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{{Short description|Technology for conversion of waste}}
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'''Solar reforming''' is the sunlight-driven conversion of diverse carbon waste resources (including solid, liquid, and gaseous waste streams such as [[biomass]], [[plastic]]s, industrial by-products, [[Carbon dioxide in Earth's atmosphere|atmospheric carbon dioxide]], etc.) into sustainable fuels (or energy vectors) and value-added chemicals.<ref name=":0">{{Cite journal |last1=Bhattacharjee |first1=Subhajit |last2=Linley |first2=Stuart |last3=Reisner |first3=Erwin |date=2024-01-30 |title=Solar reforming as an emerging technology for circular chemical industries |url=https://www.nature.com/articles/s41570-023-00567-x |journal=Nature Reviews Chemistry |language=en |volume=8 |issue=2 |pages=87–105 |doi=10.1038/s41570-023-00567-x |pmid=38291132 |s2cid=267332161 |issn=2397-3358}}</ref> It encompasses a set of technologies (and processes) operating under ambient and aqueous conditions, utilizing solar spectrum to generate maximum value.<ref name=":0" /> Solar reforming offers an attractive and unifying solution to address the contemporary challenges of [[climate change]] and [[Pollution|environmental pollution]] by creating a sustainable [[Circular economy|circular network]] of waste upcycling, clean fuel (and chemical) generation and the consequent mitigation of [[Greenhouse gas emissions|greenhouse emissions]] (in alignment with the [[Sustainable Development Goals|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 [[Titanium dioxide|TiO<sub>2</sub>]] [[semiconductor]] [[Photocatalysis|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 donor]]s to drive the generation of [[Hydrogen economy|hydrogen gas]] over TiO<sub>2</sub> photocatalyst composites.<ref>{{Cite journal |last1=Kawai |first1=Tomoji |last2=Sakata |first2=Tadayoshi |date=1980-07-31 |title=Conversion of carbohydrate into hydrogen fuel by a photocatalytic process |url=https://www.nature.com/articles/286474a0 |journal=Nature |language=en |volume=286 |issue=5772 |pages=474–476 |doi=10.1038/286474a0 |bibcode=1980Natur.286..474K |s2cid=4356641 |issn=1476-4687}}</ref><ref>{{Cite journal |last1=Kawai |first1=Tomoji |last2=Sakata |first2=Tadayoshi |date=January 1981 |title=Photocatalytic hydrogen production from water by the decomposition of poly-vinylchloride, protein, algae, dead insects, and excrement |url=http://dx.doi.org/10.1246/cl.1981.81 |journal=Chemistry Letters |volume=10 |issue=1 |pages=81–84 |doi=10.1246/cl.1981.81 |issn=0366-7022}}</ref> In 2017, Wakerley, Kuehnel and Reisner at the [[University of Cambridge]], [[United Kingdom|UK]] demonstrated the photocatalytic production of hydrogen using raw [[lignocellulosic biomass]] substrates in the presence of [[Visible spectrum|visible-light]] responsive CdS|CdO<sub>x</sub> [[quantum dot]]s under alkaline conditions.<ref>{{Cite journal |last1=Wakerley |first1=David W. |last2=Kuehnel |first2=Moritz F. |last3=Orchard |first3=Katherine L. |last4=Ly |first4=Khoa H. |last5=Rosser |first5=Timothy E. |last6=Reisner |first6=Erwin |date=2017-03-13 |title=Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst |url=https://www.nature.com/articles/nenergy201721 |journal=Nature Energy |language=en |volume=2 |issue=4 |pages=1–9 |doi=10.1038/nenergy.2017.21 |s2cid=100128646 |issn=2058-7546}}</ref> This was followed by the utilization of less-toxic, carbon-based, visible-light absorbing photocatalyst composites (for example [[Carbon nitride|carbon-nitride]] based systems) for biomass and plastics photoreforming to hydrogen and organics by Kasap, Uekert and Reisner.<ref>{{Cite journal |last1=Kasap |first1=Hatice |last2=Achilleos |first2=Demetra S. |last3=Huang |first3=Ailun |last4=Reisner |first4=Erwin |date=2018-09-19 |title=Photoreforming of Lignocellulose into H 2 Using Nanoengineered Carbon Nitride under Benign Conditions |url=https://pubs.acs.org/doi/10.1021/jacs.8b07853 |journal=Journal of the American Chemical Society |language=en |volume=140 |issue=37 |pages=11604–11607 |doi=10.1021/jacs.8b07853 |pmid=30153420 |s2cid=52111870 |issn=0002-7863}}</ref><ref>{{Cite journal |last1=Uekert |first1=Taylor |last2=Kasap |first2=Hatice |last3=Reisner |first3=Erwin |date=2019-09-25 |title=Photoreforming of Nonrecyclable Plastic Waste over a Carbon Nitride/Nickel Phosphide Catalyst |journal=Journal of the American Chemical Society |language=en |volume=141 |issue=38 |pages=15201–15210 |doi=10.1021/jacs.9b06872 |issn=0002-7863 |pmc=7007225 |pmid=31462034}}</ref> In addition to variations of carbon nitride, other photocatalyst composite systems based on [[Graphite oxide|graphene oxides]], [[MXenes]], [[Coordination polymer|co-ordination polymers]] and [[Metal chalcogenide|metal chalcogenides]] were reported during this period.