electroreduction with water have a greater
1
8-23
separate CO
2
reduction electrocatalyst at the cathode,
and
With a
2
from water-splitting. Solar
2
4-29
some have used photocathodes with a co-catalyst.
2
sophisticated tandem III-V photoanode in combination with a
noble metal Pd cathode, up to 10% solar-energy-conversion to
sustainability as a carbon-neutral, seasonal-length storage
technology in which the liquid products can be utilized at night,
during off-peak demand periods, or in transportation
30
formic acid has been achieved. The equilibrium potential for
the reduction of CO to formic acid is close to that for hydrogen
3, 4
2
applications.
Although other liquid products such as ethanol have been
1
6
evolution, and thus high efficiencies should be achievable with
two-junction tandem photoelectrodes similar to that modeled
produced through CO
faradaic efficiency,
achieved for formic acid. Sn and SnO
in particular have been prevalent electrocatalysts for the high
faradaic efficiency formation of HCOOH.
HCOOH include stability and low volatility, and it is competitive
with current state-of-the-art H
volumetric capacity of 53.4 g H
2
electroreduction at low to moderate
31
for unassisted water-splitting. Si, as a well-developed, low-
cost semiconductor with a band gap of 1.12 eV, is a prominent
candidate for a bottom subcell photocathode in such a tandem.
Photocathodes of p-Si integrated with Cu and Ag catalysts have
5-7
much greater selectivity has been
8
x
foils and nanostructures
9-12
The benefits of
previously been demonstrated to provide light-enhanced CO
2
reduction, with the resulting faradaic efficiencies and product
distribution offset by the Si photovoltage but otherwise largely
2
storage methods with a viable
/L at ambient temperature and
2
32
unchanged.
Recently, we reported electrochemically reduced porous
SnO nanowire (Sn-pNW) catalysts produced by a highly scalable
1
3, 14
pressure.
formic acid fuel cells. Moreover, the high market value of
HCOOH per CO consumed makes it one of the most
HCOOH can also be consumed as a fuel in direct
x
2
12
plasma synthesis. An acid etching step during the catalyst
processing was observed to introduce the nanoporosity as well
as an increase in crystalline grain boundaries, which resulted in
2
notably enhanced CO reduction current density and faradaic
a.Crystal Growth and Nano-Science Research Center, Department of Physics,
Government College (A), Rajamahendravaram, Andhra Pradesh, India, 533 105.
Conn Center for Renewable Energy Research, University of Louisville, Louisville,
efficiency for HCOOH. The enhanced activity was much greater
than could be attributed to increased surface area in the Sn-
pNWs alone, and the improved performance was credited to
the introduction of more effective active sites at the grain
b.
Kentucky, 40292, USA. *Email - joshua.spurgeon@louisville.edu
Department of Technology, Elizabeth City State University, Elizabeth City, NC
c.
2
7909
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ARTICLE
boundaries. Herein we have incorporated these catalysts on Si uncompensated solution resistance, R
Journal Name
u
, and the potentiostat
View Article Online
DOI: 10.1039/C7TA09672K
photocathodes for the photoelectrochemical conversion of CO
to HCOOH at high selectivity.
2
u
subsequently compensated for 85% of R during electrolysis.
The results are reported versus the reversible hydrogen
electrode (RHE) scale according to VRHE = VAg/AgCl + 0.197 +
-
2
0
.059*pH. Simulated sunlight at an intensity of 100 mW cm at
Experimental
Electrode preparation
normal incidence to the working electrode was generated with
a 300 W Xe lamp (Newport 6258) coupled with an AM1.5 global
filter (Newport 81094) and calibrated in the electrolyte with a
Si photodiode (Thorlabs FDS100-CAL).
