Electrifying interfaces

Transcription

Downloaded from http://rsta.royalsocietypublishing.org/ on October 24, 2016
10.1098/rsta.2004.1461
By F r a n k M a r k e n†
Department of Chemistry, Loughborough University,
Loughborough LE11 3TU, UK
Published online 14 September 2004
Electrochemical processes at electrified or polarized phase boundaries and methods for charging phase boundaries are discussed. An important type of interface,
the three-phase junction, is introduced and processes at three-phase junctions are
compared with those occurring at two-phase junctions. Finally, pairing charged interfaces is considered as a methodology for a more efficient use of electrical energy in
microscale electrochemical processes. Chemical processes triggered by currents in the
ultimate type of paired electrode system, a tunnel junction, are discussed.
Keywords: electrochemistry; charged phase boundaries; ion transfer;
triple junctions; electrosynthesis; tunnelling
1. Introduction
The inter-conversion of chemical energy and electrical energy is of critical importance
in natural processes as well as in technical processes. The photovoltaic generation and
the storage of electricity, as well as the use of electricity in electrochemical synthesis, are typical examples. Electricity and chemistry are closely linked and electrical
charges play an important and fundamental role in all aspects of chemistry.
In homogeneous media, e.g. in solution, the presence of free charges is minimized
by the strong requirement of electro-neutrality (as a consequence of Coulomb’s law).
The two main reasons for deviation from electro-neutrality in homogeneous media
are thermal fluctuations and molecular structures, which are imposed by stronger
quantum laws. In contrast, at phase boundaries the requirement of electro-neutrality
is removed and, indeed, it is difficult to find phase boundaries, which are uncharged.
The potential drop across an interface (typically less than 1 V) depends on the chemical composition of the adjacent phases and can be controlled by externally applied
potentials. A potential drop of 1 V across a phase boundary of ca. 1 nm results in
an enormous electrical tension of 109 V m−1 . A charge travelling through this potential gradient has enough energy (1 eV) to break chemical bonds, to drive chemical
reactions or to result in processes accompanied by the emission of light (Zu et al .
1999). At phase boundaries, the formation and separation of charges, positive and
negative, can occur. In nature, charge separation at phase boundaries and charged
membranes are the key components in, for example, energy conversion and photosynthetic processes (Stryer 1995). Figure 1 shows a schematic describing the cascade
† Present address: Department of Chemistry, University of Bath, Claverton Campus, Bath BA2 7AY,
UK ([email protected]).
One contribution of 17 to a Triennial Issue ‘Chemistry and life science’.
Phil. Trans. R. Soc. Lond. A (2004) 362, 2611–2633
2611
c 2004 The Royal Society
2612
F. Marken
stroma
lumen
Figure 1. Schematic of light-driven processes in a oxygenic
photosynthetic membrane. (Adapted from Blankenship (2002).)
of reactions involved in oxygenic photosynthesis occurring, for example, in cyanobacteria or in chloroplasts. Light energy is absorbed and converted in two steps (in the
photosystems 1 and 2) into a proton gradient across the membrane. At the same
time energy is stored chemically in two reagents of opposite electron affinity: dioxygen, O2 , a powerful oxidizing reagent and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), which remains in the organism as a versatile
reducing agent.
Biological systems use ion-exchange equilibria across membranes to ‘electrify’
phase boundaries and, in the case of photosynthesis, proton transport is driven by
light absorption from stroma to lumen. The resulting proton gradient across the
membrane or interface may be regarded as stored electrical energy and drives the
ATP synthesis in a unique ATP synthase rotor mechanism (‘oxidative phosphorylation’; see figure 1).
It is possible to express and calculate the energy stored in the form of electrical
charges in conjunction with chemical energy. Chemical equilibria across phase boundPhil. Trans. R. Soc. Lond. A (2004)
2613
counter
electrode
membrane
working
electrode
Figure 2. Schematic of a simple two-terminal electrochemical cell.
aries are balanced by electrical equilibria and the sum of terms defining the energy
stored in a molecule may be expressed as the ‘electrochemical potential’ (Schmickler
1996)
∂G
µ̃i =
= µi + zi e0 φ.
(1.1)
∂Ni p,T
In this equation, the electrochemical potential for a molecule of type i, µ̃i , which
is understood here as the molecular Gibbs free energy, is related to the chemical
potential, µi , the number of charges on the molecule, zi , the elementary charge, e0 ,
and the electrical potential across the phase boundary to vacuum, φ. In equilibrium,
the electrochemical potentials on both sides of a phase boundary have to be equal.
