Graphene lifecycle: how sustainable are emerging nanotechnologies?
Graphene@Manchester is taking a serious look at the impact – positive and negative – of graphene technologies on sustainability, writes Liam Britnell.
A team led by Cinzia Casiraghi has recently published a review of sustainable graphene production, which includes a study of lifecycle assessments (LCAs) of graphene based materials by Rosa Cuellar Franca. This builds on the ongoing work at The University of Manchester in this area.
Graphene and sustainability have been in the news a fair amount of late, with much attention focused around reduction in embodied carbon in the built environment. The GEIC has been collaborating with Nationwide Engineering on a new concrete admixture that improves cement and which could lead to significant CO2 savings in the build of new structures.
In November, alongside the Institution of Civil Engineers, the GEIC hosted a seminar focusing on benefits that graphene and 2D materials can bring to building sector as a whole, with advanced construction (eg. reduction in concrete, fire retardancy solutions) allied to the potential for smart-building technologies to reduce operational energy expenditure.
This expands on the variety of projects undertaken at GEIC, where a large proportion are focused on lightweighting and improving durability of products, both of which have been identified as materials-based opportunities for reaching net-zero.
At the end of 2019, the GEIC established a sustainability working group – led by my colleague and Printing Lab applications specialist Jane Harper – to start the process of looking at the benefits and issues arising from graphene and graphene-based technologies on the environment. The context is that, as graphene moves towards higher production and usage volumes, we should begin to consider and collaborate on these issues now, particularly with the trend towards producer responsibility for both production impacts (eg. greenhouse-gas emissions) and end-of-life (recyclability).
At the time we identified a number of unknowns and open questions:
Graphene from bio-based feedstocks
While there may be merit in using feedstocks which are renewable, many of the examples appeared to suffer from at least one of the following issues.
- Limitation of usage to a single application. The method used to convert a carbon material into graphene produces a form that is not generally applicable in such a way as that it could be easily applied to multiple applications (composites, supercapacitors and so on). We have been looking to undertake projects with partners to improve the tuneability of these processes to increase the diversity of the materials produced.
- Stability of feedstock materials. Often these processes rely on a feedstock that is unlikely to have controllable properties to an extent in which graphene properties will be stable or reliable batch to batch. We already know that the properties of graphene are highly sensitive to the graphite feedstock used and this is likely to be more challenging for naturally derived sources or those collected as post-consumer waste. Post-industrial waste may suffer fewer issues in this regard but will still be of concern. We’re keen on looking at other sectors to see how better sorting of the feedstocks might reduce variability.
- (Non-)biodegradability of graphene. A circular economy requires us to create a produce-use-dispose loop that doesn’t lose significant material on each cycle. If graphene is not biodegradable or cannot be recovered (by depolymerisation of a composite for example) then regardless of whether graphene is derived from bio-based feedstock it will only survive one loop before it needs to be incinerated or landfilled. We’ve got an eye on developments in composites recycling and chemical recycling of plastics to see how these might be fed into recovery and reuse of graphene.
- No quantitative comparison to graphite. The majority of studies that make inferences to sustainability do not make an argument against alternative graphene feedstocks with merit. In order to promote these feedstocks, we must have credible data as evidence. We’ve identified a number of specific processes where a lack of LCAs are available and plan to collaborate with our colleagues at the University to fill in the gaps.
Recovery of graphene from composites
Graphene is likely to see high-volume usage in composite materials. In addition to the commonly used definition, recyclability also includes the requirement of being widely collected. The fact that a material that can ‘in principle’ be recycled, does not mean that in reality, it will be. This is potentially a short-sighted statement but one cannot ignore the difficulties in recycling carbon-black-containing plastics. Graphene may suffer the same fate if processes for its recovery from composites are not developed.
Of course, graphene has an intrinsic value in composite applications to enhance the properties in some form. However, if graphene composites enter a recycling loop with other non-graphene containing plastics, they are likely to be treated as contaminants.
