Final Report Summary - POLYMAP (Mapping and Manipulating Interfacial Charge Transfer in Polymer Nanostructures for Photovoltaic Applications)
The realization of efficient and practical photoelectrochemical systems for energy conversion requires the continued development and understanding of new materials that can be used on a large scale, while remaining economically viable. In addition to the use of photoelectrochemical devices for the direct generation of electricity from sunlight, there is an increasing interest in their use to drive the conversion of abundant chemical reactants (H2O, CO2, etc) to generate solar fuels. The main difference between these two technologies is that one converts sunlight directly into useable electricity, whereas the latter drives an electrocatalytic chemical reaction which stores the energy in the chemical bonds of solar fuels to be consumed at a later time for electricity generation. To improve overall device efficiency, high surface area nanostructured electrode materials are being investigated intensely for potential low cost, high throughput photoelectrochemical energy generation for large scale exploitation. The interfacial activity of nanomaterials, central to their efficiency, is dependent upon several factors including crystal phases/orientations, composition, morphology, dopant distribution, and defects. However, our understanding of these systems typically relies on the interpretation of measurements using macroscopic electrodes containing a large distribution of shapes and sizes, which can be arranged in a complex hierarchal fashion, making it challenging to decouple the individual contributions to the overall electrochemical response. Recently, a new high-resolution electrochemical scanning probe technique, scanning electrochemical cell microscopy (SECCM), was developed at the Host Institution, the University of Warwick by Professor Pat Unwin in the Warwick Electrochemistry and Interfaces Group. This technique has the capability to access electrode materials at the nanoscale with unprecedented spatial resolution (~ 100 nm) and high-sensitivity (a few fA), overcoming the challenges associated with studying nanostructured electrodes.
The goal of this project was to unravel the relationship between the nanoscale morphology and electrochemical reactivity of electroactive materials for electrocatalytic and photovoltaic devices, by transferring knowledge and expertise of the Fellow in these materials (from Canada) to the Electrochemistry and Interfaces Group at the University of Warwick (UK) using the globally unique and innovative high resolution electrochemical imaging techniques and expertise for structure-dynamics investigations. Specifically, this project focused on using nanoscale electrochemical scanning probe techniques to: (a) expand upon the high resolution electrochemical imaging techniques at the Host Institution, to include high resolution photoelectrochemical imaging, where we focused on dye-sensitized electrode materials as an exemplar system, and (b) create and study new nanostructured architectures consisting of conjugated polymers, carbon nanotubes, and metal oxide nanoparticles for electrocatalytic and photoelectrochemical applications.
One of the most promising next generation photovoltaic technologies are dye sensitized solar cells (DSSCs), which typically consists of a nanostructured titanium dioxide (TiO2) electrode coated with a monolayer of a light absorbing dye; solvent; electrolyte; redox mediator (eg. I-/I3-); and counter electrode. The TiO2 scaffold is made up of tiny nanoparticles that are only tens of nanometers in size that form a mesostructured thin film that is several micrometres thick. To date, the simultaneous mapping of electrode topography and photoelectrochemical activity has not been achieved. Furthermore, as the spatial resolution increases upon moving to the nanoscale, the electrochemical current generated becomes increasingly minute, because the conversion of these devices is quite low, reaching down to tens of femtoamperes. By coupling scanning electrochemical cell microscopy with photoillumination we were able to map, with submicrometer spatial resolution, variations in the photoelectrochemical activity of a mesostructured TiO2 aggregate coated with a light absorbing dye, for the first time. The tiny electrochemical currents measured at such high spatial resolution are a salient feature of SECCM due to the small contact areas formed with the electrode, which dramatically minimizes the background noise. Furthermore, we were able to implement modulation of the light intensity, opening up opportunities for the use of non-steady state techniques such as intensity modulated photocurrent spectroscopy, which can be used to study loss processes in these devices that limit their conversion efficiencies.
There is tremendous interest in the preparation of nanostructured conjugated organic and polymer materials for electrocatalytic and, increasingly, for photoelectrochemical applications. Using SECCM we were able to create microscale arrays of these materials in a controlled and predetermined manner through the electropolymerization of conjugated organic monomers and subsequently characterizing their photoelectrochemical activity in-situ immediately following their growth. By subsequently using even smaller probes we could access variations in activity at higher resolution due to the heterogeneous structure of the electrodeposited films. A significant advantage of the SECCM approach was that we were able to make a large number of measurements rapidly on a single substrate, while changing the deposition parameters as the nanopipette was moved to a fresh position on a surface. Additionally, the films were further investigated with complementary techniques including electron microscopy, atomic force microscopy (AFM) and Raman micro-spectroscopy to provide multi-functional information. Using this multi-microscopy approach we were able to directly correlate the structure-activity relationships of electrodeposited organic films that ranged in thickness from as small as 5 nm up to 500 nm. This part of the project involved a collaboration with the University of Burgos (Spain).