<ref>{{Cite journal |last1=Wang |first1=Jiu |last2=Kumar |first2=Pawan |last3=Zhao |first3=Heng |last4=Kibria |first4=Md Golam |last5=Hu |first5=Jinguang |date=2021 |title=Polymeric carbon nitride-based photocatalysts for photoreforming of biomass derivatives |url=http://xlink.rsc.org/?DOI=D1GC02307A |journal=Green Chemistry |language=en |volume=23 |issue=19 |pages=7435–7457 |doi=10.1039/D1GC02307A |issn=1463-9262 |s2cid=238644248}}</ref><ref>{{Cite journal |last1=Wu |first1=Xinxing |last2=Zhao |first2=Heng |last3=Khan |first3=Mohd Adnan |last4=Maity |first4=Partha |last5=Al-Attas |first5=Tareq |last6=Larter |first6=Stephen |last7=Yong |first7=Qiang |last8=Mohammed |first8=Omar F. |last9=Kibria |first9=Md Golam |last10=Hu |first10=Jinguang |date=2020-10-19 |title=Sunlight-Driven Biomass Photorefinery for Coproduction of Sustainable Hydrogen and Value-Added Biochemicals |url=https://pubs.acs.org/doi/10.1021/acssuschemeng.0c06282 |journal=ACS Sustainable Chemistry & Engineering |language=en |volume=8 |issue=41 |pages=15772–15781 |doi=10.1021/acssuschemeng.0c06282 |issn=2168-0485 |s2cid=225149072}}</ref><ref>{{Cite journal |last1=Rao |first1=Cheng |last2=Xie |first2=Maoliang |last3=Liu |first3=Sicong |last4=Chen |first4=Runlin |last5=Su |first5=Hang |last6=Zhou |first6=Lan |last7=Pang |first7=Yuxia |last8=Lou |first8=Hongming |last9=Qiu |first9=Xueqing |date=2021-09-22 |title=Visible Light-Driven Reforming of Lignocellulose into H 2 by Intrinsic Monolayer Carbon Nitride |url=https://pubs.acs.org/doi/10.1021/acsami.1c10842 |journal=ACS Applied Materials & Interfaces |language=en |volume=13 |issue=37 |pages=44243–44253 |doi=10.1021/acsami.1c10842 |issn=1944-8244 |pmid=34499461 |s2cid=237472526}}</ref><ref name=":2">{{Cite journal |last1=Pichler |first1=Christian M. |last2=Bhattacharjee |first2=Subhajit |last3=Rahaman |first3=Motiar |last4=Uekert |first4=Taylor |last5=Reisner |first5=Erwin |date=2021-08-06 |title=Conversion of Polyethylene Waste into Gaseous Hydrocarbons via Integrated Tandem Chemical–Photo/Electrocatalytic Processes |journal=ACS Catalysis |language=en |volume=11 |issue=15 |pages=9159–9167 |doi=10.1021/acscatal.1c02133 |issn=2155-5435 |pmc=8353629 |pmid=34386271}}</ref><ref>{{Cite journal |last1=Guan |first1=Lijiang |last2=Cheng |first2=Guang |last3=Tan |first3=Bien |last4=Jin |first4=Shangbin |date=2021 |title=Covalent triazine frameworks constructed via benzyl halide monomers showing high photocatalytic activity in biomass reforming |url=http://xlink.rsc.org/?DOI=D1CC01102B |journal=Chemical Communications |language=en |volume=57 |issue=42 |pages=5147–5150 |doi=10.1039/D1CC01102B |issn=1359-7345 |pmid=33899846 |s2cid=233400735}}</ref><ref>{{Cite journal |last1=Nguyen |first1=Van-Can |last2=Nimbalkar |first2=Dipak B. |last3=Nam |first3=Le D. |last4=Lee |first4=Yuh-Lang |last5=Teng |first5=Hsisheng |date=2021-05-07 |title=Photocatalytic Cellulose Reforming for H 2 and Formate Production by Using Graphene Oxide-Dot Catalysts |url=https://pubs.acs.org/doi/10.1021/acscatal.1c00217 |journal=ACS Catalysis |language=en |volume=11 |issue=9 |pages=4955–4967 |doi=10.1021/acscatal.1c00217 |issn=2155-5435 |s2cid=233564941}}</ref><ref>{{Cite journal |last1=Cao |first1=Bingqian |last2=Wan |first2=Shipeng |last3=Wang |first3=Yanan |last4=Guo |first4=Haiwei |last5=Ou |first5=Man |last6=Zhong |first6=Qin |date=2022-01-01 |title=Highly-efficient visible-light-driven photocatalytic H2 evolution integrated with microplastic degradation over MXene/ZnxCd1-xS photocatalyst |url=https://www.sciencedirect.com/science/article/pii/S002197972101167X |journal=Journal of Colloid and Interface Science |volume=605 |pages=311–319 |doi=10.1016/j.jcis.2021.07.113 |issn=0021-9797 |pmid=34332406}}</ref><ref>{{Cite journal |last1=Nagakawa |first1=Haruki |last2=Nagata |first2=Morio |date=2021-12-02 |title=Highly Efficient Hydrogen Production in the Photoreforming of Lignocellulosic Biomass Catalyzed by Cu,In-Doped ZnS Derived from ZIF-8 |url=https://onlinelibrary.wiley.com/doi/10.1002/admi.202101581 |journal=Advanced Materials Interfaces |language=en |volume=9 |issue=2 |doi=10.1002/admi.202101581 |issn=2196-7350 |s2cid=244880250}}</ref> 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 polymer|condensation plastics]], accounting for more than 80% of the operation costs.