Could carbon nanomaterials replace scarce metals?

Materials World magazine
,
1 Nov 2017

Khai Trung Le talks to Rickard Arvidsson about his study of how adopting carbon nanomaterials could help reduce the use of scarce metals and risk of depletion.

The discussion around scarce metals remains divisive (see Materials World, June 2017). Some claim the Earth will reach the point of exhaustion for numerous essential minerals within decades, while others place faith in self-governance to prevent disaster. But what if the problem could be avoided altogether? A recent study argues that carbon nanomaterials (CNM) can be a potential substitute, after collecting existing examples for 14 metals found on the EU Critical Minerals list or considered a conflict mineral by the US Securities and Exchange Commission.

Rickard Arvidsson and Björn Sandén, environmental systems analysis researchers at Chalmers University of Technology, Sweden, and co-authors of the paper, Carbon nanomaterials as potential substitutes for scarce metals, published in the Journal of Cleaner Production, see CNM replacement of scarce metals as a solution to mineral exhaustion. Arvidsson told Materials World, ‘The main strategy to tackling this problem is by recycling and reusing, summarised under dematerialisation. But in this paper, we discuss another strategy – transmaterialisation, to go from scarce metals to CNMs such as graphene, carbon nanotubes and fullerenes.’

Arvidsson and Sandén reviewed published scientific papers and patent databases to collect examples of successful uses of CNMs for 13 of the 14 rare metals and their principal usage. For seven scarce metals, replacement CNM technologies were found to be described in so many papers that the researchers could not read all in detail. The outlier, gold, is primarily used as jewellery, accounting for 50% of global use. While CNMs may fulfil some of the desirable properties of gold, including resistance to corrosion and chemical reactions, the researchers found no examples for the use of CNMs for jewellery in scientific papers. 

Although many within the materials community do not endorse theories of mineral exhaustion, Arvidsson and Sandén argue that 'from a geochemical standpoint, all metals except for silicon, aluminium, iron, calicum magnerisum, sodium, potassium and titanium are scarce in the sense that they are mined from high-grade ores in risk of depletion'. Arvidsson said, ‘It seems to me that there are a number of sustainability challenges with scarce metals – there is not only the depletion, but a risk of short supply, as well as political concerns.’

The paper is sparse on details of the specifics of CNMs used as replacement materials. Arvidsson said, ‘In most cases, there were several different designs of CNM proposed. Often together with other materials and sometimes functionalised. For strong materials, potentially replacing cobalt, nickel and tungsten, bulk graphene could be appropriate. In flame retardants such as antimony, mixing graphene and plastics has been investigated. For transparent electrodes such as indium, chemical vapour deposition is the most promising production technology. It varies between the metals.’ 

The lack of detailed information persists throughout the paper. Arvidsson and Sandén specify that ‘this is not a predictive study'. Several obstacles to establishing CNMs as replacements for scarce metals are acknowledged, including the complexities of substitution options and ensuring substitution doesn’t impact technological progression, pointing to the transition from photographic film, previously the largest user of silver in the USA, to digital cameras.

But ultimately, Arvidsson’s goal for the study was resoundingly straightforward. ‘Our hope was to find promising nanomaterial technologies that give some hope regarding transmaterialisation as a complementary strategy to dematerialisation. Which, in most cases, we did.’

To view the paper, Carbon nanomaterials as potential substitutes for scarce metals, visit bit.ly/2feJ0Vl