Arvind K Bhakta
email: arvind-kumar.bhakta@u-paris.fr

Roberto Fiorenza

Khouloud Jlassi

Zineb Mekhalif

Aboubakr M Abdullah

Mohamed M Chehimi
email: mohamed.chehimi@cnrs.fr

The emerging role of biochar in the carbon materials family for hydrogen production

Keywords: Carbon allotropes, Nanohybrid, Trash to treasure, Sustainable development, (Photo)catalysis, Green carbon

The energy crisis is at the top of global priorities. It left us with an option to progress with highly efficient materials for energy conversion, production, and storage systems. Carbon nano-allotropes have more accessible, and modifiable surface with excellent surface area, conductivity, catalytic activity, and other features. Biochar synthesized from renewable organic material viz. biomass (plants, animal waste), has beneficial physicochemical features (surface functionalities, porosity, adsorption capacity, etc.) and agronomic properties. They are influenced by several factors like the type of thermal treatment and feedstocks. Modification of biochar is an excellent option to obtain synergistic effects and sustainable materials. The current comprehensive review article will focus on biochar, selected nanocarbons (fullerene, carbon dots, nanotubes, nanodiamonds, graphene, and graphdiyne), and their hybrids for hydrogen evolution. Particular emphasis will be on biochar as these materials could be easily generated from different wastes. Biochar can have graphitized or non-graphitized carbon atoms. They can have carbon fragments resembling fullerene. This can be a potential low cost biocarbon alternative to graphene and fullerene and we are well aware of the importance of graphene and fullerene.

Future projections in this paper may lead to an added foundation for "trash to treasure"/ "waste to wealth" and "waste to fuel" research and also contributes towards real-world applications in the clean fuel and sustainable energy sector.

Introduction and Scope of the Review

A list of ten world's problems includes (i) energy, (ii) water, (iii) food, (iv) environment, (v) poverty, (vi) terrorism and war, (vii) disease, (viii) education, (ix) democracy, and (x) population in which energy is on the top. Energy can play a significant role in solving all of the mentioned issues [START_REF] Smalley | Future Global Energy Prosperity: The Terawatt Challenge[END_REF]. When it comes to clean fuel and energy, hydrogen production and storage come into the picture. In view of functional material commercialization, several parameters must be addressed, such as reproducibility, ease of synthesis, toxicity, abundance of precursors, cost-effectiveness, stability and recyclability [START_REF] Blay | Research frontiers in energy-related materials and applications for 2020 -2030[END_REF].

