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# Photo- and electrochemical processes to convert plastic waste into fuels and high-value chemicals

Xinxin Liang, Ximing Li, Qibing Dong, Ting Gao, Mengxin Cao, Ke Zhao,
Eric Lichtfouse, Antonio Otavio T Patrocinio, Chuanyi Wang

► To cite this version:

Xinxin Liang, Ximing Li, Qibing Dong, Ting Gao, Mengxin Cao, et al. Photo- and electrochemical processes to convert plastic waste into fuels and high-value chemicals. Chemical Engineering Journal, 2024, 482, pp.148827. 10.1016/j.cej.2024.148827. hal-04407701

HAL Id: hal-04407701

https://hal.science/hal-04407701v1

Submitted on 21 Jan 2024

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Review

Photo- and electrochemical processes to convert plastic waste into fuels and
high-value chemicals

Xinxin Liang<sup>a</sup>, Ximing Li<sup>a</sup>, Qibing Dong<sup>a</sup>, Ting Gao<sup>a</sup>, Mengxin Cao<sup>a</sup>, Ke Zhao<sup>a</sup>, Eric Lichtfouse<sup>b</sup>, Antonio Otavio T. Patrocinio<sup>c</sup>, Chuanyi Wang<sup>a,*</sup>

ARTICLE INFO

Keywords:
Electroreforming
H₂ evolution
Photoreforming
Plastic upcycling
Waste plastics

A B S T R A C T

The massive waste of plastics is resulting in serious environmental issues, yet plastics can be regarded as raw carbon materials to produce value-added chemicals by catalytic methods. In particular, recently-developed photocatalytic and electrocatalytic processes are promising for plastic conversion. Recent reviews about plastic recycling focus on traditional thermochemical methods, however, there have been few reported reviews on advanced photo- and electrochemical processes. Therefore, here we review clean catalytic technologies to convert plastics into fuels, materials and fine chemicals, with focus on photo-, electro-, photoelectrochemical catalysis and solar thermal electrochemical processes. The catalytic mechanisms, processes, conditions as well as pretreatment methods for various plastics upcylcing are discussed and summarized, challenges are explained, and directions for upgrading and transforming plastics in the future are proposed.

1. Introduction

Since the phenolic resin was first synthesized in 1907, many types of
plastics derived from fossil fuels have been widely applied in various
fields of human production and life, because of their low cost, high
stability, and processability [1,2]. More than 6000 Mt of plastic waste
have been worldly produced from 1950 to 2015, and the annual pro-
duction of plastic waste continues to increase at a rate of 4 % [3,4].
Plastic waste causes serious pollution issues [5,6]. For example, plastic
waste in soils affect the absorption of water and nutrients by crops and
lead to the reduction of the crop yield [7]. Moreover, nano- and
microplastics can integrate ecological cycles from water to plants and
animals, and thus end up in human bodies, damaging human health [8].
Therefore, plastics should be recycled as much as possible, yet plastic
recycling is actually very challenging [9].

Actually, the main methods of plastic disposal are landfilling,
incineration and secondary recycling (Fig. 1). Plastic landfilling is a
simple and low-cost technology, but this method requires a large
amount of land, changes the soil pH, and affects its sustainable use [10].
Incineration of waste plastics can generate electricity and heat [11],
however, toxic compounds such as NO₂, SO₂ and dioxins are produced.

Secondary recycling refers to the mechanical processing of waste plastics into products different from the original raw material. Compared with landfilling and incineration, secondary recycling is conducive to the management and resource utilization of waste plastics [12]. Nonetheless, current technologies of secondary recycling mainly convert waste plastics into raw polymeric materials of lower quality by shredding and grinding. For this reason, secondary recycling is called “downcycling”, and the secondary plastic produced will still ultimately end up in landfill or incineration [13]. Overall, the current treatments of waste plastic are unsustainable, thus calling for more circular technologies.

In recent years, catalytic upcycling of waste plastics has emerged as a
way to produce fuels or valuable chemicals, and also reduce environ-
mental pollution [14]. A Web of Science search with 'upcycling' and
'plastic' as keywords shows that research on plastic upcycling increased
exponentially from 2014 to 2023 (Fig. 2). The three main methods of
catalytic upcycling are photocatalysis, electrocatalysis and thermoca-
talysis (Fig. 1). Thermocatalysis is done by pyrolysis, gasification, and
hydrothermal treatments, which are conducted under high temperature
and pressure to break chemical bonds and produce higher-value prod-
ucts such as fuels, e.g. H₂ and CH₄, and chemicals such as ethylene and

<footnote><sup>a</sup> School of Environmental Science and Engineering, Shaanxi University of Science and Technology, 710021 Xi'an PR China</footnote>

<footnote><sup>b</sup> State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, 710049 Xi'an Shaanxi PR China</footnote>

<footnote><sup>c</sup> Laboratory of Photochemistry and Materials Science - LAFOT-CM, Universidade Federal de Uberlandia, 38400-902, Uberlandia, MG, Brazil</footnote>

<footnote>* Corresponding author.
E-mail address: wangchuanyi@sust.edu.cn (C. Wang).</footnote>
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terephthalic acid [15,16]. However, the use of expensive catalysts and large energy inputs in the process of thermocatalysis are considered as major obstacles to economic and environmental operation, especially in the context of reducing the utilization of fossil fuels. Besides the intensive energy demand, thermocatalysis has also the disadvantage of generating many by-products. Compared with thermocatalysis, photocatalysis and electrocatalysis appear to be more sustainable as these methods can be carried out under environmental conditions, employing sunlight or electricity produced from renewable sources [17,18]. Moreover, photocatalysis and electrocatalysis can activate and split chemical bonds more precisely, thus preserving other polymeric functional groups, and therefore, improving product selectivity [19,20]. Given this, Xie's group reviewed advanced plastic processing techniques, which are conducted under violent or mild conditions, including pyrolysis, hydropyrolysis, solvolysis, photocatalysis, electrocatalysis, and biocatalysis [6]. Zhao and co-workers summarized the investigation progress and critical design principles of photo-/electro-catalyst for the efficient conversion of plastics waste by breaking the chemical bonds selectively [21]. Combined processes, which are also driven by renewable energy such as photoelectrochemical (PEC) and solar thermal electrochemical process (STEP), have been studied in the catalytic field for decades as powerful tools [22–25], while few of reported reviews focus on PEC and STEP processes of plastics upcycling. Therefore, these processes, which appear promising for plastic upcycling, should be summarized in detail. In addition, the strategies of various raw plastic pretreatment are also significantly important, which are the prerequisite step of plastics conversion.

Herein, we review pretreatment methods for plastics upcycling, and

the state-of-the-art plastic upcycling strategies by photocatalysis, electrocatalysis, photoelectrochemical catalysis and solar thermal electrochemical process. These methods allow to convert plastics such as polyethylene terephthalate [26,27], polyethylene [28], poly(lactic acid) [29], polyurethane [26] and polypropylene [28] into fuels and high-value chemicals. In addition, the catalytic mechanisms and conditions for various plastics upcycling are discussed and summarized. Finally, we propose a short conclusion, challenges and future perspectives on more feasible strategies for the plastics upcycling. We hope that this review enables a profound understanding of the concept and conversion mechanism in plastic disposal and provides some reference value for the design of subsequent catalytic materials.

## 2. Plastic pretreatment

Plastics are polymers and thus commonly present the characteristics of insolubility and chemical inertness, which hinder the interaction between the catalysts and plastics, further limiting the catalytic activity in photo-/electrocatalytic processes [30,31]. Therefore, before plastics upcycling, the pretreatment processes of plastics are essential. Plastics waste can be divided into four main types, including polyolefins, polyesters, polyurethanes and polyamides, which are linked by C—C, C—O, N—C—O and C—N, respectively.

Polyethylene (PE) is typical polyolefin plastic, and the main treatment methods for PE waste are based on physical methods and hydrolysis. Physical methods, such as cutting and ball milling, increase the specific surface area of plastics and thus favor the contact of plastics and catalysts. Physical pretreatment methods for PE plastics are simple to

<figure><img src="image_2.png" /><figcaption>Fig. 1. Strategies to dispose waste plastics.</figcaption></figure>
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implement, exhibit high processing efficiency, and minimal environ-
mental impact, but they still face the challenge of plastics’ insolubility
and chemical inertness. The insolubility in water and chemical inertness
of PE results from the high dissociation energy of the C—C bond (~372
KJ/mol). Therefore, the conversion of PE pretreated by physical
methods should use the photocatalysts with high oxidation potential, e.
g., ZnO, Nb₂O₅, and BiVO₄. Given this, Xie and co-workers mixed pul-
verized commercial single-use bags or food wrap films with Nb₂O₅ for
promoting the conversion of real-world plastics into CH₃COOH [28].
Besides, the main chemical methods for PE depolymerization involve
elevated levels of acids and bases. For instance, Qiu’s group reported
that C—C bond of PE can be selectively broken to generate a range of
carboxylic acids at 180 °C, owing to the strong oxidizing property of
HNO₃. Afterwards, the generated acetic acid was oxidized selectively by
photogenerated holes to produce methane [32].

Polyethylene terephthalate (PET) and polyurethane (PUR) are linked by C—O and N—C—O bonds, respectively. Depolymerization of these polymers are achieved by nucleophilic attack of the carbonyl to generate monomers. Reisner reported PET can be broken at ester bonds and converted into terephthalate (TPA) and ethylene glycol (EG) monomers in alkaline solution [26,29,33]. Terephthalate can then be directly recycled as an industrial raw material. Ethylene glycol can also be converted into formate and glycolic acid through photocatalytic and photoelectrochemical conversion. Besides, PUR is mainly hydrolyzed to 2,6-diaminotoluene and propylene glycol, whereas the propylene glycol components are converted into formate and pyruvate by photocatalysis. 6-Nylon, a polyamide linked by C—N bonds, can be depolymerized using ionic liquids to give monomeric lactam at 300 °C with a high yield [34].

