Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

9.3 Bottom-Up Strategy for Organic Electrode Fabrication 325

case of 2,2,6,6 - tetramethylpiperidine - 1 - oxyl ( TEMPO ) derivatives [23, 24] . A variety

of polymer backbones have been employed to bind the radicals, such as

poly(methacrylate)s, polystyrene derivatives, poly(vinyl ether)s, polyethers, and

poly(norbornene)s. The unpaired electron density of the polymers, as determined

by superconducting quantum interference device ( SQUID ) measurements, has

revealed the presence of radicals substantially per repeating unit. Theoretical redox

capacities based on the formula mass of the repeating unit are in the range of 50

to 150 mAh g

− 1

.

For mesoscale electrode fabrication, the radical polymers are conveniently

placed on the surface of an electrode or a current collector by a solution - based

wet process such as the spin - coating method. The contact stylus proﬁ le near a

scratched edge provides a layer thickness under dry conditions in the range of 10

to 10

3

nm, according to the concentration of the polymer in the mother liquor and

the spinning velocity, and has revealed a ﬂ at surface with a roughness of less than

several nanometers. The polymer backbones are suitably modiﬁ ed by photo -

crosslinking, for example, thus, to be swollen but not to be dissolved in the elec-

trolyte solutions, as typically shown for the TEMPO - substituted polynorbornene 1

in Scheme 9.2 [47] .

The capability of forming stable ﬁ lms by the crosslinking method has allowed

a novel design of a mesostructure fabrication processes, in which the polymer is

coated on the surface of conducting mesoscale matrices as the support. The scan-

ning electron microscopy ( SEM ) image of the resulting electrode surface, prepared

on a Au quartz crystal microbalance ( QCM ) assembly for simultaneous electro-

chemical and resonant frequency - based gravimetric analysis ( vide infra ), demon-

strates the ordered three - dimensional ( 3 - D ) structure successfully fabricated on

the mesoscale (Figure 9.5 ) and which is suitable for ion transport throughout the

polymer layer.

The unpaired electron density of the radical polymers amounts to a number

of several molar in the bulk of the swollen polymer equilibrated in electrolyte

Scheme 9.2 Photo - crosslinking of the TEMPO - substituted

polynorbornene 1 to form a polymer layer of 2 , which is

insoluble but swellable in electrolyte solutions. The spin

density by SQUID measurements amounts to 10

21

spin per

gram of 2 , indicating that most of the radicals survive during

the photo - crosslinking reaction.

326 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

Figure 9.5 Scanning electron microscopy image of the

photo - crosslinked polymer 2 /carbon composite electrode

coated on the surface of a QCM electrode assembly.

solutions. Under these conditions, charge propagation within the polymer layer is

sufﬁ ciently fast, leading to high - density charge storage, because the redox sites are

so populated that the electron self - exchange reaction goes to completion within a

ﬁ nite distance of the polymer layer attached to the carbon ﬁ ber. The concentration

gradient - driven charge propagation is accomplished by the electron self - exchange

reaction. The apparent diffusion coefﬁ cient, D , of the charge produced at the

surface of the electrode is determined by the bimolecular rate constant, k

ex

, for the

self - exchange reaction. Thus, when the sites are immobilized in the layer, allowing

only diffusional collision of the neighboring sites to undergo an electron self -

exchange reaction, this process involves an electron - hopping mechanism with a

diffusion coefﬁ cient formulated by D = k

ex

δ

2

C * /6, where k

ex

is the bimolecular rate

constant, δ is the site distance, and C * is the site concentration in the polymer

layer [65, 66] . The electron self - exchange reaction is sufﬁ ciently fast for diffusion -

limited outer - sphere redox reactions, resulting in the efﬁ cient transfer of charge

produced at the surface of the electrode (Figure 9.6 ).

The electrochemical behavior of the radical polymer layers attached to an elec-

trode have revealed an efﬁ cient charge propagation process with diffusion coefﬁ -

cients in the order of 10

− 8

to 10

− 10

c m

2

s

− 1

. The electrolytes examined are typically

tetrabutylammonium or the lithium salts of perchlorate or hexaﬂ uorophosphate,

dissolved in conventional organic solvents such as CH

3

CN and propylene carbon-

ate. The cyclic voltammogram obtained for the electrode (as in Figure 9.5 ), with a

sufﬁ cient crosslinking density of 11%, persists without change for several days of

continued charging/discharging cycling in CH

3

CN, in which the layer appears

completely insoluble (Figure 9.7 ). Wave shapes with a peak - to - peak separation of

Δ E

p

= ca. 200 mV suggest some contribution from diffusion across the thick layer

(Figure 9.7 b). In the QCM response, the resonant frequency decreases during the

9.3 Bottom-Up Strategy for Organic Electrode Fabrication 327

Figure 9.6 Electrode and chemical reactions related to the

charge transfer in radical redox polymers.

