Potassium-chemical synthesis of 3D graphene from CO2 and its excellent performance in HTM-free perovskite solar cells

Article abstract of DOI:10.1039/c7ta01768e

The conversion of greenhouse gas CO₂ into novel materials is the most promising approach to solve greenhouse gas issues. Herein, we report for the first time the reaction of potassium with CO₂ to synthesize three-dimensional honeycomb-like structured graphene (3DHG). Furthermore, 3DHG exhibited excellent performance as a counter electrode for hole transport material (HTM)-free perovskite solar cells, leading to a power conversion efficiency of 10.06%. This work constitutes a new aspect of potassium chemistry for material synthesis from a greenhouse gas and the generation of electrical energy from sunlight.

Full text of DOI:10.1039/c7ta01768e

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
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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Potassium-Chemical Synthesis of 3D Graphene from CO and Its
2
Excellent Performance for HTM-free Perovskite Solar Cells
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
b
d
e
Wei Wei, Baoyun Hu, Fangming Jin, Zhenzi Jing, Yuexiang Li, Andres Alberto García Blanco,
f
a
Dario J. Stacchiola, and Yun Hang Hu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The conversion of greenhouse gas-CO
most promising approach to solve greenhouse gas issues. Herein, no attempt has been made to exploit potassium metal for the
it is the firtst time to report the reaction of potassium with CO to conversion of a greenhouse gas to novel solid materials. In this
synthesize three-dimensional honeycomb-like structured communication, we demonstrate that potassium can react
graphene (3DHG). Furthermore, 3DHG exhibited excellent with CO to form a new type of 3D graphene sheets.
performance as counter electrodes for hole transport material Furthermore, these graphene sheets exhibited excellent
HTM)-free perovskite solar cells, leading to a power conversion performance as a counter electrode for HTM-free perovskite
efficiency of 10.06%. This work consitututes a new aspect of solar cells with a high efficiency up to 10.06%.
2
to novel materials is the nerve functions of humans. However, to our best knowledge,
2
2
(
potassium chemistry for the material synthesis from a greenhouse
gas and the generation of electrical energy from sunlights.
In this work, the following potassium reaction is designed to
convert CO to graphene-structured carbon.
K+ 3CO C (graphene) + 2K
2
4
2

2
CO
3
(1)
This reaction was examined by thermodynamic calculations. As
shown in Fig. 1, the Gibbs free energy change is negative,
indicating its thermodynamically favorable feature. Its
negative enthalpy change reveals that it is exothermic.
2
The content of CO , which is a major greenhouse gas causing
global warming, has been increasing steadily in the
atmosphere over the past centuries. Several technologies are
2 3
Furthermore, K CO , which are formed simultaneously with
1
-3
commercially available to capture CO
storage is still a challenge due to its leakage from storage
media and high sequestration costs.4-6 CO
conversion is a
2
.
However, CO
2
graphene, can play two important roles during the process: (1)
to prevent graphite formation by isolating graphene sheets
from each other and (2) to control the shape of graphene as an
in-situ template, as shown in Fig. S1. This novel strategy was
further confirmed experimentally as follows.
2
promising way to solve this issue with various strategies, such
as catalyzed chemical, thermochemical, electrochemical,
photochemical, and photoelectrochemical.7 In the recent
,8
years, Hu’s group has explored the possibility of CO
2
conversion directly into novel solid materials, such as carbon
nitride, lithium cyanamide, 3D cauliflower-fungus-like
graphene (CFG), and crape myrtle flower-like graphene.9-11
Potassium, which is the second least dense metal, is of great
importance for soil health, plant growth and animal nutrition.
Furthermore, it plays a critical role in the fluid system and
2 2 3
Fig. 1 Reaction between K and CO to graphene and K CO : (a)
changes of Gibbs free energy and enthalpy vs temperature for
the reaction. (b) XRD patterns of solid products from the
reaction. (c) XRD patterns of the carbon material obtained by
treating the solid products with HCl.
a.Department of Materials Science and Engineering, Michigan Technological
University, 1400 Townsend Dr. Houghton, MI, 49931, USA. Email:
yunhangh@mtu.edu
b.