In conclusion, pretreatment of plastics is crucial in the process of
plastics upcycling. However, most methods involve the depolymeriza-
tion of plastics in strong alkali, strong acid conditions or organic sol-
vents, which will greatly increase the cost and may cause additional
environmental issue. Therefore, conversion of plastics by biological
methods is attracting more attention owing to their eco-friendly capacity
based on the decomposition ability of proteases. For instance, using a
modified PET protease, 94.9 % TPA was recycled with a purity of 97 % at
50 °C [35]. Accordingly, depolymerization reactions for polymers by
biological methods with high conversion efficiency and selectivity to
corresponding monomer products, presenting enormous potential for
converting plastics waste to value-added molecules. Afterwards, the
conversion of various plastics through photocatalysis, electrocatalysis,
photoelectrochemical catalysis and solar thermal electrochemical pro-
cess will be discussed in the following sections.

3. Photocatalysis

Heterogeneous photocatalytic systems consist basically of three
steps: first, the semiconductor material is photoexcited yielding elec-
trons and holes, which then migrate to the surface of the photocatalyst,
thus allowing redox reactions to occur [13,36]. There are two main
types of photocatalytic conversion of plastics: photodegradation and
photocatalytic upcycling.

The ultimate products of plastic photodegradation are CO2 and H2O
[9,37,38]. Nonetheless, in a typical photocatalytic upcycling process of
plastics waste, which are selectively converted into value-added chem-
icals, meanwhile, H2O is usually reduced into H2 by the photoelectrons
generated in photocatalysts [32,39]. Therefore, compared with photo-
degradation, photocatalytic upcycling of waste plastics both alleviates
plastic pollution, and has huge economic benefits. For example, research
in plastic upcycling has allowed to produce fuels, e.g. H2, chemicals
[40], e.g. formic acid, acetic acid, ethylene, and ethane, along other
materials [41,42] such as functionalized plastics by photocatalytic.
Table 1 presents reaction conditions for photocatalytic conversion of
polyethylene terephthalate, polyethylene, polylactic acid, poly-
propylene, polyvinyl chloride, polyethylene glycol, polyurethane and
polyacrylamide into H2, a clean fuel, along with a valuable oxidation
product.

3.1. *Photoreforming to produce hydrogen*

Photocatalytic water splitting to produce molecular hydrogen (H₂), has been explored for half a century [49,50]. Hydrogen is viewed as an ideal fuel because the only product of H₂ combustion is water, with few pollution [51–53]. The required potential energy for the production of H₂ by plastic photoreforming is nearly energy neutral and much lower than that for photocatalytic water splitting (1.6 eV) [54,55]. This is due to the lower potential needed for plastic oxidation in contrast to H₂O oxidation into O₂ [56,57]. Therefore, plastics can be regarded as sacrificial agents that are preferentially oxidized in the process of photocatalytic water splitting, thus reducing the integral potential needed for H₂ production [58,59].

In 1981, an early study by Kawai and Sakata has employed a plati-
nized TiO2 photocatalyst to reform PVC into H2 at ambient conditions
[46]. Although the catalyst activity was low for H2 generation, this study
provided inspiration for subsequent studies. Reisner et al. emphasized
that photoreforming requires four components: a photocatalyst, a sub-
strate, sunlight and water [26,29]. In 2018, they have used CdS/CdOx
quantum dots for photoreforming waste plastics to produce H2 under
visible light irradiation. Various plastics were tested, such as PET, PLA,

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Number of publications related to plastic upcycling, from a search in the Web of Science using the keywords 'upcycling' and 'plastic' from 2014 to 2023 on December 1, 2023.</figcaption></figure>
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<table><caption>**Table 1**
Conversion of plastics into fuels and chemicals.</caption><thead><tr><th>Plastics</th><th>Catalyst</th><th>Solvent</th><th>Reduction product</th><th>Activity/ µmol g<sup>-1</sup>h<sup>-1</sup></th><th>Oxidation product</th><th>Activity/ µmol g<sup>-1</sup>h<sup>-1</sup></th><th>Ref.</th></tr></thead><tbody><tr><td>Pre <sup>a)</sup>-PET<sup>b)</sup></td><td>CdS/CdO<sub>x</sub></td><td>10 M NaOH</td><td>H<sub>2</sub></td><td>3.42 × 10<sup>3</sup></td><td>formate</td><td>–</td><td>[26]</td></tr><tr><td>PET</td><td>Co-Ga<sub>2</sub>O<sub>3</sub></td><td>Deionized water</td><td>H<sub>2</sub></td><td>0.384 × 10<sup>3</sup></td><td>CO</td><td>100.6</td><td>[43]</td></tr><tr><td>Pre-PET</td><td>CN<sub>x</sub>|Ni<sub>2</sub>P</td><td>1 M KOH</td><td>H<sub>2</sub></td><td>25.7</td><td>formate</td><td>–</td><td>[29]</td></tr><tr><td>Pre-PET</td><td>Ag<sub>2</sub>O/Fe-MOF <sup>c)</sup></td><td>Deionized water</td><td>H<sub>2</sub></td><td>1.9 × 10<sup>3</sup></td><td>–</td><td>–</td><td>[44]</td></tr><tr><td>Pre-PET</td><td>MXene/ Zn<sub>x</sub>Cd<sub>1-x</sub>S <sup>d)</sup></td><td>10 M NaOH</td><td>H<sub>2</sub></td><td>14.17 × 10<sup>3</sup></td><td>formate</td><td>–</td><td>[45]</td></tr><tr><td>Pre-PET</td><td>MoS<sub>2</sub>/CdS</td><td>10 M KOH</td><td>H<sub>2</sub></td><td>3.90 × 10<sup>3</sup></td><td>formate</td><td>5.96 × 10<sup>3</sup></td><td>[32]</td></tr><tr><td>PE <sup>e)</sup></td><td>Co-Ga<sub>2</sub>O<sub>3</sub></td><td>Deionized water</td><td>H<sub>2</sub></td><td>0.647 × 10<sup>3</sup></td><td>CO</td><td>158.3</td><td>[43]</td></tr><tr><td>Pre-PE</td><td>Ag<sub>2</sub>O/Fe-MOF</td><td>Deionized water</td><td>H<sub>2</sub></td><td>1.7 × 10<sup>3</sup></td><td>–</td><td>–</td><td>[44]</td></tr><tr><td>Pre-PE</td><td>Pt/TiO<sub>2</sub></td><td>5 M NaOH</td><td>H<sub>2</sub></td><td>31</td><td>–</td><td>–</td><td>[46]</td></tr><tr><td>Pre-PE</td><td>MoS<sub>2</sub>/CdS</td><td>6 wt% HNO<sub>3</sub></td><td>H<sub>2</sub></td><td>1.13 × 10<sup>3</sup></td><td>CH<sub>4</sub></td><td>113.2</td><td>[32]</td></tr><tr><td>PE</td><td>Nb<sub>2</sub>O<sub>5</sub></td><td>Deionized water</td><td>CH<sub>3</sub>COOH</td><td>0.79 × 10<sup>3</sup></td><td>CO<sub>2</sub></td><td>–</td><td>[28]</td></tr><tr><td>Pre-PE</td><td>P25|Pt</td><td>6 wt% HNO<sub>3</sub></td><td>H<sub>2</sub></td><td>66</td><td>C<sub>2</sub>H<sub>6</sub></td><td>0.26</td><td>[47]</td></tr><tr><td>PE</td><td>VPOM/CNNS <sup>f)</sup></td><td>CH<sub>3</sub>CN</td><td>–</td><td>–</td><td>HCOOH</td><td>24.66</td><td>[48]</td></tr><tr><td>Pre-PLA <sup>g)</sup></td><td>CdS/CdO<sub>x</sub></td><td>10 M NaOH</td><td>H<sub>2</sub></td><td>64.3 × 10<sup>3</sup></td><td>pyruvate</td><td>–</td><td>[26]</td></tr><tr><td>Pre-PLA</td><td>CN<sub>x</sub>|Ni<sub>2</sub>P</td><td>1 M KOH</td><td>H<sub>2</sub></td><td>55.7</td><td>CO<sub>3</sub><sup>2-</sup></td><td>–</td><td>[29]</td></tr><tr><td>Pre-PLA</td><td>MoS<sub>2</sub>/CdS</td><td>10 M KOH</td><td>H<sub>2</sub></td><td>6.68 × 10<sup>3</sup></td><td>formate</td><td>5.37 × 10<sup>3</sup></td><td>[32]</td></tr><tr><td>PP <sup>h)</sup></td><td>Co-Ga<sub>2</sub>O<sub>3</sub></td><td>Deionized water</td><td>H<sub>2</sub></td><td>64</td><td>CO</td><td>147.2</td><td>[43]</td></tr><tr><td>PP</td><td>Nb<sub>2</sub>O<sub>5</sub></td><td>Deionized water</td><td>CH<sub>3</sub>COOH</td><td>0.68 × 10<sup>3</sup></td><td>CO<sub>2</sub></td><td>–</td><td>[28]</td></tr><tr><td>PP</td><td>VPOM/CNNS</td><td>CH<sub>3</sub>CN</td><td>–</td><td>–</td><td>HCOOH</td><td>26.68</td><td>[48]</td></tr><tr><td>Pre-PVC <sup>i)</sup></td><td>Pt/TiO<sub>2</sub></td><td>5 M NaOH</td><td>H<sub>2</sub></td><td>15</td><td>–</td><td>–</td><td>[46]</td></tr><tr><td>PVC</td><td>Nb<sub>2</sub>O<sub>5</sub></td><td>Deionized water</td><td>CH<sub>3</sub>COOH</td><td>0.66 × 10<sup>3</sup></td><td>CO<sub>2</sub></td><td>–</td><td>[28]</td></tr><tr><td>PVC</td><td>VPOM/CNNS</td><td>CH<sub>3</sub>CN</td><td>–</td><td>–</td><td>HCOOH</td><td>29.85</td><td>[48]</td></tr><tr><td>PEG <sup>j)</sup></td><td>Ag<sub>2</sub>O/Fe-MOF</td><td>Deionized water</td><td>H<sub>2</sub></td><td>2.48 × 10<sup>3</sup></td><td>HCOOH</td><td>–</td><td>[39]</td></tr><tr><td>PEG</td><td>VPOM/CNNS</td><td>CH<sub>3</sub>CN</td><td>–</td><td>–</td><td>HCOOH</td><td>208.65</td><td>[48]</td></tr><tr><td>Pre-PUR <sup>k)</sup></td><td>CdS/CdO<sub>x</sub></td><td>10 M NaOH</td><td>H<sub>2</sub></td><td>0.85 × 10<sup>3</sup></td><td>formate</td><td>–</td><td>[26]</td></tr><tr><td>PAM <sup>l)</sup></td><td>VPOM/CNNS</td><td>H<sub>2</sub>O</td><td>–</td><td>–</td><td>HCOOH</td><td>156.57</td><td>[48]</td></tr></tbody></table>

a) Pre: Pretreated.

b) PET: polyethylene terephthalate.

c) MOF: metal–organic-framework.

d) M represents transition metal, X represents C/N element.

e) PE: polyethylene.

f) VPOM: H<sub>5</sub>PMo<sub>10</sub>V<sub>2</sub>O<sub>40</sub>; CNNS: g-C<sub>3</sub>N<sub>4</sub> nanosheets.

g) PLA: polylactic acid.

h) PP: polypropylene.

i) PVC: polyvinyl chloride.

j) PEG: polyethylene glycol.

k) PUR: polyurethane.

l) PAM: polyacrylamide.