The synthesis of Sn-pNW catalysts has been described in detail
1
2, 33
previously.
used in which bulk SnO
weight ratio and exposed to an atmospheric plasma of Ar and
Briefly, a solvo-plasma synthesis method was
2
powder was mixed with KOH in a 3:1
Product quantification
air at 1 kW for 2 min. This process resulted in a potassium-rich CO reduction products were measured by gas chromatography
2
2
(~7%) SnO nanowire (Sn-NW) form which was separated from (GC, SRI 8610) for gaseous products and by nuclear magnetic
the larger particles by centrifugation. The as-synthesized Sn- resonance (NMR, Bruker 400 MHz) spectroscopy for liquid
NWs were then immersed in 0.1 M HCl for 1 h to etch the products. Both instruments were calibrated with standard gases
potassium-rich nanowires. The acid etch removed most of the K or liquid solutions. The gas outlet from the catholyte was
atoms and produced the porous SnO nanowires (Sn-pNWs) connected to the GC which used an automatic valve injection (1
2
12
with a high density of grain boundaries. The Sn-pNWs were mL sample) and a thermal conductivity detector (TCD) and
subsequently calcined in air at 500 °C for 7 h.
flame ionization detector (FID). Nitrogen (99.99%, Specialty
Three types of electrode substrate were used throughout Gases) was used as the carrier gas to enable accurate hydrogen
the work. Fluorine-doped tin oxide (FTO) coated glass (TEC 15, quantification. For potentiostatic conditions at each measured
Hartford Glass Company, Inc.) substrates were used to measure potential, the gas was injected after 5 min and again twice at 18
+
1
the electrocatalyst properties by itself. Degenerately doped n - min intervals. For H NMR spectroscopy analysis of the liquid
Si(100) (doped with As to a resistivity of 0.001 – 0.005 Ω cm, phase, samples were prepared by mixing D O and electrolyte
2
University Wafer) substrates were used to measure the dark aliquots in a 1:1 volume ratio. Dimethyl sulfoxide (DMSO) was
electrocatalytic behavior of the Si semiconductor surface as well added at a known low concentration for internal calibration.
as the Si-supported Sn-pNW electrocatalyst behavior. Faradaic efficiency for each product was calculated by
+
Photoactive substrates consisted of a buried-junction n p-Si determining the charge required to produce the measured
wafer (p-Si(100) doped with B to 1 – 10 Ω cm, University Wafer, product concentration and dividing by the total charge passed
+
with n emitter layer from P thermal diffusion to a junction during the potentiostatic electrolysis measurement.
34
depth of ~ 200 nm following an established procedure ).
Before drop-casting Sn-pNW catalysts on Si substrates, the Si Materials characterization
native oxide layer was removed with a 10 s dip in 10% HF. To
The Sn-pNW catalysts were characterized with scanning
load catalyst on the electrodes, 5 mg of Sn-pNWs were
sonicated in 1 mL of isopropanol, and then drop-cast onto the
substrate in three separate intervals (with 15 min in between
for drying) to an approximate loading of ~2 mg cm , followed
by 2 h on a hot plate at 70 °C. An ohmic back contact to Si
substrates was made using Ga/In eutectic (Alfa Aesar).
electron microscopy (SEM) using a NOVA FEI microscope at an
accelerating voltage of ~ 10 – 15 kV and with transmission
electron microscopy (TEM) using a Tecnai FEI microscope
equipped with a Gatan 2002 GIF system at an accelerating
voltage of 200 kV. The samples were prepared by depositing Sn-
pNWs on a TEM grid (PELCO Center-Marked Grids) in powder
form. An HR XRD Bruker D8 Discover diffractometer was used
for X-ray diffraction (XRD) analysis. The XRD operated at 40 kV
-2
Photoelectrochemical measurements
Current density vs. potential (J-E) photoelectrochemical energy- and 40 mA with Cu Kα (λ = 0.1548 nm) radiation at a scan speed
conversion behavior for all electrodes was measured in CO
of 6 s per step with a step size of 0.02°. The samples were
saturated 0.1 M KHCO (pH 6.8, made with 18 MΩ cm H O) scanned between 10 - 90° using the θ - 2θ method. EVA
under stirring with active bubbling of CO (99.99%, Specialty software and the Powder Diffraction File were used to analyze
2
-
3
2
2
Gases) at a flow rate of 10 sccm at room temperature. The FTO the material crystallinity and phases.
or Si electrode served as a working electrode with a Ag/AgCl
(saturated KCl) reference electrode (CH Instruments, Inc.) along
with a Pt gauze counter electrode separated from the main cell Results and discussion
compartment by an anion exchange membrane (Selemion
AMV) in a glass cell with a flat quartz window for illumination.