Changing the potential across an interface or ‘electrifying’ the interface therefore
creates an imbalance and may be used to drive a chemical process. We review and
discuss some electrochemically driven processes at interfaces of different types and
complexity.
2. Electrochemical processes at electrified interfaces
(a) The electrochemical circuit
Electrochemical experiments are conducted by completing an electrical circuit across
two contacts (electrodes) immersed into, for example, a solution phase (see figure 2).
Once a sufficient potential is applied between the two electrodes, a current will start
to flow. It is helpful to consider the two electrodes separately and to denote them as
‘working electrode’ (where the process of interest occurs) and ‘counter electrode’ (an
auxiliary electrode). In simple polarization experiments, at the electrode–solution
interface, the flow of electrons in the electrode is transformed into a flow of cations
and anions in the solution (Faraday’s law). Chemical reactions are required to couple
electron flow and ion flow and they occur simultaneously at both electrodes. Usually
only the process at the working electrode is of interest. A membrane can be used to
avoid unwanted products from the counter electrode interfering with the workingelectrode process.
In practice, a reference electrode is often included (in a three-terminal electrochemical cell) to provide a well-defined potential with respect to which all measurements are made. To give an example relevant to bulk-scale industrial electrochemistry
Phil. Trans. R. Soc. Lond. A (2004)
2614
F. Marken
(b)
(a)
A
A
e−
e−
B−
C+
B−
(d)
(c)
A
e−
B−
A
e−
C+
B
e−
C
Figure 3. Schematic of four types of electrochemical interfaces. (a) The flow of charge drives
a chemical process at a metal–solution interface. (b) The flow of charge occurs across a
metal–solution interface and drives the transfer of a C+ ion at a liquid–liquid phase boundary. (c) At a solid-electrode–liquid–liquid triple interface the flow of charge is driving both a
chemical process and simultaneously the ion transfer. (d) At a paired electrode system the flow
of charge is driving two complementary processes.
(Pletcher & Walsh 1993), metal deposition, refining or electrowinning processes such
as the production of aluminium (equation (2.1)) are driven by potentials applied to
suitable electrodes:
2Al2 O3 (cryolite) + 3C (counter electrode) → 4Al (metal) + 3CO2 (gas).
(2.1)
In this process the metal cation in solution (here Al in molten cryolite at 1030 ◦ C)
is reduced at the working electrode at sufficiently negative potential (giving Al metal)
and a complementary process at the counter electrode (here the formation of CO2 )
occurs at a positive applied potential. The case of a reduction (consumption of electrons) or oxidation (release of electrons) process occurring at the electrode–solution
(electrolyte) interface is conceptually relatively simple, but there are other more
complex types of interfaces and potential-driven processes.
3+
(b) Electrochemical interfaces
A metal electrode in contact with an ionic conductor creates a phase boundary,
which in the presence of an applied potential is connecting the flow of electrons
with a flow of ions via a chemical reaction (see figure 3a). In this conventional electrochemical process, effectively, at the interface, electrical energy (supplied from an
2615
outside reservoir with energy = Faraday constant × applied potential) is converted
into chemical energy (in the form of products) and vice versa. In addition, there are
energy losses, usually in the form of heat.
A thin film of liquid deposited onto the surface of the electrode can be employed
to drive a second interfacial process at the phase boundary of the two immiscible
liquids (see figure 3b). Shown in the schematic is the case of a cation transfer from
the outer to the inner phase driven by the applied potential. The condition for
this process to occur is a sufficient ionic conductivity in the thin film phase. In
figure 3c, the alternative case of a triple-phase junction process between an electrode
and two phases is shown. This more versatile kind of process also allows chemical
reactions to be driven in non-ion-conducting media. This ‘triple-phase junction effect’
has important implications for processes involving solid-state reactions and for the
development of novel ion-sensing methodology. In contrast to the thin film case (see
figure 3b), only a line interface is active.
Finally, in figure 3d two electrochemical interfaces are paired to drive a chemical
process. The potential benefits of this configuration are the use of both working and
counter electrode simultaneously within a very small space and without unwanted
side reactions or waste products. In practice, this paired electrosynthesis approach
requires a careful design of a more complex reaction sequence and has not yet been
realized for a commercial process. The relevance, fundamentals and application of
these four different types of electrified interfaces are discussed in more detail below.
3. Electrochemical processes at triple interfaces
(a) Processes at conventional liquid–liquid interfaces
In bulk solution, compositional gradients lead to transport phenomena and relaxation
towards a homogeneous state. However, at phase boundaries, a stable compositional
gradient exists and this manifests a spatial change in chemical potential balanced by
a gradient in electrical potential (see equation (1.1)). The potential gradient leads
to the formation of a charged interfacial (double) layer (Bockris et al . 2000). In
order to generate the gradient in electrical potential, dipolar orientation and, more
importantly, ion exchange occur.
