Chapter 2 Literature Review
2.2 Methanol reforming
Hydrocarbon fuels have very high hydrogen contents, e.g. methane (CH4) has
hydrogen content as high as 25 wt%.[34] They may be of use for automobile and
and diesel.[35] There are three ways to transform hydrocarbons into hydrogen, i.e. (i)
direct decomposition, (ii) partial oxidation, and (iii) steam reforming.[11] The direct
decomposition and partial oxidation of hydrocarbons require an elevated
temperature and, at the same time, they produce a considerable amount of CO as a
product or a byproduct (Table 2.1).[11] In a hydrogen FC, even a trace of CO can
deteriorate the Pt electrode, e.g. on introducing 20 ppm CO into the polymer
electrolyte membrane FC (PEMFC), the current density decreases to about 18%
within 210h. In comparison with other impurities, the poisoning effect of CO is
found to exert the largest impact on FC performance.
Steam reforming of hydrocarbons produces a lower CO content relative to the
direct decomposition and partial oxidation of hydrocarbons, as listed in Table 2.1;
hence, of the above three methods, steam reforming is a promising method to
generate hydrogen for automobile and portable applications.[11] As the reforming of
methanol requires a low temperature and produces one order of magnitude less CO
than the other hydrocarbons (0.8% vs 10%-20%; see Table 2.1), it has attracted
considerable attention in the past few decades in this research field.[35] In fact, the
U.S. army is focusing on the development of the technology of reforming methanol
to generate hydrogen for portable PEMFCs, which are used as energy source for
battlefield.[36]
In the following, the reforming of methanol is specifically introduced. Methanol is
a single chemical compound (CH3OH) and commercially is primarily formed from
natural gas through a syngas route. It is a liquid under ambient conditions and has a
low boiling point of 65℃, which allows for facile vaporization in roughly the same
temperature range as that for water. For the typical range of operation in methanol
reforming (45-60 wt% methanol), the freezing point of the fuel mixture ranges from
-44 to -74℃, a distinct advantage for cold-weather development of methanol-fueled
systems. However, the high toxicity of methanol may pose a problem for large-scale
applications. Unlike gasoline or diesel, methanol does not cause vomiting when
ingested. This means that any ingestion that is not deal with quickly will result in the
formic-acid metabolism route internally.[35]
Methanol reacts with steam in the presence of Cu-based catalysts, e.g.
Cu/ZnO/Al2O3, at temperatures higher than 150℃ to form a hydrogen-rich gas.[13]
The main products are H2, CO2, and CO. The formation of methane is
thermodynamically favored, but Cu-based catalysts usually do not promote the
formation of this byproduct. The methanol steam reforming process involves the
following reactions:[11]
CO + H2O→CO2 + H2, △H0298 = -41.1 kJ/mol (3)
Equation (1) generates 12 wt% hydrogen, which is the algebraic summation of
Eqs. (2) and (3). Equation (2) represents methanol decomposition. Equation (3)
represents a water-gas shift (WGS) reaction, which is an important process to reduce
CO content in the reformate gas. The steam reforming reaction is endothermic, and
an external heat supply is required to maintain the reaction. Steam reforming of
methanol over Cu-based catalysts was originally thought to have involved the above
process, i.e. methanol decomposition, followed by WGS. However, in recent years,
this is considerable evidence to suggest another pathway including a methyl formate
intermediate.[35] At the same time, the mechanism for the formation of the CO
byproduct is a controversial topic. Even so, it is generally observed that CO can be
minimized by decreasing the contact time, increasing the steam to carbon ratio to
facilitate the WGS reaction, and decreasing the temperature, which acts to suppress
CO thermodynamically.[35] But the increase of the steam to carbon ratio requires
more heating for vaporizing the additional water in the feed.[11]
As the CO tolerance of PEMFCs is about 10-20 ppm at the normal working
temperature of 80℃ in portable applications, further processing is needed to remove
the small amount of CO (~0.8 vol%) from the reformate gas of methanol.[37]
selective CO methanation.[35] Figure 2.7 displays a schematic representation of the
integrated methanol steam reformer system, which shows the main steps and
components of steam reforming of methanol to generate hydrogen for portable
applications.[11,38]
Compared with DMFC, steam reforming of methanol does not have the methanol
crossover and the high catalyst usage problems. Current DMFCs typically use a Pt
alloy at a loading of 2-8 mg/cm2 on the anode, which is much higher than 0.2-0.3
mg/cm2 used for both the anode and cathode in PEMFCs.[35] The drawback of
steam reforming of methanol is its thermal management, because the reactor works
at temperature above 150℃ and sometimes up to 300℃. For portable FCs, the
device not only needs to be well insulated to eliminate hot surface but also the
exhaust needs to be cooled sufficiently so that it does not burn anyone.[35] As
external heat is required for steam reforming (Figure 2.7), recently an
autothermal-reforming technology was developed, which is a combination of partial
oxidation and steam reforming processes.[35] In this technology, methanol reacts
with a mixture of steam and oxygen, and the partial oxidation and steam reforming
are carried out simultaneously, where the exothermic oxidation supplies the energy
for the endothermic reforming.
