[1] Solar reforming offers an attractive and unifying solution to address the contemporary challenges of climate change and environmental pollution by creating a sustainable circular network of waste upcycling, clean fuel (and chemical) generation and the consequent mitigation of greenhouse emissions (in alignment with the United Nations Sustainable Development Goals).
Kawai and Sakata from the Institute for Molecular Science, Okazaki, Japan in the 1980s reported that the organics derived from different solid waste matter could be used as electron donors to drive the generation of hydrogen gas over TiO2 photocatalyst composites.
[2][3] In 2017, Wakerley, Kuehnel and Reisner at the University of Cambridge, UK demonstrated the photocatalytic production of hydrogen using raw lignocellulosic biomass substrates in the presence of visible-light responsive CdS|CdOx quantum dots under alkaline conditions.
[4] This was followed by the utilization of less-toxic, carbon-based, visible-light absorbing photocatalyst composites (for example carbon-nitride based systems) for biomass and plastics photoreforming to hydrogen and organics by Kasap, Uekert and Reisner.
[5][6] In addition to variations of carbon nitride, other photocatalyst composite systems based on graphene oxides, MXenes, co-ordination polymers and metal chalcogenides were reported during this period.
[7][8][9][10][11][12][13][14] A major limitation of PC reforming is the use of conventional harsh alkaline pre-treatment conditions (pH >13 and high temperatures) for polymeric substrates such as condensation plastics, accounting for more than 80% of the operation costs.
[15] This was circumvented with the introduction of a new chemoenzymatic reforming pathway in 2023 by Bhattacharjee, Guo, Reisner and Hollfelder, which employed near-neutral pH, moderate temperatures for pre-treating plastics and nanoplastics.
[16] In 2020, Jiao and Xie reported the photocatalytic conversion of addition plastics such as polyethylene and polypropylene to high energy-density to C2 fuels over a Nb2O5 catalyst under natural conditions.
[17] The photocatalytic process (referred to as PC reforming; see Categorization and configurations section below) offers a simple, one-pot and facile deployment scope, but has several major limitations, making it challenging for commercial implementation.
[15] In 2021, sunlight-driven photoelectrochemical (PEC) systems/technologies operating with no external bias or voltage input were introduced by Bhattacharjee and Reisner at the University of Cambridge.
[18] These PEC reforming (see Categorization and configurations section) systems reformed diverse pre-treated waste streams (such as lignocellulose and PET plastics) to selective value-added chemicals with the simultaneous generation of green hydrogen, and achieving areal production rates 100-10000 times higher than conventional photocatalytic processes.
[18] In 2023, Bhattacharjee, Rahaman and Reisner extended the PEC platform to a solar reactor which could reduce greenhouse gas CO2 to different energy vectors (CO, syngas, formate depending on the type of catalyst integrated) and convert waste PET plastics to glycolic acid at the same time.
[22] In 2025, Andrei, Roh and Yang demonstrated solar-driven hydrocarbon synthesis by interfacing copper nanoflower catalysts on perovskite-based artificial leaves at the University of California, Berkeley.
Devices can produce ethane and ethylene at high rates by coupling CO2 reduction with glycerol oxidation into value-added chemicals, which replaces the thermodynamically demanding O2 evolution.
[23][24] These developments has led solar reforming (and electroreforming, where renewable electricity drives redox processes; see Caterogization and configurations section) to gradually emerge as an active area of exploration.
Solar reforming is the sunlight-driven transformation of waste substrates to valuable products (such as sustainable fuels and chemicals) as defined by scientists Subhajit Bhattacharjee, Stuart Linley and Erwin Reisner in their 2024 Nature Reviews Chemistry article where they conceptualized and formalized the field by introducing its concepts, classification, configurations and metrics.
[1] It also includes the subset of 'photoreforming' encompassing utilization of high energy photons in the UV or near-UV region of the solar spectrum (for example, by semiconductor photocatalysts such as TiO2).
Solar thermal reforming, on the other hand, exploits the infrared (IR) region for waste upcycling to generate products of high economic value.
It offers a less energy-intensive and low carbon alterative to methods of waste reforming such as pyrolysis and gasification which require high energy input.
[1] This results in better performance in terms of higher production rates, and also translates to other similar processes which depend on water oxidation as the counter reaction such as CO2 splitting.
[1] The added economic advantage of forming two different valuable products (for example, gaseous reductive fuels and liquid oxidative chemicals) simultaneously makes solar reforming suitable for commercial applications.
[1] Solar reforming encompasses a range of technological processes and configurations and therefore, suitable performance metrics can evaluate the commercial viability.
In artificial photosynthesis, the most common metric is the solar-to-fuel conversion efficiency (ηSTF) as shown below, where 'r' is the product formation rate, 'ΔG' is the Gibbs free energy change during the process, 'A' is the sunlight irradiation area and 'P' is the total light intensity flux.
[1] Therefore, a more adaptable and robust metric is the solar-to-value creation rate (rSTV) which can encompass all these factors and provide a more holistic and practical picture from the economic or commercial point of view.
Solar reforming depends on the properties of the light absorber and the catalysts involved, and their selection, screening and integration to generate maximum value.
On the other hand, the residual, non-absorbed low-energy IR photons may be used for boosting reaction kinetics, waste pre-treatment or other means of value creation (for example, desalination,[36] etc.).
It is also now understood that sustainable fuel/chemical producing technologies of the future will rely on biomass, plastics and CO2 as key carbon feedstocks to replace fossil fuels.
[37] Therefore, with sunlight being abundant and the cheapest source of energy, solar reforming is well-positioned to drive decarbonization and facilitate the transition from a linear to circular economy in the coming decades.