Two-dimensional materials – turning ideas into reality
There has been much talk of the potential energy applications of graphene and related two-dimensional materials, and these applications are increasingly being realised in academic and industrial R&D laboratories the world over. Graphene manufacturers are coming up with new ways of producing the material in quantity, tailored for specific needs and contexts, and researchers are integrating graphene into novel devices and systems, continually assessing and refining their performance.
Information exchange and the Graphene Flagship
Taking stock of progress in energy applications for graphene and related materials, researchers and industrialists recently came together to listen, learn and exchange ideas. The Graphene Connect – Energy Applications
workshop, which took place in Dresden from 20-21 October 2014, was organised by Europe’s Graphene Flagship
, the world’s premier research and development initiative devoted to graphene and related two-dimensional nanomaterials.
The Graphene Flagship is a science-driven, academic-industrial partnership which addresses grand scientific and technological challenges relating to graphene and related materials. Its vision is long term, bringing together scientists and engineers from across various countries and disciplines, all of whom share a unifying goal, and an ambitious plan on how to achieve it. The roadmap includes regular exchanges of ideas and information, and workshops such as Graphene Connect – Energy Applications
are one of the primary means through which this dialogue takes place.
Graphene in energy – an overview
Following an introduction from Helena Theander, deputy leader of the Graphene Flagship’s innovation work package, the scene was set by Etienne Quesnel
, a senior materials scientist at CEA
in Grenoble, and leader of the flagship’s energy work package.
In his keynote presentation, Dr Quesnel spoke of the key application areas for graphene in the energy sector. When it comes to energy generation, these include photovoltaic cells, thin film photovoltaics, dye-sensitised solar cells and quantum dot solar cells.
Dr Quesnel emphasised the potential of graphene in lowering device production costs, and as a replacement for indium tin oxide in photovoltaics. ITO is a brittle compound, which means that it cannot be used in flexible solar cells. Indium is also relatively scarce.
Graphene is a promising material for solar cells, but it does not yet match the performance of ITO. Nevertheless, graphene can be functionalised and finely tuned for specific applications, and employed in combination with other materials. One may, for example, mix graphene nanoflakes from solution into the photo-anodes of solar cells, and process these electrodes at a temperature of less than 150 degrees, which is much lower than usual. Graphene also has the potential for replacing costly platinum in dye-sensitised solar cells.
Batteries, supercapacitors and fuel cells
Graphene as an additive in battery cathodes could lead to higher capacities, and the material improves electron migration during the charge-discharge process. Graphene anodes will give us higher specific capacities than with graphite, with a maximum measured value to date of 750 mAh per gramme, which is twice that of graphite anodes.
Supercapacitors can also benefit from the use of graphene. High energy and power densities result from electrodes made of materials with high specific surface areas and conductivities, which makes graphene a most suitable material for this purpose. Other carbon nanomaterials such as nanotubes and fullerenes display similar properties, but one particularly interesting type of graphene comes in the form of petals grown perpendicularly to substrates over large surface areas. With graphene petals we can achieve specific capacitances in excess of 1,200 farads per gramme, and current densities of up to 100 amps per gramme.
Cost is key when it comes to commercial exploitation of novel materials for energy storage, just as with other applications. Dr Quesnel outlined a low-cost process for producing graphene-based supercapacitors, which involves coating a DVD disc with a graphene-oxide layer supported on a flexible substrate, and chemically reducing this with the laser in a LightScribe drive. The laser-scribed graphene film is then peeled from the disc and transferred to the device substrate.
Fuel cells can also benefit from graphene. The key here is in reducing if not removing the need for the precious metal platinum, used as a catalyst in the chemical reaction between hydrogen and oxygen which turns hydrogen fuel into electricity, leaving water as a byproduct. Dr Quesnel showed how graphene can be used to build a tuneable catalyst support which promotes better platinum adhesion and dispersion. Another strategy is to use an entirely metal-free cathode, with doped graphene as the catalyst. Doping polarises adjacent carbon atoms, and this facilitates catalytic activity.
From proofs-of-concept to products
In his summary, Dr Quesnel highlighted the collaboration between academic laboratories, applied research labs and materials producers in developing graphene-based proof-of-concept devices for energy applications. Converting these demonstrators into commercial products requires that researchers connect with other industrial players, and workshops such as Graphene Connect – Energy Applications
provide the space for such collaborations.
Overview to scientific detail
Following Etienne Quesnel’s presentation, workshop participants heard from a number of other energy specialists from both academia and industry.Emmanuel Kymakis
, an electrical engineer at the Technological Educational Institute of Crete, spoke of graphene-based organic solar cells, and the development of organic photovoltaics with improved efficiencies and lowered production costs. On the technical side, the exploitation of graphene in solar cells requires the tuning of material work function, for example with chlorine or nitrogen doping. In this way, graphene can lead to enhanced photo-generation and charge carrier transport. Dr Kymakis also discussed production processes, focusing on graphene ink printing and roll-to-roll manufacturing of photovoltaic films.