<ref name=":3" /> 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.<ref>{{Cite journal |last1=Bhattacharjee |first1=Subhajit |last2=Guo |first2=Chengzhi |last3=Lam |first3=Erwin |last4=Holstein |first4=Josephin M. |last5=Rangel Pereira |first5=Mariana |last6=Pichler |first6=Christian M. |last7=Pornrungroj |first7=Chanon |last8=Rahaman |first8=Motiar |last9=Uekert |first9=Taylor |last10=Hollfelder |first10=Florian |last11=Reisner |first11=Erwin |date=2023-09-20 |title=Chemoenzymatic Photoreforming: A Sustainable Approach for Solar Fuel Generation from Plastic Feedstocks |journal=Journal of the American Chemical Society |language=en |volume=145 |issue=37 |pages=20355–20364 |doi=10.1021/jacs.3c05486 |issn=0002-7863 |pmc=10515630 |pmid=37671930}}</ref> In 2020, Jiao and Xie reported the photocatalytic conversion of addition plastics such as [[polyethylene]] and [[polypropylene]] to high energy-density to C<sub>2</sub> fuels over a [[Niobium pentoxide|Nb<sub>2</sub>O<sub>5</sub>]] catalyst under natural conditions.<ref>{{Cite journal |last1=Jiao |first1=Xingchen |last2=Zheng |first2=Kai |last3=Chen |first3=Qingxia |last4=Li |first4=Xiaodong |last5=Li |first5=Yamin |last6=Shao |first6=Weiwei |last7=Xu |first7=Jiaqi |last8=Zhu |first8=Junfa |last9=Pan |first9=Yang |last10=Sun |first10=Yongfu |last11=Xie |first11=Yi |date=September 2020 |title=Photocatalytic Conversion of Waste Plastics into C 2 Fuels under Simulated Natural Environment Conditions |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201915766 |journal=Angewandte Chemie International Edition |language=en |volume=59 |issue=36 |pages=15497–15501 |doi=10.1002/anie.201915766 |pmid=32003512 |s2cid=210983540 |issn=1433-7851}}</ref>

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.<ref name=":3">{{Cite journal |last1=Uekert |first1=Taylor |last2=Pichler |first2=Christian M. |last3=Schubert |first3=Teresa |last4=Reisner |first4=Erwin |date=2020-11-30 |title=Solar-driven reforming of solid waste for a sustainable future |url=https://www.nature.com/articles/s41893-020-00650-x |journal=Nature Sustainability |language=en |volume=4 |issue=5 |pages=383–391 |doi=10.1038/s41893-020-00650-x |bibcode=2020NatSu...4..383U |s2cid=227236618 |issn=2398-9629}}</ref> In 2021, sunlight-driven [[Photoelectrochemical process|photoelectrochemical]] (PEC) systems/technologies operating with no external bias or voltage input were introduced by Bhattacharjee and Reisner at the [[University of Cambridge]].<ref name=":4">{{Cite journal |last1=Bhattacharjee |first1=Subhajit |last2=Andrei |first2=Virgil |last3=Pornrungroj |first3=Chanon |last4=Rahaman |first4=Motiar |last5=Pichler |first5=Christian M. |last6=Reisner |first6=Erwin |date=2021-10-27 |title=Reforming of Soluble Biomass and Plastic Derived Waste Using a Bias-Free Cu 30 Pd 70 {{!}}Perovskite{{!}}Pt Photoelectrochemical Device |journal=Advanced Functional Materials |language=en |volume=32 |issue=7 |doi=10.1002/adfm.202109313 |issn=1616-301X|doi-access=free }}</ref> These PEC reforming (see ''Categorization and configurations'' section) systems reformed diverse pre-treated waste streams (such as [[Lignocellulosic biomass|lignocellulose]] and [[Polyethylene terephthalate|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.<ref name=":4" /> In 2023, Bhattacharjee, Rahaman and Reisner extended the PEC platform to a solar reactor which could reduce [[greenhouse gas]] CO<sub>2</sub> to different energy vectors ([[Carbon monoxide|CO]], [[syngas]], [[formate]] depending on the type of catalyst integrated) and convert waste PET plastics to [[glycolic acid]] at the same time.