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www.npl.co.uk

Multi-scale Imaging of Electrocatalytic Activity

Andrew J. Wain and Mike A. O’Connell National Physical Laboratory, Teddington, UK, TW11 0LW

andy.wain@npl.co.uk

Acknowledgements This work was financially supported by the UK National Measurement System and the European Metrology Research Programme (EMRP, Ind 15 - SurfChem). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. The authors also wish to thank Christoph Richter at NanoWorld Services GmbH for supplying the cantilever probes.

Introduction The ability to characterize the electrocatalytic behavior of heterogeneous surfaces on a localized scale is critical to our understanding of interfacial processes and impacts a multitude of applications, notably the development of energy conversion technologies such as fuel cells and electrolyzers. Scanning electrochemical microscopy (SECM) is a powerful approach to undertaking localized electrochemical characterization and significant advances have been made recently in both spatial resolution and breadth of application. This poster highlights the latest developments at NPL in the multi-scale electrochemical imaging of surfaces decorated with electrocatalyst nanomaterials.

Scanning Electrochemical Microscopy (SECM) In conventional SECM the sample is immersed into an electrolyte solution and a microelectrode probe is used to monitor processes locally in solution and scanned laterally to map corresponding surface reactivity.

SECM at the Nanoscale Major requirements for high-resolution electrochemical imaging:• Nanoscopic electrode probe (< 100 nm) • Positional feedback to track surface (working distance < 100nm)a Dual Function Probes

Conclusion: Increase in O2 reduction electroactivity with decreasing particle size can be rationalized by the systematic exposure of more active Au(110) facets with decreasing particle size, in accordance with the established view that this is a structure sensitive reaction. [A. J. Wain, Electrochim.Acta, 92 (2013) 383].

Macroscopic Electrocatalyst Screening

Gold Nanoparticle MicroarraysGold nanoparticle (AnNP) ensembles were immobilized in an array pattern on a glassy carbon substrate by dispensing colloidal AuNPs using a piezo controlled inkjet device.

Oxygen Reduction Activity ScreeningAuNP microarray scanned using SECM in redox competition mode:

• Array immersed in 0.1 M H2SO4 solution• Platinum SECM tip positioned ~10 μm from substrate• Diffusion controlled O2 reduction electrochemically driven

at tip • Substrate biased to drive O2 reduction at AuNPs• Tip current monitored as probe is scanned laterally • Depletion of O2 close to AuNPs causes local drop in tip current a Current drop increases with AuNP coverage within array spots a Au(110)/Au(111) ratio systematically decreased

with increasing particle diameter

Depositing electrocatalyst ensembles on a microarray platform enables their relative assessment under identical conditions within a single experiment.

Conclusion: SECM-AFM enables excellent quality Faradaic current mapping alongside high resolution topographical imaging. This has the potential to yield new insights into structure-activity relationships. [A. J. Wain et al., Anal. Chem., 86 (2014) 5143].

Capillary Probes • Combine SECM with Scanning Ion Conductance Microscopy (SECM-SICM)• Quartz θ-capillary pulled using laser puller (aperture ~50 to 500 nm)• One barrel filled with solid carbon by pyrolysis of a hydrocarbon feed• Carbon electrode platinized using electrodeposition

Pt Nanoparticle Imaging • Pt nanoparticles (PtNPs, diameter ~ 200 nm) electrodeposited on glassy carbon • Individual particles topographically resolved• Electrochemical imaging in O2 redox competition mode indicates local O2 consumption in the region of PtNPs • Electrocatalytic activity measured at the single particle level

Gold Disk Imaging• Au disks (150 nm diameter × 50 nm high) on SiO2• SICM current tracks topography faithfully• Positive feedback (FcMeOH) observed over Au disks

Conclusion: SECM-SICM is a powerful approach to high resolution electrochemical imaging in which the probes are relatively easy to fabricate. We have demonstrated imaging of individual PtNPs, taking us a step closer to determining electrokinetics at the single particle level.

Graphene Imaging • Exfoliated graphene on SiO2 substrate• Positive feedback over carbon

(rapid electron transfer)• Graphene/graphite regions show similar

behaviour• Some electrochemical non-uniformities do not

correlate with topography

Gold Band Imaging • Au bands (50 nm high) on SiO2 substrate• Good quality topographical imaging• Faradaic current enhancement over Au

(positive feedback)

Cantilever Probes • Combine SECM with Atomic Force Microscopy

(SECM-AFM)• Batch fabricated probes with conical

Pt nanoelectrode (radius ~ 100 nm)• Imaging undertaken in Lift-Mode (dual pass)• Aqueous ferrocenemethanol (FcMeOH aFcMeOH+) test system

Approach Curves • SICM current decays with proximity

to surface (positional feedback)• SECM current sensitive to the nature

of the surface (i.e. active or inert)

Size EffectsAverage AuNP particle diameter varied within the range 50 – 5 nm (AuNP coverages normalized to give same total Au surface area in each spot):

a O2 reduction activity increases with decreasing particle diameterThe ratio of exposed Au(110)/Au(111) facets was estimated by electrochemical fingerprinting, using lead underpotential deposition voltammetry (1 mM Pb2+ in 0.1 M NaOH):