A carbon atom can exist in sp, sp 2 , and sp 3 hybridizations [START_REF] Bianco | A carbon science perspective in 2018: Current achievements and future challenges[END_REF][START_REF] Georgakilas | Broad family of carbon nanoallotropes : classification , chemistry , and applications of fullerenes , carbon dots , nanotubes , graphene , nanodiamonds , and combined superstructures[END_REF]. There are numerous carbon allotropes (Figure 1), such as graphite, diamond, fullerene, carbon nanotubes (CNTs), graphene, carbon nanohorns (CNHs) and carbon nanoonions (CNOs). Biochar is an amorphous, carbon-rich porous material [START_REF] Hidalgo | Synthesis of carbon nanotubes using biochar as precursor material under microwave irradiation[END_REF], obtained by thermochemical treatment of the biomass, and which could be ranked among the large carbon allotrope series. The latter can be employed as such, chemically/physically modified or hybridized with other materials. They possess fascinating high surface area, electrical and thermal conductivity, catalytic activity and chemical stability [START_REF] Georgakilas | Broad family of carbon nanoallotropes : classification , chemistry , and applications of fullerenes , carbon dots , nanotubes , graphene , nanodiamonds , and combined superstructures[END_REF]. For these reasons, they are utilized in several fields, including the energy sector [START_REF] Tiwari | Magical allotropes of carbon: prospects and applications[END_REF], encompassing heterogeneous catalytic hydrogen production needed to meet the demand for a greener 21 st century [START_REF] Yi | Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production[END_REF]. From the economic and environment point of view, it is interesting to utilize wastes as sources of carbon (nano)materials [START_REF] Janas | From Bio to Nano : A Review of Sustainable Methods of Synthesis of Carbon Nanotubes[END_REF] which can be used for several applications [START_REF] El-Nahas | Aluminum building scrap wire, take-out food container, potato peels and bagasse as valueless waste materials for nitrate removal from water supplies[END_REF][START_REF] Lopes | Biochar as a support for nanocatalysts and other reagents : Recent advances and applications[END_REF]. In this regard, biochar is today's emerging important carbonaceous material, and synthesized via thermochemical process (in the absence (pyrolysis) or presence of limited O2) using biomass derived from e.g. plants and animals wastes [START_REF] Wu | A scientometric review of biochar research in the past 20 years (1998-2018[END_REF][START_REF] Kumar | Hydochar and biochar: Production, physicochemical properties and techno-economic analysis[END_REF]. It is a representative of a "trash to treasure" [START_REF] Huang | Sewage sludge-derived carbon-doped manganese as efficient cathode catalysts in microbial fuel cells[END_REF] or "waste to wealth" and/ or "waste to fuels" [START_REF] Wang | Waste to Wealth": Lignin as a renewable building block for energy harvesting/storage and environmental remediation[END_REF] concept. It is also an excellent example of accomplishing a "circular bioeconomy" [START_REF] Ghodake | Review on biomass feedstocks, pyrolysis mechanism and physicochemical properties of biochar: State-ofthe-art framework to speed up vision of circular bioeconomy[END_REF][START_REF] Yaashikaa | A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy[END_REF]. Biochar shows unique chemical (high pH, carbon sequestration, cation exchange capacity, nutrient exchange site, and surface functionalities: C=O, -COOH, -R-OH, and OH), physical (high porosity, water holding capacity, surface area, and surface charge), and agronomic (nutrients, salinity, etc.) properties [START_REF] Wu | A scientometric review of biochar research in the past 20 years (1998-2018[END_REF][START_REF] Weber | Properties of biochar[END_REF][START_REF] Patel | A critical literature review on biosolids to biochar: an alternative biosolids management option[END_REF][START_REF] Tomczyk | Biochar physicochemical properties : pyrolysis temperature and feedstock kind effects[END_REF][START_REF] Manolikaki | Agronomic potential of biochar prepared from brewery byproducts[END_REF][START_REF] Chen | Past, present, and future of biochar[END_REF][START_REF] Oliveira | Environmental application of biochar: Current status and perspectives[END_REF][START_REF] Méndez | Physicochemical and agronomic properties of biochar from sewage sludge pyrolysed at different temperatures[END_REF][START_REF] Cely | Agronomic properties of biochars from different manure wastes[END_REF]. Biochars were utilized in many fields including the green hydrogen generation [START_REF] Wang | Biochar-driven simplification of the compositions of cellulose-pyrolysis-derived biocrude oil coupled with the promotion of hydrogen generation[END_REF].

Figure 2 plots the number of publications on carbon materials with particular emphasis on biochar for hydrogen production. Whilst hydrogen production using carbon is known for decades, the takeoff of research on biochar for hydrogen production dates back to 2005.

According to the web of science, 4.47% of papers on hydrogen production using carbon allotropes in 2021 were dedicated to biochar, that is about ten-fold the 0.49% in 2011. In onedecade, remarkable interest has been expressed by materials scientists in the domain of hydrogen production using biochar. A thorough literature survey has been carried out to explore the recent findings related to biochar and carbon allotropes for hydrogen production. We have first screened the literature and hand-picked available reviews, gathered in Table 1. It features the variety of carbon materials and their hybrids utilized for H2 evolution. Some articles deal with only one type of allotropes such as graphene and related materials, whereas some focus on a few carbon allotropes. They lack detailed information on the recent update in which biochar, nanocarbon, nanoparticles and their hybrids are discussed together in the single article specifically to hydrogen production. Some of them deal with several relevant applications, for example energy and the environment. There are also contributions describing the broad family of carbon nanoallotropes and their classifications; however, they do not tackle hydrogen production. As far as papers dedicated to hydrogen production are concerned, they deal with nanoparticles, but most are not focused exclusively on the broad series of carbon nano-allotropes. This created a literature gap in this field. Nasir et al. [START_REF] Nasir | Carbon-Based Nanomaterials / Allotropes : A Glimpse of Their Synthesis , Properties and Some Applications[END_REF] focused on carbon allotropes and carbon-based materials for biomedical and energy storage applications. The authors consider the topic of biomass-derived carbon. Here, more focus is on the historical chemical background, synthesis, and properties, but the application (energy storage) is described briefly. To the best of our knowledge, no review has summarized the recent advancements in applying both biochar and the broad family of nanocarbons for H2 production via different processes, hence the motivation for this contribution. It is significant to indicate that a recent review by Reza et al. [START_REF] Reza | Hydrogen Production from Water Splitting through Photocatalytic Activity of Carbon-Based Materials[END_REF] has taken into account nanocarbon and activated carbon, but it focused on a few carbon allotropes (graphene, graphite, CNTs, and activated carbon). Bhavani et al. [START_REF] Bhavani | Recent advancements on the sustainable biochar based semiconducting materials for photocatalytic applications : A state of the art review[END_REF] have discussed the role of biochar in photo-catalytic applications.