PUR, PE and PVC. PLA, PET and PUR exhibited high activities with H<sub>2</sub> generation of 64.3, 3.42 and 0.85 mmol g<sub>CDs</sub>h<sup>-1</sup> in 10 M aq. NaOH, respectively. In addition, the monomers formed by alkaline hydrolysis of plastics were oxidized into high-value chemicals such as pyruvate, formate, ethanol, acetate and lactate. In the experiment of a real-world PET water bottle, the activity towards H<sub>2</sub> generation was 4.13 mmol g<sub>CDs</sub>h<sup>-1</sup> with a rate of conversion of 5.15 % and an external quantum yield of 2.17 % in 6 days. In this work, none of the polymers gave a conversion rate higher than 40 %, but this represented a proof-of-concept system to recycle waste plastics.

Because of the toxicity of Cd<sup>II</sup> ions, the same research group designed a CN<sub>x</sub>|Ni<sub>2</sub>P co-catalyst [29], made with cyanamide-functionalized carbon nitride (CN<sub>x</sub>) and nickel phosphide (Ni<sub>2</sub>P) for the photoreforming of plastics (Fig. 3a). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed Ni<sub>2</sub>P nanoparticles with a diameter of 9.4 ± 0.6 nm distributed across the CN<sub>x</sub> surface (Fig. 3b). This allowed higher electron-hole separation due to a shift of electron density from CN<sub>x</sub> to Ni<sub>2</sub>P in CN<sub>x</sub>|Ni<sub>2</sub>P. Therefore, CN<sub>x</sub>|Ni<sub>2</sub>P showed high photoreforming efficiency. For example, H<sub>2</sub> evolution varied from 82.5 ± 7.3 and 178 ± 12 µmol g<sup>-1</sup> after 50 h of photoreforming of PET and PLA, respectively (Fig. 3c). Here, H<sub>2</sub> was derived from water molecules rather than from plastics as evidenced by the isotope labeling. The <sup>1</sup>H NMR spectra proved ethylene glycol (EG) and lactate were hydrolysates of PET and PLA, which were mainly converted into formate and CO<sub>3</sub><sup>2-</sup>, respectively. (Fig. 3d-f). Moreover, the practical efficacy was evaluated through long-term photoreforming of polyester microfibers, a PET bottle, and oil-contaminated PET. SEM showed developed cracks and pits on the microfibers. After 5 days of illumination, H<sub>2</sub> production was 104,

22, and 11.4 µmol g<sup>-1</sup> for polyester microfibers, PET bottles, and oil-contaminated PET, respectively. For future large-scale applications, they upscaled the photoreforming device from 2 to 120 mL. When the conditions were optimized, the biggest setup exhibited a H<sub>2</sub> production of 0.53 µmol cm<sup>-2</sup>, which was higher than that for the smallest setup (0.26 ± 0.03 µmol cm<sup>-2</sup>). That is to say, a 15 m<sup>2</sup> reactor would be enough to power a smartphone.

To stimulate the maximum activity of the photocatalyst and prevent clumping, Reisner's group designed a photocatalyst panel [60], which could attach the CN<sub>x</sub>|Ni<sub>2</sub>P power to frosted glass (Fig. 3g). The yield of H<sub>2</sub> was positively correlated with the amount of catalyst at low load, and the yield tended to be flat at high load (Fig. 3h). Interestingly, the photocatalytic panel showed higher H<sub>2</sub> production activity when light irradiated from the back side, which is explained by an inner filter effect promoted by the solution in case of the front side irradiation [60]. Finally, the photocatalytic panel along with CN<sub>x</sub>|Ni<sub>2</sub>P could convert high concentrations of PET-colored solutions in the flow photoreforming system (Fig. 3i). This work overcomes the use of toxic materials, and analyzes the photoreforming process of PET and PLA, systematically. Moreover, the development of photocatalytic panel made photoreforming of plastic coloring solution possible, laying a foundation for the practical application of subsequent plastic upcycling.

Recently, metal organic frameworks (MOF) have been widely explored in photocatalysis, because of their large surface area and excellent light absorption properties [61]. However, the low charge separation efficiency limits the application of MOF materials. For this reason, Zhang et al. constructed a Ag<sub>2</sub>O/Fe-MOF heterojunction to enhance the separation efficiency during photocatalytic conversion of
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<figure><img src="image_4.png" /><figcaption>Fig. 3. (a) Photoreforming plastic and (b)TEM image of the CNₓ|Ni₂P (2 wt%). (c) The stability experiment of photoreforming PET and PLA in 1 M KOH under simulated sunlight. ¹H NMR spectra of photoreforming PET (d) and (e) PLA. (f) Chemical structures and peak assignments. Reproduced with permission [29]. Copyright 2019, American Chemical Society. (g) Photoreforming upscaled panel for 25 cm⁻² loading with CNₓ|Ni₂P. (h) The H₂ activity in different catalyst loading. (i) Schematic diagram of expanded photoreforming device in the flow phase. Reproduced with permission [60]. Copyright 2020, John Wiley and Sons.</figcaption></figure>

microplastics (PEG, PET, PE) into valuable chemicals and H₂ (Fig. 4a and b) [44]. The H₂ production of pure Ag₂O, Fe-MOF, Ag₂O/Fe-MOF, Ag₂O@Fe-MOF (physical mixed) and Ag₂O-Fe-MOF was evaluated by the photocatalytic conversion of PEG and Ag₂O/Fe-MOF (0.2 wt%) presented the highest H₂ production of 6.2 mmol g⁻¹, and 27.5 mg of PEG microplastics were converted in 3 h (Fig. 4c and 4d). Mechanistic investigation showed that when Ag₂O and Fe-MOF were in close contact, the Fermi level of Ag₂O increased and the Fermi level of Fe-MOF decreased until the Fermi levels of Ag₂O and Fe-MOF converge. This led to the overall shift of Ag₂O energy band and the formation of p-n junction along with an internal electric field. Under irradiation, the electrons of Ag₂O and Fe-MOF were excited to their respective

conduction bands, and the electrons of the Ag₂O conduction band migrated to the conduction band of Fe-MOF. At the same time, the holes of Fe-MOF were transferred to the valence band of Ag₂O in the action of the internal electric field. Finally, electrons accumulated in the conduction band of Fe-MOF to reduce water into H₂, and holes accumulated in the valence band of Ag₂O to oxidize water to •OH radicals and further oxidize microplastics to short-chain organic molecules (Fig. 4e). This work constructs a Ag₂O/Fe-MOF heterojunction through a novel in-situ synthesis method, enables Ag₂O nanometer-sized particles were confined into the framework of MOFs with uniform size ~ 6 nm, provides more active sites for photoreforming of plastics.

Syngas is made of CO and H₂, which are important industrial
<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) Ag₂O/Fe-MOF photoreforming of various plastics and (b) in situ synthesis processes of Ag₂O/Fe-MOF. (c) The H₂ production and (d) weight loss in the process of photoreforming polyethylene glycol (PEG) under simulated sunlight with different catalysts. (e) The reaction mechanism of Ag₂O/Fe-MOF photoreforming microplastics [44]. Copyright 2022, Elsevier.</figcaption></figure>

chemicals since they could act as feedstocks for synthesizing hydrocarbon fuels [62,63]. Xie et al. put forward utilizing Co-Ga₂O₃ nanosheet photocatalysis to convert waste plastics (PE, PP and PVC) into H₂ and CO under ambient conditions (Fig. 5a). The electrons and holes of Ga₂O₃ have strong redox performance, due to its negative conduction (−1.62 V vs. NHE) and positive valence band potential (+3.16 V vs. NHE), so that the photogenerated carriers of Ga₂O₃ could satisfy the thermodynamic

demand for promoting most of the redox reactions of interest [43,64]. Compared with pure Ga₂O₃, Co-Ga₂O₃ had more efficient photocarrier separation efficiency (Fig. 5b), stronger light absorption (Fig. 5c) and stronger adsorption performance for CO₂, which were conducive to the reduction process of carbon dioxide. Thus, Co-Ga₂O₃ nanosheets showed higher photoconversion activity of PE, PP and PVC. For example, the activities of Co-Ga₂O₃ converted PE plastic bags into H₂, CO and CO₂

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) Photoconversion of various plastics into syngas. (b) PL spectra and (c) UV-vis diffuse reflectance spectra for the Ga₂O₃ nanosheets and the Co-Ga₂O₃ nanosheets. (d) The activity of H₂, CO, CO₂ in photoreforming commercial PE plastic bags in 100 mL deionized water. The Gibbs free energies (ΔG) of Co-Ga₂O₃ and the Ga₂O₃ nanosheets (e) photocatalytic CO₂ to CO and (f) H₂O to H₂ [43]. Copyright 2022, Oxford University Press.</figcaption></figure>
were 647.8, 158.3 and 419.3 µmol g⁻¹h⁻¹ (Fig. 5d), individually. Meanwhile, the activities of pure Ga₂O₃ were 404.9, 83.3 and 262.1 µmol g⁻¹h⁻¹. Moreover, the total reaction could be divided into three steps:

- Step 1: H₂O was split into H₂ and O₂ by part of the photogenerated charge carriers.