energy into chemical energy that can be stored in molecules, such as hydrogen,
through electrochemical reactions, e.g. water splitting. The generation of hydrogen
from PEC water splitting was first demonstrated in 1972 by Honda and
Fujishima.[23] Besides electrolytes, the key components in a PEC cell are the
electrodes (cathode and anode) on which redox chemical reactions involving
electron transfer take place; at least one electrode should be a semiconductor (SC).
Figure 2.8 shows a simple schematic of a typical PEC device. A conventional PEC
cell is established with a SC photoanode and a Pt electrode as the cathode in the
electrolyte solution. Under irradiation with the photon energy equal to or exceeding
the band-gap energy of the SC photoanode, the electrons are excited and promoted
from the valence band to the unoccupied conduction band. n-type SCs are preferred
for the photoanode, and the depletion layer formed at the n-type SC-electrolyte
interface will lead to energy-band bending as shown in Figure 2.9, which facilitates
the separation of photogenerated electrons and holes. The electrons transport to the
cathode and react with photons to generate hydrogen, while the holes accumulate on
the surface of the photoanode and react with water molecules to produce oxygen. In
the presence of sacrificial hole scavengers such as alcohols in the electrolyte
solution, the photoexcited holes can oxidize these reducing reagents without
To achieve efficient splitting of water, the SC photoanode should meet the
following criteria: (i) photochemically stable with good corrosion resistance in
aqueous solution; (ii) with a conduction band edge more negative than hydrogen
evolution potential and a valence band edge more positive than the oxygen evolution
potential; (iii) strong absorption in the solar spectrum region; (iv) high-quality
material with low density of defects for efficient charge transfer and reduction of the
electron-hole recombination; and (v) low cost.[41-43] Unfortunately, to date, there is
no such material that can meet all the requirements simultaneously. Among the
various candidates for the photoelectrode, SC metal oxides are relatively
inexpensive and have a better photochemical stability. Many metal oxides have been
extensively studied and considerable progress has been made in recent years.[26]
For a PEC cell, the conduction band of most metal oxide material is less negative
than the hydrogen evolution potential; thus, a small external potential needs to be
applied to facilitate the PEC reactions.
The SCs for PEC water splitting can be generally classified as metal oxide and
conventional photovoltaic (PV) material. The SC photoelectrode can be n-type
(Figure 2.10a), p-type (Figure 2.10b) or coupling of n-type and p-type (Figure 2.10c).
This can be a single photosystem as in the n-type (TiO2) or p-type (InP), but for the
solar spectrum or several p-types can also be done the same way (Figure 2.10d).
When involving more than one photosystem, it is important to match the currents
generated by the different layers to obtain better efficiency and this is achieved by
aligning complimentary band gaps and controlling the thickness or active area.[21]
In PEC water splitting, metal oxide and conventional PV material or their
combination are used. The anode and cathode are usually physically separated, but
can be combined into a monolithic structure either using a metal structure by
depositing the anode on one side and cathode on the other and sealing the edges
(Figure 2.10e) or stacking the anode on its own substrate with the cathode on its own
substrate and providing an electrical connection between the two (Figure 2.10f).[21]
2.3.1 PEC hydrogen generation based on nanomaterial photoelectrodes
The use of nanomaterial-based PEC for water splitting dates back to 1997.