Figure 9.7 (a) QCM response and (b) cyclic

voltammogram obtained for the photo -

crosslinked polymer 2 /carbon composite

electrode coated on the surface of a QCM

electrode assembly. The Au electrode surface

area A was 0.196 cm

2

. The content of 2 in the

composite electrode was 10 wt%. The

crosslinking density was 11%. The electrolyte

was a solution of 0.1 mol l

− 1

tetrabutylammonium hexaﬂ uorophosphate in

CH

3

CN. Scan rate = 20 mV s

− 1

.

oxidation and attains its original value after the reduction, indicating reversibility

to the mass change (Figure 9.7 a). The galvanostatic E – t curves show a plateau

region, which agrees with the formal potential. The amount of charge consumed

during the potential scan and the constant - current electrolysis both coincide well

with the formula weight - based redox capacity, which reveal that almost all of the

sites in the layer undergo the redox reaction.

328 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

The redox process of the polymer layer is accompanied by the rapid injection

and rejection of solvated counterions that compensate the charges produced by

the redox reaction. Electroneutrality requires that the removal of each electron

from the polymer layer results in the insertion of one electrolyte anion into the

layer and/or the Donnan exclusion of the electrolyte cation. The mass change, Δ m ,

associated with the redox reaction is obtained from the resonant frequency change

Δ f in Figure 9.7 a using the Sauerbrey equation ( Δ f = − C

f

Δ m ), where the sensitive

factor C

f

is 0.183 Hz cm

2

n g

− 1

for the QCM assembly employed in this study. The

redox capacity Q is determined from the area under the waves in Figure 9.7 b. The

mass change relative to the redox capacity ( Δ mF / Q ∼ 71 g mol

− 1

) is smaller than

the formula mass of the PF

6

−

anion, indicating the slight contribution from the

nonpermselective incorporation of the electrolyte cation during the mass transfer

process.

Nitroxide radicals are potentially reducible to aminoxy anions by a one -

electron - transfer process, which corresponds to the n - type doping of the neutral

radical polymer (Scheme 9.3 ).

Attempts have been made to synthesize the n - dopable polymers and to use them

as the anode - active material, with a view to employing only nitroxide radicals at

both electrodes [48] . It has been demonstrated that suitable electron - withdrawing

group s ( EWG )s such as triﬂ uoromethyl and cyano substituents, bound proximally

to the NO redox center (Scheme 9.4 a), shift the n - doping potential positively up

to approximately − 0.8 V. Acylnitroxides (Scheme 9.4 b) are also potentially useful

as the anode material. The nitroxide radical polymer can thus be switched from

the p - type material for use in cathodes to the n - type anode - active material, by

tuning the substituent effect.

Scheme 9.3 Redox couples related to nitroxide radicals.

Scheme 9.4 n - Type nitroxide polymers.

9.3 Bottom-Up Strategy for Organic Electrode Fabrication 329

The radical polymers act as both cathode - and anode - active materials, because

of their capability to tune the redox potentials by the bottom - up strategy based on

the molecular design. A couple of polymers, different in redox potentials, are used

as the electroactive materials in the organic radical battery. The charging process

corresponds to the oxidation of the radical to the cation at the cathode, and the

reduction of the radical to the anion at the anode (Figure 9.8 ). The electromotive

force is close to the potential gap between the two redox couples, which typically

amounts to 0.5 – 1.5 V. The radical polymers are reversibly converted to the corre-

sponding polyelectrolytes during the charging process. The rapid electrode reac-

tion of the radicals and the efﬁ cient charge propagation within the polymer layer

lead to a high rate performance, allowing rapid charging and large discharge

currents without any substantial loss of output voltage. The amorphous radical

polymers also allow the fabrication of ﬂ exible, thin - ﬁ lm devices. A curious feature

is the capability of forming a “ see - through ” battery, due to the absence of signiﬁ -

cant chromophores in the radical polymer. Here, use could also be made of the

slight color changes accompanied by radical redox reactions as an indicator of the

charging level.

Efforts have also been directed towards further increasing the theoretical redox

capacity of the radical polymer. For this purpose, radical polymers with even

smaller formula masses per repeating unit have been designed (Scheme 9.5 ). The

dinitroxide - functionalized polystyrene in Scheme 9.5 with a theoretical capacity of

193 mAh g

− 1

has been typically characterized by a notably high radical density of

4.3 × 1 0

21

unpaired electrons per gram, which leads to the high capacity charging/

discharging characteristics of the polymer membrane.

Figure 9.8 Charging (left) and discharging (right) processes of the organic radical battery.

Scheme 9.5 Typical radical polymers designed for high redox capacity.

330 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

9.4

Conclusions

Radical polymers have been identiﬁ ed as a new class of redox polymers, which are

characterized by a fast kinetics of electrode reactions. The suitably designed

polymer backbone allows efﬁ cient swelling in conventional electrolyte solutions,

which in turn enables an efﬁ cient charge propagation by a site - hopping mecha-

nism within the mesostructured polymer layer. The use of radical polymers as

electroactive materials leads to the ﬁ rst fabrication of organic electrode - active

materials ordered in a mesoscale. The organic radical batteries have several advan-

tages over the Li - ion batteries, such as higher safety, adoptability to wet fabrication

processes, easy disposability, and capability of fabrication from less - limited

resources. While organic batteries have an intrinsically lower volumetric energy

density, this limitation can surely be overcome in the near future so that they can

be designed to be compatible with, and installable in, a wide variety of electronic

equipment.