A liquid–liquid interface in equilibrium can be perturbed by the addition of chemical reagents or by changing the electrical potential in electrochemical experiments
(Girault 1993). Figure 4a shows a schematic of a system in which two electrodes
have been introduced into two separate liquid phases (water and oil).
The potential applied to these electrodes is known and equal to the sum of potentials across the cell. For electrodes which cannot sustain an applied potential (nonpolarizable electrodes, e.g. reference/counter combinations or electrodes immersed
in redox-active material; see figure 4b), the resulting potential gradient is localized
or ‘focused’ at the liquid–liquid interface only. Therefore, a potential can be applied
across the liquid–liquid phase junction and chemical reactions or ion-exchange processes can be driven. A prerequisite for this type of process is the ability of the two
liquids to conduct electrical signals. This can be achieved for example by adding
dissociating salts (the supporting electrolyte).
In practice, well-defined liquid–liquid interfaces can be investigated at microholes
(Wilke & Zerihun 1998), at expanding droplets (Slevin & Unwin 1999) or at the outlet
of micropipettes (Shao & Mirkin 1997). Over recent years, elegant experimental
2616
F. Marken
(a)
(b)
working
electrode
counter
electrode
e−
e−
Β− C−
A
B
liquid–liquid
interface
Figure 4. Schematic of (a) a simple liquid–liquid electrochemical cell with a liquid–liquid phase
junction separating two electrodes and (b) a cycle of charge transfer processes which lead to the
polarization of the liquid–liquid interface upon externally applying a potential.
techniques have been developed for the study of static liquid–liquid phase boundaries
and also for dynamic (flowing) liquid–liquid phase boundaries (Yunus et al . 2002). A
beautiful example of an electrochemical process driven at the liquid–liquid interface
is the formation of metal clusters by interfacial reduction of metal cations. This
has been demonstrated for gold (Cheng & Schiffrin 1996) and for silver (Guo et al .
2003a) and may be compared with the electrochemical reduction and deposition of
these metals at conventional electrode surfaces. Compared with a conventional solid
surface with defects and imperfections, the liquid surface is much more uniform and
free of catalytic nucleation sites and therefore these liquid–liquid metal deposition
processes are unique.
(b) Processes at solid–liquid–liquid triple interfaces
A triple interface—or line interface (Rowlinson & Widom 1982)—commonly forms
where three immiscible phases are in contact. Figure 5a shows a droplet of, for
example, oil sessile on an electrode surface and immersed in aqueous electrolyte
solution. The triple interface is located in the form of a ring at the oil–water–electrode
three-phase junction.
An example of triple interface processes at microdroplets is shown in figure 6. An
oily and entirely water insoluble compound, N,N,N ,N -tetrahexylphenylenediamine
(THPD) (Marken et al . 1997), is deposited in the form of microdroplets onto an electrode and immersed into an aqueous solution containing only supporting electrolyte.
The potential of the electrode is scanned from 0.0 V towards a positive potential
of 0.6 V versus SCE. A deposit on a transparent electrode placed under a microscope
clearly shows a colour change commencing from the triple-phase junction (figure 6b).
Chemical microanalysis revealed that, during this oxidation process, anions have been
forced from the water phase into the organic oil phase. Thus, during the potentialPhil. Trans. R. Soc. Lond. A (2004)
(a)
2617
water
oil
electrode
X−
X−
(b)
oxidation
X−+
X−
e−
(c)
reduction
−
X− X + X−
X−+ +X− X−+ −
X−+ X−+ +X−++ X +
X−
X−
e−
Figure 5. Schematic of (a) an oil droplet forming a three-phase junction with an aqueous phase
and the electrode; (b) an oxidation process within the droplet, triggering anion transfer from the
aqueous into the oil phase; (c) the reverse reduction process, triggering the expulsion of anions
into the aqueous phase.
driven oxidation of the droplet, anions have to cross the liquid–liquid phase boundary
(see figure 5b). This process is fully reversible and, after applying a negative potential,
expulsion of the anions (figure 5c) is observed. Changing the type of anion and pH
in the water phase causes a tell-tale shift in the potential at which the oxidation
occurs (see figure 6c, d). This potential shift for different anions is dominated by the
contribution from the energy required to remove the anion from the aqueous phase
(or more correctly by the Gibbs free energy of ion transfer across the liquid–liquid
phase boundary) and the shift with pH (in agreement with the appropriate Nernst
equation (Schröder et al . 2001)) is indicative of a transition from anion exchange to
proton exchange at sufficiently high proton activities.