Fitzmaurice and coworkers reported the studies of charge separation in a
nanostructured TiO2 membrane sensitized with Ru complexes.[44] The long-lived
charge separation observed was an important finding that suggests that water
splitting is practically feasible on nanostructured TiO2 material. In the following few
years, Khan and Akikusa reported the photoresponse of nanocrystalline n-TiO2,
photoanodes are stable for water splitting. Significantly, the nanocrystalline n-Fe2O3
films showed higher photoresponse compared to those prepared by compression of
n-Fe2O3 powder or by thermal oxidation of metallic Fe sheets, indicating that
high-quality nanostructured materials could improve the overall efficiency of the
water splitting reaction. Since then, the PEC performance of different
nanocrystalline metal oxide films has been studied.[26]
Besides 0D nanostructures, 1D nanostructures, such as nanotubes, nanowires, and
nanorods, are expected to exhibit much improved transport properties than
nanoparticles. The first demonstration of PEC water splitting using 1D nanostructure
as photoelectrodes was reported by Lindquist and coworkers in 2000.[47,48] They
reported the photoelectrochemistry of hematite nanorod arrays for the photoanode. It
is generally accepted that recombination of electrons and holes, trapping of electrons
by oxygen deficiency sites, and low mobility of the holes cause the low
photoresponse for hematite films. In comparison to nanoparticles, nanorods improve
the transportation of carriers and thus reduce the recombination losses at grain
boundaries, as illustrated in Figure 2.11a. Moreover, the small diameter of the
nanorods minimizes the distance for holes to diffuse to the SC/electrolyte interface,
as shown in Figure 2.11b. Ideally, when the nanorod radius is smaller than the hole
well beyond the efficiencies reported for polycrystalline and nanocrystalline films of
hematite. This work demonstrated that nanorod arrays could potentially address
some of the fundamental PEC issues and increase the photon-to-current yield of
hematite.
Figure 2.1 Schematic operating principles of DMFC.[27]
Figure 2.2 Breakdown of anode, cathode, and electrolyte-related performance
losses in a DMFC.[29]
Figure 2.3 Schematic illustration of the synthesis procedure of the composite.[30]
Figure 2.4 TEM images of Pt/CNT (inset: enlarged image).[30]
Figure 2.5 TEM images of 20 wt% PtRu/C nanocatalyst prepared with different
molar ratios of Pt:Ru (a) LRTEM of 1:1, (b) HRTEM of 1:1.[31]
Figure 2.6 Typical TEM image of the CNT/PyPBI/Pt. Pt nanoparticles are loaded
Table 2.1 Reactor temperature ranges for initial processing of different fuels and
CO content of different process streams after the initial reaction.[11]
Figure 2.7 Schematic diagram of the integrated methanol steam reformer
system.[11,38]
Figure 2.8 Schematic of a typical PEC device and its basic operation mechanism
for hydrogen generation from water splitting.[40]
Figure 2.9 Energy diagram of a PEC cell consisting of an n-type SC photoanode
and a metal cathode for water splitting.[40]
Figure 2.10 Type of photoelectrode for PEC water splitting (SC-semiconductor;
M-metal).[21]
Figure 2.11 (a) Schematic representation of the electron transport through
spherical particles and nanorods. (b) A schematic of a Fe2O3 photoanode for water
splitting. The small dismeter of the nanowires ensures a short hole diffusion
length.[40,48]
1D & 3D nanoarchitectures
Carbon nanotube ZnO nanorod ZnO-CuO
inverse opal
Electrochemicalmeasurement
1D & 3D nanoarchitectures
Carbon nanotube ZnO nanorod ZnO-CuO
inverse opal
Electrochemicalmeasurement
Chapter 3
Experiment Methods
3.1 Flowchart of experiment process
3.2 Preparation of nanomaterials
3.2.1 Synthesis of CN
xNTs and Pt NPs deposition
For the synthesis of the CNx NTs, an iron catalyst layer was deposited on Ti/Si
substrates by ion beam sputtering prior to the NT growth step. Then CNx NTs were
grown on the precoated substrates by MPECVD method (Figure 3.1), which has
been reported in our previous paper.[7] For Pt NPs deposition, a potential of -0.1 V
vs Ag/AgCl in 0.5M H2SO4 & 0.0025M H2PtCl6 mixture solutions was performed
for 15 sec as shown in Figure 3.2. Other details for the electrochemical deposition
process are reported by Quinn and co-workers.[65]
3.2.2 Fabrication of ZnO NRs and metallic Cu NPs deposition
For ZnO array preparation, a thin film of ZnO was first deposited on the inner
surface of microchannels prior to the NR growth step by solution method, which
acted as a seed layer. This was followed by growth of aligned ZnO NRs on the
precoated substrates by chemical bath deposition (CBD) method. The CBD growth
was performed using equimolecular mixtures of zinc nitrate hexahydrate (99.5 %,
Aldrich) and hexamethylenetetramine (99 %, Aldrich) as source precursors and a
reaction temperature of 90 oC for 7 h (Figure 3.3). Other details of the NR array
growth process were same as those described in the procedure proposed by
precursor salts (copper nitrate trihydrate, 99.5 %, Aldrich) that were subsequently
reduced (Figure 3.4). This reducing step is the boiling under reflux, which leads to
Cu/Cu+ colloids. The synthesis of size-selected Cu NPs immobilized on ZnO NRs
was carried out in ethylene glycol (EG; as reducing agent) solutions containing
different amounts of sodium hydroxide (0.5 M NaOH in EG) with pH value between
9 and 11.[85] The decoration concentrations were controlled between 0.5 and 3 mM.