<ref name=":5">{{Cite journal |last1=Bhattacharjee |first1=Subhajit |last2=Rahaman |first2=Motiar |last3=Andrei |first3=Virgil |last4=Miller |first4=Melanie |last5=Rodríguez-Jiménez |first5=Santiago |last6=Lam |first6=Erwin |last7=Pornrungroj |first7=Chanon |last8=Reisner |first8=Erwin |date=2023-01-09 |title=Photoelectrochemical CO2-to-fuel conversion with simultaneous plastic reforming |url=https://www.nature.com/articles/s44160-022-00196-0 |journal=Nature Synthesis |language=en |volume=2 |issue=2 |pages=182–192 |doi=10.1038/s44160-022-00196-0 |s2cid=255686581 |issn=2731-0582}}</ref> This further inspired the direct capture and conversion of CO<sub>2</sub> to products from [[flue gas]] and air ([[direct air capture]]) in a PEC reforming process (with simultaneous plastic conversion).<ref>{{Cite journal |last1=Kar |first1=Sayan |last2=Rahaman |first2=Motiar |last3=Andrei |first3=Virgil |last4=Bhattacharjee |first4=Subhajit |last5=Roy |first5=Souvik |last6=Reisner |first6=Erwin |date=2023-07-19 |title=Integrated capture and solar-driven utilization of CO2 from flue gas and air |url=https://doi.org/10.1016/j.joule.2023.05.022 |journal=Joule |volume=7 |issue=7 |pages=1496–1514 |doi=10.1016/j.joule.2023.05.022 |issn=2542-4351}}</ref> Choi and Ryu demonstrated a [[Polyoxometalate|polyoxometallate]]-medated PEC process to achieve biomass conversion with unassisted hydrogen production in 2022.<ref>{{Cite journal |last1=Choi |first1=Yuri |last2=Mehrotra |first2=Rashmi |last3=Lee |first3=Sang-Hak |last4=Nguyen |first4=Trang Vu Thien |last5=Lee |first5=Inhui |last6=Kim |first6=Jiyeong |last7=Yang |first7=Hwa-Young |last8=Oh |first8=Hyeonmyeong |last9=Kim |first9=Hyunwoo |last10=Lee |first10=Jae-Won |last11=Kim |first11=Yong Hwan |last12=Jang |first12=Sung-Yeon |last13=Jang |first13=Ji-Wook |last14=Ryu |first14=Jungki |date=2022-10-03 |title=Bias-free solar hydrogen production at 19.8 mA cm−2 using perovskite photocathode and lignocellulosic biomass |journal=Nature Communications |language=en |volume=13 |issue=1 |pages=5709 |doi=10.1038/s41467-022-33435-1 |pmid=36192405 |pmc=9529942 |issn=2041-1723}}</ref> Similarly, Pan and Chu, in 2023  reported a PEC cell for renewable formate production from sunlight, CO<sub>2</sub> and biomass-derived sugars.<ref>{{Cite journal |last1=Pan |first1=Yuyang |last2=Zhang |first2=Huiyan |last3=Zhang |first3=Bowen |last4=Gong |first4=Feng |last5=Feng |first5=Jianyong |last6=Huang |first6=Huiting |last7=Vanka |first7=Srinivas |last8=Fan |first8=Ronglei |last9=Cao |first9=Qi |last10=Shen |first10=Mingrong |last11=Li |first11=Zhaosheng |last12=Zou |first12=Zhigang |last13=Xiao |first13=Rui |last14=Chu |first14=Sheng |date=2023-02-23 |title=Renewable formate from sunlight, biomass and carbon dioxide in a photoelectrochemical cell |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=1013 |doi=10.1038/s41467-023-36726-3 |pmid=36823177 |pmc=9950059 |bibcode=2023NatCo..14.1013P |issn=2041-1723}}</ref> 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.<ref name=":0" /> It generally operates without external heating and pressure, and also introduces a thermodynamic advantage over traditional [[green hydrogen]] or CO<sub>2</sub> reduction fuel producing methods such as [[water splitting]] or CO<sub>2</sub> splitting, respectively. Depending on solar spectrum utilization, solar reforming can be classified into two categories: “solar catalytic reforming” and “solar thermal reforming”.<ref name=":0" /> Solar catalytic reforming refers to transformation processes primarily driven by [[ultraviolet]] (UV) or [[Visible spectrum|visible light]].<ref name=":0" /> It also includes the subset of ‘photoreforming’ encompassing utilization of high energy [[photon]]s in the UV or near-UV region of the solar spectrum (for example, by semiconductor photocatalysts such as TiO<sub>2</sub>). Solar thermal reforming, on the other hand, exploits the [[infrared]] (IR) region for waste upcycling to generate products of high economic value.<ref name=":0" /> 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.<ref name=":0" /> 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).<ref name=":0" />

=== 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.<ref name=":0" /> Solar reforming also provides several benefits over traditional green hydrogen production methods such as [[water splitting]] (H<sub>2</sub>O → H<sub>2</sub> + ½O<sub>2</sub>, ΔG° = 237 kJ mol<sup>−1</sup>). 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<sub>x</sub>H<sub>y</sub>O<sub>z</sub> + (2''x''−''z'')H<sub>2</sub>O → (2''x''−''z''+''y''/2)H<sub>2</sub> + ''x''CO<sub>2</sub>; ΔG° ~0 kJ mol<sup>−1</sup>).<ref name=":0" /> 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 CO<sub>2</sub> splitting. Furthermore, concentrated streams of hydrogen produced from solar reforming is safer than [[Oxyhydrogen|explosive mixtures of oxygen and hydrogen]] (from traditional water splitting), that otherwise require additional separation costs.<ref name=":0" /> 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.<ref name=":0" />