A schematic depicting the electrochemically driven ion-exchange process at the
three-phase junction is shown in figure 5. Electron transfer occurs close to the triple
interface and the resulting charge generated in the organic phase causes the transfer
of anions from the water phase. Coating the electrode with a thin film of organic oil
rather than with microdroplets causes only electrode blocking, and no currents are
detected. Only in regions where the two interfaces (solid–liquid and liquid–liquid)
are sufficiently proximate does the redox process commence. Interestingly, additional
convective transport effects (Marangoni convection) have been proposed to be partly
responsible for the high efficiency of the process (Ball et al . 2000).
This development of an electrochemically driven ion-exchange process at microdroplets was exploited and developed further. The Gibbs free energy of transfer
2618
F. Marken
(a)
H2C
H2C
H2C
H2C
H2C
H2C
(b)
CH3
H3C
CH2
H2C
CH2
H2C
N
N
CH2
H2C
CH2
H2C
CH3
H3C
CH2
CH2
CH2
CH2
CH2
CH2
(d)
(c)
10 µA
pH4
pH1
pH2
pH3
pH7
0
0.3
E/V (vs. SCE)
0.6
Emid in V versus SCE
0.6
0.5
0.4
NO3−
0.3
SCN−
CIO4−
PF6−
0.2
0.1
0
1
2
3
4
5
pH
6
7
8
Figure 6. (a) Structure of THPD. (b) Microscopic image of THPD microdroplets on the surface of
a transparent electrode and immersed in aqueous 0.1 M NaClO4 being converted into dark blue
THPD+ ClO−
4 microdroplets. The process commences at the three-phase junction. (c) Cyclic
voltammograms for the oxidation of THPD droplets immersed in aqueous 0.1 M KNO3 as a
function of pH. (d) Plot of the midpoint potential observed for different types of anions. (Adapted
from Marken et al . (1997).)
is a key parameter in biology and physiology, where ion transfer between aqueous
fluid domains and membranes or fat domains is crucial. For instance, the interaction
between pharmaceuticals and the body is strongly affected by this parameter. In the
past, data for these Gibbs free energies have been very difficult to obtain. However, in
recent work by Scholz and co-workers it is demonstrated how voltammetric measurements at triple-phase junctions provide access to these data. A wide variety of anions
(Komorsky-Lovric et al . 2002) including amino acids and oligo-peptides (Mirceski et
al . 2002; Gulaboski & Scholz 2003), anionic drugs (Bouchard et al . 2003), as well as
cations (Scholz et al . 2003) have been studied.
In the presence of microdroplets, electrode surfaces also provide femtolitre microreactor environments for catalytic (Davies et al . 2004) or photochemical (Wadhawan
et al . 2003a) processes. Compton and co-workers (Wadhawan et al . 2003b) have
demonstrated distinct types of electrocatalysis with processes at the droplet surface or processes within the droplets. Now, electrocatalysts can be placed at electrode surfaces in microdroplet environments to overcome solubility problems and
2619
(b)
(a)
(iii)
(ii)
(i)
n-C6H13
N
n-C6H13
N
n-C6H13
n-C6H13
N
n-C6H13
100 µA
0
E/V (versus SCE)
0.7
(d)
(c)
n-C6H13
50 µA
N
n-C6H13
−0.3
0
E/V (versus SCE)
N
n-C6H13
S
n-C6H13
N
n-C6H13
0.5
Figure 7. (a) Structure of N1 -[4-(dihexylamino)phenyl]-N1 ,N4 ,N4 -trihexyl-1,4-phenylenediamine
(DPTPD). Cyclic voltammograms obtained for DPTPD microdroplets (b) in the absence and
(c) in the presence of sulphide anions. (d) Structure of the proposed reaction product. (Adapted
from Marken et al . (1999).)
immobilization effects. Figure 7 shows the case of a sensor microdroplet redox system chemically responding to a particular type of anion, here hydrogen sulphide,
HS− (Marken et al . 1999). Microdroplets of N1 -[4-(dihexylamino)phenyl]-N1,N4 ,N4 trihexyl-1,4-phenylenediamine, when oxidized in the presence of HS− , give a product
shown in figure 7d, with a new voltammetric signature. A review of electrochemically
driven microdroplet processes is given in Banks et al . (2003).