The solutions were stirred for 30 min at room temperature, subsequently heated
under reflux to 190 ℃ for 2 h, and then cooled in air. Dark brown solutions
containing Cu NPs were formed in this manner, referred to as colloidal solutions in
this work.
3.2.3 Synthesis of microwave-activated CuO NT/ZnO NR nanocomposites
For synthesis of ZnO nanorods inside the microchannels, chemical bath deposition
growth was performed using the equimolecular mixtures of zinc nitrate hexahydrate
(99.5 %, Aldrich) and hexamethylenetetramine (99 %, Aldrich) as source precursors,
at a reaction temperature of 90 oC for 7 h. Other details for the NR array growth
process were similar to the procedure proposed by Vayssieres.[84] Typically, the
one-step direct impregnation method involved impregnation of ZnO
mixing Cu(NO3)2‧3H2O (99.5 %, Aldrich) in deionized water (18.2 MΩ).
Cu(NH3)42+ complex cations were prepared by adding a concentrated ammonia
solution (28-30 wt %) dropwise into the above aqueous solution until the pH value
reached 11 under vigorous stirring. The resulting homogeneous solutions were then
gently heated in an oven at 90 oC for over 2 h. Finally, CuO nanotip/ZnO nanorod
nanoarchitectures on the inner surface of microchannels were formed.
The as-derived CuO nanotip/ZnO nanorod catalyst precursors were processed in a
microwave oven (2.45 GHz; TMO-17MA; TATUNG CO., Ltd.; Taiwan) for 10 min
to obtain the final microwave-irradiated CuO nanotip/ZnO nanorod catalyst
precursors. The microwave chamber was 452 mm in length, 262 mm in width, and
325 mm in height. During the microwave irradiation, the substrate temperature,
environmental atmosphere and microwave power were maintained at room
temperature, air and 600 W, respectively. For comparison, the as-derived CuO
nanotip/ZnO nanorod catalyst precursors were also treated in a conventional
annealing furnace at 450 oC for 1 h to obtain the conventional thermal-treated CuO
nanotip/ZnO nanorod catalyst precursors
3.2.4 Preparation of O
2plasma-activated CuO-ZnO inverse opals
For synthesis of CuO-ZnO inverse opals, 10 wt % of polystyrene (PS) colloidalwell-defined structures were formed inside the microchannels. These PS
particle-packed microchannels were heated at 90 ℃ for 3-6 h in order to improve
the connectivity between the neighboring particles. The copper-zinc precursor
solution containing 0.5 M Cu(NO3)2‧3H2O (99.5%, Aldrich) and 0.5 M Zn(NO3)2‧
6H2O (98%, Aldrich) in ethanol was then applied dropwise over the surface of the
PS layer. These infiltrated samples were then placed in air at room temperature for
2-4 h. Finally, the resulting CuZnO/PS composites were heated in air to 300 ℃ to
obtain the CuO-ZnO inverse opals (Figure 3.6).
These pristine CuO-ZnO inverse opals were then exposed to O2 plasma for a short
duration of 3-15 min to obtain O2-plasma treated CuO-ZnO inverse opals. The
process of plasma-chemical surface modification was performed in a parallel-plate
reactor with a DC flowing discharge (PCD-150; ALL REAL TECHNOLOGY CO.,
Ltd.; Taiwan). The distance between ground electrode and powered electrode was
about 6 cm. The plasma chamber was 250 mm in diameter and 140 mm in height.
The electrode was in the form of disk with around 6 inch in size. For the O2 plasma
treatment, the substrate temperature, total gas pressure and DC power were
maintained at room temperature, 200 mTorr and 50 W, respectively.
