Diamond is one of the world's foremost research facilities, enabling world-changing science every day. Diamond's 9000th publication clearly demonstrates the role science can play in tackling some of the world's most pressing problems. A team of researchers from Aalto University in Finland have developed a scalable and straightforward technique for turning lignin from renewable biomass into hybrid nanocapsules that show great promise as phase change materials (PCMs). Their results, recently published in Chemical Engineering Journal, are a step towards the sustainable expansion of PCMs in thermal energy storage applications, an important technology to reduce our greenhouse gas emissions and mitigate climate change.
According to The World Meteorological Organization (WMO), 2019 was the warmest year on record in Europe, with Europe heating faster than the global average. Eleven of the 12 warmest years in Europe have occurred since 2000. We know that failure to tackle climate change will threaten human well-being, ecosystems and economies for centuries. However, on an individual level, the response to increased temperatures is often to buy energy-intensive air conditioning units that increase carbon emissions.
With 27% of global energy consumption occurring in the residential sector, and demand growing (particularly in developing countries), it is becoming increasingly important to develop methods for harvesting and storing thermal energy. One of the most promising options is based on phase-change materials (PCMs) that absorb or release large amounts of heat when they change state, e.g. from solid to liquid and vice versa. PCMs act, in effect, like rechargeable batteries, but storing thermal energy rather than electrical energy.
One of the most significant potential applications for PCMs is building heating and cooling. PCMs incorporated into building materials could remove excess heat during the day and release it at night, with minimum carbon emissions. Thermal energy storage (TES) can also help bridge the gap between peak energy demand and the fluctuating supply from renewable sources. However, the development of affordable TES is hampered by a reliance on fossil hydrocarbons and synthetic polymers and a lack of environmentally benign and scalable PCMs.
Dynamic systems require stable PCM suspension, and one approach to maintain PCM stability is nanoconfinement in core-shell structures. A team of researchers from Aalto University in Finland has been exploring simple, low-cost, and scalable colloidal synthesis methods to produce hybrid nanomaterials from renewable resources.
Inspired by recent work into the encapsulation of organic molecules in lignin microparticles and nanoparticles, the team decided to investigate the use of lignin as a component of nanoconfined PCMs. Lignin is a class of complex organic polymers that occur as structural materials (particularly in wood and bark) in many plants. Lignin is widely available as it is a byproduct of processing wood into paper, but is not currently used as PCM for thermal energy storage applications.
The team developed a scalable and straightforward way to produce hybrid nanocapsules using kraft lignin and fatty acids as renewable building blocks. They characterised the nanoscale morphology of the capsules using small angle X-ray scattering (SAXS) on Diamond's B21 beamline, in combination with thermoporometry-differential scanning calorimetry, transmission electron microscopy, atomic force microscopy and dynamic light scattering. Insights gained from the SAXS analysis and porosity measurements improves our understanding of the formation mechanism and packing patterns of various lignin morphologies.
Profs Monika Österberg and Mika Sipponen said that they were “very satisfied with the levels of cooperation and communication with the staff at Diamond in all stages of the process from the sample send-in service to the delivery of data for their detailed analyses”.
Dr Paavo Penttilä explained:
The SAXS experiments allowed us to see the structure of the particles in their native state, that is, in the aqueous solution and without drying. In this way, we could ensure that what we saw in the electron microscopy images was indeed the real structure of the samples.
The researchers investigated whether the nanocapsules had potential for use in PCMs for thermal energy storage. Their results showed that their water-accessible shells allowed the nanocapsules to overcome the poor thermal conductivity of lignin. They were also able to extract the hybrid capsules to obtain highly porous capsules containing only lignin.
The 'one-pot fabrication' the team has developed allows for simple preparation of hybrid capsules with a predictable concentration of tall oil fatty acid, oleic acid, or lauric acid in core-shell particles stabilised by softwood kraft lignin. The resulting hybrid nanocapsules show outstanding performance as phase change materials, paving the way for sustainable expansion of PCMs in thermal energy storage applications. Porous lignin nanocapsules could also be explored as catalyst supports and gas separation materials.
To find out more about the B21 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Nathan Cowieson: firstname.lastname@example.org.
Sipponen MH et al. Lignin-fatty acid hybrid nanocapsules for scalable thermal energy storage in phase-change materials. Chemical Engineering Journal 124711 (2020). DOI:10.1016/j.cej.2020.124711.
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