(c) Processes at solid–solid–liquid triple interfaces
In contrast to microdroplet-based electrochemical processes, electrochemical reactions at microparticles have been studied for a considerable time (Grygar et al . 2002;
Brainina & Neyman 1993). The triple-phase junction in this case is formed at the
contact point between the particle (here assumed to be electrically insulating), the
electrode surface and the aqueous electrolyte phase (see figure 8a). In comparison
with the situation above where material in a droplet is electrochemically converted,
the conversion of a solid requires a solid-to-solid reaction step. This is indicated in
figure 8a in terms of the formation of a new product, phase 2, from the starting material, phase 1, and a moving reaction front separating the two. For many types of solids
2620
F. Marken
(b)
(a)
phase 3
phase 1
phase 2
phase 4
2 µm
(d)
(c)
100 µA
−0.7
NaCl
0
0.7
50 µA
−0.6
0
0.6
0
0.3
KPF6
20 µA
−0.7
K
Fe
Fe
NaClO4
20 µA
−0.7
Cl
NaF
0
Fe
Cl
K
Fe
0.3
E/V versus Ag/AgCl
Figure 8. (a) Schematic of a solid–liquid–electrode triple-phase junction system. (b) SEM image
of decamethylferrocene microparticles imbedded in a graphite electrode. (c) Cyclic voltammetric
data for the oxidation of decamethylferrocene microparticles in aqueous 0.1 M electrolyte solution with different types of salts. (d) Elemental analysis of decamethylferrocene particles before
and after oxidation in the presence of ClO−
4 , proving the uptake of anions into the solid during
oxidation. (Adapted from Bond & Marken (1994).)
this reaction step is extremely slow and prevents any reaction progress through bigger particles. However, for very small particles, electrochemical processes are readily
detected even for highly insoluble materials. The oxidation of decamethylferrocene
deposited onto carbon electrodes and immersed in aqueous electrolyte solution is
an instructive example (see figure 8). The one-electron oxidation of the solid was
detected (Bond & Marken 1994) and it was shown by elemental analysis (see figure 8d) that the transfer of anions into the solid phase indeed occurs.
A wide range of solid microparticles have been studied electrochemically (Scholz
& Meyer 1998; Bond 2002) and two distinct types of processes may be distinguished.
Some solid materials allow the uptake and expulsion of ions from the aqueous phase to
proceed without a significant activation-energy barrier. For these ‘electrochemically
open’ materials (e.g. the dinuclear ruthenium complex shown in figure 9 (Bond et
2621
n+
(PF6)n− with n = 2,3,4
N
N
N Ru
N O
N
N
O
N
N
N Ru N
N
N
100 µA
0
0.5
E/V (versus SCE)
−0.5
1.0
Figure 9. The structure of [{Ru(bipy)2 }2 (µ-1,4-dihydroxy-2,5-bis(pyrazol-1-yl)benzene dianion)]
(PF6 )n with n = 2, 3 or 4 and cyclic voltammograms for the oxidation of microparticles of this
metal complex immobilized at a graphite electrode and immersed in aqueous 0.1 M KPF6 .
(Adapted from Bond et al . (2000).)
NC
Iox
CN
400 µA IIox
IIred
NC
CN
Ired
0.5
1.0
−1.0 −0.5
0
E/V versus Ag(s)|AgCI(s)|KCI(aq)
3m
Figure 10. Structure of TCNQ and voltammetric data for the reduction of TCNQ microparticles
immobilized at the surface of a graphite electrode and immersed in aqueous 0.1 M KCl. (Adapted
from Bond et al . (1996).)
2622
F. Marken
(a)
(b)
+
anode
starting
materials
O2
ATP
products ADP
cathode
NADH
−
Figure 11. Schematics of (a) a paired electrochemical process and
(b) the electron flow in the biological oxidative phosphorylation process.
al . 2000)), voltammetric responses are well defined and the peak-to-peak separation
for complementary oxidation and reduction processes is small.
In contrast, ‘electrochemically closed’ materials prevent ion uptake into the host
lattice, presumably due to stronger lattice forces and/or the need for substantial
lattice expansion. These materials require a nucleation step and the formation of a
new phase (phase 2 in figure 8a), which is associated with a considerable activation
energy barrier (in the form of interfacial tension between phase 1 and phase 2 (Bond
et al . 1996)). As a result a characteristic gap or ‘inert zone’ between reduction
and oxidation (see, for example, process I for tetracyanoquinodimethane (TCNQ)
in figure 10) is observed (Bond et al . 1998).
