[Turning food and plastic waste into valuable nanomaterials for energy applications ]

Schematic illustration of the formation of 2D Mo2C layers from the recycling of coconut husk (CH) fruit waste. Stage 1: synthesis of carbonaceous materials derived from CH, denoted as CCH. Stage II: synthesis of 2D Mo2CCH layer by carbonization of CCH with Mo

(Nanowerk Spotlight) Our society generates staggering amounts 
of waste in all areas of economic activities. Foremost among them, 
  apart from energy waste, are the food and plastic sectors.
Data gathered by the FAO (pdf), the Food and Agriculture Organization 
  of the United Nations, estimates that around 931 million tonnes of 
  food waste was generated in 2019, 61% of which came from households, 
  26% from food service and 13% from retail. This suggests that 17% of 
  total global food production may be wasted. More than half of that is 
  made up of fruit waste.
According to a 2022 report by the World Economic Forum, the world 
  produces about 400 million tons of plastic waste each year, but 
  only 9% of that plastic is being recycled; 12% is incinerated 
  and a whopping 79% is dumped in landfills or the environment.

However, both food and plastic wastes are potentially valuable 
sources of carbon. In previous reporting we have covered 
various approaches by research teams around the world to 
turn food and plastic waste into feedstock for making 
nanomaterials or even make nanomaterials like graphene 
directly via flash synthesis.

Edison H. Ang, an Assistant Professor at Nanyang 
Technological University Singapore, and his group are 
working on upcycling of waste materials to high-value 
carbon by combining materials science and nanotechnology 
approaches to develop functional nanostructures for 
advanced energy storage, catalysis, water purification, 
and biosensor applications.

The group recently published two papers where they 
describe routes for the sustainable production of MXene 
from fruit waste (Chemistry - A European Journal, 
"Sustainable Production of Molybdenum Carbide (MXene) 
from Fruit Wastes for Improved Solar Evaporation") and 
the sustainable development of graphitic carbon 
nanosheets from plastic wastes (Journal of Materials 
Chemistry A, "Sustainable development of graphitic 
carbon nanosheets from plastic wastes with efficient 
photothermal energy conversion for enhanced solar 
evaporation").
"Both MXene and graphite are conductive in nature and 
their 2D structure makes them attractive to be used in 
energy storage applications," Ang tells Nanowerk. 
"Our primary goal with this research has been to 
create innovative and sustainable materials for 
constructing solar evaporators. Our aim here has 
been to use environmentally friendly methods to 
produce freshwater using solar energy. However, 
the challenge lies in identifying suitable renewable 

aterials for this purpose. Hence, our focus has 
shifted to waste materials that can undergo 
carbonization and upcycling to create solar 
evaporators that are both environmentally 
friendly and more efficient."
In their report in Chemistry - A European 
Journal, the team presents a straightforward, 
two-stage calcination process that enables the creation 
of two-dimensional (2D) layered molybdenum carbide (Mo2C) 
materials using fruit waste as the carbon source. 

The chosen fruit waste materials for this study were 
coconut husk, orange peel, and banana peel. These were 
selected because a significant proportion of the fruit 
(50-65% of the total mass) is inedible and is typically 
discarded as waste.Schematic illustration of the formation 
of MXene layers from the recycling of coconut husk waste
Schematic illustration of the formation of 2D Mo2C layers 
from the recycling of coconut husk (CH) fruit waste. 
Stage 1: synthesis of carbonaceous materials derived from CH, 
denoted as CCH. Stage II: synthesis of 2D Mo2CCH layer by 
carbonization of CCH with Mo precursor. A common setup of 
Mo2CCH solar evaporator consists of three components, 
including the simulated seawater, the thermal insulator 
(i.e., polystyrene foam) along with the photothermal layer 
comprises of the 2D Mo2CCH layer deposited on the air-laid 
paper. (Reprinted with permission from Wiley-VCH GmbH)
According to the researchers' preliminary findings, different 
types of fruit wastes have different water evaporation rates 
and photothermal conversion efficiency (PTCE) in solar water 
evaporators. The photothermal layer made from coconut husk 
has the highest PTCE of 94% and the highest evaporation 
rate of 1.52 kg m-2h-1 under one sun illumination 
(i.e., the amount of solar radiation that reaches the 
Earth's surface under normal conditions when the 
sun is directly overhead).

"The large specific surface area of 555.1 m2g-1 and 
wide solar absorption band ranging between 300 to 1600 
nm results in enhanced PTCE, while the better wetting 
ability and presence of a broad group micro- and 
mesopores enable rapid water transportation," Ang 
explains the results. "When compared to prior published 
data, this is the first time that such enhanced PTCE 
and evaporation rates are attained."
In their report in Journal of Materials Chemistry A, 
the team demonstrates a simple two-step method 
involving acid treatment and carbonization to 
synthesize ultrathin (with a thickness of less 
than 1 nm) honeycomb-structured 2D graphitic carbon nanosheets 
(g-CNS) from plastic waste such as 
plastic bags and bottles.

Schematic illustration of the formation of 2D 
graphitic carbon nanosheets from upcycling of 
plastic bag waste
(a) Schematic illustration of the formation of 
2D graphitic carbon nanosheets (g-CNS) from 
upcycling of plastic bag (PB) waste. Stage I: 
growth of sulfonated carbon black derived from 
plastic bag (s-CBPB). Stage II: formation of 
2D g-CNSPB by carbonization of s-CBPB. 
(b) The schematic shows a typical setup of a 
solar evaporator and the unique features of 
the 2D g-CNSPB consisting of: (1) simulated 
seawater, (2) a thermal insulator (i.e., polystyrene, 
PS foam), and (3) a photothermal layer made up of 
2D g-CNS on an air-laid paper support. (Reprinted 
with permission from The Royal Society of Chemistry)
"We believe this is the first time these graphitic 
2D CNS were fabricated from plastic waste," says Ang. 
"The unique graphitic-like and 2D structures do not 
appear in previously reported carbonaceous materials 
originated from plastic waste. Because of the merits 
of the graphitic-like characteristics and the 2D 
interlayer channel architecture this can improve the 
light-to-heat conversion as well as the water transport 
for solar evaporation, respectively."
In the next stage of their investigations, the team will 
work on extending the MXenes and graphite nanosheets 
recycled from organic wastes to other possible 
applications such as electrode materials for energy 
storage devices.
"The challenges we face in extending this work to 
energy storage applications is to remove the impurities 
in the fruit and plastic waste since they may affect 
the performance of battery electrodes," Ang concludes. 
"We therefore need to develop methods to purify the 
feedstock materials in order to produce high-quality 
MXenes and graphite suitable for energy applications."
By Michael Berger – Michael is author of three books 
by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible

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