=== 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<sub>STF</sub>) 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.<ref name=":0" /><ref name=":6" /> The η<sub>STF</sub> 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<sup>‒1</sup>), this makes the η<sub>STF</sub> per definition close to zero, despite the high production rates and [[quantum yield]]s. However, replacing the ΔG for product formation (during solar reforming) with that of product utilisation (|ΔG<sub>use</sub>|; such as [[combustion]] of the hydrogen fuel generated) can give a better representation of the process efficiency.<ref name=":0" />

<math>\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)}</math>

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<sub>areal</sub>) as shown, where ‘n’ is the [[Mole (unit)|moles]] of product formed, ‘A’ is the sunlight irradiation area and ‘t’ is the time.<ref name=":0" />

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

Although r<sub>areal</sub> 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.<ref name=":0" /> Therefore, a more adaptable and robust metric is the solar-to-value creation rate (''r''<sub>STV</sub>) which can encompass all these factors and provide a more holistic and practical picture from the economic or commercial point of view.<ref name=":0" /> The simplified equation for ''r''<sub>STV</sub> is shown below, where ''C<sub>i</sub>'' and ''C<sub>k</sub>'' are the costs of the product ‘i’ and substrate ‘k’, respectively. ‘''C<sub>p</sub>''’ is the pre-treatment cost for the waste substrate ‘k’, and ‘''n<sub>i</sub>''’ and ‘''n<sub>k</sub>''’ 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.<ref name=":0" />

<math>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)}{}</math>

=== 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:<ref name=":0" />

* Photocatalytic (PC) reforming - PC reforming is a one-pot process involving homogeneous or heterogenous [[Photocatalysis|photocatalyst]] suspensions (or immobilized photocatalysts on sheets<ref name=":2" /><ref name=":1">{{Cite journal |last1=Uekert |first1=Taylor |last2=Bajada |first2=Mark A. |last3=Schubert |first3=Teresa |last4=Pichler |first4=Christian M. |last5=Reisner |first5=Erwin |date=2021-10-05 |title=Scalable Photocatalyst Panels for Photoreforming of Plastic, Biomass and Mixed Waste in Flow |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202002580 |journal=ChemSusChem |language=en |volume=14 |issue=19 |pages=4190–4197 |bibcode=2021ChSCh..14.4190U |doi=10.1002/cssc.202002580 |issn=1864-5631 |pmid=33156562 |s2cid=226271147}}</ref><ref name=":6">{{Cite journal |last1=Andrei |first1=Virgil |last2=Wang |first2=Qian |last3=Uekert |first3=Taylor |last4=Bhattacharjee |first4=Subhajit |last5=Reisner |first5=Erwin |date=2022-12-06 |title=Solar Panel Technologies for Light-to-Chemical Conversion |journal=Accounts of Chemical Research |language=en |volume=55 |issue=23 |pages=3376–3386 |doi=10.1021/acs.accounts.2c00477 |issn=0001-4842 |pmc=9730848 |pmid=36395337}}</ref> or floating materials<ref>{{Cite journal |last1=Linley |first1=Stuart |last2=Reisner |first2=Erwin |date=2023-05-12 |title=Floating Carbon Nitride Composites for Practical Solar Reforming of Pre-Treated Wastes to Hydrogen Gas |journal=Advanced Science |language=en |volume=10 |issue=21 |pages=e2207314 |doi=10.1002/advs.202207314 |issn=2198-3844 |pmc=10375181 |pmid=37171802}}</ref> 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 CO<sub>2</sub>, challenging catalyst/process optimization and harsh pre-treatment conditions.<ref name=":3" /><ref>{{Cite journal |last1=Djurišić |first1=Aleksandra B. |last2=He |first2=Yanling |last3=Ng |first3=Alan M. C. |date=2020-03-01 |title=Visible-light photocatalysts: Prospects and challenges |journal=APL Materials |volume=8 |issue=3 |page=030903 |doi=10.1063/1.5140497 |bibcode=2020APLM....8c0903D |issn=2166-532X|doi-access=free }}</ref><ref>{{Cite journal |last1=Kou |first1=Jiahui |last2=Lu |first2=Chunhua |last3=Wang |first3=Jian |last4=Chen |first4=Yukai |last5=Xu |first5=Zhongzi |last6=Varma |first6=Rajender S. |date=2017-02-08 |title=Selectivity Enhancement in Heterogeneous Photocatalytic Transformations |journal=Chemical Reviews |language=en |volume=117 |issue=3 |pages=1445–1514 |doi=10.1021/acs.chemrev.6b00396 |pmid=28093903 |issn=0009-2665|doi-access=free }}</ref>
