(447e) Scaling-up, Scaling-out, and on-Line Monitoring of Graphene Production

Future technologies will be enabled by advanced materials that possess outstanding physico-chemical properties. One example of this is graphene, a two-dimensional monolayer material that exhibits high electronic and thermal conductivity, optical transmittance, and mechanical strength. These characteristics have resulted in a variety of applications in opto-electronics, semiconductors, biomedical sensors, tissue engineering, drug delivery, concrete, composites, energy conversion and storage, and many more. For some of these applications, graphene offers superior product qualities at similar cost economics to many traditional chemical additives. While the advantages of graphene are being demonstrated at laboratory and pilot scales, much of the industrial demand for efficient, flexible and sustainable production of pristine graphene products such as Few Layers Graphene (FLG) and Graphene Nano-Platelets (GNP) have not been met at present.

Numerous approaches exist for producing graphene-related materials via graphite exfoliation including mechanical, chemical and electrochemical methods. Each one of these techniques offers technological solutions with its own pros and cons in terms of process efficiency, product quality, production throughput and yield, overall process sustainability, and feasibility of scale-up production. To date, liquid phase exfoliation (LPE) technologies that utilise mechanical exfoliation mechanisms have been proven to have the potential to produce pristine graphene with minimal or no adverse oxidative effects, with global production capacity exceeding 1000's of tonnes per annum. This is due to LPE processes typically relying on shear-assisted exfoliation mechanisms that separate and disperse graphene materials in solution. These approaches provide an opportunity to select and control favourable process parameters which are key in achieving high-quality and large-scale production of graphene. Such parameters include: (i) usage of different types graphite precursors typically characterised by their grade, purity and size, (ii) selection of suitable solvents on the basis of matching Hansen solubility parameters and surface tension to that of graphite, (iii) optimal hydrodynamics governed by solvent viscosity, temperature, fluid dynamics, initial graphite concentration, additives (e.g., surfactants) and exfoliation mechanisms employed. Identifying suitable combinations of these parameters may allow for pristine graphene to be produced sustainably and to the required specifications at mass-scale. This optimisation process requires graphene characterisations using labour, cost and technology-intensive methods that are, in most cases, conducted ex situ e.g., spectroscopy (UV-Vis and Raman), and microscopy (AFM, TEM). This approach is not conducive to large-scale manufacturing. To enable process controllability, adherence to ISO standards and stringent batch-to-batch Quality Assurance tests, it is imperative to implement alternative production monitoring tools that are compatible with Industry 4.0.

In the present work [1] (see Figure 1), we have developed in-house turbulent Taylor-Couette (TC) flow systems that generate the necessary conditions (strain rates) for effective graphite exfoliation. We demonstrate exfoliation of various precursors for a range of different initial concentrations dispersed in sustainable and eco-friendly solvents, namely Cyrene and water/surfactant solution. We have employed two independent TC flow systems to investigate 'scale-up' and 'scale-out' production methods as follows: (1) multiple exfoliation units connected in series demonstrating 'scale-out' production approach; (2) single exfoliation unit being much larger than those used in (1) demonstrating 'scale-up' production approach. In parallel, we present an on-line monitoring system based on optical spectroscopy that measures key information for 2D material production processes in real-time (i.e. concentration and average number of atomic layers). This provides a solution amenable to Industry 4.0, while also enabling control over the whole value chain ensuring traceability, minimal waste, and on-demand materials by design. In addition, quality of FLG is assessed not only through various characterisation techniques, i.e., UV-Vis, AFM, but also through direct application of FLG as an additive to fabricate graphene films, membranes, batteries and reinforced concrete.

The initial results show distinct differences in terms of FLG concentration, thickness and lateral size distributions for various graphite precursors with the most superior materials clearly identified. For instance, same graphite type milled into two sizes differentiated by a factor of ten can result in production of FLG with concentration ten times higher, five times thinner and twice smaller for bigger initial precursor size. Two types of 'green' solvents used herein show a remarkable difference in FLG specification, yet, both are favourable for specific end-use applications. Through examination of 'scale-out' and 'scale-up' production configurations, we identify optimal size and process conditions for each exfoliation unit, and provide performance estimations for our in-house LPE technology to manufacture high-quality graphene at pilot- and large- scales. Lastly, we demonstrate the capability of our low-cost multispectral transmission-reflectance spectroscopy device to perform on-the-fly monitoring of concentration and nanosheet layer number with RMS uncertainty typically being <15-20~%.

Acknowledgements: This research has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No. 707340, and support from the EPSRC CDT in Advanced Characterisation of Materials (2018 NPIF grant EP/S515085/1).

References

[1] J. Stafford, U. Nwachukwu, U. Farooq, W. S. Favero, Silvia AND, H.-H. Chen, A. L’Hermitte, C. Petit, and O. Matar, “Real-time monitoring and hydrodynamic scaling of shear exfoliated graphene,” 2D Materials, vol. 8, no. 2, 2021.