Herein, we discuss recent progress on biochar, different carbon forms like 0D (fullerene), 1D

(CNTs), 2D (graphene, and graphdiyne) and 3D (nanodiamonds) and their hybrids for hydrogen (H2) production. The cited literature is mostly from 2018-2022. For the sake of simplicity, detailed production methods of carbon nano-allotropes are not discussed in this review article.

The scope of this review article is the carbon-based materials for hydrogen generation using different techniques with a particular emphasis placed on the biochar production, structure, and its application in the hydrogen generation.

We aim at fulfilling the gap using both carbon nano-allotropes and biochar in the energy production sector, more specifically hydrogen (green fuel) synthesis. In addition, a comparative study has been carried out between biochar and other carbon allotropes in terms of hydrogen production efficiency. Finally, the future of using biochar is highlighted. We will finish by future perspectives and conclusions.

Different Methods of Hydrogen Production

Hydrogen is considered the third most plentiful element on Earth. Due to its unavailability in pure form, it is not an energy source but an energy carrier synthesized from other sources.

Fascinatingly, it is denoted by several colors depending on its production methodology such as grey (or brown/black), blue, turquoise, green, and yellow (or purple) hydrogen. Further, hydrogen is also termed renewable hydrogen / clean hydrogen/ low-carbon hydrogen [START_REF] Noussan | The role of green and blue hydrogen in the energy transitiona technological and geopolitical perspective[END_REF]. H2 can be generated from fossil fuels, H2O, and waste [START_REF] Fu | Towards hydrogen production from waste activated sludge : Principles , challenges and perspectives[END_REF][START_REF] Ibrahim | Pyrolysis of hydrothermal liquefaction algal biochar for hydrogen production in a membrane reactor[END_REF]. Solar water splitting [START_REF] Burton | Increasing the efficiency of hydrogen production from solar powered water electrolysis[END_REF] can be an efficient method for a vast quantity of hydrogen production [START_REF] Hisatomi | Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts[END_REF]. The process involving H2O dissociation into H2 (HER) and O2 (OER), electrocatalytically is called electrolysis. The following equation describes this process [START_REF] Kagkoura | Carbon nanohorn-based electrocatalysts for energy conversion[END_REF]:

H2O H2 + 1/2O2 (1)
Reaction at cathode (HER):

4H + + 4e - 2H2 (2)
The scheme indicating photocatalytic water splitting is shown in [START_REF] Maeda | Photocatalytic water splitting: Recent progress and future challenges[END_REF]. Regarding the green hydrogen (i.e. hydrogen produced without the use of fossil fuels), the solar water splitting, the photo-reforming or the photo-electrocatalysis can be fascinating routes to obtain hydrogen in a sustainable way. In particular, with the photo-catalytic solar water splitting, a crucial renewable source such as the solar energy is valorized and the use of semiconductors efficiently designed for the solar light harvesting allowed a promising H2 yields [START_REF] Tachibana | Artificial photosynthesis for solar watersplitting[END_REF][START_REF] Li | Engineering heterogeneous semiconductors for solar water splitting[END_REF]. The photoreforming reaction is another valuable strategy that provides the addition into the water of a little amount of an organic compound that acts as holes scavenger avoiding in this way the photoelectrons/holes recombination (generated during the photocatalytic process on the surface of the photocatalyst) that is a common reason of the photo-catalyst deactivation [START_REF] Fiorenza | Photocatalytic H2 production over inverse opal TiO2 catalysts[END_REF][START_REF] Shimura | Heterogeneous photocatalytic hydrogen production from water and biomass derivatives[END_REF]. With the photo-reforming process it is possible to increase the sustainability of the process with the use of different green scavengers as bio-alcohols, the glycerol or, also carbohydrates such as glucose [START_REF] Bahadori | Photoreforming of Glucose over CuO/TiO2[END_REF][START_REF] Lakhera | A review on particulate photocatalytic hydrogen production system: Progress made in achieving high energy conversion efficiency and key challenges ahead[END_REF][START_REF] Fiorenza | Solar photocatalytic H2 production over CeO2-based catalysts: Influence of chemical and structural modifications[END_REF].

Finally, the photo-electrocatalysis (PEC) process, similarly to photocatalysis, provides the use of 2 photo-active semiconductor electrodes such as: n-type (photo-anode) and p-type (photocathode). It is possible to excite it by photons (energy levels ≥ band gaps). A constant bias potential / a appropriate constant current supplied to a semiconductor anode are applied in solar / UV-vis facilitated electrocatalysis. This is to facilitate photo-induced e -s movement towards an external electrical circuit continuously. The external potential enhances the charge carrier separation on the electrodes surface via photogenerated enabled charge-transfer reactions on Reaction at anode (OER):

2H2O O2 + 4H + + 4e - (3)
the semiconductor surface. Compared to the bare photocatalysis a higher H2 yield was detected with this multi catalytic approach [START_REF] Rajput | Photoelectrocatalysis as a high-efficiency platform for pulping wastewater treatment and energy production[END_REF][START_REF] Lian | Efficient Self-Driving Photoelectrocatalytic Reactor for Synergistic Water Purification and H2 Evolution[END_REF].