* 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]]).<ref name=":4" /><ref name=":5" /> A photoelectrode consists of a light-absorber and additional charge transport and [[Catalysis|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).<ref name=":4" /><ref name=":0" /> 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.<ref name=":4" /><ref name=":5" /> 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.<ref name=":5" /> They can also benefit from better spectral utilization such as using [[Concentrated solar power|solar concentrators]] or [[Thermoelectric generator|thermoelectric modules]] to harvest heat, thereby improving reaction kinetics and performance.<ref>{{Cite journal |last1=Wang |first1=Qian |last2=Pornrungroj |first2=Chanon |last3=Linley |first3=Stuart |last4=Reisner |first4=Erwin |date=2021-11-19 |title=Strategies to improve light utilization in solar fuel synthesis |url=https://www.nature.com/articles/s41560-021-00919-1 |journal=Nature Energy |language=en |volume=7 |issue=1 |pages=13–24 |doi=10.1038/s41560-021-00919-1 |s2cid=256726357 |issn=2058-7546}}</ref> 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 [[Photovoltaics|photovoltaic]] panels (and therefore driven by sunlight) to drive electrochemical ([[electrolysis]]) reactions for waste reforming.<ref>{{Cite journal |last1=Jacobsson |first1=T. Jesper |last2=Fjällström |first2=Viktor |last3=Edoff |first3=Marika |last4=Edvinsson |first4=Tomas |date=2014-06-19 |title=Sustainable solar hydrogen production: from photoelectrochemical cells to PV-electrolyzers and back again |url=https://pubs.rsc.org/en/content/articlelanding/2014/ee/c4ee00754a |journal=Energy & Environmental Science |language=en |volume=7 |issue=7 |pages=2056–2070 |doi=10.1039/C4EE00754A |issn=1754-5706}}</ref> 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.<ref name=":2"/><ref>{{Cite journal |last1=Zhou |first1=Hua |last2=Ren |first2=Yue |last3=Li |first3=Zhenhua |last4=Xu |first4=Ming |last5=Wang |first5=Ye |last6=Ge |first6=Ruixiang |last7=Kong |first7=Xianggui |last8=Zheng |first8=Lirong |last9=Duan |first9=Haohong |date=2021-08-17 |title=Electrocatalytic upcycling of polyethylene terephthalate to commodity chemicals and H2 fuel |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=4679 |doi=10.1038/s41467-021-25048-x |pmid=34404779 |pmc=8371182 |issn=2041-1723}}</ref><ref>{{Cite journal |last1=Wang |first1=Jianying |last2=Li |first2=Xin |last3=Wang |first3=Maolin |last4=Zhang |first4=Ting |last5=Chai |first5=Xinyu |last6=Lu |first6=Junlin |last7=Wang |first7=Tianfu |last8=Zhao |first8=Yixin |last9=Ma |first9=Ding |date=2022-06-03 |title=Electrocatalytic Valorization of Poly(ethylene terephthalate) Plastic and CO 2 for Simultaneous Production of Formic Acid |url=https://pubs.acs.org/doi/10.1021/acscatal.2c01128 |journal=ACS Catalysis |language=en |volume=12 |issue=11 |pages=6722–6728 |doi=10.1021/acscatal.2c01128 |s2cid=249026599 |issn=2155-5435}}</ref><ref>{{Cite journal |last1=Zhao |first1=Hu |last2=Lu |first2=Dan |last3=Wang |first3=Jiarui |last4=Tu |first4=Wenguang |last5=Wu |first5=Dan |last6=Koh |first6=See Wee |last7=Gao |first7=Pingqi |last8=Xu |first8=Zhichuan J. |last9=Deng |first9=Sili |last10=Zhou |first10=Yan |last11=You |first11=Bo |last12=Li |first12=Hong |date=2021-03-31 |title=Raw biomass electroreforming coupled to green hydrogen generation |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=2008 |doi=10.1038/s41467-021-22250-9 |pmid=33790295 |pmc=8012647 |issn=2041-1723}}</ref><ref>{{Cite journal |last1=Pichler |first1=Christian M. |last2=Bhattacharjee |first2=Subhajit |last3=Lam |first3=Erwin |last4=Su |first4=Lin |last5=Collauto |first5=Alberto |last6=Roessler |first6=Maxie M. |last7=Cobb |first7=Samuel J. |last8=Badiani |first8=Vivek M. |last9=Rahaman |first9=Motiar |last10=Reisner |first10=Erwin |date=2022-11-04 |title=Bio-Electrocatalytic Conversion of Food Waste to Ethylene via Succinic Acid as the Central Intermediate |journal=ACS Catalysis |language=en |volume=12 |issue=21 |pages=13360–13371 |doi=10.1021/acscatal.2c02689 |issn=2155-5435 |pmc=9638992 |pmid=36366764}}</ref>

