Solar reforming

Definitions 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 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]

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 dictate 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][25][26] or floating materials^[27] 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][28][29]
• 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 using 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.^[30] The versatility and high performance of these new PEC arrangements has a wide scope of further exploitation and research ^{[citation needed]}.
• 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.^[31] 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][32][33][34][35]}

Solar reforming metrics

Solar reforming encompasses a range of technological processes and configurations, and because of this, suitable performance metrics can evaluate the commercial viability ^{[citation needed]}. 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][26] 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]

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

Advantages over conventional processes

Solar reforming offers several advantages over conventional methods of waste management or fuel/chemical production through less energy-intensive and low-carbon alternative 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 + ⁠1/2⁠O₂, ΔG° = 237 kJ mol⁻¹) because of a thermodynamic advantage which circumvents the energetically and kinetically demanding water oxidation half reaction (E⁰ = +1.23 V vs. reversible hydrogen electrode (RHE)) by energetically neutralizing 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 are safer than explosive mixtures of oxygen and hydrogen (from traditional water splitting), which 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, which simultaneously make solar reforming suitable for commercial applications.^[1]