- Step 2: The plastic was degraded into CO₂, while O₂ was gradually reduced into O₂⁻, H₂O₂ and H₂O.

- Step 3: CO₂ was reduced to •COOH intermediates and further to CO, while H₂O was oxidized to O₂.

Among these three steps, the formation of •COOH and H* intermediates are the rate-limiting steps and both of the energy barriers of Co-Ga₂O₃ to form •COOH intermediates and H* were lower than that for pristine Ga₂O₃ by calculating the Gibbs free energy of the reaction (Fig. 5e and 5f). Therefore, Co-Ga₂O₃ exhibited higher activity than Ga₂O₃ during the photocatalytic conversion of PE. Although the conversion efficiency of PE in this work is still far from practical application, the photoreforming of plastics to produce syngas in water solution under environmental conditions is successfully proven. Moreover, the rate limiting step of the process is obtained through a mechanism exploration, which provides reference and guidance for the design of high-efficient catalysts in the future.

## 3.2. Photocatalytic conversion of plastics into carbon-containing chemicals

Most plastics are derived from petroleum-based hydrocarbons [65,66]. Therefore high-value chemicals can be easily produced by cracking waste plastic [67]. Photocatalytic conversion of waste plastics into chemical monomers not only reduces environmental pollution but also can alleviate the demand for fossil fuels [68]. Compared to photo-reforming of waste plastics for H₂ production, photocatalytic conversion of waste plastics to high-value chemicals mainly depends on the oxidation processes of substrates [69]. The main barrier is the high stability of the C—C bonds in the polymer, which typically leads to slow

kinetics and the low selectivity of the target product [70]. These problems can be overcome by selecting the appropriate band structure of the photocatalyst or adjusting the band position to obtain suitable redox potentials [71].

Formic acid is a common chemical, utilized as a disinfectant and preservative, as well as in fuel cells [72,73]. Li and co-workers reported a VPOM/CNNS Z-scheme, which was consisted of H₅PMo₁₀V₂O₄₀ (VPOM) and ultrathin g-C₃N₄ nanosheets (CNNS) through electrostatic interaction for photocatalytic conversion of plastics (PE, PP, PVC) into formic acid (HCOOH) under visible light conditions [48]. The morphology of VPOM/CNNS was still a curved sheet-like structure, after the introduction of VPOM, and it could be observed by aberration-corrected high-angle annular dark field TEM (AC-HAADF-STEM). VPOM nanoparticles were evenly distributed on the surface of CNNS to form a 0D/2D structure (Fig. 6a). In situ electron spin resonance (ESR) measurements confirmed that under the action of the internal electric field of VOPM/NNS, photogenerated electrons and holes were accumulated in the conduction band of CNNS and the valence band of VPOM, further reacted with O₂ and H₂O to produce O₂⁻ and •OH, respectively (Fig. 6b and 6c). The valence band (E<sub>VB</sub>) and conduction band (E<sub>CB</sub>) potentials of CNNS and VPOM were 1.92, -1.04 V and 3.04, 0.85 V, respectively. Therefore, electrons could be transferred spontaneously between the interface of CNNS and VPOM to form the internal electric field of CNNS pointing towards VPOM (Fig. 6d and 6e). Consequently, VPOM/CNNS exhibited higher electron-hole separation efficiency and higher photocatalytic activity and the optimal VPOM/CNNS-15 sample showed the activity for HCOOH production was 24.66 µmol h⁻¹ g⁻¹ towards PE conversion. Moreover, the reaction mechanism was proposed, the C—C bond of PE was oxidized cleavage to form formaldehyde and alkyl radical. The alkyl radical was further oxidized into alkyl peroxide groups, long-chain alcohol and then participated in the next reaction of C—C bond cleavage. On the other hand, the aldehyde was oxidized into formic acid by O₂⁻ (Fig. 6f). Briefly, this work realizes the high-efficient photocatalytic upcycling of plastic through constructing Z-scheme which exhibits efficient spatial separation and strong redox ability of photoexcited charge carriers, and expands the roads of Z-

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (a) Aberration-corrected high-angle annular dark field TEM image of VPOM/CNNS-15 material. In situ ESR signal of (b) O₂⁻ and (c) •OH of materials after 5 min of light. (d) VPOM/CNNS Z-scheme heterojunction after contact. (e) Diagram of photocatalytic upcycling waste plastic and (f) the reaction mechanism schematic of photocatalytic oxidative upcycle PE to formic over VPOM/CNNS Z-scheme heterojunction [48]. Copyright 2022, Elsevier.</figcaption></figure>
scheme heterojunction in upcycling of plastic wastes.

Morphology is an important characteristic of photocatalysts because an appropriated morphology is beneficial to improve the separation efficiency of photogeneration carriers [74–76]. Qiu’s group [32] realized the high-efficiency conversion of PET, PLA and PE to chemicals by MoS₂/CdS composite catalyst. MoS₂ covered one end of CdS nanorods to form an electron accumulation area and the other end formed a hole accumulation area (Fig. 7a). So that the electron-hole separation efficiency of MoS₂/CdS was improved. The anisotropic morphology of MoS₂/CdS was observed by SEM, while Pt nanoparticles and MnOₓ nanosheets were selectively deposited on the end of CdS, which was covered by MoS₂, and the other end (Fig. 7c and 7d). The H₂ activity was 6.68 ± 0.10 mmol g⁻¹h⁻¹ and 3.90 ± 0.07 mmol g⁻¹h⁻¹ in the PLA and PET hydrolysate (10 M KOH) with the optimal load capacity of 21.8 wt %.

On the other hand, the oxidation half-reactions over MoS₂/CdS of PLA and PET were the conversion of the hydrolyzed monomers into formate and exhibited 5.37 ± 0.67 mmol/L and 5.96 ± 0.02 mmol/L in 5 h of photoreforming, respectively. Different from PLA and PET, PE was hydrolyzed to succinic acid and glutaric acid, in 6 wt% HNO₃ solution at 180 °C for 5 h (Fig. 7d). Moreover, CH₄ was generated by photo-oxidation decarboxylation of CH₃COOH achieved 196.2 µmol g⁻¹h⁻¹, meanwhile, ethane, propane and n-pentane were also produced due to the generation of carbon-centered radical species in the process of photoreforming PE. In this paper, composite photocatalysts are covered for the first time from the perspective of charge separation behavior for the upcycling of plastics, further paving the way for further study on plastic upcycling.

Reisner et al. also converted PE hydrolysate into alkane chemicals through photocatalysis [47]. The whole reaction could be divided into two steps. The first step was the hydrolysis of PE to produce 44 % succinic acid, 22 % glutaric acid, 22 % acetic acid, 9 % lipid and 4 % propionic acid in 6 % HNO₃ aqueous solution at 180 °C for 4 h. The second step was photocatalytic decarboxylation of succinic acid by hole to form propanoic acid intermediates which further react with Hads/H₂ to produce ethane and CO₂. Similarly, glutaric acid was converted to propane. When pure succinic acid as substrate, P25|Pt and NCN CNx|Pt on

the activity of ethane were 56.3 and 7.2 µmol g⁻¹h⁻¹ (Fig. 7e), respectively. Besides, the PE hydrolysate solution was also tested with a P25|Pt catalyst and the products were ethane (0.25), ethylene (0.02), propane (0.14) and propylene (0.007 mmol g⁻¹). Moreover, they investigated the activity of a photocatalyst in a flow phase using a photocatalyst panel, which was fixed to a ground glass surface and irradiated on the back of the photocatalyst panel to reduce the absorption of light by the substrate solution. The NCN CNx|Pt exhibited higher activity for ethane (77.9 µmol m⁻²), ethylene (69.2 µmol m⁻²), propane (40.7 µmol m⁻²) and propylene (19.1 µmol m⁻²). This work provides a breakthrough to closed-loop recycling of plastic through the conversion of plastic into gaseous alkane, which can be separated from the reaction solution, readily.

CH₃COOH is also a commonly used chemical product [77–79]. Xie et al. transformed PE, PP and PVC into CH₃COOH by a two-step method involving photocatalytic C—C bond cleavage and photoinduced C—C coupling, under simulated natural environmental conditions (Fig. 8 a) [28]. Because of the high stability and suitable band position Nb₂O₅ was selected as a photocatalyst, with the valence band maximum and conduction band minimum at + 2.5 V and at – 0.9 V (vs. NHE, pH = 7), respectively [80,81]. That is meaning the redox ability of photo-generated carriers of Nb₂O₅ can meet the potential demand for the conversion of CO₂ to CH₃COOH and the generation of ●OH in water (Fig. 8 b). Taking PE as an example to explore its reaction mechanism, AgNO₃ was added into the reaction systems, then only CO₂ could be detected. It could infer that the C—C bond was oxidized to form CO₂, meanwhile, CH₃COOH and CO stemmed from the photoreduction processes. Moreover, ●OH and O₂⁻ radicals exhibited strong electron spin resonance (ESR) signals (Fig. 8c and 8d). On the other hand, a characteristic peak of H₂O₂ at 436 nm was observed by UV–vis absorption spectrum, indicating that photogenerated electrons of Nb₂O₅ could gradually reduce O₂ into O₂⁻, H₂O₂ and H₂O. Furthermore, in situ FTIR spectroscopy demonstrated C=O and COOH were formed by the reduction of CO₂. On the whole, photocatalytic converted PE into CH₃COOH could be divided into two Steps (Fig. 8e):

-Step 1: PE was converted into CO₂ by ●OH oxidation to break the C—C bond; in the meantime, O₂ was reduced by photogenerated electrons to O₂⁻, H₂O₂ intermediates, and finally H₂O was formed.