School of Materials Science and Engineering, Tongji University, 4800 Cao’an Road,
Shanghai 201804, P. R. China.
School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Table 1. Graphene composition.
c.
8
00 Dongchuan Road, Shanghai, 200240, P. R. China.
Samples
Elemental Analysis
EDS
C (%) O (%)
96.51
96.75
96.79
d.
e.
Department of Chemistry, Nanchang University, Nanchang, 330031, P. R. China.
Instituto de Física Aplicada-INFAP, Departamento de Física, CONICET, UNSL
C (%)
DHG-12 91.33
DHG-24 92.74
O (%)
8.67
7.26
5.86
(Ejercito de Los Andes 950, D5700HHW), San Luis, Argentina.
3
3
3.49
3.25
3.21
f.
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY
1973, USA.
1
Electronic Supplementary Information (ESI) available: DOI: 10.1039/x0xx00000x
3DHG-48 94.14
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.1mol of potassium (from Aldrich) reacted with CO
2
in a
The oxygen functional groups and defects of 3DHG were
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DOI: 10.1039/C7TA01768E
o
batch ceramic-tube reactor at temperature of 550 C and an evaluated by Raman and XPS spectra. As shown in Fig. 3a,
initial pressure of 50 psi for a selected time (12, 24, or 48h). Raman spectra of 3DHG exhibit a clear G peak at 1580 cm ,
The products were subjected to X-ray diffraction (XRD) providing evidence of a sp bonded carbon. A strong D peak at
evaluations. Fig. 1b shows the formation of K
-
1
2
-1
2 3
CO , confirming about 1350 cm was also observed, indicating rich defects in
the proposed reaction. After K CO was removed by HCl graphene sheets. Fig. S2 shows XPS spectra for 3DHG. The
2
3
treatment, the obtained black powder was identified as carbon deconvolution of the C1s peak revealed four components
with oxygen functional groups by elemental analysis and centered at 284.6, 286.4, 288.5, and 290.5 eV, which would be
energy dispersive X-ray spectrometry (EDS) (Table 1). The XRD associated with C-C, C-O, C=O, and O-C=O, respectively. The
2
patterns of the carbon presents a diffraction peak at 2θ main component is sp bonded carbon (81.4%). Furthermore,
o
=
25.91 , which can be indexed to the (002) plane of graphitic the surface area and pore structure of 3DHG were determined
o
structure. Another peak at 43.22 is associated with (100) by N
2
adsorption. Figs. 3b-3d show a type II isotherm with a
plane. The broad peaks were observed due to the wrinkled type H3 hysteresis loop, indicating that 3DHG sheets possess
and folded structure of carbon, which was further confirmed the mesopore structure. This was supported by the pore size
by FESEM and TEM images in the following discussion. The distribution ranged from 5 to 90 nm (Figs. S3a-3c). The specific
thickness of the primary crystal layer, which was calculated surface areas, which were calculated with BET model, are 946,
2
-1
with Scherrer equation from XRD peak widths at half heights, 906, and 630 m g for 3DHG-12, 3DHG-24, and 3DHG-48,
is about 2.8 nm. Furthermore, the average interlayer space of respectively. Furthermore, the sheet resistances of 3DHG films,
the material is 3.44 Å, which is larger than the interlayer space which were also measured by four-point probe analysis, are
-1
of graphite. This indicates that each of the layer consists of 8 5.13, 4.81, and 3.97 kΩ sq for 3DHG-12, 3DHG-24, and 3DHG-
graphene sheets. Furthermore, field emission scanning 48, respectively. This indicates that 3DHG materials possess
electron microscope (FESEM) images show that the high conductivity and large surface area. In other words, they
synthesized carbon is a three-dimensional honeycomb-like would be promising electrode materials for energy devices.