Before each measurement with a Si electrode, the native oxide
was removed with a 10 s dip in 10% HF. A potentiostat (Bio-Logic
SP-200) with electrochemical impedance spectroscopy (EIS) was
used for all measurements. Potentiostatic EIS measurements
were performed before every experiment to determine the
Catalyst characterization
The Sn-pNWs used as
a
co-catalyst for the
were characterized via
electron microscopy and XRD (Fig. 1). As seen in the SEM of Fig.
a, the ~ 50 nm diameter, 400 – 700 nm long nanowires were
photoelectrochemical reduction of CO
2
1
deposited with a dense coverage at the electrode surface. The
2
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ARTICLE
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DOI: 10.1039/C7TA09672K
Fig. 1. (a) SEM image of Sn-pNWs drop-cast on a Si substrate, scale bar is 300 nm. (b) XRD data for as-synthesized Sn-NWs before etching, and Sn-pNWs after
etching. (c-d) TEM images of a Sn-pNW with (c) scale bar 100 nm, and (d) scale bar 5 nm.
+
drop-casting deposition process resulted in a film of Sn-pNWs ~
For Si electrodes, degenerately doped n -Si substrates were
2
– 3 μm thick across most of the area, however, agglomeration used to measure the dark electrocatalytic properties owing to
of particles in some spots led to imperfect catalyst layer their metallic character and abundance of majority-carrier
uniformity (Fig. S1). Before electrochemical reduction of the electrons available for driving reduction under cathodic
+
catalyst oxide during electrolysis, XRD showed a polycrystalline conditions. In contrast, for a buried-junction n p-Si or a
2
material of the SnO phase (Fig. 1b). The various phases of photoelectrochemical liquid junction to p-Si, reverse bias
potassium-tin oxides observed before the acid etch step in the conditions for the resulting diode behavior in the dark prevent
as-synthesized Sn-NWs were absent after the acid etch to form the flow of electrons across the interface and prohibit the
the Sn-pNW material. TEM micrographs of individual Sn-pNW measurement of exponentially increasing current density
3
4-37
catalysts show the nanoscopic pores throughout the wires (Fig. characteristic of electrocatalytic Butler-Volmer kinetics.
1
The
+
c), and the high-resolution images show the misaligned lattice degenerate n -Si substrates were thus used to capture this dark
fringes of nanocrystallite grains, where the interface between kinetic behavior of the Si surface to enable the photogenerated
each crystallite marks a grain boundary (Fig. 1d).
performance of the buried-junction electrodes to be compared
to the dark Si cathodic overpotential behavior. For all Si
electrodes, the surface was chemically etched prior to
measurement to prevent the ~ 1 – 2 nm native oxide from
impeding charge transfer across the Si/water or Si/Sn-pNW
Photoelectrochemical behavior
2
The photoelectrochemical CO reduction current density vs.
potential (J-E) behavior of the electrodes are shown in Fig. 2.
The dark electrocatalytic performance of Sn-pNWs in the
absence of substrate effects from the Si semiconductor was first
established on flat FTO/glass substrates. Blank FTO is a
kinetically poor surface for the reaction, while the addition of
Table 1. Photoelectrochemical energy-conversion parameters.