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** (a) MoS₂/CdS photoreforming plastics. The investigation of the redox sites of MoS₂/CdS by selective light deposition of (b) MnOₓ and (c) Pt. (d) Schematic diagram of PE hydrolysis process in HNO₃ solution [32]. Copyright 2022, American Chemical Society. The different products yield of (e) photocatalytic convert succinic acid and PE decomposition solution using a flow device over P25|Pt and NCN CNx|Pt [47]. Copyright 2021, American Chemical Society.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** (a) Photocatalytic conversion of plastics into C₂ chemicals by C—C bond cleavage and C—C coupling. (b) The band position of Nb₂O₅ and redox relative energy levels of CO₂, H₂O and O₂. ESR signal of (c) ●OH and (d) O₂· during photoconversion PE process in pure water and methanol/water mixed solution. (e) The reaction mechanism of photoconversion PE process for C—C bond cleavage and coupling [28]. Copyright 2019, John Wiley and Sons.</figcaption></figure>

### -Step 2: Two ●COOH intermediates reduced from CO₂ were photo-induced to form HOOC–COOH and were further reduced into CH₃COOH.

Pure PE, PP and PVC were treated in a simulated natural environment the total degradation within 40, 60, and 90 h, and the average activities of CH₃COOH in the degradation process were 47.4, 40.6, and 39.5 µg g⁻¹h⁻¹, respectively. It should be noted that when CO₂ was passed directly into the solution as a substrate, the efficiency of photo-conversion of PE, PP and PVC to CH₃COOH is similar to it, further implying that CH₃COOH was reduced from CO₂ by photocatalysis. Compared with other studies of plastics waste upcycling, this two-step method allows the coupling of oxidation and reduction to produce C₂ chemicals in water solution without further disposal of plastics and gives a new approach to upgrading and utilizing plastics.

Given unstable adsorption of the PE monomer, Xie and coworkers designed a photocatalyst exhibited charge-asymmetrical dual sites by doping Zr atom over CoFe₂O₄ [82]. The in situ FTIR spectra confirmed the formation of CH₃COOH was originated from the degradation of PE, directly. DFT calculations proved the charge-asymmetrical dual sites (Zr–Fe) were constructed by doping Zr atom which exhibited low-electro-negativity over CoFe₂O₄. As a result, the optimized showed the CH₃COOH evolution rate of 1.10 mmol g⁻¹h⁻¹ with the help of 3 % H₂O₂ which could promote the generation of ●OH radicals. This work constructs charge-asymmetrical dual sites photocatalyst through doping different negativity atoms, realizes the conversion of PE into CH₃COOH with high formation rate and 100 % selectivity under normal temperature and pressure.

### 3.3. Photocatalytic functionalization of plastics

In addition to the depolymerization of plastics into small molecular monomers, the post-polymerization functionalization captured the researcher's attention [83–85]. The advantage of the functionalization of plastics is that the process does not require the breaking of the polymer backbone chain, and the installation of functional groups directly onto the polymer long chain molecules gives the polymer unique properties

[86].

Polystyrene (PS) is difficult to dispose of, due to its strong chemical stability [88–90]. Leibfarth [87] reported a method for the C—H functionalization of PS by the use of their innate reactivity with electrophilic radical intermediates. The fluoroalkylation of PS could be divided into two steps in which, the first was the electronic reduction of Ru(bpy)₃Cl₂ trifluoroacetic anhydride into electrophilic trifluoroalkyl radicals, pyridine and CO₂. The second step, trifluoroalkyl radicals would be installed on the aromatic rings of PS by replacing H (Fig. 9a). In optimal, trifluoromethylation of PS reached 32 ± 2 mol %, and by changing the stoichiometric ratio of pyridine-N-oxide to trifluoroacetic anhydride (TFAA) the degree of trifluoromethylation of PS can be adjusted while the molecular weight distribution (MWD) of original PS was preserved (Fig. 9b). Besides, the contact angle between pure PS and water was 94° and the highest was 111° after fluoridation (Fig. 9c), indicating that fluoridation of PS increased its hydrophobicity, and fluoroalkylation could be applied to other aromatic-containing polymers like polyesters and polycarbonates.

However, the content of ruthenium (Ru) found in the crust is only one billionth of the total mass, therefore it is an extremely precious metal element. Therefore, the utilization of Ru-based catalysts is not economically viable [91,92]. Leibfarth and co-workers took advantage of organic photocatalysts namely phenoxazine, phenothiazine, and phenazine instead of Ru(bpy)₃Cl₂ to achieve the fluoroalkylation of aromatic-containing polymers under environmental conditions (Fig. 9d) [42]. Phenoxazines and phenothiazine showed a high proportion of fluoroalkylation (20 % and 19 %), but both of them accompanied by chain-coupling side reactions resulted in the MWD of the products was changed. Meanwhile, phenazine photocatalysts not only achieved 18 % fluoroalkylation but also did not affect the MWD of the product when the reactant concentration was 0.4 M and the catalyst load was 1 mol % (Fig. 9e and 9f). Moreover, by adjusting the stoichiometric ratio of pyridine N-oxide and TFAA, the functionalization of the polymer reached 33 mol %. C-H fluoroalkylation could also be achieved by organic photocatalysis of post-consumption and post-industrial PS waste, and this method could be used for the fluoroalkylation of
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** (a) Schematic diagram of photocatalytic trifluoromethylation of PS over Ru(bpy)₃Cl₂. (b) The trifluoromethylation of PS in different proportions of pyridine N-oxide and trifluoroacetic anhydride. (c) Contact Angle between water and PS at different degrees of trifluoromethylation [87]. Copyright 2019, The Royal Society of Chemistry. (d) Phenazine catalysts were used in the study. GPC traces from the trifluoromethylation of different (e) catalyst loading and (f) concentrations of reactants [42]. Copyright 2020, The Royal Society of Chemistry.</figcaption></figure>

polyester, proved the applicability of C–H in various plastics. This work demonstrates the value of the methods developed and the structural-reactivity relationships identified in facilitating the continued adoption of C–H functionalization methodologies that improve the properties of commercially available polymers.

# 4. Electrocatalytic

## 4.1. Electrocatalytic for plastics upcycling

Although electric energy is secondary energy, with the gradual maturity of clean energy generation technology, electric energy is also considered to be sustainable energy [93–96]. Thus, electrocatalysis technology has attracted extensive research [97,98], such as electro-catalytic water splitting and the reduction of CO₂ into CO, HCOOH, CH₃OH, etc [99,100]. Recently, more reports focus on electrocatalytic upcycling of plastics waste and exhibited great potential for the industrial application. In a typical electrocatalytic upcycling process of plastics waste, an external bias is applied to facilitate the redox reaction, allowing access to the reaction pathways. Generally, the oxidation of plastics waste and the reduction reaction of water occur at anode and cathode electrodes, which are connected by an external electric circuit and an electrolyte in-between. In view of this, electrocatalytic technology exhibits some obvious advantages for the conversion of plastic waste, including a controllable electrode potential operation process, recyclable supporting electrolytes to reduce overall chemical consumption. Therefore, the utilization of electrocatalytic technologies to upcycle the plastics waste is promising [101–104]. The products

obtained by electrocatalysis can be divided into solid carbonaceous materials and value-added chemicals. In this part, we would focus on assessing the study about value-added chemicals, because in the formation of solid carbonaceous materials pyrolysis plays a major role, which is not in line with the concept of clean energy.

Table 2 shows some of the previous examples of the electrocatalytic upcycling of plastics. In general, the anode material is optimized first, because the upgrading and conversion of plastics are mainly oxidation reactions [110]. Therefore, Chen et al. constructed two-electrode electrolysis cell using a Ni foam (NF) catalyst modified by Pd as an anode, and NF as a cathode [107]. The PET bottle could be oxidized to carbonate and terephthalate at the anode, meanwhile, the water was reduced to H₂ at the cathode in 1 M KOH solution. The Pd/NF-10 showed network structure and exhibited better electrochemical performance because Pd/NF-10 exposed more catalytically active sites than others. The electro-reforming experiment was conducted in the optimum condition, all the PET was converted into terephthalate and carbonate after 20 h and the pathway of reaction was proposed for EG oxidation. The yield of terephthalate reached up to 99 % and the faradaic efficiency of carbonate reached 93 %. Moreover, the stability also was tested, and after 4 cycles it retained high selectivity and FE. Besides, the real-world PET bottle was also converted into terephthalate and carbonate with high selectivity and faradaic efficiency (FE). At the same time, H₂ was generated at the cathode with high FE reached 98 %. Although this work can achieve a complete conversion of PET, the added-value of carbonate, one of the products, is not enough.

Compared with carbonate, the added value of formic acid is higher due to it is one of the basic organic chemical raw materials widely used
<table><caption>**Table 2**
Examples of electrocatalytic upcycling of waste plastics.</caption><thead><tr><th>Plastic type</th><th>Electrolyte</th><th>Applied potential</th><th>Anode/Cathode</th><th>Anode product</th><th>Faradaic efficiency</th><th>Cathode product</th><th>Faradaic efficiency</th><th>Ref.</th></tr></thead><tbody><tr><td><sup>a)</sup>Pre-PET</td><td>0.1 M KOH</td><td>1.45 V</td><td>CuO/Pt</td><td>formate</td><td>88 %</td><td>H<sub>2</sub></td><td>–</td><td>[105]</td></tr><tr><td>Pre-PET</td><td>1 M NaOH/1 M NaHCO<sub>3</sub></td><td>1.9 V</td><td>NiCo<sub>2</sub>O<sub>4</sub>/SnO<sub>2</sub></td><td>formate</td><td>85 %</td><td>formate</td><td>70 %</td><td>[106]</td></tr><tr><td>Pre-PET</td><td>1 M KOH</td><td>0.7 V</td><td>Pd-NF/NF<sup>a)</sup></td><td>CO<sub>3</sub><sup>2-</sup></td><td>91 %</td><td>H<sub>2</sub></td><td>98 %</td><td>[107]</td></tr><tr><td>Pre-PET</td><td>2 M KOH</td><td>1.5 V</td><td>CoNi<sub>0.25</sub>P/CoNi<sub>0.25</sub>P</td><td>formate</td><td>96.7 %</td><td>H<sub>2</sub></td><td>–</td><td>[108]</td></tr><tr><td>Pre-PET</td><td>1 M KOH</td><td>1.2 V</td><td>Pd NTs/LC-CoOOH</td><td>glycolic acid</td><td>81.17 %</td><td>NH<sub>3</sub></td><td>94.47</td><td>[109]</td></tr><tr><td>Pre-PE</td><td>NaOH pH = 10</td><td>5 V</td><td>Carbon Paper/Pt</td><td>C<sub>2</sub>H<sub>4</sub></td><td>3.8 %</td><td>H<sub>2</sub></td><td>100 %</td><td>[47]</td></tr><tr><td>Pre-PE</td><td>2:1 Methanol pH = 10</td><td>5 V</td><td>Carbon Paper/Pt</td><td>C<sub>2</sub>H<sub>4</sub></td><td>9.0 %</td><td>H<sub>2</sub></td><td>65.3 %</td><td>[47]</td></tr></tbody></table>

a) Pre: Pretreated.