4. Electrochemical processes at paired interfaces
(a) Beneficially pairing electrochemical processes
All electrode processes discussed up to this point were driven by the potential applied
to one electrode, the working electrode. However, in an electrochemical circuit there
are always at least two terminals and a complementary chemical process is always
occurring at the second terminal, the counter electrode. It is interesting to ask
whether these two electrode processes can be beneficially coupled without Faradaic
losses and with a minimum of resistive losses or unwanted side products. A comparison with natural redox processes shows that highly effective reaction cycles have
evolved by pairing complementary redox systems. A case of considerable importance
is depicted schematically in figure 11. ‘Redox fuels’ for biological processes, dioxygen
(O2 , taken from the atmosphere) and NADH (reduced nicotinamide adenine dinucleotide, formed in glycolysis and in the citric acid cycle) provide a 1.14 V driving
2623
force for diverse biological processes. In a ‘paired’ process (oxidative phosphorylation) both compounds are consumed and ATP (adenosine triphosphate) is formed.
ATP may be regarded as a universal biological source of chemical energy, which,
for example, then drives biosynthesis in the Calvin cycle (Stryer 1995). Analogously,
it should also be possible to beneficially couple anodic and cathodic electrode processes with energy from both electron transfer processes going into the formation of
the desired products (see figure 11).
Currently, technical electrode processes are far less complex compared with biological redox chains, and a few attempts to pair electrode processes have been reported
(Lund & Baizer 1991). The first step towards paired electrosynthesis is the use of
an undivided cell in which both anode and cathode contribute to the overall reaction. A typical example, the use of a complementary counter electrode process in a
‘convergent’ process forming glyoxalic acid from oxalic acid and glyoxal (Jalbout &
Zhang 2002), is given by
COOH
redu
ction
COOH
CHO
ion
oxidat
CHO
COOH
CHO
(4.1)
‘Divergent’ paired electrode processes (one starting material yielding two products;
for example, glucose (Yu et al . 1988)) and the elegant use of hydrogen peroxide
formed at the cathode (Li et al . 1999) have been proposed. In contrast to these
processes, more intimately paired electrode reactions may result at a very small
anode to cathode separation. For example, the formation of anthraquinone (ANQ)
from anthracene (AN) and dioxygen has been reported (Amatore & Brown 1996):
O2
ANQ
.−
ANQ.− O2
AN.+
cathode +e−
ANQ
−e− anode
AN
H2
(4.2)
The distance between the two electrodes involved in the paired process is a crucial parameter. For electrodes at a considerable distance from each other, the electrode processes remain decoupled and independent (see equation (4.1)). However,
upon bringing the two electrodes into closer proximity, new reaction pathways may
become available. Figure 12 shows a graph with the distance between two charges (or
electrodes) on the horizontal axis and the approximate time-scale for inter-diffusion
on the vertical axis. For a large distance, conventional electrochemical processes are
expected. In contrast, for a very small gap, of the order of some angstroms, direct
tunnelling is expected without the formation of useful products (similar to photoPhil. Trans. R. Soc. Lond. A (2004)
F. Marken
photochemical
processes
time scale (s)
105
101
10−3
10−7
'transition zone'
conventional
electrochemical
processes
2624
10−11
1
1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm
distance of charge separation
Figure 12. Plot indicating the time-scale of diffusion for a given distance of charge separation.
chemical processes which involve charge separation by excitation followed by annihilation and heat generation). Located between these two extremes is a transition
zone, where reaction layers of the two electrodes overlap and where novel coupled
processes are possible. Continuously varying the distance between a probe electrode
and a substrate electrode immersed in a suitable solution should result in a characteristic current—distance or potential—distance profile indicative of zones in which
coupled chemical processes occur.
From this discussion it can be seen that the distance between anode and cathode,
and the ‘design’ of the chemical process, are the key factors in making paired electrochemistry a viable option to replace conventional bulk-scale synthetic routes. The
use of a two-phase systems within the electrode gap to control the reaction pathway
of the paired process may further contribute to this development. Beneficially pairing
two electrode processes represents a considerable challenge.
(b) The electrode design for paired electrosynthesis
Chemical systems for paired electrosynthesis are rare and of far lower complexity
than biological processes. Paired electrosynthesis methodology has been developed by
Lund & Baizer (1991) and recently pursued, for example, by Li & Nonaka (1999a, b),
Hu et al . (1995), Steckhan et al . (2001), Ferrigno et al . (1998) and Belmont et al .
(1998). In some cases entirely new products, such as the anthracene electro-oxidation
product anthracenedione or a C–C coupling product formed in a zone between the
anode and cathode (Kim et al . 2001), can be isolated after bulk electrolysis. Propylene oxidation (Belmont & Girault 1995) and sea-water-electrolysis processes have
been realized, for example, at closely spaced interdigitated band-array electrodes.
In the following section, types of electrodes for paired electrosynthesis and masstransport parameters are discussed.
