Solar reforming

Solar reforming is the sunlight-driven conversion of diverse carbon waste resources (including solid, liquid, and gaseous waste streams such as biomass, plastics, industrial by-products, atmospheric carbon dioxide, etc.) into sustainable fuels (or energy vectors) and value-added chemicals. ^[1] It encompasses a set of technologies (and processes) operating under ambient and aqueous conditions, utilizing solar spectrum to generate maximum value. ^[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).

Background

The earliest sunlight-driven reforming (now referred to as photoreforming or PC reforming which forms a small sub-section of solar reforming; see Definition and classifications section) of waste-derived substrates involved the use of TiO₂ semiconductor photocatalyst (generally loaded with a hydrogen evolution co-catalyst such as Pt). 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|CdO_x 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. ^[19] This further inspired the direct capture and conversion of CO₂ to products from flue gas and air (direct air capture) in a PEC reforming process (with simultaneous plastic conversion). ^[20] Choi and Ryu demonstrated a polyoxometallate-medated PEC process to achieve biomass conversion with unassisted hydrogen production in 2022. ^[21] Similarly, Pan and Chu, in 2023 reported a PEC cell for renewable formate production from sunlight, CO₂ and biomass-derived sugars. ^[22] 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.

Concept and considerations

Definition and classifications

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 generally operates without external heating and pressure, and also introduces a thermodynamic advantage over traditional green hydrogen or CO₂ reduction fuel producing methods such as water splitting or CO₂ splitting, respectively. Depending on solar spectrum utilization, solar reforming can be classified into two categories: “solar catalytic reforming” and “solar thermal reforming”. ^[1] Solar catalytic reforming refers to transformation processes primarily driven by ultraviolet (UV) or visible light. ^[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. ^[1] An important aspect of solar reforming is value creation, which means that the overall value creation from product formation must be greater than substrate value destruction. ^[1] In terms of deployment architectures, solar catalytic reforming can be further categorized into: photocatalytic reforming (PC reforming), photoelectrochemical reforming (PEC reforming) and photovoltaic-electrochemical reforming (PV-EC reforming). ^[1]

Advantages over conventional waste recycling and upcycling processes

Solar reforming offers several advantages over conventional methods of waste management or fuel/chemical production. 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] Solar reforming also provides several benefits over traditional green hydrogen production methods such as water splitting (H₂O → H₂ + ½O₂, ΔG° = 237 kJ mol⁻¹). It offers a thermodynamic advantage over water splitting by circumventing the energetically and kinetically demanding water oxidation half reaction (E⁰ = +1.23 V vs. reversible hydrogen electrode (RHE)) by energetically neutral oxidation of waste-derived organics (C_xH_yO_z + (2x−z­)H₂O → (2x−z+y/2)H₂ + xCO₂; ΔG° ~0 kJ mol⁻¹). ^[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. Furthermore, concentrated streams of hydrogen produced from solar reforming is safer than explosive mixtures of oxygen and hydrogen (from traditional water splitting), that otherwise require additional separation costs. ^[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 metrics

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 (η_STF) 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] [23]} The η_STF can be adopted as a metric for solar reforming but with certain considerations. Since the ΔG values for solar reforming processes are very low (ΔG ~0 kJ mol^‒1), this makes the η_STF per definition close to zero, despite the high production rates and quantum yields. However, replacing the ΔG for product formation (during solar reforming) with that of product utilisation (|ΔG_use|; such as combustion of the hydrogen fuel generated) can give a better representation of the process efficiency. ^[1]

${\displaystyle \eta _{\mathrm {STF} }={\frac {\mathrm {r} _{\mathrm {SR} }\left(\mathrm {mol} \cdot \mathrm {s} ^{-1}\right)\times \Delta \mathrm {G} _{\mathrm {SR} }\left(\mathrm {J} \cdot \mathrm {mol} ^{-1}\right)}{\mathrm {P} _{\text{total }}\left(\mathrm {W} \cdot \mathrm {m} ^{-2}\right)\times \mathrm {A} \left(\mathrm {m} ^{2}\right)}}}$

Since solar reforming is highly dependent on the light harvester and its area of photon collection, a more technologically relevant metric is the areal production rate (r_areal) as shown, where ‘n’ is the moles of product formed, ‘A’ is the sunlight irradiation area and ‘t’ is the time. ^[1]

${\displaystyle \mathrm {r} _{\text{areal}}={\frac {\mathrm {n} _{\text{product}}(\mathrm {mol} )}{\mathrm {A} \left(\mathrm {m} ^{2}\right)\times \mathrm {t} (\mathrm {h} )}}}$

Although r_areal is a more consistent metric for solar reforming, it neglects some key parameters such as type of waste utilized, pre-treatment costs, product value, scaling, other process and separation costs, deployment variables, etc. ^[1] Therefore, a more adaptable and robust metric is the solar-to-value creation rate (r_STV) which can encompass all these factors and provide a more holistic and practical picture from the economic or commercial point of view. ^[1] The simplified equation for r_STV is shown below, where C_i and C_k are the costs of the product ‘i’ and substrate ‘k’, respectively. ‘C_p’ is the pre-treatment cost for the waste substrate ‘k’, and ‘n_i’ and ‘n_k’ are amounts (in moles) of the product ‘i’ formed and substrate ‘k’ consumed during solar reforming, respectively. Note that the metric is adaptable and can be expanded to include other relevant parameters as applicable. ^[1]

${\displaystyle r_{\mathrm {STV} }={\frac {{\textstyle \sum _{i=1}^{M}\displaystyle C_{i}(\$mol^{-1})\times n_{i}(mol)}-{\textstyle \sum _{k=1}^{N}\displaystyle {\bigl (}C_{k}+C_{p}{\bigr )}(\$mol^{-1})\times n_{k}(mol)}}{A(m^{2})\times t(h)}}{}}$