framework (Figs. 2a-2c). The thickness of each layer estimated
from FESEM images is about 3.0 nm, which is consistent with
that (about 2.8nm) from XRD. As a general definition,
graphene material is a carbon layer consisting of 10 or less
graphene sheets. Therefore, the synthesized carbon is the 3D
honeycomb-structured graphene (3DHG). The 3DHG samples
obtained with reaction times of 12h, 24h, and 48h are denoted
as 3DHG-12, 3DHG-24, and 3DHG-48, respectively. The
microstructures of 3DHG were further evaluated by
transmission electron microscopy (TEM). As shown in Fig. 2d,
one can see that one graphene sheet exhibits a typical
wrinkled structure with corrugation and scrolling like crumped
silk veil waves. Furthermore, the curved shapes were further
supported by electron diffraction that shows poly-crystalline
ring patterns.
Fig. 3 Characterization of 3DHG: (1) Raman spectrum (a) and (2)
Adsorption/desorption curves of 3DHG-12 (a), 3DHG-24 (b),
and 3DHG-48 (c).
Solar cells have been recognized as one of the most
promising technologies to relieve the global energy crisis and
warming effect. Perovskite solar cells (PSCs) are attracting
intensive attention due to their high power conversion
efficiency (PCE), which has surpassed that of both dye-
sensitized and organic solar cells and is approaching that of
inorganic solar cells such as CdTe, CIGS, and even crystalline Si
12-14
solar cells.
In the conventional structure of PSCs, small
organic molecules (such as Spiro-OMeTAD) or organic
polymers (such as P3HT, PEDOT:PSS, and PTAA) are employed
as hole transport materials (HTMs), and noble Au or Ag are
used as counter electrodes. The main issues that prohibit the
commercial application of PSCs are their instability and high
Fig. 2 Microstructures of graphene sheets: (1) FESEM images of
graphene sheets obtained with reaction times of 12h (a), 24h
(b), and 48h (c) and (2) TEM image of graphene sheets with the
electron diffraction pattern in the inset (d).
2
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fabrication cost. A simple way to cut down fabrication cost is forward bias at one of the interfaces. 3DHG-4V8iewbaArstieclde OPnlCinSe
the development of HTM free perovskite solar cells due to the shows the smallest radius of the semDOicIi:r1c0l.e103i9n/Ct7hTAe01h76ig8hE
ambipolar charge transfer property of the methylammonium frequency (Fig. S5b), which indicates the lowest charge
1
5-17
30,31
lead iodide perovskite.
heterojunction FTO/TiO
Etgar et al. fabricated the first transfer resistance at the CH
3 3 3
NH PbI /carbon interface.
2
/CH
3
NH PbI /Au cell, achieving initial Capacitance-voltage (Cap-V) analysis was further conducted to
3
3
18
PCE of 5.5%. They improved the PCE to 8.1% using a 300 nm evaluate depletion region in the cell. The corresponding Mott-
film and tuning the perovskite layer1 and Schottky curves are displayed in Fig. 5d. According to the
9
mesoporous TiO
2
2
0
then to 10.85% using the two-step deposition technique. The junction capacitance, the built-in potential was extrapolated
elimination of unstable organic HTM can also remarkably using the intercept of the linear regime with the x-axis of the
increase the stability of PSCs. Furthermore, noble metal Mott-Schottky plot and equal to 0.98V, which is consistent
counter electrodes can be replaced by earth abundant, low- with the Voc of the photovoltaic device. The presence of a
cost, and chemical stable carbon materials.
stimulated us to explore 3DHG as counter electrodes for HTM- photogenerated carriers and suppress the back reaction of
free PSCs. To fabricate the device of HTM-free perovskite solar electrons from the TiO film to the CH NH PbI layer. Finally,
cell, 3DHG was coated on the CH NH PbI layer as a counter the excellent preparation-reproducibility of cells was
21-26
Those have built-in field can efficiently force the separation of
2
3
3
3
3
3
3
electrode with doctor-blade method. To improve the interface, demonstrated by the histogram of efficiencies for 10 cells (Fig.