When compared to various methods of hydrogen production (gasification, steam reforming, dark-fermentation, pyrolysis, and photo-biolysis), water electrolysis has a definite advantage of using H2O (feedstock) which is clean and readily available. As a matter of fact, the product of electrolysis is pollution-free which is very important for sustainable development [START_REF] Kannah | Techno-economic assessment of various hydrogen production methods -A review[END_REF].

It is with no surprise that even just biomass is involved in hydrogen production through microalgae as shown in reference [START_REF] Wang | Fermentative hydrogen production using pretreated microalgal biomass as feedstock[END_REF]. Nanocarbon derived from biomass is a potential candidate for futuristic energy devices (energy storage and conversion) [START_REF] Gupta | Biomass-derived functional carbon nanomaterials for the development of futuristic energy devices: Challenges and Opportunities[END_REF][START_REF] Mugadza | Synthesis of Carbon Nanomaterials from Biomass Utilizing Ionic Liquids for Potential Application in Solar Energy Conversion and Storage[END_REF]. For example, human hair waste biomass is a source of activated-carbon nano-bundles (partially graphitized)

for HER [START_REF] Sekar | Excellent Electrocatalytic Hydrogen Evolution Reaction Performances of Partially Graphitized Activated-Carbon Nanobundles Derived from Biomass Human Hair Wastes[END_REF]. It is also possible to produce nanocarbon and hydrogen from the common plastic waste [START_REF] Williams | Hydrogen and Carbon Nanotubes from Pyrolysis -Catalysis of Waste Plastics : A Review[END_REF].

Physical and chemical modifications of carbon allotropes were shown to be important when it comes to hydrogen production. This can be well understood by the example of graphene. Pristine graphene (flat material) is catalytically inactive for HER and their curvature strongly affects the HER performances positively [START_REF] Qu | Effect of curvature on the hydrogen evolution reaction of graphene[END_REF]. It is to note that their modifications, provide them with some unusual properties [START_REF] Li | Graphene and their hybrid electrocatalysts for water splitting[END_REF][START_REF] Luis-Sunga | Non-precious metal graphene-based catalysts for hydrogen evolution reaction[END_REF]. Also, modifications are considered for unbundling carbon nanotubes [START_REF] Bensghaïer | The Molecular and Macromolecular Level of Carbon Nanotube Modification Via Diazonium Chemistry: Emphasis on the 2010s Years[END_REF], exfoliating graphene [START_REF] Amiri | A review on liquidphase exfoliation for scalable production of pure graphene, wrinkled, crumpled and functionalized graphene and challenges[END_REF] or dispersing nanodiamonds, 0D carbons such as fullerenes [START_REF] Speranza | The role of functionalization in the applications of carbon materials : an overview[END_REF].

Application of Carbon Allotropes for Hydrogen Production

In this work, we have mainly focused on the photo-catalytic hydrogen production, with emphasis on biochar. It is clear from the literature that biochar could serve as a catalyst to produce hydrogen. However, the biochar catalytic performances increase upon activation that could be achieved via simple methods. For example, Liu et al. [START_REF] Liu | Role of Ocontaining functional groups in biochar during the catalytic steam reforming of tar using the biochar as a catalyst[END_REF] have demonstrated that activation of biochar in H2O (15 vol%) under Ar, increases the o-containing functional groups, mostly aromatic C-O groups, with a result of enhanced catalytic steam reforming of tar. It is also interesting to note that biochar incorporation in composite materials improves its thermal, mechanical and electrical properties [START_REF] Das | Incorporation of Biochar to Improve Mechanical[END_REF]. The presence of nitrogen and phosphorus in the biomass is reflected in the biochar composition (self-doping) as these important elements impart photocatalytic properties to the biochar in HER [START_REF] Zhou | P Self-Doped Porous Carbon Material Derived from Lotus Pollen for Highly Efficient Ethanol -Water Mixtures Photocatalytic Hydrogen Production[END_REF].