in pesticide, leather, dye, medicine and rubber industries [111–113]. Recently, Zhao and co-workers [105] reported CuO nanowires (NW) electrocatalysis conversion of PET hydrolysate into formate (Fig. 10a). TEM images demonstrated that CuO NW was synthesized by an in situ self-growth method (Fig. 10b), successfully. Electrochemical tests showed that CuO NWs-200 °C electrode exhibited the optimal electrochemical performance, which was manifested in lower onset oxidation potentials, Tafel slope, transfer resistance and higher double-layer capacitance compared with other electrodes. In a typical experiment, 1 M KOH with 0.1 M PET hydrolysate was electrolyzed, formic acid was generated on the anode CuO, and H₂ was generated on the cathode Pt. Moreover, because ethylene glycol (EG) was the main product of PET hydrolysate, 10 mM pure EG was electrolyzed in 0.1 M KOH solution. After 62C, the faradaic efficiency and yield of formate were 88 % and 86.5 %, respectively. Besides, the DFT calculation suggested a possible reaction pathway, first EG adsorbed on the surface of CuO through a bidentate configuration, then the two O-H bonds of EG were broken to form *OCH<sub>2</sub>CH<sub>2</sub>O* intermediates, further formed *OCH – HCO*, and finally the C—C bond cleavage to generate formic acid (Fig. 10c and

10d). This work presents a novel way of electro-reforming PET waste through utilizing CuO NWs electrode to produce valued chemicals and H₂, and proposes the possible pathway of pretreated PET oxidation reaction.

Besides, Duan's group reported electrocatalytic upcycling of PET into potassium diformate (KDF) and terephthalic acid (TPA) [108]. PET was decomposed into PTA and EG in 1 M KOH solution, and then the EG was further oxidized into formate. The next step was the acidification of PET hydrolysate by adding formic acid to obtain the PTA and KDF. Ni₂P and CoP nanoparticles were interconnected with each other observed through HRTEM, indicating that heterojunctions were formed in the CoNi<sub>0.25</sub>P electrode [114,115]. Compared with other traditional electrodes (Pt/C, RuO₂ and Ni-Fe/NF), CoNi<sub>0.25</sub>P/NF electrode had an excellent electrochemical performance in all aspects, and the comprehensive performance is optimal in EG oxidation and H₂ evolution. In a three-electrode system, with or without EG in the electrolyte (1 M KOH) along with no bubble and O₂ bubbles were observed on the anode, because of lower thermodynamically for EG oxidation than water (Fig. 10e and 10f). On the contrary, H₂ bubbles in both conditions

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** (a) Electroreforming of PET hydrolysate into HCOOH and H₂. (b) TEM, and HRTEM images of CuO NWs-200 °C. (c) The reaction pathways of EG oxidation in the alkaline electrolyte. (d) The reaction free energy in different pathways during the EG oxidation process [103]. Copyright 2022, The American Chemical Society. Photograph of H-cell configuration for (e) HER//EG oxidation and (f) HER//OER over CoNi<sub>0.25</sub>P/NF at cathode and anode. (g) The stability experiment of CoNi<sub>0.25</sub>P/NF at 1.7 V vs RHE and 1.5 vs RHE for 39 h and 33 h, respectively. (h) The structure of anode and cathode CoNi<sub>0.25</sub>P after electroreforming of PET hydrolysate. Reproduced with permission [108]. Copyright 2021, Springer Nature.</figcaption></figure>
existed on the cathode surface. When pure glycolic aldehyde and gly-
colic acid as the substrate, they exhibited higher and lower activities
than EG for the formate, respectively, indicating that glycolic aldehyde
was more possibly to be the intermediate in the oxidation process of EG.
Moreover, 389.2 g formate, 818.5 g PTA, and 16.9 g H₂ could be ob-
tained by electrolysis 1 kg PET after being calculated. In the stability
experiment, CoNi₀.₂₅P/NF at 1.7 V and 1.5 V (vs RHE) both showed high
FE (80–95 %) and selectivity (75–95 %) of formate within 39 h and 33 h,
severally (Fig. 10g). In addition, the crystalline structure of the
CoNi₀.₂₅P catalyst was well maintained and only one CoNi₀.₂₅Oₓ(OH)ᵧ
shell was formed after HER at the cathode. In contrast, because of the
surface reconstruction of CoNi₀.₂₅P at the anode, it was oxidized into
low-crystalline CoNi₀.₂₅Oₓ(OH)ᵧ (Fig. 10h) conducted to EG oxidation.
In a word, this work realizes the conversion of PET into fine chemicals
with high efficiency and selectivity, and separates them from reaction
solution, successfully. The economic feasibility of the catalytic process is
evaluated from a commercial perspective, providing guidance for future
research work.

Reisner and coworkers not only converted PE into alkanes by pho-
tocatalysis, but also reported the conversion of PE into olefin compounds
by electrocatalysis, based on both electrocatalysis and photocatalysis
can provide suitable oxidation potential of PE [47]. In the three-
electrode system, varying electrode material (carbon Paper, graphite
rod or FTO-coated glass sheet) as the working electrode, Ag/AgCl as the
reference electrode and Pt as a counter electrode. The activity of
ethylene increased with the increase of electrolyte pH values, which was

derived from decarboxylation reactions of succinic acid in 0.1 M HNO₃
solution. The faraday efficiency of carbon paper electrode for ethylene
generation was 28.6 % at pH 10 and applied potential 2 V vs RHE
(Fig. 11a). Besides, a two-electrode electrolyzer was constructed (carbon
paper and Pt foil as the working electrode and counter electrode) to
assess the conversion rate of succinic acid in methanol and 0.1 M HNO₃
mixed solution (Fig. 11b). The FE of ethylene was 38 % achieved 21.0
µmol cm⁻²h⁻¹. Moreover, the PE hydrolysate was also tested in a mixed
solution, ethylene and propylene were the main products with FE 4 %
and 9 %, achieving 6.0 and 5.0 µmol cm⁻²h⁻¹, respectively (Fig. 11c).
On the contrary, the FE of H₂ was lower in the mixed solution and higher
in the HNO₃ aqueous. This work realizes the conversion of PE into
ethylene, which was the molecular monomers in PE synthesis and pro-
vides a breakthrough to closed-loop recycling of plastic.

Ma et al. [106] did the electrocatalytic conversion of PET into formic acid coupling with reduction of CO₂ to formic acid (Fig. 11a), so that the faraday efficiency of formic acid was up to 155 %. NiCo₂O₄ displayed nanosheet and nanowire spinel structure, while SnO₂ displayed nano-sheet structure (Fig. 11b and 11c). Besides, NiCo₂O₄/carbon fiber paper (CFP) and SnO₂ carbon cloth (SnO₂/CC) electrodes were investigated in the three-electrode system. NiCo₂O₄/CFP electrode showed a lower onset potential in PET hydrolysate compared with pure water (Fig. 11d). On the other hand, SnO₂/CC electrode exhibited excellent activity for the reduction of CO₂ into formic acid in the 1 M NaHCO₃ solution with saturated CO₂ (Fig. 11e). Both electrodes exhibited high Faradaic effi-ciency (FE) for formic acid achieving 90 % and 82 % with the applied

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** (a) Electroreforming of PET coupling with reduction of CO₂. SEM images of (b) NiCo₂O₄ and (c) SnO₂/CC electrode. (d) LSV curves of electrocatalytic with and without 0.1 M PET in the electrolyte NiCo₂O₄/CFP electrode as the anode. (e) LSV curves of electrocatalytic in Ar- or CO₂-saturated 1 M NaHCO₃ solutions SnO₂/CC electrode as the cathode. (f) The faradaic efficiencies of PET hydrolysate oxidation and CO₂ reduction for formic acid generation at different applied potentials in the two-electrode system [106]. Copyright 2022, The American Chemical Society. (g) The imagine of LC-CoOOH/CF and Pd NTs/NF electrodes. (h) Schematic diagram of CoOOH/CF||Pd NTs/NF two-electrode system. (i) FE of NH₃ and GA in CoOOH/CF||Pd NTs/NF two-electrode system with various voltages [109]. Copyright 2023, The American Chemical Society.</figcaption></figure>
potential was 1.45 V and – 0.9 V (vs RHE), respectively. Moreover, the two-electrode system was designed for evaluating the total activity to coproduce formic acid. The anode was NiCo₂O₄/CFP electrode, which oxidized EG in PET hydrolysate to formic acid with FE of 80 %, meanwhile, the cathode was SnO₂/CC electrode which reduced CO₂ to formic acid with FE of 75 % (Fig. 11f), at an applied potential of 1.9 V (vs RHE). In summary, this work not only realizes the coupling between electro-reforming of PET and the reduction of CO₂ to produce formic acid at both electrodes, but also gives the inspiration for generating more valuable chemicals by half-reaction of reduction.

Glycolic acid as a kind of value-added chemicals is utilized to textile and daily chemical industry, expected to reach US $531.5 million by 2027. Given this, Xu et al integrated the plastics oxidation reaction with nitrate reduction reaction by a LC-CoOOH/CF |Pd NTs/NF PEC system [109]. When the applied potential was – 0.25 V (vs RHE), low-crystalline CoOOH (LC-CoOOH/CF) with the rough surface was utilized as cathode which promoted the adsorption of nitrate and provided abundant active sites, leading to a high ammonia faradic efficiency (FE) of 97.38 ± 1.0 % (Fig. 11g). Meanwhile, Pd nanothorns (Pd NTs/NF) with the unique nanothorn morphology was utilized as anode, which facilitated the selective conversion of EG into glycolic acid (GA) with the FE of 87.87 ± 4.0 % at 0.774 V vs RHE. Moreover, the two-electrode coupling system could be conducted at a low applied potential (0.5 V vs RHE) to obtain the co-production, which was much lower than traditional nitrate electrolysis process (1.4 V) (Fig. 11h). Besides, the optimal FE of GA and NH₃ were 90.12 ± 3.6 % and 94.47 ± 2.8 %, and the corresponding applied potential were 1.0 and 1.2 V vs RHE, respectively (Fig. 11i). This study presents a new strategy for plastics reforming through coupling with nitrate reduction to ammonia reaction, further expanding the utilization path-way of plastics waste resource.

