Some Fundamental Challenges in Electrocatalysis

Some Fundamental Challenges in Electrocatalysis

David J. SchiffrinChemistry DepartmentUniversity of LiverpoolUK

Summary, or what is going on? And why?Instrumentation and techniquesSurface spectroscopiesIn situ analysis e.g., mass spectroscopy, XAFSAvailability of large scale facilitiesSurface science techniques, high resolution XPSImaging, surface probe microscopies, STEM, ultrahigh resolution TEMTheoryTheoretical advances in electron transfer and reactivity Computational methods e.g., quantum chemical calculationsMaterialsAlloysNanoparticlesNon Pt based electrocatalystsApplicationsElectroanalysisFuel cellsElectrosynthesisEconomic and social driversEnvironmental issues (“green” chemistry)Hydrogen transportWater treatment

Some scientific issues

Nanoparticles Alloy properties: how to predict structures and properties Importance of size effectsPrediction of reactivity of different surfacesSynthesis: control of size and geometry

ReactivityUse biologically inspired synthetic strategiesTransposition of single crystal studies to nanostructured surfacesQuantum chemical calculations for the prediction of reactivityExtension of theory of electron transfer to reactions on nanostructured surfacesMetal-reactant interactions

Reactions of interest Oxygen reductionOxygen evolution Nitrate reductionCarbon dioxide reduction

Fuel cell buses in Europe and North America

A fuel cell engine

Running a car on nanoparticles

CarbonCarbon

O2

H2O + 4 e-

2 H2

4H+ + 4 e-

Cathode (+) Anode (-)Pt Nanoparticles

What about the Pt-carbon contact? How do we study the properties of well-defined nanoparticle-electrode systems?

Preparation of nanoparticulate

electrode surfaces

Nanoparticles at surfaces

Problem: in order to study the electrochemical properties of nanoparticles (e.g., size effects), we need to attach them to an electrode surface.

Two approaches:

1. Synthesise and then fix them

2. In-situ growth

Attachment by ligands of nanoparticles

Dithiols: Stability problems with Pt

Diamines: Poor long term stability

Alternative strategy: In situ growth

Electrochemical growth of nanoparticles on carbon surfaces

R Penner, J. Phys. Chem. B 2002, 106, 3339-3353

Separate control of nucleation and growth to achieve uniform size distribution

In-situ nucleation and growth on HOPG

AFM image of Pt nanoparticles prepared using the potential pulse sequence shown. Average particle height = 26 nm.

Average particle height = 16 nm.

Bayati, Abad, Nichols, Schiffrin, in preparation

Guojin Lu and Giovanni Zangari, J. Phys. Chem. B 2005, 109, 7998

Creation of growth sites by oxidation

Bayati, Abad, Nichols, Schiffrin, in preparation

Oxidation in H2SO4 leads to a large disruption of the surface. Bad news

AFM image of electrodeposited nanoparticles on electrochemically oxidized HOPG. Size between 2 and 5 nm depending on growth time.

Oxidised HOPG at 2.0 V for 1 s

Chemical growth of nanoparticles on

electrode surfaces

Chemical functionalisation strategy of carbon surfaces

Dao-jun Guo, Hu-lin Li, Electrochemistry Communications 6 (2004) 999–1003

Waje etg al., Nanotechnology 16 (2005) S395–S400

NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2

NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2

Electrochemical or chemical growth

Pt nuclei

24PtCl

Redn

Construction of 2-D arrays of Pt particles. Strategy followed

Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem., in press

Construction of 2-D arrays of Pt particles. (I) Amino termination

+

e-

+ N2

N+

N

NO2

NO2 NO 2 NO 2

Diazonium chemistry

NO2 NO 2 NO 2 NH2 NH 2 NH 2

Sn(II), HCl Chemical reduction

396 398 400 402 404 406

0

50

100

150

200

103 x

Co

un

ts

Binding Energy / eV

XPS spectra of the N1s region for Pt/-NH2 modified HOPG after background subtraction.