Fullerene

Fullerene [66] or C60 resembles a soccer ball in structure. The rate of hydrogen synthesis over pristine g-C3N4 photocatalyst has been increased by four times after making a hybrid with C60 molecule (hydrogen production rate = 266 μmol h -1 g -1 ) [START_REF] Chen | A facile mechanochemical route to a covalently bonded graphitic carbon nitride (g-C3N4) and fullerene hybrid toward enhanced visible light photocatalytic hydrogen production[END_REF]. Water and urea splitting has been successfully carried out at fullerene quantum dot in association with sheet of CoNi layered double hydroxides present on nickel foam (porous material) [START_REF] Feng | Decorating CoNi layered double hydroxides nanosheet arrays with fullerene quantum dot anchored on Ni foam for efficient electrocatalytic water splitting and urea electrolysis[END_REF]. In the recent years, significant advancements in the field of photocatalysis using fullerene and their derivatives for hydrogen production have been noticed [START_REF] Pan | Advances in photocatalysis based on fullerene C60 and its derivatives: Properties, mechanism, synthesis, and applications[END_REF]. In this context, there are reports on WO3/fullerene@Ni3B/Ni(OH)2 [START_REF] Shahzad | Engineering the performance of heterogeneous WO3/fullerene@Ni3B/Ni(OH)2 Photocatalysts for Hydrogen Generation[END_REF], MoS2 "inorganic fullerene"/TiO2 [START_REF] Afanasiev | MoS2 "inorganic fullerenes" combined with TiO2 in water-methanol suspensions: Highly active hydrogen production photo catalysts operating via transfer of accumulated electrons[END_REF], fullerene-WO3 composites [START_REF] Tahir | Role of fullerene to improve the WO3 performance for photocatalytic applications and hydrogen evolution[END_REF], C60/graphene/g-C3N4 hybrid photocatalyst [START_REF] Song | C60/graphene/g-C3N4 composite photocatalyst and mutually-reinforcing synergy to improve hydrogen production in splitting water under visible light radiation[END_REF], etc. A composite between boron carbon nitride and C60 has proven outstanding performance in the electrocatalytic reduction of H + ion [START_REF] Ahsan | Tuning the Intermolecular Electron Transfer of Low-Dimensional and Metal-Free BCN/C60 Electrocatalysts via Interfacial Defects for Efficient Hydrogen and Oxygen Electrochemistry[END_REF], where metal-free 10% C60/BCN nanohybrid synergized that the intermolecular e -transfer.

Carbon Dots

Carbon dots [START_REF] Xu | Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments[END_REF][START_REF] Sun | Quantum-sized carbon dots for bright and colorful photoluminescence[END_REF] are considered one of the essential materials in the nanoscience world [START_REF] Gayen | Carbon dots : a mystic star in the world of nanoscience[END_REF]. There is a debate about their specialty over fullerene. Carbon dots are remarkable eacceptors and donors [START_REF] Liang | Carbon dots: zero-dimensional carbon allotrope with unique photoinduced redox characteristics[END_REF]. They are potential candidates for solar hydrogen production due to their excellent electrical, optoelectronic and photophysical properties [START_REF] Luo | Carbon dots in solar-tohydrogen conversion[END_REF]. It is well known that carbon dots are photoluminescent [START_REF] Carbonaro | On the emission properties of carbon dots : reviewing data and discussing models[END_REF]. This property can be exploited to tune the wavelength to affect the hydrogen production. A case study was carried out using various modified carbon dots [START_REF] Bhattacharyya | Effect of nitrogen atom positioning on the trade-off between emissive and photocatalytic properties of carbon dots[END_REF] as depicted in Figure 5. It is found that there is an enhancement in hydrogen evolution with the duration of irradiation. Maximum was found for the CD2 sample (2g of BPEI is used in the synthesis procedure) (Figure 3a). Stability and retention of the photocatalytic efficiency is depicted through cyclic experiment (Figure 3b). Advantage of this method is that it is best suited at pH close to neutral (Figure 3c). The role of hole scavenger is shown in Figure 3d.

Carbon Nanotubes

CNTs [START_REF] Iijima | Helical microtubules of graphitic carbon[END_REF] are long hollow tubular cylindrical structures in shape. They possess a high surface-to-volume ratio. They have sp 2 -hyridized C-atom. They can be single-walled (SWCNTs), double-walled (DWCNTs), or multi-walled (MWCNTs). In the carbon materials family, CNTs continue to raise much hope worldwide, in the domain of e.g. electronic devices, biosensors, catalysis and nanocomposites [START_REF] Bensghaïer | The Molecular and Macromolecular Level of Carbon Nanotube Modification Via Diazonium Chemistry: Emphasis on the 2010s Years[END_REF][START_REF] Bensghaïer | Painted CNT"@Au nanoparticles: a nanohybrid electrocatalyst of direct methanol oxidation[END_REF].