Reactions at electrodes are distinct from reactions in homogeneous solutions, in
that mass transport to and away from the reaction zone is crucial and often rate
limiting. The transport of reactants may occur via diffusion, but it is usually aided
by controlled convection. This can be achieved by stirring a solution, by directing
a liquid jet towards the electrode surface (Alden et al . 1999), or by placing the
electrode system into a high-intensity ultrasound field (Compton et al . 1997). Each
type of mass-transport enhancement affects the distribution of the current density
across the electrode surface in a specific way. For example, with a laminar flow of
liquid across an electrode surface, the current density at the upstream edge is the
(a)
(b)
2625
width
length
electrode 1
electrode 2
flow
height 2h
(c)
starting
materials
reaction
products
porous Au
SiO2
conducting
substrate
Figure 13. Schematic of three-electrode systems for paired electrochemical processes.
highest, and products from this zone can reduce the efficiency of the electrode further
downstream.
Figure 13a shows an interdigitated electrode system, which can now be produced
routinely and in extremely small dimensions via advanced lithographic techniques.
Each electrode has a band geometry with a width of typically 0.1–100 µm and a gap
of similar size between the electrodes. It is instructive to consider two limiting cases
of processes at this type of electrode: a fully reversible process,
Α
cathode
+e−
Α−
Α
−e− anode
Α−
(4.3)
and a chemically irreversible paired electrode process,
P
B
cathode +e−
A
C
−e− anode
D
S
(4.4)
2626
F. Marken
A fully reversible process causes diffusion-controlled feedback currents between the
generator and collector electrodes, which are strongly size and geometry dependent.
For the interdigitated band electrode (see figure 13a), the steady-state feedback current has been determined (Aoki et al . 1988; Fosset et al . 1991) and the approximate
diffusion layer thickness, δdiffusion (the layer of solution affected by the process), can
be equated (in good approximation) to the width of the gap between electrodes, w:
δdiffusion ≈ w.
(4.5)
Under feedback conditions (equation (4.3)), the magnitude of the current for the
reversible process is high, and effects of externally applied convection are low. In
contrast, for an irreversible paired process (equation (4.4)), reactants will be immediately depleted and the convection-assisted transport of reactants toward the electrode becomes crucial. Solution flowing across the electrode surface will improve the
process based on the following approximation:
δdiffusion =
nF DAc
.
Ilim
(4.6)
In this expression δdiffusion , the diffusion layer thickness, is related to n, the number of electrons transferred per molecule, F , Faraday’s constant, D, the diffusion
coefficient, A, the electrode area, c, the bulk concentration of starting material and
Ilim , the current for the process. Convection reduces the diffusion layer thickness and
therefore increase the current. By comparing equations (4.5) and (4.6) it can be seen
that the irreversible process will remain paired as long as the diffusion layer thickness
achieved by convection remains larger compared with the electrode and gap dimensions of the interdigitated array. Therefore, reducing the dimensions of the electrode
system improves the process. On the other hand, for a particular reaction with a
chemical rate constant kc the smallest electrode dimension
(or reaction layer thickness) avoiding feedback conditions is given by δreaction = D/kc . Reducing the gap
between the electrodes below the reaction-layer thickness will cause simple feedback
currents to flow without the formation of products.
An alternative approach to paired electrochemical processes can be based on thin
layer flow-through channel cells (figure 13b). This geometry allows coupling of the
two electrodes located vis-à-vis and mass-transport enhancement via flowing the
solution (Paddon et al . 2002). For a paired (irreversible) electrode process, this type
of electrode assembly brings two big disadvantages: in upstream and downstream
regions of the cell different current densities and processes will occur and the volume of solution pumped through the cell is severely limited. Finally, an alternative
electrode system shown in figure 13c is based on a sandwich of two electrodes and
a ceramic membrane of high porosity, and convective transport of reactants towards
the surface. The sandwich electrode system is proposed to combine simple design
and operation and high efficiency. All three types of electrode system allow a further problem of electrosynthesis methodology to be addressed: the need to support
the electrolyte. Processes within very small gaps between electrodes are less affected
by conductivity problems and generally these processes are ‘self-supported’ due to
charged species being generated in situ.
2627
x
y
variable-height
working electrodes
z
substrate
Figure 14. Schematic of the experimental geometry for SECM or STM.
(c) Nanoscale paired electrochemical and tunnelling processes
A family of techniques has been developed in which a sharp metal tip or electrode
is lowered towards a substrate surface (see figure 14) and the current is detected as
a function of distance and position. In this way, electrochemical processes can be
monitored within a gap of typically 10–100 µm in scanning electrochemical microscopy (SECM) and tunnelling processes can be monitored within a gap of 0.1–1 Å
in scanning tunnelling microscopy (STM).
