(186aj) Hierarchical Zeolite Templated-Carbon Materials for Clean Energy Applications

A major challenge the world faces today is the ability to provide sustainable energy sources to meet the demands for quality of life and economic growth. Clean energy is essentially important as part of the sustainable development goals and strongly linked to achieve other global goals and resolve environmental issues like global warming and climate change.

As most of the energy today is provided from fossil fuels, huge amounts of CO₂ and other greenhouse gases (GHG) are released into the atmosphere, with negative effects on the environment. Recently, it has been reported CO₂ emissions has increased by about 1.5 ppm (8 billion tons) per year, whereas the average annual increase was nearly 0.45 ppm (2.4 billion tons) in about 200 years since 1800 [1]. This global increase in CO₂ concentrations resulting as by-products of burning fossil fuels can be mitigated by deploying the use of renewable energy sources with more efficient electrical energy storage or by producing clean energy from fossil fuels while capturing the produced CO₂, to avoid its emission into the atmosphere.

Current technologies for CO₂ capture and storage (CCS) are available but still quite expensive. When CCS are installed the electricity production unit cost increases by 50%, and only the capture of CO₂ (including separation and purification, but not compression, transport, and storage) accounts for 75% of the total CCS investment. [2-3] At industrial scale, the main drawbacks rely on the highly energy-intensive regeneration process of the amine aqueous solution at elevated temperatures for CO₂ desorption, and other issues such as O₂-based oxidation, irreversible reaction with SOx and other impurities, and amine volatilization. [4] To overcome these limitations, a wide range of emerging technologies are being investigated, selective adsorption being one of the most attractive alternatives. [5-6] Hence, the development of energy efficient CO₂ capture systems requires low-cost and well-engineered solid adsorbents. Carbon-based materials have earned much attention because they are easy to regenerate, do not require moisture removal (unlike most zeolites and some MOFs), present a high CO₂ capacity at ambient pressures, are mechanically and chemically stable (unlike most MOFs), and are cheaper (compared to zeolites and MOFs) as they can be produced from the carbonization of biomass (e.g. wood, coconut shell granules etc.). [7-8]

An alternative approach to lower CO₂ emissions is to decarbonize the transportation sector by adopting electric and hybrid vehicles. Electric cars can use electricity generated from renewables or clean fossil fuel. The main limitation to the penetration of electric vehicles into the market is the driving distance hindered by the limited specific energy density of electric batteries in comparison to gasoline. Nowadays, lead-acid and lithium-ion batteries (LIBs) are the most prominently batteries used in the automotive and electronics sectors and have reached their theoretical energy limits.[9-10] LIB could deliver a theoretical specific energy density of 387 Wh kg^-1 which is quite low compared to gasoline which is estimated to be around 13000 Wh kg^-1.²⁵ However, the actual accessible energy density is only about 1700 Wh kg^-1 due to the low efficiency of internal combustion engines, in comparison to nearly 90% efficiency for electric motors.²⁴Lithium-Air (Li-A) batteries are electrochemical cells which typically consist of a lithium metal anode and a carbon cathode known as the “air” electrode due to the utilization of oxygen as a reactant. Theoretically, lithium-air batteries have the highest specific energy density among batteries of about 3500 Wh kg^-1(including the mass of the cathodic product) [11]. This allows Li-air batteries to store 5 to 10 times as much energy when compared to current state-of-the-art LIBs. Lithium ions and oxygen react at the cathode during discharge to form lithium peroxide (Li2O2). Porous carbon (PC) cathodes are most widely utilized in Li-A batteries due to their stability under high oxidation potentials, their light weight compared to other materials like nanoporous gold, their high surface area and low cost.[11]

Porous carbon (PC) materials with well-defined ordered structures are attracting attention, given their high surface area and large pore volume that they offer which make them suitable for a wide range of clean energy applications. Zeolite Templated-Carbons (ZTC) are especially attractive since they have been found to achieve the highest gravimetric and volumetric surface area among any carbon materials, as well as superior capacity for energy storage (energy density). [12] The quality of the ZTC depends on the synthesis conditions, which might result in ZTCs consisting of the three-dimensionally ordered framework replicated by the zeolite template, a mixture of the previous one and extra carbonaceous components formed outside the zeolite template, or mainly carbonaceous product without an ordered-structure as of the first one.[13]

This works aims at developing novel hierarchical ZTC with a 3D network of curved and single nanographene from faujasite (FAU) zeolites as hard templates. The synthesis of these ordered carbon structures is achieved by means of liquid phase impregnation in which carbon is introduced into the zeolite nanochannels using a liquid carbon source and polymerized by heat treatment. This step is followed by carbonization and/or chemical vapour deposition at variable high temperatures up to ca. 1073 - 1273 K. Subsequently, the zeolite removal under different etching methods are tried in order to obtain hierarchical ZTC as a negative ordered-structure replica of the parent FAU-zeolite.

We aim to achieve very high gravimetric and volumetric surface area, avoid stacking of graphene layers on the outer 3D structure, and obtain significant mesoporous volume. Also, through modification of the structure, crystallinity and surface chemistry as well as tailor-made properties to enhance their performance in CO₂ capture by adsorption and electrical energy storage in lithium- air batteries.

References

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[10] D. Aurbach, B. D. McCloskey, L. F. Nazar, and P. G. Bruce, “Advances in understanding mechanisms underpinning lithium-air batteries,” Nat. Energy, vol. 1, no. 9, pp. 1–11, 2016.

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