SWCNT/fullerodendron/Pt(II) supramolecular nanocomposites based on a photosensitizer/co-catalyst interconnecting system have been developed, which can generate hydrogen (QY = 0.015) from H2O in the presence of light (λ = 600 nm). In nano-coaxial photocatalysts, both chiral indices (n, m) and the individuality of these CNTs affect the photocatalytic efficiency [START_REF] Murakami | SWCNT photocatalyst for hydrogen production from water upon photoexcitation of (8, 3) SWCNT at 680-nm light[END_REF]. Enhanced hydrogen synthesis was observed in the case of Mn0.5Cd0.5S/CNTs in which nanotubes serve as an e -acceptor which inhibit the recombination of charges and holes at this composite photocatalyst [START_REF] Meng | Hydrothermal preparation of Mn 0.5Cd0.5S / carbon nanotubes nanocomposite photocatalyst with improved H2 production performance[END_REF]. A study about the Ni/α-Al2O3 catalyst for toluene reforming (for hydrogen production) has revealed that the occurrence of self-growth amorphous carbon on Ni surface decreases the activity of the catalyst by blocking the active site (reactant are not able to reach active site of the catalyst) while self-growth carbon nanotubes increase the stability and activity of the catalyst [START_REF] He | Opposite effects of self-growth amorphous carbon and carbon nanotubes on the reforming of toluene with Ni /α-Al2O3 for hydrogen production[END_REF].

When five samples were compared for their hydrogen production efficiency, it was noticed that the combination of TiO2, Pt, and functionalized CNTs have superior performance. Pt (noble metal) has played a significant role in separating photogenerated charged species (electron: e - and hole: h + ) resulting In high photocatalytic activity [START_REF] Abdulrazzak | Sonochemical/hydration-dehydration synthesis of Pt-TiO2 NPs/decorated carbon nanotubes with enhanced photocatalytic hydrogen production activity[END_REF]. The mechanism of the reactions is shown in Figure 4A.

Interestingly, incorporation of MWCNTs in the ZnO composite resulted in 5-fold improved PEC performance. Under solar light irradiation, this porous structure (Figure 4B) caused 458% enhancement of the photocurrent density [START_REF] Prasadam | CNT-ZnO Core-Shell Photoanodes for Photoelectrochemical Water Splitting[END_REF].

Graphene

Graphene [START_REF] Novoselov | Electric Field Effect in Atomically Thin Carbon Films[END_REF] is a one atom thick planar structure in which C-atom is in an sp 2 hybridized state. For the past decades, graphene has dominated the carbon family for many applications, including clean fuel [START_REF] Shah | Current progress on 3D graphene-based photocatalysts: From synthesis to photocatalytic hydrogen production[END_REF]. There are numerous reports on hydrogen production utilizing graphene and its composites, such as CdS@hexagonal boron nitride on reduced graphene oxide (rGO) [START_REF] Li | CdS@h-BN heterointerface construction on reduced graphene oxide nanosheets for hydrogen production[END_REF], and CuO/rGO [START_REF] Yadav | Spongy ball-like copper oxide nanostructure modified by reduced graphene oxide for enhanced photocatalytic hydrogen production[END_REF]. A computational study has been carried out on nitrogen-doped graphene [START_REF] Nishat | Ab initio study of oxygen evolution reaction and hydrogen evolution reaction via water splitting on pure and nitrogen-doped graphene surface[END_REF] in order to understand the characteristics responsible for efficient HER. Density functional theory (DFT) calculations suggested that effective H2O splitting is due to the more conductive system. This is happening because of the Fermi level movement towards conduction band and the intrinsic electronegativity of 'N' moiety.

In an experiment, GO was reduced to RGO and simultaneous impregnation with Zn0.8Cd0.2S

NPs. Figure 6c has the sample labelled as GS followed by some numbers. These numbers are indicative of GO to Zn0.8Cd0.2S weight percentage ratio. The maximum rate of H2 production was obtained at GS0.25. Indeed, as noted from Figures 5a-b, after solar irradiation, sp 3 VB electrons get excited to the conduction band. Further, these electrons in CB of Zn0.8Cd0.2S are transferred to RGO, avoiding or lengthening the recombination step of the photogenerated charges (electrons and holes). RGO also acts as co-catalyst where H + is reduced to H2 [START_REF] Zhang | Noble metal-free reduced graphene oxide-ZnxCd1-xS nanocomposite with enhanced solar photocatalytic H2-production performance[END_REF]. Reprinted from reference [START_REF] Zhang | Noble metal-free reduced graphene oxide-ZnxCd1-xS nanocomposite with enhanced solar photocatalytic H2-production performance[END_REF], with permission of the American Chemical Society.