3.2.5 Preparation of carbon-modified ZnO inverse opals
After room temperature evaporation of water from the suspension, PS opals with
well-defined structures were formed on the ITO substrate. These PS particle-packed
templates were heated at 90 ℃ for 3-6 h in order to improve the connectivity
between the neighboring particles. The zinc precursor solution containing 0.5 M
Zn(NO3)2‧6H2O (98%, Aldrich) in ethanol was then applied dropwise over the
surface of the PS layer. These infiltrated samples were then placed in air at room
temperature for 2-4 h. Finally, the resulting ZnO/PS composites were heated in air to
300 ℃ to obtain the carbon-modified ZnO inverse opals. For comparison, the ZnO
noninverse opals could be prepared by the same fabrication procedure only without
using PS particle-packed templates.
3.3 Experimental measurements and characteristics analysis
For methanol reforming reaction, the Al-alloy (6061) chip of the microreactor was
made by ourselves through a laser machining method. Ten microchannels per
Al-alloy chip were separated by 800 μm fins. The width, depth and length of the
microchannels were 500μm, 200μm and 4.3 cm, respectively. For the calculation of
the catalyst weight for the steam reforming reactions, we measure the total weight
by precision electronic balance (± 0.1 mg; Sartorius). After reduction of catalysts in
a H2/N2 (5/95) at a flow rate of 50 mL min-1 at 200 ℃ for 1 hr, premixed water,
gas streams were analyzed online with a thermal-conductivity-detector gas
chromatograph (TCD-GC; China Chromatography CO., LTD.) and CO detector
(Gastech CO., Ltd.; GTF200).
For photoelectrochemical measurement, a water-splitting photoelectrode was used
as the working electrode with surface area of 0.5-1 cm2, a platinum plate as counter
electrode, and Ag/AgCl as reference electrode. All PEC studies were operated in a
1M Na2SO4 (pH7.0) solution as supporting electrolyte medium by using
Electrochemical Multichannel Solartron Analytical System. The water-splitting
photoelectrode was illuminated with a xenon lamp equipped with filters to simulate
the AM1.5 spectrum.
For material characterization, XRD analyses were performed on a Bruker D8
Advance diffractometer with Cu (40 kV, 40 mA) radiation. SEM measurements were
made on a JEOL 6700 filed-emission SEM. XPS spectra were obtained using a
Microlab 350 system. For obtaining TEM images, the products on the substrate were
scratched and dispersed on a carbon-coated Cu grid, and analyzed using a JEOL
JEM-2100 TEM system. Micro-Raman analyses were performed on a Jobin Yivon
Labram HR800 spectrometer. XAS analyses were performed on a beamline BL17C1
and BL20A1 at the National Synchrotron Radiation Research Center (NSRRC),
Cooling lines
Dummy Load
A-5000 Microwave power source
Mechanical Pump
Figure 3.1 Schematic diagram of microwave-plasma enhanced chemical-vapor
deposition (MPECVD) facility.
Figure 3.2 Schematic diagram of electrochemical deposition facility.
Zn2+
HMT Heating by oven
Arrayed
Arrayed ZnOZnONRsNRs
9Concentration
Arrayed ZnOZnONRsNRs
9Concentration
Cu NPs/ZnO ZnO NRs NRs
9pH Value
Cu NPs/ZnO ZnO NRs NRs
9pH Value
9Temp. Ramp Rate Figure 3.3 ZnO array preparation.
Figure 3.4 Cu NP/ZnO NR nanocomposites preparation.
Cu2+
Figure 3.5 Cu NT/ZnO NR nanocomposites preparation.
Figure 3.6 Procedure for the preparation of CuO-ZnO inverse opals using
polystyrene colloidal crystal templates.
Chapter 4
Effects of Nitrogen-Doping on the Microstructure, Bonding and Electrochemical Activity of Carbon Nanotubes
4.1 Introduction
Carbon nanoscience and nanotechnology have been developed very rapidly over
the past decade since the discovery of carbon nanotubes (CNTs). Miniaturization of
electronic and electrochemical (EC) devices using the single-walled CNTs has been
demonstrated.[49-51] Meanwhile, the remarkable structure of CNTs offers attractive
scaffolds for further anchoring of nanoparticles (NPs) and biomolecules, which is
highly desirable for energy conversion/storage and molecular sensing
applications.[6,52-62] For these applications, surface modification of the CNTs or
attaching functional groups on the sidewall become a key issue. In particular,
surface modification offers an opportunity to improve the EC reactivity of CNTs
through facilitating an efficient route for their electron-transfer (ET) kinetics with
ambient species or specific biomolecules. Therefore, understanding the ET behavior
between the CNT surface structures and the active entities are essential.