Potential
-2
Onset Potential
(V vs. RHE)
-0.36
@10 mA cm
(V vs. RHE)
-1.00
Electrode
Condition
Dark
Dark
FTO/Sn-pNWs
-
2
Sn-pNWs led to ~ 10 mA cm at -1.0 V vs. RHE (Table 1). At the
chosen catalyst loading, this performance was comparable to
that previously reported on structured porous carbon gas
diffusion layer substrates, which was competitive with many
n+-Si
-0.75
-1.30
n+-Si/Sn-pNWs
n+p-Si
n+p-Si/Sn-pNWs
Dark
-0.42
-0.15
0.32
-0.98
-0.64
-0.25
1 Sun, AM1.5
1 Sun, AM1.5
1
2
2
other reported Sn-based CO reduction catalysts.
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ARTICLE
Journal Name
ohmic overpotential. However, the SnO
2
conduction band edge
View Article Online
DOI: 10.1039/C7TA09672K
is significantly lower (i.e., more positive) than the Si conduction
40
band edge, which should promote photoexcited electron
transfer to the co-catalyst for photoelectrochemical CO
2
reduction.
+
The bare buried-junction n p-Si photoelectrode under 1 Sun
AM1.5 illumination showed the expected behavior relative to
the bare n -Si. Illuminated bare n p-Si exhibited an onset for
+
+
-
2
cathodic current at ~ - 0.15 V vs. RHE, achieving 10 mA cm at -
0
3
.64 V vs. RHE, and reaching a light-limited current density of ~
3.0 mA cm . Coating the n p-Si with Sn-pNW co-catalyst, the
-2
+
onset potential shifted to ~ 0.32 V vs. RHE and exhibited 10 mA
-
2
+
cm at -0.25 V vs. RHE. The illuminated n p-Si/Sn-pNWs
photoelectrodes displayed a reduced light-limited current
-
2
density (~ 19.5 mA cm ) relative to the bare photoelectrodes
due to parasitic light absorption within the Sn-pNW co-catalyst
layer. The reduced photocurrent was consistent with the
measured catalyst layer transmittance which averaged ~ 60% at
+
wavelengths above the Si bandgap (Fig. S4). The n p-Si/Sn-
pNWs electrode also maintained > 90% of its photocurrent after
+
>
3 h of operation (Fig. S5). On illuminated buried-junction n p-
-
2
Si photoelectrodes at 10 mA cm , the Sn-pNW catalyst thus
decreased the overpotential by ~ 390 mV. Moreover, the
positive shift in potential at 10 mA cm from dark Si
electrocatalytic behavior to the illuminated performance
reflects a 1-Sun generated photovoltage of ~ 660 mV for the
-
2
+
n p-Si. This is a high photovoltage for Si, indicating a quality
junction and low overall recombination. For Sn-pNW catalyzed
electrodes, the light-driven positive shift in potential at 10 mA
-
2
cm was even greater at 730 mV. This shift is perhaps too large
to attribute to a buried-junction Si photovoltage alone, and we
attribute the remainder to a slightly better catalyst interface
+
with less series resistance compared to the dark n -Si/Sn-pNW
electrode.
Fig. 2. Current density vs. potential (J-E) behavior for electrodes in CO -
2
saturated 0.1 M KHCO . (a) Dark electrocatalytic behavior for FTO/glass with
3
and without Sn-pNW catalyst. (b) Dark electrocatalytic behavior for
CO
2
reduction product distribution
+
degenerate n -Si with and without Sn-pNW catalyst and illuminated 1 Sun
+
The faradaic efficiencies for CO reduction products measured
2
AM1.5 behavior for buried-junction n p-Si photocathodes with and without
Sn-pNW catalyst.
at the cathode are shown in Fig. 3a for FTO/Sn-pNWs in the dark
and for n p-Si/Sn-pNWs under 1-Sun illumination. The only
+
+
interface. In the dark, bare n -Si electrodes exhibited a cathodic
products observed at these conditions by GC were H
and the only liquid product observed by NMR was HCOOH. At
low applied potential (> -0.4 V vs. RHE) in the dark on FTO, H
2
and CO,
-
2
current onset at ~ -0.75 V vs. RHE and reached 10 mA cm at -
.30 V vs. RHE. With the addition of Sn-pNW co-catalyst, the
cathodic current onset increased to ~ -0.42 V vs. RHE and
1
2
was the dominant product with > 95% faradaic efficiency.
