{"id":9435,"date":"2021-02-16T12:59:24","date_gmt":"2021-02-16T12:59:24","guid":{"rendered":"https:\/\/www.innovationnewsnetwork.com\/?p=9435"},"modified":"2021-02-16T12:59:24","modified_gmt":"2021-02-16T12:59:24","slug":"metal-free-porous-carbons-innovations-for-energy-and-environmental-applications","status":"publish","type":"post","link":"https:\/\/www.innovationnewsnetwork.com\/metal-free-porous-carbons-innovations-for-energy-and-environmental-applications\/9435\/","title":{"rendered":"Metal-free porous carbons: innovations for energy and environmental applications"},"content":{"rendered":"
Metal-based catalysis is ruling the world\u2019s most important industrial processes, including energy generation, conversion and storage. However, metal-based catalysis faces multiple disadvantages related to the high cost of precious metals (very often most active metal-based catalysts), low durability due to poisoning during the reaction operation, limited and uneven geographical distribution of the reserves of precious metals, and low sustainability. Overcoming such drawbacks with metal-free materials based on abundant renewable resources has become a priority of most research programmes, aiming to boost the large-scale commercial applications of sustainable and renewable energy generation\/conversion\/storage technologies (e.g., fuel cells, batteries, supercapacitors, water splitting, carbon dioxide conversion into fuels, nitrogen fixation), as promising solutions towards current energetic and environmental scenarios.<\/p>\n
The proof-of-concept of the potentialities of metal-free carbon-based materials as alternatives to metallic catalysts was first demonstrated in 2009 for the electrochemical reduction of oxygen (Science 2009, 323, 760). Since then, many different forms of carbon (graphene, carbon nanotubes, carbon nitride, porous carbons) or their combinations have demonstrated high catalytic activities (with better performance and durability for certain carbons) than state-of-the-art, non-precious metal catalysts for energy conversion\/storage and environmental protection. These findings have garnered the interest on the use of carbon-based catalysts. Indeed, owing to the versatility and availability of carbons with varied morphology, composition, and pore architecture, as well as the abundance of precursors, it is highly feasible to push the use of carbons as promising sustainable alternatives to metallic catalysts.<\/p>\n
Among the variety of carbon materials, porous carbons \u2013 also known as activated carbons \u2013 have attracted increasing attention in the last decades in the field of catalysis. Porous carbons are some of the oldest materials known to human beings, with a long history of applications going back to BC times (metal age, inks in cave paintings, Chinese ink, water purification, etc.). Their fabrication has ancient origins, and current manufacturing processes have been known for more than a century. Due to their low cost and large-scale production, porous carbons are essential materials in the technological applications of multidisciplinary fields, although they have been traditionally applied as adsorbents in liquid and gas purification processes, where they have almost no competitors.<\/p>\n
The discovery of graphene (and its derivatives) and the poor performance of defect-free graphene sheets in numerous applications \u2013 including catalysis \u2013 motivated the interest for porous carbons from a new perspective in certain catalytic domains. Indeed, the reactivity of the highly defective graphenic layers present in the pores of disordered porous carbons has been revealed to be the key towards novel applications. Other advantages of porous carbons are their porosity, surface chemistry, and a good level of conductivity \u2013 even if lower than in graphite or graphene \u2013 which can be modulated to fulfil the requirements of a specific application.<\/p>\n
The combination of all these features is unique to porous carbons, and a powerful advantage of the capabilities of porous carbons in new fields of application. The challenge lies in rationalising their behaviour in cutting edge applications related to energy storage\/conversion, environmental remediation, catalysis, etc.<\/p>\n
In a world that urgently needs to redress the balance of energy derived from fossil fuels, the efficient conversion of sunlight into chemical energy is the key towards combating climate change, environmental pollution, and natural resource challenges. With solar energy being the primary and practical source of energy in the world, it should take a step forward in the energetic scenario in the near future to provide the clean, affordable, multi-source and multi-purpose fuels needed to achieve the intertwined goals of economic growth, environmental conservation, and energy security (the so-called \u20183 Es\u2019), which are the pillars of green development.<\/p>\n
Contrary to solar electricity, solar fuels have the gigaton-scalability and multi-functionality to substitute coal and oil in major energy sectors (industry, transportation, and petrochemicals), accomplishing the transition to a future powered by sunlight. Among the different options to convert solar energy into easily storable chemicals (solar fuels), artificial photosynthesis \u2013 solar-powered water splitting to produce hydrogen\/oxygen, and the solar-driven photo-reforming of carbon dioxide \u2013 appears to be a key solution. However, this process is technically complex and a number of scientific and technological challenges remain to be solved in order to successfully scale-up existing lab-prototypes towards commercialisation. The ideal concept of a direct conversion of light into chemical energy has several limitations, and practical applications lie at the crossroads of basic physics and the chemistry of materials and energy technology. Materials must efficiently harvest sunlight in natural environments, and the majority of systems for solar fuels production use metal oxides and noble metal catalysts (which are scarce, expensive and unsustainable).<\/p>\n
Recent studies have pointed out the potential of certain carbons to provide an affordable alternative to replace metallic catalysts for the production of solar fuels, overcoming technical challenges, complying with sustainability, and capable of bringing down costs to competitive levels. Carbon catalysts are capable of promoting the in-situ reduction of carbon dioxide and the oxidation of water to produce energy carriers (i.e., from C1 \u2013 methane, formaldehyde, and methanol \u2013 to C2-C4 fuels) through appropriate modifications (hole\/electron donor doping, chromophore sites, porosity) to tailor the catalytic centres. Despite such promising results, the area is still in its infancy.<\/p>\n
Owing to the complexity of carbon catalysts, the exploitation of such abilities for the production of solar fuels is not straightforward and much work needs to be done in the appropriate direction. The optimisation of carbon catalysts and interfaces can be performed by creating separate sites for oxidation and reduction reactions, while the selectivity can be controlled by tailoring the nanopore confinement and hydrophobic nature to commensurate redox potentials and affinity of the target molecules.<\/p>\n
An adequate choice of the carbon characteristics may render materials with a high light harvesting ability that would overcome the low photonic yield of certain semiconductors under sunlight, as well as increasing the overall efficiency. As photocatalysts, porous carbons also offer the advantage of coupling their high adsorption capability to a tuneable photoactivity; the challenge is in further enhancing the photochemical activity by balancing the surface composition, porosity, and change-carrier mobility.<\/p>\n