A heated exchange
The birthplace of graphene has sought to utilise the material in increasing fuel efficiency in cars. But not everyone is convinced by the University of Manchester’s latest efforts. Khai Trung Le reports.
Noting that the average car loses around 70% of energy generated through fuel consumption to heat, a University of Manchester team led by Professors Ian Kinloch and Robert Freer and research associate Yue Lin added a small amount of graphene to a lanthanum strontium titanium oxide composite, looking to improve its thermoelectric properties to convert heat into energy.
The addition of graphene – which comprises less than one weight percent of the composite – reduced the thermal operating window far below the standard operating temperatures of current oxide thermoelectric materials, around 700˚C, to room temperature, although it currently only converts 3-5% of the heat into energy.
Freer is optimistic about the potential of graphene as an additive, ‘At this first stage, it is all very promising. We have demonstrated that the composite is good to the low temperature range, and shows an encouraging window in the high temperature range. As yet, we don’t have the peak performance to compete with the best of existing traditional materials, but also doesn’t have the disadvantages of being toxic and reliance on a very rare supply of materials.’
But given the current results, Freer was cautious about speculating on its immediate impact on industry. ‘I would like to see better performance out of these materials first. The next step is to work with other materials. We started with strontium titanium as a very basic beginning. We have other work ongoing at the moment, and determine increased performance that would really offer an alternative to some of the traditional materials.’
From Thermoelectric power generation from lanthanum strontium titanium oxide at room temperature through the addition of graphene
The LSTO composites incorporated one percent or less of graphene and were sintered under an argon/hydrogen atmosphere. The resultant materials were reduced and possessed a multiphase structure with nanosized grains. The thermal conductivity of the nanocomposites decreased upon the addition of graphene, whereas the electrical conductivity and power factor both increased significantly. These factors, together with a moderate Seebeck coefficient, meant that a high power factor of -2,500 μWm–1 K–2 was reached at room temperature at a loading of 0.6 wt % graphene. The highest thermoelectric figure of merit (ZT) was achieved when 0.6 wt % graphene was added (ZT = 0.42 at room temperature and 0.36 at 750 °C), with >280% enhancement compared to that of pure LSTO. A preliminary 7-couple device was produced using bismuth strontium cobalt oxide/graphene-LSTO pucks. This device had a Seebeck coefficient of -1,500 μV/K and an open voltage of 600 mV at a mean temperature of 219°C.
Professor Andrea Ferrari, Cambridge Graphene Centre, was keen to downplay the significance of Manchester’s findings to industry.
‘I’m not sure it is a discovery. It’s a paper reporting slight improvement in the thermolytic properties of a certain material when you add graphene to it. I think the paper is interesting, but there are so many thermolytic materials available, and with far greater performance than this one, such as bismuth telluride or bismuth selenide. There are better choices even among other two-dimensional, layered materials.’
Nonplussed by the 3–5% heat into energy transfer rate, Ferrari was unconvinced of the choice of using graphene in Manchester’s efforts. ‘What makes graphene special – its high thermal conductivity – is also what makes it very bad as a thermolytic material. You would have to do something special to reduce the thermal conductivity while somehow retaining the high electrical conductivity, possibly needing to nanostructure your material.’
Despite this, Ferrari believes that graphene will be significant in the future of the automotive industry.
‘Graphene is not a material that is applied in the automotive industry, but it certainly has potential as an anticorrosion material, to improve the electrical or thermal properties of steel, or in combination to create batteries. When it comes to this paper, however, interesting as it is, it is not a game changer.’