Graphdiyne

Graphdiyne [START_REF] Li | Architecture of graphdiyne nanoscale films[END_REF] comprises an sp and sp 2 co-hybridized C network [START_REF] Xue | 2D graphdiyne materials: challenges and opportunities in energy field[END_REF]. It has a porous structure (rapid mass transfer and uncover active sites) with a direct band-gap and elevated carrier mobility (better for light absorption) [START_REF] Song | Using graphdiyne ( GDY ) as a catalyst support for enhanced performance in organic pollutant degradation and hydrogen production : A review[END_REF][START_REF] Huang | Graphdiyne: The Fundamentals and Application of an Emerging Carbon Material[END_REF][START_REF] Jia | Synthesis and Properties of 2D Carbon -Graphdiyne[END_REF]. Interestingly, GDY also exhibits antibacterial properties [START_REF] Zhu | Antibacterial Activity of Graphdiyne and Graphdiyne Oxide[END_REF]. GDY itself plays a crucial role in photo-electrocatalysis or act as a support for catalyst [START_REF] Sun | The preparation and use of γgraphdiyne, a superb new photoelectrocatalyst[END_REF]. Each unit cell in GDY consists of two acetylenic linkages. It is comparatively softer than graphene or graphyne [START_REF] Peng | New materials graphyne , graphdiyne , graphone , and graphane : review of properties , synthesis , and application in nanotechnology[END_REF].

Cu atoms anchored on GDY act as an efficient catalyst for HER with selectivity and activity greater than commercial one (20wt.% Pt/C), 18 times higher turnover frequency (30.52 s -1 ) was found [START_REF] Hui | Loading copper atoms on graphdiyne for highly efficient hydrogen production[END_REF]. H2 generation using CuI (156.5 μmol/5 h) and GDY (29.4 μmol/5 h) is less than the In-situ photocatalytic H2 generation over GDY-CuI (465.9 μmol/5 h) composite photocatalyst, viz. 3 and 15.8 times of pure CuI and GDY, respectively. This is an indication of a synergistic effect [START_REF] Li | Distinctive improved synthesis and application extensions graphdiyne for efficient photocatalytic hydrogen evolution[END_REF].

Thus, GDY and related structures are increasingly raising interest in the field of energy. For example: GDY ultrathin nanosheets [START_REF] Liu | Graphdiyne ultrathin nanosheets for efficient water splitting[END_REF], GDY@NiOx(OH)y heterostructure [START_REF] Zhang | Graphdiyne@NiOx(OH)y heterostructure for efficient overall water splitting[END_REF], GDYnickel boron oxide hybrid [START_REF] Yin | In situ synthesis of a nickel boron oxide/graphdiyne hybrid for enhanced photo/electrocatalytic H2 evolution[END_REF], self-active metal-free GDY [START_REF] Hui | Efficient hydrogen evolution on nanoscale graphdiyne[END_REF], porphyrin-based GDY [START_REF] Pan | Crystalline porphyrin-based graphdiyne for electrochemical hydrogen and oxygen evolution reactions[END_REF], and so on for hydrogen production.

Integrated PEC cells are shows that after irradiation with a Xe lamp, it shows photocurrent with applied potential, and H2 generation is observed simultaneously. Gas chromatography (GC)

studies have revealed that H2 production is directly proportional to the time at CdSe QDs/GDY photocathode (0.1M Na2SO4, -0.20 V vs NHE). Also, stable photoelectrochemical activity is observed during 12h test [START_REF] Li | Graphdiyne: a metal-free material as hole transfer layer to fabricate quantum dotsensitized photocathodes for hydrogen production[END_REF].

In another work, graphdiyne was impregnated with CdS. A schematic depiction of the materials synthesis and their photocatalytic mechanism is shown in Figure 6c. The effect of graphdiyne composition on CdS photocatalytic activity is clearly seen in Figure 6b. Another experiment set was carried out with just graphdiyne and a physical mixture of CdS and graphdiyne.

Graphdiyne shows no hydrogen production (Figure 6a), indicating its inactivity towards visible light reaction in the current circumstances. The physical mixture (Figure 7b) does not boost the photo-catalytic activity of CdS, indicating the need for compositing rather than physical contact [START_REF] Lv | In situ synthesis of CdS/graphdiyne heterojunction for enhanced photocatalytic activity of hydrogen production[END_REF]. Reprinted from reference [START_REF] Lv | In situ synthesis of CdS/graphdiyne heterojunction for enhanced photocatalytic activity of hydrogen production[END_REF], with permission of the American Chemical Society.