the 3DHG was introduced into the solution of perovskite S4).
precursors, followed by the in-situ synthesis of the perovskite
layer. As a result, a transition layer, in which part of
3 3 3
CH NH PbI perovskite molecules were highly dispersed on
3
3
DHG, was formed between the perovskite layer and the
DHG counter electrode. Fig. 4a shows the configuration
2 3 3 3
(HTM-free TiO /CH NH PbI /3DHG//3DHG) of fabricated
perovskite solar cells, in which the HTM layer was eliminated
and the novel metal electrode was replaced by 3DHG. Fig. 4b
shows the current density (I)-voltage (V) characteristics of
HTM-free 3DHG-based PSCs. The short-circuit photo-current
(Isc), open-circuit voltage (Voc), fill factor (ff), and power
conversion efficiency ( ) for 3DHG-12-based cell are 17.79 mA
cm , 0.87 V, 0.50, and 7.71%, respectively. When the
synthesizing time increased to 48h, power conversion

Fig. 4 HTM-free perovskite photovoltaic performance: (a) cell
configuration of 3DHG-based perovskite solar cell and (b) I–V
curves of PSCs with 3DHG counter electrodes.
-
2
efficiency () increased to 10.06% with enhanced Isc (18.11 mA
-
2
cm ), Voc (0.89 V), and ff (0.63). This happened probably
because the increase in the synthesis time of 3DHG enhanced
its electrical conductivity. It is well-known that some
perovskite solar cells suffer an issue that a photocurrent
hysteresis occurs at certain voltage scanning rate and scan
direction. This issue is usually caused by either charge traps in
low quality perovskite films or ferroelectric properties of the
perovskite materials and/or the electromigration of ion in
2
7-29
perovskites.
for the perovskite solar cell with the 3DHG counter electrode
Fig. 5a). The excellent performance of the 3DHG-based
However, such a hysteresis was not observed
(
perovskite solar cells was further supported by the incident-
photon-to-current conversion efficiency (IPCE). As shown in Fig.
5
b, one can see the IPCE curve with a strong spectral response
in the range from 350 to 750 nm. The integrated current from
the IPCE curve is very close to the Isc obtained from the I-V
curve. EIS measurement was carried out in the frequency
range of 1 Hz to 1 MHz with 10 mV AC amplitude under dark
condition. Fig. 5c depicts the Nyquist plot of solar cell with Fig. 5 Characterization of a HTM-free PSC with 3DHG-48
DHG CEs at a forward bias of 900 mV. The first semi-circle in counter electrode: (a) I–V curves, (b) IPCE curve, (c) Nyquist
3
the high frequency region is assigned to the charge exchange plot, and (d) Mott–Schottky plot.
process at the perovskite/CE interface, while the one at low
frequencies represents the TiO /perovskite interface. The
2
negative capacitance appears in the low frequency region
corresponding to an additional recombination pathway at high
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1
1
2 Best
Research
accessed September 2016).
Cell
Efficiencies,
View Article Online
DOI: 10.1039/C7TA01768E
Table 2. Photovoltaic performance of PSCs with 3DHG CE.
http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
CEs
I
(
sc
V
oc
ff
Efficiency
(%)
(
mA cm-2) (V)
3 M. He, X. Pang, X. Liu, B. Jiang, Y. He, H. Snaith, and Z. Lin,
3
3
3
DHG-12 17.79
DHG-24 16.75
DHG-48 18.11
0.87
0.87
0.89
0.50
0.62
0.63
7.71
9.01
10.06
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,
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1
2
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Acknowledgements
This work was supported by the ACS Petroleum Research Fund
2
2
3
3
6
(PRF-51799-ND10). WW and YHH thank Charles and Carroll
2
7
McArthur for their great support.
2
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