## 4.2. *Photoelectrochemical catalysis for plastics upcycling*

Photoelectrochemical catalysis is the combination of photocatalysis and electrocatalysis, which can maximize the advantages of both technologies. Compared with photocatalysis, the solar energy can be utilized by photoelectrodes to generate photo-generated electrons, improving reaction activity and catalytic efficiency. Compared with electro-catalysis, photoelectrochemical catalysis reactions greatly reduce the injection of external energy, effectively reducing energy consumption. In addition, photoelectrochemical can also apply catalysts that are not suitable for photocatalysis due to band structure mismatch under appropriate applied voltage conditions. Recently, photoelectrochemical catalysis has been widely studied in CO₂ reduction and the valorization of biomass derived compounds [116–118]. In addition, polymer plastics contain abundant organic groups and could also depolymerize into small organic molecules through hydrolysis [119]. Therefore, it is completely feasible to utilize PEC to transform plastics into high-value chemical feedstocks, which provides a broad prospect for the upcycling of plastic waste in real life.

Reisner et al. designed a Cu₃₀Pd₇₀|Perovskite|Pt photo-electrochemical (PEC) catalysis device (Fig. 12a) [120]. The introduction of Pd drove the dendritic Cu to take on a flower-like appearance, which was prepared by electrodeposition procedure on the Ni foam (Fig. 12b). Subsequently, the two-electrode PEC system was constructed by Ni foam|Cu₃₀Pd₇₀ and perovskite|Pt as the anode and photocathode, respectively, and the perovskite layer was back-illuminated through FTO-glass. When glycerol, ethylene glycol, and glucose as substrates, the production rates of H₂ were 1083,1316, and 1415 µmol H₂ cm⁻² in 10 h, respectively. On the other hand, the oxidation products were glyceric acid, glycolic acid and gluconic acid, corresponding to 111 ± 28 µmol, 163 ± 45 µmol and 206 ± 46 µmol, respectively and without any applied potential (Fig. 12d and 12e). The high selectivity of the products could be attributed to the high open-circuit voltage (V<sub>oc</sub>) of 1 V vs. RHE

<figure><img src="image_13.png" /><figcaption>**Fig. 12.** (a) The Cu₃₀Pd₇₀|perovskite|Pt PEC system. (b) SEM image of Cu₃₀Pd₇₀. (c) “artificial leaf” configurations. (d) The production yield of H₂ and (e) the amount of oxidation product with different substrates in the PEC system without any applied potential in 10 h. (f) The production yield of oxidation product with different real-world substrates (PET powder, PET bottle, and cellulose) for PEC device in both two-compartment and “artificial leaf” device in 1 M KOH at room temperature without any applied potential for 10 h [120]. Copyright 2021, Wiley.</figcaption></figure>
provided by the perovskite, the overlap potentials for the different substrates and high competition for active sites. Moreover, to explore the real-world convert ability, the PET powder and a real-world commercial PET bottle were reformed by a two-electrode PEC system (Ni foam|Cu₃₀Pd₇₀ and perovskite|Pt), the H₂ production was 776 and 748 µmol cm⁻², and the selectivity was above 90 %. Furthermore, the PEC system was integrated as a compact system and named “artificial leaf” (Fig. 12c), which could realize redox reactions without applied voltage and wiring. PET powder and PET bottle were then converted into glycolic acid, and at the same time H₂O was reduced to H₂ by artificial leaf without applied voltage (Fig. 12f). In short, the PEC system makes plastic upcycling problems break through the bottleneck, the selectivity of products achieves 60–90 % and the activity are10²–10⁴ times higher than conventional photoreforming systems. Besides, the construction of “artificial leaf” further laying a foundation for the practical application.

Yi's group prepared TiO₂ nanorods photoanode modified by P-Ni alloy thin film (nanoNi-P) and nanoNi-P with carbon nanotubes (CNT-nanoNi-P) cathode to construct nanoTiO₂|nanoNi-P//CNT-nanoNi-P<sub>op</sub> two-electrode PEC system for converting PET into formic acid and produce H₂, simultaneously (Fig. 13a) [121]. The optimal electrodeposition condition of nanoNi-P was applied current densities (I<sub>dep</sub>) = -20,000 µA cm<sup>-2</sup> and C<sub>hypophosphite</sub> = 0.04 M. On the other hand, in a three-electrodes system, nanoNi-P<sub>op</sub>(CV) (the optimal nanoNi-P after CV-activation) presented higher activity and Faradaic efficiency (FE) than those for nanoNi<sub>(CV)</sub>, achieved 244.6 µmol cm<sup>-2</sup>h<sup>-1</sup> for EG oxidation

(Fig. 13b). It is attributed to more β-NiOOH being reserved by the P element as the active species. Moreover, the nanoNi-P<sub>op</sub>(CV) was integrated onto nanoTiO<sub>2</sub> as the photoanode (C = 0.02C cm<sup>-2</sup>) showed higher performance than bare nanoTiO<sub>2</sub> photoanode under irradiation (0.5 V vs. RHE). In addition, the two-electrode PEC system was constructed by nanoTiO<sub>2</sub>|nanoNi-P<sub>op</sub>(CV) and CNT-nanoNi-P<sub>op</sub>(CV) as the photoanode and cathode, respectively. The yield of formate and H<sub>2</sub> were 6.4 µmol and 12.5 µmol, much higher than the PEC system constituted by TiO<sub>2</sub>|Pt in 4 h. Finally, the possible mechanism was two hydroxyl groups of EG were oxidized by β-NiOOH to form glyoxal, which was further oxidized into formic acid (Fig. 13c). This study utilizes a photoelectrochemical platform based on nanoNi-P<sub>(CV)</sub>-modified TiO<sub>2</sub> nanorods photoanode and CNT/nanoNi-P cathode to achieve efficient and selective generation of H<sub>2</sub> and formate during the PET reforming process.

Subsequently, Zhao and coworkers designed a PEC system constructed by Fe₂O₃/Ni(OH)ₓ photoanode, while Ag/AgCl and platinum gauze electrode as the reference electrode and counter electrode, respectively [19]. This PEC system realized the conversion of PET to formic acid and produce H₂ (Fig. 13d). Fe₂O₃/Ni(OH)ₓ was synthesized by different concentrations of Ni(NO₃)₂, and named Fe₂O₃-xNi. The Fe₂O₃-xNi photoanode represented more excellent PEC performance than bare Fe₂O₃, where Fe₂O₃-10Ni was the optimal. Because of the competition between the oxidation of EG and water splitting at high potentials, the Fe₂O₃-10Ni electrode exhibited an FE of about 100 % at a

<figure><img src="image_14.png" /><figcaption>**Fig. 13.** (a) The nanoTiO₂|nanoNi-P<sub>op</sub>(CV)//CNT-nanoNi-P<sub>op</sub> two-electrode PEC system for reforming PET hydrolysate into formic acid and H₂. (b) The production rate and FE of formic acid over nanoNi-P<sub>op</sub>(CV) and nanoNi-P<sub>(CV)</sub> at 1.5 V vs. RHE for EG oxidation in KOH solution. (c) The reaction mechanism of EG oxidation over nanoNi-P<sub>op</sub>(CV) photoanode [121]. Copyright 2022, Elsevier. (d) The upcycling of PET over Fe₂O₃/Ni(OH)ₓ photoanode. (e) The formic acid yield and FE over Fe₂O₃-10Ni at different applied bias and (f) different Ni loading of Fe₂O₃-xNi photoelectrodes at the applied bias of 1.2 V<sub>RHE</sub> [19]. Copyright 2022, the American Chemical Society. (g) Schematic diagram of Cu₂₇Pd₇₃||PVK|CO₂R<sub>cat</sub> PEC system. (h) The amount and corresponding FEs of oxidation product formed at the anode with the pretreated PET after a 10 h PEC reforming without applied voltage [33]. Copyright 2022, Springer Nature.</figcaption></figure>
lower potential, and with the increase of potential, the FE of formic acid decreased to less than 80 % (Fig. 13e). Moreover, at the potential of 1.2 V<sub>RHE</sub>, the FE of Fe<sub>2</sub>O<sub>3</sub>-10Ni and Fe<sub>2</sub>O<sub>3</sub>-20Ni photoanodes were higher than Fe<sub>2</sub>O<sub>3</sub>-5Ni and Fe<sub>2</sub>O<sub>3</sub>-0Ni (Fig. 13f). Furthermore, Fe<sub>2</sub>O<sub>3</sub>-10Ni was then studied in the higher concentration of PET hydrolysate, and retained photoelectrocatalytic performance in KOH electrolyte. Finally, the possible working mechanism of Fe<sub>2</sub>O<sub>3</sub>/Ni(OH)<sub>x</sub> photoanode was proposed, with the Ni<sup>2+</sup> being oxidized into Ni<sup>3+</sup> by the photogenerated holes from the Fe<sub>2</sub>O<sub>3</sub>. The Ni<sup>3+</sup> centers promptly react with EG to yield formate. This work investigates the mechanism based on Ni<sup>2+</sup>/Ni<sup>3+</sup> redox couple and suggests that the introduction of Ni(OH)<sub>x</sub> cocatalyst can effectively regulate the oxidation ability of the photogenerated holes, thereby improving the selectivity towards the desired formic acid products during the photoelectrochemical catalytic process of PET waste.