There is a widely held belief that carbon nanotubes (CNTs) are highly inert towards (electro)catalytic processes unless (un)intentionally modified or doped. Using SECCM we were able to visualize the (electro)chemical reactivity along the length of an individual CNT. Focusing on the oxygen reduction reaction (ORR) (hydrogen peroxide generation) at single walled carbon nanotubes (SWNTs) as a key example, we showed that pristine defect-free SWNTs electrocatalyze the ORR just as effectively as gold (a standard catalyst for hydrogen peroxide electrogeneration). We further showed that the architecture of a SWNT had a significant impact on activity with kinked regions enhancing the activity several-fold, and the activity of straight, pristine regions being enhanced by an order of magnitude by pre-electrooxidizing the SWNT. This clearly demonstrated the sidewalls of SWNTs are far from being inert and are actually good electrocatalysts themselves for hydrogen peroxide generation. Significantly, this provides the basis of new structurally-engineered metal-free electrocatalysts. On the other hand, the use of SWNTs in fuel cell technologies, and for some healthcare applications, where hydrogen peroxide (or peroxy species) has to be avoided, is potentially a major issue. This work is a major scientific advance. This, again, is a major advance in functional imaging.
A key reaction in the generation of solar fuels is the oxidation of water to oxygen. Metal oxide nanoparticles (NPs) are being extensively explored in electrocatalysis for this application but key knowledge that could improve their utilization is missing, as measurements have tended to be made at bulk electrodes involving millions of NPs, which have subtly different structures that can significantly impact reactivity. One advantage of bulk measurements is the ability to create voltammograms by measuring the electrochemical current as a function of the electrode potential over a range of values. Voltammograms provide insight into reaction kinetics (including reaction intermediates) as well as electrode material properties. However, one limitation of high-resolution imaging is that images are recorded at only a few electrode potentials and miss some of the information contained within a voltammogram. By introducing a new scanning algorithm it was possible to dramatically increase the scanning speed achieved in SECCM and acquire many electrochemical substrate images at small potential steps to build up a visual representation of a voltammogram that could be played back frame-by-frame as a function of electrode potential to visualize electrode reactivity. Using iridium oxide nanoparticles that were electrodeposited onto a supporting substrate it was possible to visualize the local electrochemical activity over a range of substrate potentials. It was found that ensembles of nanoparticles displayed notable variations in their activity as a function of electrode potential, where for example some regions exhibited consistently high activity across a range of potentials, while others only began to show activity at higher overpotentials.
In summary, photoelectrochemical measurements have been introduced into the SECCM platform at the Host Institution. Photoelectrochemical and electrocatalytic systems were studied at high spatial resolution with high-sensitivity providing key insight into the influence of local structure (conjugated polymers), morphology (DSSCs), distributed activity (metal oxide NPs), and defects (single walled carbon nanotubes) that significantly impact electrochemical activity. The new capability developed during the course of this project offer significant opportunity for future nanomaterials characterization. The success of this project is evident in the Fellow (Josh Byers) obtaining a faculty position at a Canadian university immediately at the end of the Marie Curie International Incoming (IIF) Fellowship. The Natural Sciences and Engineering Research Council of Canada (NSERC) spends approximately US $ 1 billion dollars a year on scientific research, with funding opportunities available for international collaboration. This commitment to the continued development of collaborative research combined with the relationships created during the IIF ensure continued growth that will develop further in the future creating long-term mutually beneficial cooperation between the European Union and Canada.
The goal of this project was to unravel the relationship between the nanoscale morphology and electrochemical reactivity of electroactive materials for electrocatalytic and photovoltaic devices, by transferring knowledge and expertise of the Fellow in these materials (from Canada) to the Electrochemistry and Interfaces Group at the University of Warwick (UK) using the globally unique and innovative high resolution electrochemical imaging techniques and expertise for structure-dynamics investigations. Specifically, this project focused on using nanoscale electrochemical scanning probe techniques to: (a) expand upon the high resolution electrochemical imaging techniques at the Host Institution, to include high resolution photoelectrochemical imaging, where we focused on dye-sensitized electrode materials as an exemplar system, and (b) create and study new nanostructured architectures consisting of conjugated polymers, carbon nanotubes, and metal oxide nanoparticles for electrocatalytic and photoelectrochemical applications.
One of the most promising next generation photovoltaic technologies are dye sensitized solar cells (DSSCs), which typically consists of a nanostructured titanium dioxide (TiO2) electrode coated with a monolayer of a light absorbing dye; solvent; electrolyte; redox mediator (eg. I-/I3-); and counter electrode. The TiO2 scaffold is made up of tiny nanoparticles that are only tens of nanometers in size that form a mesostructured thin film that is several micrometres thick. To date, the simultaneous mapping of electrode topography and photoelectrochemical activity has not been achieved. Furthermore, as the spatial resolution increases upon moving to the nanoscale, the electrochemical current generated becomes increasingly minute, because the conversion of these devices is quite low, reaching down to tens of femtoamperes. By coupling scanning electrochemical cell microscopy with photoillumination we were able to map, with submicrometer spatial resolution, variations in the photoelectrochemical activity of a mesostructured TiO2 aggregate coated with a light absorbing dye, for the first time. The tiny electrochemical currents measured at such high spatial resolution are a salient feature of SECCM due to the small contact areas formed with the electrode, which dramatically minimizes the background noise. Furthermore, we were able to implement modulation of the light intensity, opening up opportunities for the use of non-steady state techniques such as intensity modulated photocurrent spectroscopy, which can be used to study loss processes in these devices that limit their conversion efficiencies.