However, the selectivity for formic acid increased greatly at
more cathodic potentials, reaching as high as 65.9% at -0.8 V vs.
RHE, consistent with previously reported results on porous
-2
reached 10 mA cm at -0.98 V vs. RHE, a decrease of ~ 320 mV
in overpotential compared to the bare Si. However, the
resulting linear ohmic character of the J-E curve for dark n -
Si/Sn-pNWs indicates an appreciable increase in series
resistance, which we attribute to the interfacial series
resistance from the contiguous Sn-pNW layer (Fig. 1a). If it was
directly contacting a p-Si photocathode, this unbroken layer of
Sn-pNWs would establish the barrier height at the Sn/Si
interface and restrict the achievable photovoltage in a liquid-
+
1
2
carbon electrodes. The CO faradaic efficiency for FTO/Sn-
pNWs at this potential was limited to 9.2%. The same trend in
+
product distribution was observed under illumination on n p-
2
Si/Sn-pNWs with H being the sole product detected at
potentials > -0.1 V vs. RHE, while the peak HCOOH faradaic
efficiency was 59.2% at -0.4 V vs. RHE with a corresponding CO
faradaic efficiency of 11.4%. Although the light-driven positive
potential shift was less than observed in the J-E curves, the
overpotential for peak formic acid selectivity decreased by at
least 400 mV. The energy of the incident light was thus
34, 38
junction photoelectrochemical cell.
By using a buried-
+
junction n p-Si photoelectrode instead, the Si homojunction can
maintain maximum photovoltage with minimal recombination
3
4, 39
at the junction interface.
Si/Sn-pNW interface may also contribute to the increased
Charge-transfer resistance at the
4
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Journal Name
converted to assist in the reduction of CO
ARTICLE
2
to a useful liquid
View Article Online
DOI: 10.1039/C7TA09672K
+
product as HCOOH. Without the Sn-pNW catalyst, n p-Si
electrodes produced primarily H
for CO (Fig. S6).
2
with < 10% faradaic efficiency
The partial current density of each product is displayed in
+
Fig. 3b for illuminated n p-Si/Sn-pNW photoelectrodes. The
partial current density of formic acid, JHCOOH, achieved 10.0 mA
-
2
cm at -0.4 V vs. RHE, increasing only slightly at more negative
potentials as the photoelectrode reached its light-limited
current density. For dark FTO/Sn-pNWs at -0.8 V vs. RHE, the
potential measured for peak HCOOH faradaic efficiency, JHCOOH
-2
was only 1.64 mA cm . Only some of the best literature Sn-
-
2
based catalysts have reported JHCOOH ≥ 10 mA cm . For instance,
-
SnO
x
on graphene was reported to produce JHCOOH ~ 10 mA cm
2
10
at -1.15 V vs. RHE. Hierarchical mesoporous SnO
2
nanosheets
on carbon cloth are among the highest current density formic-
acid-selective CO electroreduction systems reported to date,
but they required a cathodic potential of at least -0.8 V vs. RHE
2
-2 41
to achieve JHCOOH ≥ 10 mA cm . Thus a solar-driven JHCOOH ~ 10
-
2
mA cm at -0.4 V vs. RHE makes this Si photoelectrode quite
competitive for Sn-catalyzed HCOOH electrosynthesis from CO
To estimate the system efficiency for the
2
.
photoelectrochemical energy-conversion behavior of the
+
illuminated n p-Si/Sn-pNW photoelectrode for CO
2
reduction, a
photo-assisted electrolysis system efficiency, 휂푃퐴퐸, was
4
2
calculated by:
ꢀ
푓,표
휂푃퐴퐸
=
ꢀ
푠
+ ꢀ푒,푖
where
ꢀ
푓,표
is the output power density contained in the
chemical fuel produced,
ꢀ
푠
is the incident illumination power
density, and ꢀ푒,푖 is the input electrical power density.