Conductance measurements for BDMT (To find out if a phenyl group is a high conductance linker)

Van Zalinge, et al., Nanotechnology 17 (2006) 3333–3339

Zero bias conductance = 7 nS (140 MΩ

per molecule) or approx. 5x10-6 Ω cm-2

Very small resistance!! OK

Mechanisms of dediazotation reactions

N2+ + N2

N2+ + Red . + N2 + Ox (concerted)

N2 + Ox.

(stepwise)

heterolytic

homolytic

Rate of the heterolytic reaction is very slow, 1/2 of 6-9 hs!

The radical mechanism is the preferred route for the electrochemical functionalisation. Spontaneous attachment? Source of electrons?

Spontaneous Grafting of Glassy Carbon with Fast Red

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015(b) 3

2

1

I / m

A

E / V-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015(c) 2

1

I / m

AE / V

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

(d)

21

I / m

A

E / V

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015 3

21(e)

I / m

A

E / V

0.05 M H2SO4 ACETONITRILE

phosphate buffer (pH 7) H2O unbuffered

Seinberg, Kullapere, Mäeorg, Maschion, Maia, Schiffrin, Tammeveskia, J. Electroanal Chem, in press

Fast Red

NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2

NH2 NH2 NH2 NH2 NH2 NH2 NH2 NH2

Electrochemical or chemical growth

Pt nuclei

24PtCl

Redn

Construction of 2-D arrays of Pt particles. Strategy followed


XRD data analysis- Pt nanoparticles on HOPG

XRD (a) XRD patterns for Pt nanoparticles on Ar-NH2 modified HOPG of 4.0 (1), 2.7 (2) and 2.0 nm (3) average size (measured by TEM). (b) Analysis for the 2.0 nm Pt nanoparticles using the Rietveld refinement.

(b)

sp3 carbon?

Size calculations using the Scherrer equation ii

i θcosβ

λD

βi is the width in radians of the diffraction peaks measured at half their maximum intensity (FWHM) and corrected for instrumental broadening

Fitting to data

Residuals


sp2- sp3 evidence from single CNT conductance

CNTs were functionalised using diazonium chemistry. “Upon annealing, the functionalization is removed, restoring the electronic properties of the nanotubes.”

sp2 sp3 surface changes on grafting

Graphene sheet - sp2 sites

Surface functionalisation by bonding changes the sp2 sites on the graphene sheet into sp3 sites. CNTs become non-conducting on side functionalisation

40 45 50 55 600

20000

40000

60000

80000

100000

Inte

nsi

ty/ A

rb u

nit

s

2 / deg

X-Ray diffraction of a functionalised HOPG surface

sp3 sites


Pt nanoparticles prepared by chemical reduction

TEM image of Pt nanoparticles on Ar-NH2 modified HOPG after one nucleation-growth cycle. The insets show the size distribution. Average sizes: a) (2.7 ± 0.4); b) (4.0 ± 0.5) nm.


XPS of Pt nanoparticles attached to HOPG

CPS

Original data

b’ b

a’a

Fitted data

Deconvoluted XPS spectra of the 4f core level for Pt/Ar-NH2 modified HOPG electrodes. Diameter = 2 nm.

Two contributions observed: core and surface

The Pt core has a lower value of the FWHM: Uniform environment

Surface BE corresponds to Pt(II) present in a range of environments

Pt 2 nm

Bayati, Abad, Bridges, Rosseinsky, Schiffrin, J. Electroanal. Chem 623 (2008) 19

XRD analysis for 4 nm particles

2θ/deg

Inte

nsit

y/A

.U.

C

Rietveld refinement for the analysis of the XRD data for 4.0 nm Pt nanoparticles on -NH2 modified HOPG.

Fitting to data

Residuals


Size effects: Yes!

-0.6 -0.4 -0.2 0.0 0.2 0.4

Cu

rren

t D

ensi

ty /

mc

m-2

E / V vs. MSE

0.4mAcm-2

4 nm

2.7 nm

2 nm

CO stripping voltammogram in 0.5 M H2SO4 (sweep rate = 20 mV/s) at a Pt/Ar-NH2 modified HOPG containing nanoparticles with sizes of 4.0 ± 0.5 (1), 2.7 ± 0.4 (2) and 2.0 ± 0.5 nm (3).