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Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Zhi Li , Soorly G. Divakara , and Ryan M. Richards

333

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

10

10.1

Introduction

Catalysis by nanoscale materials is a rapidly growing ﬁ eld which involves the use

of nanoparticles as catalysts for a variety of organic and inorganic reactions.

Although the application of nanoscale catalysts has a long history in industry,

the ability to engineer catalysts on the nanoscale and to explore the related phe-

nomena in a controlled manner has evolved considerably in recent years. Numer-

ous reviews have been published during the past decade on both heterogeneous

catalysis in which nanoparticles are supported on solid surfaces (e.g., silica,

alumina, MgO) and “ soluble heterogeneous ” or “ quasi - homogeneous ” catalysis

with colloidal nanoparticles [1 – 8] .

The catalytic performances of nanoparticles can be ﬁ nely tuned either by their

composition, which mediates electronic structure, or by their shape, which deter-

mines surface atomic arrangement and coordination. Beyond the exciting poten-

tial for tailoring catalysts, nanostructured materials introduce additional challenges

to catalyst design. Being small, and with surface atoms of different unsaturated

valencies, nanoparticles of speciﬁ c shape are more liable to change their shape in

the harsh medium of chemical reactions; this in turn raises concerns about dura-

bility, what the true active species is, and what approaches can be taken to control

this phenomena. The surface reconstruction and/or dissolution of active atoms

on corners or edges by one or more of the reactants, or even by the solvent, is an

additional concern that requires attention. Despite these challenges, several types

of chemical reaction have been catalyzed using transition metal nanomaterials,

including crosscouplings, electron transfers, hydrogenations, fuel cell electroca-

talysis and oxidations. In this chapter, attention will be focused on oxidation reac-

tions, with particular emphasis on the Group 11 metals of the Periodic Table,

namely silver, gold, and copper.

The Group 11 metals have a long history for their uses in jewelry, ornaments

and as “ coinage metals, ” as well being particularly interesting on the nanoscale

for the colors they display as a function of size. However, as research has

334 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

progressed with regards to both size - and shape - related chemical reactivities,

the rich chemistry of this group of metals is emerging, and particularly their

application as catalysts. Gold, in its bulk state, has long been thought to be far less

catalytically active than other transition metals, such as silver, platinum, and pal-

ladium. However, many research groups [9 – 11] have reported pioneering studies

to indicate that gold could catalyze hydrogenation, hydrogen exchange, hydroc-

racking, and carbon monoxide oxidation reactions. These initial ﬁ ndings have

inspired numerous investigations into the nature of ultraﬁ ne gold particles dis-

persed on supports, using a wide variety of approaches [12, 13] .

For many years, the chemistry of silver and gold was believed to be more similar

than is now known to be the case. Silver (Ag) is the best conductor among these

metals, and so silver nanoparticles facilitate more electron transfer than do gold

(Au) nanoparticles. Silver has a reduction potential of +0.79 V (versus NHE) for

an Ag

I

(aqueous)/Ag

metal

system, but for an Ag

I

(aqueous)/Ag

atom

system it is

− 1.80 V (versus NHE). Among the many different metal nanoparticles under

investigation, silver nanoparticles are emerging as one of the most intensively

studied, largely because of the broad range of applications that they exhibit. These

properties include shape - and size - dependent optical, electronic, and chemical

properties, and present many possibilities with respect to technological applica-

tions. Currently, copper (Cu) is one of the most widely used materials worldwide,

being of major signiﬁ cance in all industries, especially in the electrical sector due

to its low cost. Moreover, Cu continues to gain importance on the basis of it being

an essential component in future nanodevices due to its excellent conductivity, its

good biocompatability, and its surface - enhanced Raman scattering ( SERS ) activity.

Further, copper nanoparticles smaller than 50 nm in size are also considered

“ superhard ” materials, as they do not exhibit the same malleability and ductility

as bulk copper.

10.2

Preparations

There are two general paradigms for the preparation of nanoscale materials,

namely “ top down ” and “ bottom up ” . Those preparations characterized by the

breaking down of larger starting materials are classiﬁ ed as “ top down, ” while those

which are built up from atomic or molecular starting materials are termed “ bottom

up. ” Chemical procedures such as alcohol reduction [14 – 16] , hydrogen reduction

[17 – 19] , and sodium borohydride reduction [20 – 22] have in the past been recog-

nized as the most common methods for synthesizing colloidal metal nanoparti-

cles. Other reduction methods such as electrochemical [23, 24] , photochemical

[25 – 27] , and sonochemical [28, 29] have also been used, but to a smaller extent.

Many different stabilizers have been used as capping agents for the synthesis of

colloidal metal nanocatalysts, including polymers [30, 31] , dendrimers [32, 33] ,

block copolymer micelles [31, 34] , and surfactants [35, 36] . Supported metal nano-

catalysts have been prepared by the adsorption of colloidal metal nanocatalysts

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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