== 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.<ref name=":0" /> An ideal solar reforming process is one where the light absorber can absorb incident [[Ultraviolet|UV]] and [[Visible spectrum|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 [[Infrared|IR]] photons may be used for boosting reaction kinetics, waste pre-treatment or other means of value creation (for example, [[desalination]],<ref>{{Cite journal |last1=Pornrungroj |first1=Chanon |last2=Mohamad Annuar |first2=Ariffin Bin |last3=Wang |first3=Qian |last4=Rahaman |first4=Motiar |last5=Bhattacharjee |first5=Subhajit |last6=Andrei |first6=Virgil |last7=Reisner |first7=Erwin |date=2023-11-13 |title=Hybrid photothermal–photocatalyst sheets for solar-driven overall water splitting coupled to water purification |url=https://www.nature.com/articles/s44221-023-00139-9 |journal=Nature Water |language=en |volume=1 |issue=11 |pages=952–960 |doi=10.1038/s44221-023-00139-9 |issn=2731-6084}}</ref> 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 economy|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 [[University of Cambridge|Cambridge]] on PC reforming of raw lignocellulosic biomass or pre-treated polyester plastics to produce hydrogen and organics attracted attention of several stakeholders.<ref>{{Cite web |title=Scientists harness solar power to produce clean hydrogen from biomass |url=http://www.biofueldaily.com/reports/Scientists_harness_solar_power_to_produce_clean_hydrogen_from_biomass_999.html |access-date=2024-02-13 |website=Bio Fuel Daily}}</ref><ref>{{Cite news |date=2020-12-22 |title=Covid: PPE 'could be recycled' with help of sunlight |url=https://www.bbc.com/news/uk-wales-55396511 |access-date=2024-02-13 |language=en-GB}}</ref><ref>{{Cite web |title=Scientists harness solar power to produce clean hydrogen from biomass |url=https://www.eurekalert.org/news-releases/713884 |access-date=2024-02-13 |website=EurekAlert! |language=en}}</ref> The recent technological breakthrough leading to the development of high-performing solar powered reactors (PEC reforming) for the simultaneous upcycling of greenhouse gas CO<sub>2</sub> and waste plastics to sustainable products received widespread acclaim and was highlighted in several prominent national and international media outlets.<ref>{{Cite news |date=2023-03-14 |title=Could waste plastic become a useful fuel source? |url=https://www.bbc.com/news/business-64703976 |access-date=2024-02-13 |work=BBC |language=en-GB}}</ref><ref>{{Cite web |last=Theil |first=Michele |date=2023-01-23 |title=Scientists have found a way to convert plastics and CO2 into sustainable fuels using solar power |url=https://www.bigissue.com/news/environment/scientists-break-through-process-convert-plastics-greenhouse-gases-sustainable-fuels/ |access-date=2024-02-13 |website=The Big Issue |language=en}}</ref><ref>{{Cite web |title=Plastic waste and CO2 converted into hydrogen and feedstock chemical using sunlight |url=https://www.chemistryworld.com/news/plastic-waste-and-co2-converted-into-hydrogen-and-feedstock-chemical-using-sunlight/4017613.article |access-date=2024-02-13 |website=Chemistry World |language=en}}</ref><ref>{{Cite web |date=2023-01-10 |title=Solar reactor converts both CO2 and plastic waste into useful products |url=https://newatlas.com/environment/solar-reactor-co2-plastic-waste-useful-products/ |access-date=2024-02-13 |website=New Atlas |language=en-US}}</ref><ref>{{Cite web |date=2023-01-09 |title=Solar-powered system converts plastic and greenhouse gases into sustainable fuels |url=https://www.cam.ac.uk/research/news/solar-powered-system-converts-plastic-and-greenhouse-gases-into-sustainable-fuels |access-date=2024-02-13 |website=University of Cambridge |language=en}}</ref><ref>{{Cite web |title=Solar-powered system converts plastic and greenhouse gases into sustainable fuels |url=https://www.sciencedaily.com/releases/2023/01/230109112706.htm |access-date=2024-02-13 |website=ScienceDaily |language=en}}</ref><ref>{{Cite web |title=Solar Powered Machine Turns CO2 and Waste Plastic Into Valuable Fuel |url=https://futurism.com/solar-machine-co2-plastic-fuel |access-date=2024-02-13 |website=Futurism}}</ref><ref>{{Cite web |title=Converting plastics and greenhouse gases into sustainable energy {{!