Potential sweep voltammograms at 20 mV/s in 0.5 M H2SO4 + 1 M methanol at a Pt/Ar-NH2 modified HOPG electrode containing nanoparticles with sizes of 4.0 ± 0.5 (1), 2.7 ± 0.4 (2) and 2.0 ± 0.5 nm (3).

CO Methanol

Control of shape, size and composition of

nanoparticles

Nanoparticle Alloys Synthesis

Giersig et al. J. Phys. Chem. B 2003, 107, 7351-7354

“Polyol” method: reduction of a Pt complex by a di-alcohol in the presence of metal carbonyls with oleic acid or oleylamine as stabilisers. T = 200 oC

Pt

Pt-Co

Mainly (111) planes!

Shape control

Ag nanocubes formed by reduction in ethylene glycol used both as a reducing agent and solvent. Stabilising agent = poly(vinylpyrrolidone)

SunY, XiaY, Science 2002, 298, 2176–79

Nanorods -Seeded growth

C J Murphy et al J. Phys. Chem. B,. 2005 109, 13857

Nanorods-Seeded growth

C J Murphy et al J. Phys. Chem. B,. 2005 109, 13857

Nanosphere lithography

P D Van Duyne, J. Phys. Chem. B 2001, 105, 5599-5611

Insulating spheres

Conducting substrate Controlled metal deposition

Template dissolution

Synthesis of nanotriangles

For use in enhanced Raman spectroscopy: high field localisation to produce “hot

spots”, regions of high electromagnetic field

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)

Au : Polymer = 1 : 0.006 Au : Polymer = 1 : 0.03 Au : Polymer = 1 : 0.18 Au : Polymer = 1 : 0.6 Au : Polymer = 1 : 1.8 Au : Polymer = 1 : 3.6

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)


Alkanethiol

Transfer to organic solvents

Langmuir 2007, 23, 885-895

Polymer stabilised Au nanoparticles (2-5 nm)

Polymers used

Langmuir 2007, 23, 885-895

CO2H

S x9

HAuCl4 in Water

PMAA-DDT Co-polymerNaBH4

Gold nanoparticles of different sizes

Size-controlled Synthesis (Irshad Hussain, M Brust, A Cooper, Liverpool, Langmuir 2007, 23, 885-895 )

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)


300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

Wavelength (nm)


Alkanethiol

Transfer to organic solvents

Langmuir 2007, 23, 885-895

Polymer stabilised Au nanoparticles (2-5 nm)

Reduction/fixation of carbon dioxide

Electrochemical reduction of CO2

1.Direct reduction2.Transition metal ions catalysed reduction3.Biologically inspired-Calvin cycle

Direct reduction

Main products (older work): CO, formic acid, oxalic acid

Difficulties: direct electron transfer leads to reactive radicals

CO2 + e- ⇌ CO2-CO2- + H2O ⇌ COOH

COOH is readily reduced:COOH + e- HCOO- (Formate!)

Reduction from the adsorbed state

From Pt, the product is adsorbed CO, which is strongly attached to the Pt surface.

From Cu, a very large number of products are formed in low yields and very irreproducibly. Alcohols, hydrocarbons, acids, etc have been reported. The mechanism is not understood.

Reduction on complexes

Transition metals macrocylic complexes produce a large zoological garden of compounds. The mechanisms are not clear. The yields are in general low.

Calvin Cycle approach

Try to reproduce sections of the cycle by which plants reduce and fix carbon dioxide

Looking at biology-CO2 reduction

The reduction step in the Calvin Cycle

H2C-OPO32-

HC-OH

CO2 --

-

3-Phosphoglyceric acid

ATP ADP

PGK = Phosphoglycerate kinase

H2C-OPO32-

HC-OH

O=C-OPO32-

1,3-Bisphosphoglycerate

NADH NAD++ Pi

GAPDH = glyceraldehyde 3-phosphatedehydrogenase

H2C-OPO32-

HC-OH

HC=O

Glyceraldehyde 3-phosphate

HC-OH

H2C-OPO32-

C=O

HC-OH

H2C-OPO32-

Ribulose-1,5-bisphosphate

CO2

Rubisco = Ribulose-1,5- bisphosphate carboxylase/oxygenase

Rubisco

GAPDH

CarboxylationReduction of Phosphoglycerate

Regenaration of CO2 Acceptor

To glucose phosphate and other products

PGK

The real reduction step is the reduction of a carboxylate group to yield an aldehyde