Categorization and configurations

Solar reforming depends on the properties of the light absorber and the catalysts involved, and their selection, screening and integration to generate maximum value. The design and deployment of solar reforming technologies dictates the efficiency, scale and target substrates/products. In this context, solar reforming (more specifically, solar catalytic reforming) can be classified into three architectures: ^[1]

• Photocatalytic (PC) reforming - PC reforming is a one-pot process involving homogeneous or heterogenous photocatalyst suspensions (or immobilized photocatalysts on sheets ^{[10] [24] [23]} or floating materials ^[25] for easy recovery), which, under sunlight irradiation generate charge carriers (electron-hole pairs) to catalyze redox reactions (UV or near-UV based photoreforming systems generally also come under PC reforming). Despite the low cost and simplicity of PC reforming, there are major drawbacks of this approach which includes low product formation rates, poor selectivity of oxidation products or overoxidation to release CO₂, challenging catalyst/process optimization and harsh pre-treatment conditions. ^{[15] [26] [27]}
• Photoelectrochemical (PEC) reforming - PEC reforming involves the use of PEC systems/assemblies which consist of separated (photo)electrodes generally connected using a wire and submerged in solution (electrolyte). ^{[18] [19]} A photoelectrode consists of a light-absorber and additional charge transport and catalyst layers to facilitate the redox processes. While conventional PEC systems typically require a bias or voltage input in addition to the energy obtained from incident light irradiation, PEC reforming ideally operates with a single light absorber without any external bias or voltage (that is, completely driven by sunlight). ^{[18] [1]} PEC reforming can already produce clean fuels and valuable chemicals with high selectivity and achieve production rates which are 2-4 orders of magnitude higher than conventional PC processes. ^{[18] [19]} The spatial separation between the redox processes offered by PEC systems allows flexibility in the screening and integration of light-absorbers and catalysts, and also better product separation. ^[19] They can also benefit from better spectral utilization such as using solar concentrators or thermoelectric modules to harvest heat, thereby improving reaction kinetics and performance. ^[28] The versatility and high performance of these new PEC arrangements, therefore has wide scope of further exploitation and research.
• PV-EC reforming and extension to ‘electroreforming’ systems - PV-EC reforming refers to the use of electricity generated from photovoltaic panels (and therefore driven by sunlight) to drive electrochemical (electrolysis) reactions for waste reforming. ^[29] The concept of PV-EC reforming can be further extended to ‘electroreforming’ where renewable electricity from sources other than the sun (for example, wind, hydro, nuclear, among others) is used to power the electrochemical reactions achieving valuable fuel and chemical production from waste feedstocks. While traditionally most electrolysers, including commercial ones focus on water splitting to produce hydrogen, new electrochemical systems, catalysts and concepts have emerged which have started to look into waste substrates for utilisation as sustainable feedstocks. ^{[10] [30] [31] [32] [33]}

Introduction of 'Photon Economy'

An important concept introduced in the context of solar reforming is the ‘photon economy’, which, as defined by Bhattacharjee, Linley and Reisner, is the maximum utilization of all incident photons for maximizing product formation and value creation. ^[1] An ideal solar reforming process is one where the light absorber can absorb incident UV and visible light photons with maximum quantum yield, generating high charge carrier concentration to drive redox half reactions at maximum rate. 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, ^[34] etc.). Therefore, proper light and thermal management through various means (such as using solar concentrators, thermoelectric modules, among others) is encouraged to have both an atom economical and photon economical approach to extract maximum value from solar reforming processes.

Reception and media

The technological advancements in solar reforming garnered widespread interest in recent years. The works from scientists at Cambridge on PC reforming of raw lignocellulosic biomass or pre-treated polyester plastics to produce hydrogen and organics attracted attention of several stakeholders. ^{[35] [36] [37]} The recent technological breakthrough leading to the development of high-performing solar powered reactors (PEC reforming) for the simultaneous upcycling of greenhouse gas CO₂ and waste plastics to sustainable products received widespread acclaim and was highlighted in several prominent national and international media outlets. ^{[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]} Solar reforming processes primarily developed in Cambridge were also selected as “one of the eleven great ideas from British universities that could change the world” by Sunday Times (April 2020 edition) ^[53] and featured in the UK Prime Minister’s Speech on Net Zero, “Or the researchers at Cambridge who pioneered a new way to turn sunlight into fuel” ^[54] (indicating solar reforming which was a major subset of the broader research activities at Cambridge).

Outlook and future scope

Solar reforming is currently in the development phase and the scalable deployment of a particular solar reforming technology (PC, PEC or PV-EC) would depend on a variety of factors. These factors include deployment location and sunlight variability/intermittency, characteristics of the chosen waste stream, viable pre-treatment methods, target products, nature of the catalysts and their lifetime, fuel/chemical storage requirements, land use versus open water sources, capital and operational costs, production and solar-to-value creation rates, and governmental policies and incentives, among others. ^[1] Solar reforming may not be only limited to the conventional chemical pathways discussed, and may also include other relevant industrial processes such as light-driven organic transformations, flow photochemistry, integration with industrial electrolysis, among others. ^[1] The products from conventional solar reforming such as green hydrogen or other platform chemicals have a broad value-chain. 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. ^[55] 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. ^[1]

See also

• Artificial photosynthesis
• Circular economy
• Conference of the parties
• Electrochemical reduction of carbon dioxide
• Electrochemistry
• Hydrogen economy
• Net zero emissions
• Photocatalysis
• Photoelectrochemistry
• Solar fuel

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