}} Technology |url=https://www.labroots.com/trending/technology/24402/converting-plastics-greenhouse-gases-sustainable-energy |access-date=2024-02-13 |website=Labroots}}</ref><ref>{{Cite web |last=محمود |first=عبد الحكيم |title=ابتكار نظام ثنائي يعمل بالطاقة الشمسية يحول البلاستيك وغازات الاحتباس الحراري إلى وقود مستدام |url=https://www.aljazeera.net/science/2023/1/26/%d8%a7%d8%a8%d8%aa%d9%83%d8%a7%d8%b1-%d9%86%d8%b8%d8%a7%d9%85-%d8%ab%d9%86%d8%a7%d8%a6%d9%8a-%d9%8a%d8%b9%d9%85%d9%84-%d8%a8%d8%a7%d9%84%d8%b7%d8%a7%d9%82%d8%a9 |access-date=2024-02-13 |website=الجزيرة نت |language=ar}}</ref><ref>{{Cite web |last=Patel |first=Prachi |date=2023-01-12 |title=Solar powered reactor converts plastic and CO2 into fuel |url=https://www.anthropocenemagazine.org/2023/01/sunlight-converts-plastic-and-carbon-dioxide-into-useful-chemicals/ |access-date=2024-02-13 |language=en-US}}</ref><ref>{{Cite web |date=2023-01-24 |title=BREAKTHROUGH! Indian researchers at Cambridge university offer solution to global warming and plastic pollution |url=https://www.timesnownews.com/world/breakthrough-indian-researchers-at-cambridge-university-offer-solution-to-global-warming-and-plastic-pollution-article-97282783 |access-date=2024-02-13 |website=TimesNow |language=en}}</ref><ref>{{Citation |title=Video {{!}} The Importance Of Solving The Biggest Environmental Challenges Through Research & Technology |url=https://www.ndtv.com/video/news/banega-swasth-india/the-importance-of-solving-the-biggest-environmental-challenges-through-research-technology-689126 |access-date=2024-02-13}}</ref><ref>{{Cite web |last=Bhatia |first=Anisha |date=2023-03-17 |title=Two Indian Scientists On The Importance Of Solving The Biggest Environmental Challenges Through Research And Technology |url=https://swachhindia.ndtv.com/two-indian-scientists-on-the-importance-of-solving-the-biggest-environmental-challenges-through-research-and-technology-74812/ |access-date=2024-02-13 |website=NDTV-Dettol Banega Swasth Swachh India |language=en-US}}</ref><ref>{{Cite web |date=2023-01-09 |title=Scientists use 'miracle material' to convert plastic waste into sustainable fuel |url=https://www.independent.co.uk/tech/solar-power-energy-perovskite-plastic-waste-b2258781.html |access-date=2024-02-13 |website=The Independent |language=en}}</ref><ref>{{Cite web |title=In Breakthrough, 2 Indian Scientists Offer Answers To These Global Issues |url=https://www.ndtv.com/science/in-breakthrough-2-indian-scientists-offer-answers-to-these-global-issues-3719985 |access-date=2024-02-13 |website=NDTV.com}}</ref> 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)<ref>{{Cite news |last=Forster |first=Rosie Kinchen and Katherine |date=2024-02-13 |title=Reasons to be cheerful: 11 great ideas from British universities that could change the world |newspaper=[[The Times]] |url=https://www.thetimes.co.uk/article/reasons-to-be-cheerful-11-great-ideas-from-british-universities-that-could-change-the-world-32msbbdlz |access-date=2024-02-13 |language=en |issn=0140-0460}}</ref> 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''”<ref>{{Cite web |date=2023-09-20 |title=PM speech on Net Zero: 20 September 2023 |url=https://www.gov.uk/government/speeches/pm-speech-on-net-zero-20-september-2023 |access-date=2024-02-13 |website=GOV.UK |language=en}}</ref> (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.<ref name=":0" /> 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.<ref name=":0" /> 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 CO<sub>2</sub> as key carbon feedstocks to replace [[fossil fuel]]s.<ref>{{Cite web |date=2019-10-09 |title=Roadmap Chemie 2050 |url=https://www.vci.de/services/publikationen/broschueren-faltblaetter/vci-dechema-futurecamp-studie-roadmap-2050-treibhausgasneutralitaet-chemieindustrie-deutschland-langfassung.jsp |access-date=2024-02-13 |website=VCI Online |language=German}}</ref> 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.<ref name=":0" />

== 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]]

== References ==
{{reflist|30em}}

[[Category:Sustainability]]
[[Category:Sustainable energy]]
[[Category:Energy]]
[[Category:Engineering]]
[[Category:Science and technology]]
[[Category:University of Cambridge]]
[[Category:Solar energy]]
[[Category:Chemistry]]
[[Category:Materials science]]
[[Category:Chemical industry]]
[[Category:Climate change mitigation]]
[[Category:Green chemistry]]