Electron-tunnelling events between an electrode and a molecule are part of all
electrochemical processes. These fundamental and elementary reaction steps occur
very quickly and on a very small scale. Only very little detailed experimental information is available. When two biased electrical conductors are placed in very close
proximity (typically a gap of less than 1 nm) electron tunnelling occurs continuously
and tunnel currents become measurable as a function of distance. Molecules may be
placed within this gap and in this way tunnelling of electrons through the molecular
structure can be observed. Techniques able to achieve this kind of molecular conductivity measurement are ‘break junction’ or ‘crossed-wire junction’ experiments
(Weber et al . 2003) and STM (Wiesendanger 1994). Break junctions are formed by
controlled tearing of a very thin metal wire. Placing molecules into the resulting
nanogap results in characteristic current–voltage curves, which reflect the molecular
electronic structure (Reed et al . 1997) and the way the molecules are connected to
the electrodes (Tada & Yoshizawa 2002). Tunnelling through these molecules apparently has no implication in terms of chemical reactions, but inelastic tunnelling is
known to lead to the excitation of molecular vibrations or even to photon emission
(Guo et al . 2003b). If chemical reactions could be triggered under these conditions,
a new methodology would have to be invented to detect and determine the reaction
products.
In STM techniques, a microscopically sharp metal tip is lowered towards an electrically conducting surface (figure 14) and tunnelling currents are detected as a function
of distance and bias voltage. The technique allows scanning across the surface of a
sample, exploring and imaging the molecules present at the surface and monitoring
2628
F. Marken
(a)
initial
final
(b)
I (nA)
0.6
0.2
120
(c)
(d)
time (ms)
(e)
Figure 15. (a) Schematic of how a tunnel current applied from the STM tip leads to molecular
bond breaking. (b) Tunnel current changes during bond breaking. (c)–(e) A sequence of STM
images showing the iodobenzene molecule adsorbed on the Cu(111) step edge at 12 K before
and after reaction. (Adapted from Hla & Rieder (2002).)
of the progress of chemical reactions directly in situ. It has been shown that at low
temperature the STM tip can used for moving atoms and for manipulating molecular
structures (Ueba 2003).
Hla et al . (2000) and Hla & Rieder (2002) proposed and demonstrated a step-bystep chemical reaction sequence driven and monitored by the STM tip (see figure 15).
They discovered that at low temperature iodobenzene is adsorbed onto a well-defined
copper surface and a tunnel current can be employed to trigger a chemical bond
breaking process. Furthermore, the phenyl fragment from this reaction was combined
with a second phenyl fragment to give biphenyl after bond formation. Overall, this
type of reaction is known as the Ullmann process, and has been demonstrated step
by step, triggered by the STM tip. The tunnel current causes excitation of molecular
vibrations and therefore this process may be regarded as ‘molecular welding’. The
experiment demonstrates that there are novel ways of driving chemical processes at
electrified interfaces and at a very small scale.
5. Summary
Electrified interfaces are essential in many technical and natural processes, and a
better understanding of these processes is of considerable future importance. Processes described here highlight only some selected electrochemical processes, with
the emphasis on novel types of interfaces and going from large-scale processes to
molecular-scale reactions. It is instructive to compare the technical approach for
chemical energy storage or electrochemical synthesis with the way nature stores
energy and synthesizes molecular structures. A striking difference identified here
is the complexity of reaction sequences and environments. Bringing two electrodes
together for a paired electrochemical reaction might only be the beginning of a
2629
development requiring more-complex reaction sequences, more-complex interfaces
and better designs of electrochemical cells.
I thank The Royal Society for its generous support through a University Research Fellowship.
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AUTHOR PROFILE
Frank Marken
Frank Marken was born in 1964 in Oldenburg, Germany, and studied Chemistry at
RWTH Aachen, Germany, and at the University of Bristol. He graduated in 1989
and prepared his doctoral dissertation on ‘Metal cyclopentadienide complexes’ with
Professor G. E. Herberich. Periods of postdoctoral research followed with Professor
A. M. Bond in Melbourne, Australia (supported by the Alexander-von-Humboldt
Foundation) and with Professor R. G. Compton in Oxford. He has held a Royal
Society University Research Fellowship since October 1996 and has been elected to
a Stipendiary Lectureship in Physical Chemistry at New College, Oxford. He joined
the Department of Chemistry at Loughborough University as a Lecturer in 2000,
and moved to the Department of Chemistry at the University of Bath in September
2004. Research interests focus on electrochemical processes and complex interfaces.
2633

Electrifying interfaces

Transcription

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