Careful process control is needed to ensure the addition of graphene to a plastic yields improvement in mechanical properties. This means unwanted graphene in the recycling chain may not only lose its value but also devalue the whole recycled plastic batch. One way to address this issue is to focus on developing processes to recover graphene from a range of commodity and speciality plastics.
The ‘Black Plastic’ Issue
Graphene composites are universally black or shades of dark grey. The trend is towards reduction of use of black plastics (especially single use) because of issues with automated sorting by plastic type. While improvements in technology (eg. extending IR detector range in viable form) may overcome this issue, a broad-brush regulatory approach may well be applied in any case. Unless graphene composites reach a critical usage before this happens, they will be unlikely to be treated as a special case. With this in mind, a focus on long-life applications in sectors with a perceived higher value, and where waste is already segregated such as automotive composites, is key.
Textiles are not recycled in large numbers today (80% are landfilled or incinerated) due to their high level of integration of multiple materials and low perceived value. We’re unaware of any processes that exist today which can recycle eTextiles (ie. wearable tech with circuitry integrated in the fabric).
This is a known problem and does not have a huge impact while sales volumes remain low. However, there are many ongoing efforts in the graphene field to capitalise on replacement of metal conductors on fabrics with graphene. In a similar argument to that made with composites, graphene-coated fabrics are unlikely to be recycled with other textiles unless suitable recovery processes for graphene can be established and deployed widely. We need to have a holistic approach from the design stage to ensure that these issues are minimised at end of life. We’re promoting a design for reuse and recyclability mindset in the organisation.
Biodegradability and toxicology of graphene
To a great extent, many of these issues would be negated if graphene were proven to be biodegradable in a practical timeframe and under common conditions. Today’s understanding is that under certain conditions graphene may degrade in the environment but unless that is shown to have wider applicability or the ecotoxicological effects of graphene are shown be negligible, this must be a concern.
The question of hazard then becomes one of exposure and severity. As stated, the severity is unknown but the exposure (in terms of ppm) of most applications is likely to be small. This issue would likely rule out graphene as a filler in biodegradable plastics but, with the exception of the production environment, may not have greater implications.
Much work is being done in this field by colleagues developing healthcare applications for graphene and other 2D materials, notably around advanced therapeutics and diagnostics. Find out more at the Nanomedicine Lab website.
One way to simplify recycling is through reduction in the variety and heterogeneity of materials we use. However, in almost all cases, developing advanced materials means creating a greater diversity of materials and ever-more-complex integration of materials. Sustainability is now a priority and we should focus on applications for graphene that already treat the waste separately from other waste streams, applications where we can increase operational lifetime and integration with materials from which we can recover graphene most easily.
We also identified the following as key initiatives that Graphene@Manchester should undertake as part of our action-plan towards greater sustainability:
- As a community, we should seek greater engagement with experts in the fields of sustainability and lifecycle assessment to shine light on the most important issues.
- We should take note of developments in chemical recycling and develop processes to recover graphene effectively from commonly used plastics and substrates to ensure segregation of filler materials.
- We should ensure we are not promoting the use of graphene in fields that are intrinsically unsustainable.
- We should work towards a position where we can meaningfully approximate impact from mined and non-mined sources of graphene.
In the review of the available LCAs for production of graphene, we’ve provided insights that can be used by the research community to improve the production and the use of graphene-based materials by enabling graphene production to be compatible with the circular economy principles.
We’ve been collaborating more broadly in this space, including running a workshop for our academic and industry partners to start disseminating understanding, bringing together the fields of graphene and environmental assessments and meaningful discussions on data transparency.
We’re keen to build on this work and invite open discussions with our network and drive greater engagement with with other University initiatives such as Sustainable Futures and organisations such as the newly launched Sustainable Materials Innovation Hub at our neighbouring facility the Henry Royce Institute.
The future of graphene looks bright, but we must ensure that the world we live in is considered when making decisions on the direction of graphene applications.
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