There is tremendous interest in the preparation of nanostructured conjugated organic and polymer materials for electrocatalytic and, increasingly, for photoelectrochemical applications. Using SECCM we were able to create microscale arrays of these materials in a controlled and predetermined manner through the electropolymerization of conjugated organic monomers and subsequently characterizing their photoelectrochemical activity in-situ immediately following their growth. By subsequently using even smaller probes we could access variations in activity at higher resolution due to the heterogeneous structure of the electrodeposited films. A significant advantage of the SECCM approach was that we were able to make a large number of measurements rapidly on a single substrate, while changing the deposition parameters as the nanopipette was moved to a fresh position on a surface. Additionally, the films were further investigated with complementary techniques including electron microscopy, atomic force microscopy (AFM) and Raman micro-spectroscopy to provide multi-functional information. Using this multi-microscopy approach we were able to directly correlate the structure-activity relationships of electrodeposited organic films that ranged in thickness from as small as 5 nm up to 500 nm. This part of the project involved a collaboration with the University of Burgos (Spain).
There is a widely held belief that carbon nanotubes (CNTs) are highly inert towards (electro)catalytic processes unless (un)intentionally modified or doped. Using SECCM we were able to visualize the (electro)chemical reactivity along the length of an individual CNT. Focusing on the oxygen reduction reaction (ORR) (hydrogen peroxide generation) at single walled carbon nanotubes (SWNTs) as a key example, we showed that pristine defect-free SWNTs electrocatalyze the ORR just as effectively as gold (a standard catalyst for hydrogen peroxide electrogeneration). We further showed that the architecture of a SWNT had a significant impact on activity with kinked regions enhancing the activity several-fold, and the activity of straight, pristine regions being enhanced by an order of magnitude by pre-electrooxidizing the SWNT. This clearly demonstrated the sidewalls of SWNTs are far from being inert and are actually good electrocatalysts themselves for hydrogen peroxide generation. Significantly, this provides the basis of new structurally-engineered metal-free electrocatalysts. On the other hand, the use of SWNTs in fuel cell technologies, and for some healthcare applications, where hydrogen peroxide (or peroxy species) has to be avoided, is potentially a major issue. This work is a major scientific advance. This, again, is a major advance in functional imaging.
A key reaction in the generation of solar fuels is the oxidation of water to oxygen. Metal oxide nanoparticles (NPs) are being extensively explored in electrocatalysis for this application but key knowledge that could improve their utilization is missing, as measurements have tended to be made at bulk electrodes involving millions of NPs, which have subtly different structures that can significantly impact reactivity. One advantage of bulk measurements is the ability to create voltammograms by measuring the electrochemical current as a function of the electrode potential over a range of values. Voltammograms provide insight into reaction kinetics (including reaction intermediates) as well as electrode material properties. However, one limitation of high-resolution imaging is that images are recorded at only a few electrode potentials and miss some of the information contained within a voltammogram. By introducing a new scanning algorithm it was possible to dramatically increase the scanning speed achieved in SECCM and acquire many electrochemical substrate images at small potential steps to build up a visual representation of a voltammogram that could be played back frame-by-frame as a function of electrode potential to visualize electrode reactivity. Using iridium oxide nanoparticles that were electrodeposited onto a supporting substrate it was possible to visualize the local electrochemical activity over a range of substrate potentials. It was found that ensembles of nanoparticles displayed notable variations in their activity as a function of electrode potential, where for example some regions exhibited consistently high activity across a range of potentials, while others only began to show activity at higher overpotentials.
In summary, photoelectrochemical measurements have been introduced into the SECCM platform at the Host Institution. Photoelectrochemical and electrocatalytic systems were studied at high spatial resolution with high-sensitivity providing key insight into the influence of local structure (conjugated polymers), morphology (DSSCs), distributed activity (metal oxide NPs), and defects (single walled carbon nanotubes) that significantly impact electrochemical activity. The new capability developed during the course of this project offer significant opportunity for future nanomaterials characterization. The success of this project is evident in the Fellow (Josh Byers) obtaining a faculty position at a Canadian university immediately at the end of the Marie Curie International Incoming (IIF) Fellowship. The Natural Sciences and Engineering Research Council of Canada (NSERC) spends approximately US $ 1 billion dollars a year on scientific research, with funding opportunities available for international collaboration. This commitment to the continued development of collaborative research combined with the relationships created during the IIF ensure continued growth that will develop further in the future creating long-term mutually beneficial cooperation between the European Union and Canada.