Considering the energy stored in the multiple products of this
system (H
2
, CO, HCOOH):
Fig. 3. CO
Faradaic efficiency for H
FTO/Sn-pNWs in the dark (dashed lines, open markers) and for n p-Si/Sn-
pNWs under 1 Sun AM1.5 illumination (solid lines, filled markers). (b) The
current density vs. potential for illuminated n p-Si/Sn-pNWs and
2
reduction product distribution in CO
2
-saturated 0.1 M KHCO . (a)
3
2
(squares), CO (circles), and HCOOH (triangles) for
퐽표푝(휀퐻2ꢁ푓,퐻2 + 휀퐶푂ꢁ푓,퐶푂 + 휀퐻퐶푂푂퐻ꢁ푓,퐻퐶푂푂퐻
)
+
휂푃퐴퐸
=
ꢀ
푠
+ 퐽 푉
표푝 푒,푖
+
where 퐽표푝 is the operating current density at the potential corresponding partial current density for the reaction products.
under evalution, 휀푖 represents the faradaic efficiency of each
efficiency.42 To produce HCOOH with the n p-Si/Sn-pNWs
+
respective product, ꢁ푓,푖 is the potential difference
photocathode from sunlight unassisted by electrical bias, extra
corresponding to the Gibbs free-energy difference between the
series-connected solar cells or a tandem combination with
two half-reactions of the fuels being produced, and 푉푒,푖 is the
complementary bandgap subcells would be required to achieve
input voltage required to drive the electrolysis at the operating
the necessary additional photovoltage.
current density of interest. 푉푒,푖 is the total applied bias to the
system, and thus represents the voltage required in a two-
electrode electrolysis measurement. However, by separately
measuring the J-E behavior of the Pt gauze anode in the cell, the
Conclusions
High-quality Si photocathodes coupled with porous tin oxide
nanowire co-catalyst with a high density of grain boundaries
corresponding anodic overpotential at 퐽표푝 can be accounted for
42
+
to estimate 푉푒,푖
under illumination at -0.4 V vs. RHE, 푉푒,푖 was estimated to be ~
.03 V. Using literature values for the cathodic half-reaction
.
In the current system for n p-Si/Sn-pNWs
were used to harness the energy of visible light to assist in the
+
conversion of CO
2
to formic acid. The buried-junction n p-Si
2
4
3
interface permitted a contiguous layer of Sn-pNW catalyst at
the interface without limiting the effective barrier height and
corresponding photovoltage. The electrons from the cathode
were thus able to utilize the added potential from the
photogenerated quasi-Fermi-level splitting in the illuminated Si
potentials for each product, we determined the total 휂푃퐴퐸
~
1
7.4%. For the formic acid product alone, 휂푃퐴퐸,퐻퐶푂푂퐻 ~ 11.0%.
Though informative, photo-assisted electrolysis efficiency
should not be compared directly with solar-to-fuel efficiency in
the absence of electrical bias since electricity-to-fuel efficiency
is typically much higher than solar-energy-conversion
to drive the CO
2
reduction half-reaction at lower applied
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Conflicts of interest
There are no conflicts to declare.
3. G. Yin, H. Abe, R. Kodiyath, S. Ueda, N. Srinivasan, A. Yamaguchi
and M. Miyauchi, Journal of Materials Chemistry A, 2017, 5,
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The authors acknowledge support from the Conn Center for
Renewable Energy Research at the University of Louisville. K.R.
Rao also acknowledges support from the University Grants
Commission, Government of India, for providing a Raman Post-
doctoral Fellowship to conduct work at the Conn Center at the
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09672K
Full Size
4
cm Height
Caption:
High-quality Si photoelectrodes with novel Sn nanowire catalysts convert solar energy to reduce CO
formic acid with high selectivity.
2
to