Chemist Alkan Gürsel
of Sabanci University in Turkey focused on the potential of graphene in fuel cells with a low platinum content. Graphene – in particular as chemically reduced graphene oxide – would provide catalyst support in such devices.
Following Dr Gürsel came Toby Meyer, co-founder of Swiss solar cell manufacturer Solaronix
. Dr Meyer spoke passionately about photovoltaic technology, with an emphasis on dye-sensitised and Perovskite solar cells. Solaronix specialises in dye-sensitised cells based on titanium dioxide and platinum, and in his plenary talk and subsequent discussions, Dr Meyer asked a number of pertinent questions concerning the feasibility of replacing platinum with graphene.
Renault Mosdale of Grenoble-based Paxitech
was another invited speaker from industry. Paxitech, founded in 2003 as a spin-off from the French government-funded research lab CEA, develops fuel cells and related systems and technologies. Fuel cell technology is a challenge to work with, noted Dr Mosdale, but it is developing apace, and Paxitech provides fuel cell manufacturers with components such as membranes and electrodes, with a particular focus on air-breathing fuel cell modules of up to 100 watts capacity for system integrators. Paxitech also produces complete fuel cell systems from five to a few hundred watts.
Dr Mosdale told his audience in Dresden that he was initially sceptical about graphene, but has since been won over by the science and engineering arguments. He now appreciates the scope of graphene R&D, and its potential. Dr Mosdale is now talking detail, and asking about timescales for the exploitation of graphene in the fuel cell sector. On a technical level, he is looking to increase carbon support stability, and reduce voltage drops arising from the electrical resistance of cell components and interconnections.
Following the plenary presentations on the first day, the workshop broke up into smaller groups for networking and detailed discussions. Three groups met concurrently, with each focusing on a specific area such as energy conversion, technology beyond photovoltaics and fuel cells, and solar cells. Similarly, on the second day, workshop participants met to discuss energy distribution, batteries and supercapacitors, and hydrogen storage.
In all of the group discussions there was enthusiastic and constructive dialogue between researchers and industrialists. Some questions were answered, others raised, and proposals for project collaborations put forward. Participants described the business networking opportunities afforded by the workshop as invaluable.
The first day of the workshop ended with the participants continuing informally with discussions inspired by plenary presentations and group discussions earlier in the day. The second day of the workshop began with further presentations from invited speakers, this time with an emphasis on industry.Di Wei
from Nokia Research in Cambridge looked at printed batteries incorporating graphene oxide. This production method is relatively cheap and scalable, but the graphene used has a higher resistivity than that intrinsic to high-quality graphene produced through chemical vapour deposition.
Paolo Bondavalli from Thales
, a French multinational which among other things provides R&D services to the aerospace, defence, transport and security sectors, discussed supercapacitors based on a combination of carbon nanomaterials. Thales works closely with academic researchers in France, and its researchers are building capacitors formed from graphene layers interspersed with carbon nanotube spacers. This arrangement leads to increased power as a result of greater ion penetration, and it also impedes the disintegration and delamination of device components.
Further industrial perspectives were provided by Alberto Blázquez of CIDETEC
, a firm which specialises in research and knowledge transfer in the areas of materials, surfaces and energy, and Amaia Zurutuza of Graphenea
, a graphene-producing SME also based in the Basque Country of Spain.
Dr Blazquez spoke about battery production, and Dr Zurutuza on the bulk production of graphene. Micromechanical exfoliation of graphene flakes produces high quality material, but it is a low yield process, and non-scalable. Liquid-phase exfoliation is higher yield, and is particularly suitable for ink production. Silicon sublimation of graphene is very expensive, and graphene produced by chemical vapour deposition can be difficult to transfer to different substrates. Dr Zurutuza noted that in 2013 the graphene production market was valued at €10m. This will rise in time, with the market responding to demand.
A successful workshop
Reflecting on the Graphene Connect – Energy Applications
, workshop host Etienne Quesnel notes…“Feedback from participants was entirely positive. Surprising to me was the realisation that, despite intense media coverage of graphene these past few years, the industrial players present at the workshop were far from realising the full potential of graphene for energy applications. For example, substituting platinum with doped graphene into dye-sensitised solar cells or fuel cells is now more than a simple working assumption.
“In this regard, the Graphene Connect event fully reached its target, disseminating the knowledge required to drive new industrial developments.”
Upcoming Graphene Connect
events include a workshop devoted to photonics and electronics, to be held in Barcelona from 3-4 February 2015, and one on materials and production, scheduled for 9 March 2015. Check out the Graphene Flagship website
for further details.
For media enquiries relating to the research and development activities of the Graphene Flagship, please contact Dr Francis Sedgemore.