Besides H₂, other valuable chemicals, such as the products of CO₂ reduction and N₂-based synthesis of ammonia can also be achieved through reduction reaction. Taking the advantages separating oxidation and reduction processes with minimized crossover reactions of photo-electrochemical catalysis, Reisner and co-workers realized the plastics oxidation coupling with the CO₂ reduction by a Cu₂₇Pd₇₃|PVK|CO₂Rcat (CO₂Rcat: CO₂R catalysts) PEC systems (Fig. 13g) [33]. This PEC system could selectively convert the pretreated PET into glycolic acid (GA) in different pH without applied voltage. Meanwhile, when the CO₂Rcat photocathode were lipophilic, alkyl-functionalized cobalt porphyrin (CoPL), copper-indium (Cu₉₁In₉) and formate dehydrogenase from *Desulfovibrio vulgaris* Hildenborough (FDH), the main products were CO, syngas and formate, respectively. PEC analysis showed that the working potentials of CoPL, Cu₉₁In₉ and FDH were -0.7 V, -0.7 V and -0.8 V versus RHE (AM 1.5G 0.5 M KHCO₃ electrolyte) and exhibited the Faradaic efficiency (FE) of corresponding products were 85 ± 2%, 94% (FE<sub>CO</sub> = 48 ± 8% and FE<sub>H2</sub> = 46 ± 4%) and 96 ± 2%, respectively. In solar-driven PEC experiments, Cu₂₇Pd₇₃ anode and PVK|CO₂Rcat photocathodes were separated by a bipolar membrane, the anolyte and photocathode consisted of 1 M aqueous KOH and 0.5 M aqueous KHCO₃ (CO₂-saturated electrolyte for FDH). As a result, the CO, syngas (CO and H₂) and formate were produced from the PVK|CO₂Rcat (CoPL, Cu₉₁In₉ and FDH) photocathodes, and their evolution rates were 263 ± 99, 452 (CO: 212 ± 148|H₂: 240 ± 75) and 121 ± 87 µmol cm⁻², respectively. Meanwhile, the GA was produced from the Cu₂₇Pd₇₃ anode with yields of 31 ± 7, 52 ± 18 and 14 ± 8 µmol after 10 h in the three different cases (Fig. 13i). In short, this work integrates single-light-absorber Cu₂₇Pd₇₃|PVK|CO₂Rcat PEC device with effort to couple CO₂ reduction with plastics reforming and presents a unique demonstration for sustainable commercial implementation.

### 4.3. Solar thermal electrochemical conversion

Traditional pyrolysis of plastics has been broadly studied in the past decades [122,123]. Generally, pyrolysis of plastics demands a temperature of about 500 °C even higher temperature in the range of 700 °C-900 °C to improve productivity [124]. But higher temperature means more energy requirements, which is not economical and environmentally friendly. The solar thermal electrochemical process (STEP) has captured the researcher's attention from 2009 [125-128]. In a STEP system, the solar energy can be converted into electric and heat energy, thermal energy has the potential to effectively decrease the energy required for the endothermic process of electrolysis, allowing it to surpass the activation energy necessary for the redox reaction. This greatly promotes the enhancement of solar utilization and conversion efficiency of plastics waste [23]. Besides, solar energy is the sole power source in the STEP system with no necessary for any external energy source. Therefore, it is a clean and sustainable strategy to make use of STEP to upcycle waste plastics [129,130].

Wang et al. realized the conversion of polypropylene (PP) to H₂, CH₄, C₂H₆, C₃H₄, and C₅H₁₂ through the STEP system, constructed by them. In

this system, a solar thermal collector and the photovoltaic panel provided heat and electricity for the reaction process (Fig. 14a), respectively [23]. At 350 °C to 400 °C, the total conversion rate of PP by STEP not only increased with the rise of temperature, but also was higher than the conversion rate of pure heat at the same temperature, indicating that electric field played an important role in STEP system. Meanwhile, the effect of electrolysis current exhibited the same rule for the STEP transformation of PP, the total conversion rate could achieve 52.02 % when the current was 400 mA at 400 °C, while the conversion rate was 42.08 % corresponding to 0 mA at 400 °C, in an hour. The other significant difference between STEP and pyrolysis was the productivity of H₂, the yield of STEP H₂ was 10.3 times that of pure pyrolysis and 2.9 times that of pure electrolysis (Fig. 14b). Moreover, the STEP system was also studied outdoors for exploring its stability. In the optimal, the conversion of PP reached 66.64 % exhibiting excellent conversion ratio and stability, meanwhile, the conversion just was 26.37 % of the pyrolysis. In this work, the coupling between the solar concentrators and solar cell packs technology is achieved to convert the plastics waste efficiently. Moreover, the STEP is completely driven by sustainable solar energy and the equipment of STEP system is not high. In short, the STEP system lays the foundation for industrialization of plastic upcycling.

## 5. Conclusion and outlook

The massive generation of plastic waste has resulted in serious environmental and ecological crisis, yet plastics can be regarded as raw carbon materials to produce value-added chemicals by catalytic methods. This mini review summarizes the progress for the conversion of plastic into fuels, fine chemicals and materials, with focus on photocatalysis, electrocatalysis, photoelectrochemical (PEC) catalysis and solar thermal electrochemical process (STEP) as well as the pretreatment method of various plastics. Compared with the traditional treatment of plastics, the upcycling by photo- and electrochemical processes produce higher add-value products. Moreover, the photo- and electrochemical processes conduct in ambient condition, which are more environmentally friendly and sustainable compare with the thermocatalysis under harsh conditions. In short, these technologies based on clean energy from solar and electricity energy may attract more attention in the future owing to environmental sustainability.

However, these technologies for plastic upcycling are still in the early stages of development, and there is still a huge space to improve the efficiency, product selectivity and stability for practical production and real applications. Finally, we propose the worth research directions from perspective of catalytic in the field of plastic upcycling:

1) **Catalyst improvements:** Catalysts with high efficiency, low cost, easy synthesis and high selectivity are the necessary conditions for plastics waste upcycling. Till now, few catalysts have been utilized to explore plastics upcycling, suggesting that more catalysts should be studied in this process. Taking photocatalytic materials as example, attention should be paid to the development of narrow band gap semiconductors, which can absorb the solar light as much as possible, since plastic conversion is much more thermodynamically favorable than water splitting. Additionally, the study of composite catalysts should also be further emphasized, due to the performance of a single photocatalyst is often limited. Consequently, combining different catalysts are advantageous in overcoming the activity bottleneck and leveraging the benefits of multiple catalytic materials.

2) **Reaction mechanism:** Exploring the mechanism of plastics conversion is important to adjust and improve the utilized catalyst, further understanding the chemical changes on catalyst's surface, thereby enhancing the activity and selectivity towards a target product. Advanced characterization methods and theoretical calculations could be helpful to get an insight about the reaction mechanism of plastics conversion, and break the bottlenecks which limit the activity or selectivity of the used catalytic materials, further promoting the industrialization of plastics upcycling. For example, in-situ FTIR analysis has
<figure><img src="image_15.png" /><figcaption>Fig. 14. (a) The device of the outdoor experiments. (b) The production yield (H₂, CH₄ and C₂H₆) of pyrolysis, electrolysis and STEP process for reforming PP hydrolysate [23]. Copyright 2020, Elsevier.</figcaption></figure>

been employed to monitor reactive intermediates in plastics upcycling. However, its effectiveness is still far from satisfactory. Given this, it is vital to combine in-situ mass spectrometry and isotope-labeling experiments to elucidate the reaction pathway, which is mutually corroborating with in-situ FTIR analysis, further explaining the evolution of intermediates during plastics upcycling. Based on these advanced characterization analyses, we could design controlled experiment to find the main drawbacks during plastic conversion, as well as design more suitable catalysts with enhanced activity and selectivity towards a target product. On the other hand, both thermodynamic and kinetic studies are significantly important during plastic conversion and can be effectively explored through theoretical studies. In particular, for photocatalysis and electrocatalysis of plastics initiated by charge carriers, redox potential and quantity of charge carriers should be appropriately adjusted to satisfy the requirement of theoretical thermodynamic potentials for promoting the activity and selectivity of catalytic materials in plastics conversion. In addition, the pH of the solution and the rate-limiting step are key kinetic parameters, which should also be explored by the theoretical studies to pursue optimal catalytic conditions.

3) Reaction coupling: Upcycling of plastics consist of oxidation and reduction half-reactions, in which the reduction half-reaction is mainly to reduce water to H₂. However, it has the potential to generate more valuable chemicals, such as CO₂ reduction and N₂-based synthesis of ammonia, but limited work has been done in this area. Additionally, it is worthy to note that the use of coupling between existing technologies should be considered, since coupling technologies can often produce a synergistic effect, such as photoelectrochemical and photo-thermal electrochemical processes.

4) Design of reaction equipment: Up to now, plastics upcycling by clean energy technologies is mostly carried out in the laboratory trial stage, and it is necessary to design large-scale reaction equipment for future practical applications. Moreover, establishing a mobile phase reaction equipment to increase the contact between plastics and catalysts can better promote catalytic performance for photocatalysis and electrocatalysis upcycling of plastics. As for plastic photocatalysis, sufficient light source is a necessary condition, thus the collector (such as a convex lens) can be utilized to increase the light intensity in the site where the catalyst is located, further improving catalytic efficiency. Besides, the fixation of photocatalysts (frosted glass) is also crucial, which is beneficial for recycling and environmental protection. With the development of photovoltaic technology, electrocatalysis upcycling of plastics can be combined with distributed photovoltaic technology to meet its electricity demand.

CRediT authorship contribution statement

Xinxin Liang: Writing – original draft, Writing – review & editing.

Ximing Li: Writing – review & editing. Qibing Dong: Formal analysis.

Ting Gao: Resources. Mengxin Cao: Validation. Ke Zhao: Software.

Eric Lichtfouse: Writing – review & editing. Antonio Otavio T.

Patrocinio: Writing – review & editing. Chuanyi Wang: Funding acquisition, Validation, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors do not have permission to share data.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52161145409, 21976116), SAFEA of China (“Belt and Road” Innovative Talent Exchange Foreign Expert Project # 2023041004L) (High-end Foreign Expert Project # G2023041021L), and Alexander-von-Humboldt Foundation of Germany (Group-Linkage Program). AOTP is thankful to the Phosagro/IUPAC/UNESCO research program in green chemistry.

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