The real reduction step is the reduction of a carboxylate group to yield an aldehyde

ReductionStep

Accessing the electrocatalytic metal

centre of redox proteins with

nanoparticles

Does the incorporation of a nanoparticle within a molecular wire change the rate

of ET?

Jensen, Chi, F. Grumsen, Abad, Horsewell, Schiffrin, Ulstrup J. Phys. Chem. C, 2007, 111, 6124-6132

e- e-

Compare with:

Au

SH SH

C

S

OS

O

SH SH

C

S

OS

O

SH SH

C

S

OS

O

Electronic coupling of redox proteins by AuNPs

Direct Electron Transfer mediated by gold nanoparticles

Galactose oxidase

Nanoparticle–biomolecule conjugates

J. Abad, M.Gass, A. Bleloch and D. J. Schiffrin, in preparation

Redox protein studied

Firbank, S.J., Rogers, M., Hurtado-Guerrero, R., Dooley, D.M., Halcrow, M.A., Phillips, S.E.V., Knowles, P.F., McPherson, M.J., Biochem. Soc. T 2003, 31, 506–509.

(GalODox) (GalODsemi) (GalODred)

Cu2+- Tyr· Cu2+- Tyr Cu+- Tyr

O2 H2O2

R-CH2-OH R-CHO

+e-

-e-

+e-

-e-

CuII

Tyr-495

His-581

Tyr-272

His-496

H20

Galactose oxidase

Cu(II) centre

Labile ligand. Can be replaced by COO-


High Angle Annular Dark Field (HAADF) STEM Tomography

The strong Coulomb interaction of the electrons with the potential of an atom core, which leads to high angle scattering (designated as Rutherford scattering) and even to back-scattering, is employed in STEM (Z-contrast imaging) and in SEM. By the HAADF-STEM method, small clusters (or even single atoms) of heavy atoms can be imaged in a matrix of light atoms since the contrast is approximately proportional to Z2 (Z: atomic number).

HAADF detectorThe high-angle annular dark field detector is also a disk with a hole, to detect electrons that are scattered to higher angles and almost only incoherent Rutherford scattering contributes to the image. Thereby, Z contrast is achieved

http://www.microscopy.ethz.ch/STEM.htm

http://www.microscopy.ethz.ch/bse.htm

http://www.microscopy.ethz.ch/BFDF-STEM.htm

http://www.microscopy.ethz.ch/elast.htm

HAADF-STEM redox protein-nanoparticle hybrid images

Copper

5 nm

(a) (b)

HAADF-STEM image for galactose oxidase (a) and 3D molecular display of the protein (b) obtained from the Jmol http://www.jmol.org/.

Copper

(a)

5 nm

HAADF-STEM images for TA-AuNPs~Galactose oxidase hybrid systems and 3D molecular display of the protein in different orientations


Wiring Redox Proteins to AuNPs for Direct Electron Transfer

Au

SH SH

SH SH

SH SH

Cu(II) centre

-0.4 -0.2 0.0 0.2 0.4 0.6-2

-1

0

1

Cu

rre

nt

(A

)

Potential / V vs. SCE

No electron transfer!


Electronic coupling of redox proteins to AuNPs

Cyclic voltammogram shows that

electron transfer to the copper centre is

very fast when the molecular wire

connecting it to the electrode surface

contains a nanoparticle as shown

above.


Conclusions

It might be possible to either drive biological processes

electrochemically or at least to use the information form these in the

design of electrocatalytic materials

The inclusion of nanoparticles within molecular wires change the

transfer function, increasing conductance

Modern instrumental techniques and computational methods are having

a profound effect on our understanding of fundamental aspects of

electrocatalytic reactions.

Some Fundamental Challenges in Electrocatalysis

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