Nanodiamond

Nanodiamonds (NDs) are well-known for their chemical stability, easy modifications, hardness, low toxicity, thermal conductivity, narrow band-gap (broad absorption-extension to near-infrared region), high surface area, high refraction, extraordinary resistance to photobleaching, fluorescence, biocompatibility, bulk preparations, unique optical and chemical properties [START_REF] Bagheri | Nano-diamond based photocatalysis for solar hydrogen production[END_REF][START_REF] Mishra | Recent progress in nanodiamonds : Synthesis , properties and their potential applications[END_REF][START_REF] Su | Emerging applications of nanodiamonds in photocatalysis[END_REF].

Rh NPs impregnated NDs have been utilized for renewable green energy production (H2 generation) via ammonia-borane hydrolysis (a model reaction to evaluate Rh NPs/NDs catalytic efficiency). Low activation energy (Ea = 25.6 kJ mol -1 ) and high turnover frequency (TOF = 729.4 min -1 ) are observed. The high activity is due to the hydrophilicity nature (presence of oxygen-containing moieties) of NDs, which promotes well dispersion of Rh NPs on its surface. Thus, exposing more active sites for the catalysis [START_REF] Wen | Facile fabrication of Rhodium/Nanodiamond hybrid as advanced catalyst toward hydrogen production from ammonia-borane[END_REF]. Rh NPs are the main catalytic active sites, and NDs act as support. More information can be extracted from Figure 12. Hydrogen generation rate (GHT) relies on Rh content: 0.29-1.16 mM, substrate concentration: 150-300 mM and temperature: 25-40 °C with respect to reaction time.

Generally, photocatalysis is increased by high light absorption and charge transfer [START_REF] Su | Heterostructured boron doped nanodiamonds@g-C3N4 nanocomposites with enhanced photocatalytic capability under visible light irradiation[END_REF]. The large surface area, inhibits recombination of charge carriers (photo-generated), and increased charge transfer can enhance the H2 synthesis by 50% [START_REF] Haleem | Facile synthesis of mesoporous detonation nanodiamond-modified layers of graphitic carbon nitride as photocatalysts for the hydrogen evolution reaction[END_REF].

Biochar

Production of Biochar

Biochar is manufactured via thermal treatment (gasification, torrefaction, vacuum, flash, intermediate, hydro, slow or fast pyrolysis) [START_REF] Wu | Visualizing the emerging trends of biochar research and applications in 2019: a scientometric analysis and review[END_REF]. Numerous parameters [START_REF] Tripathi | Effect of process parameters on production of biochar from biomass waste through pyrolysis: A review[END_REF] govern their properties, namely: (a) type of thermal treatment [START_REF] Ippolito | Feedstock choice , pyrolysis temperature and type influence biochar characteristics : a comprehensive meta-data analysis review[END_REF], (b) residence time [START_REF] Te | Optimization of pyrolysis parameters for production of biochar from banana peels: evaluation of biochar application on the growth of ipomoea aquatica[END_REF], (c) atmosphere (air, N2, H2, Ar, their mixture, etc.), (d) temperature [START_REF] Ippolito | Feedstock choice , pyrolysis temperature and type influence biochar characteristics : a comprehensive meta-data analysis review[END_REF][START_REF] Te | Optimization of pyrolysis parameters for production of biochar from banana peels: evaluation of biochar application on the growth of ipomoea aquatica[END_REF][START_REF] Wystalska | The effect of biodegradable waste pyrolysis temperatures on selected biochar properties[END_REF][START_REF] Nzediegwu | Fuel , thermal and surface properties of microwave-pyrolyzed biochars depend on feedstock type and pyrolysis temperature[END_REF], (e) biomass (feedstock choice) [START_REF] Ippolito | Feedstock choice , pyrolysis temperature and type influence biochar characteristics : a comprehensive meta-data analysis review[END_REF][START_REF] Nzediegwu | Fuel , thermal and surface properties of microwave-pyrolyzed biochars depend on feedstock type and pyrolysis temperature[END_REF] and their mineral composition [START_REF] Zhao | Comparison of biochars derived from different types of feedstock and their potential for heavy metal removal in multiple-metal solutions[END_REF], and (f) heating rate [START_REF] Te | Optimization of pyrolysis parameters for production of biochar from banana peels: evaluation of biochar application on the growth of ipomoea aquatica[END_REF].

Biomass pyrolysis can also be carried out in the presence of a catalyst [START_REF] Norouzi | What is the best catalyst for biomass pyrolysis ?[END_REF]. The feedstock can be obtained from the forest, aquatic, industry, agriculture, human, and animal waste [START_REF] Wu | Visualizing the emerging trends of biochar research and applications in 2019: a scientometric analysis and review[END_REF].

There have been plenty of precursors-used for biochars synthesis: such as cornstalk [START_REF] Zhao | Residue cornstalk derived biochar promotes direct bio-hydrogen production from anaerobic fermentation of cornstalk[END_REF],

seaweed [START_REF] Cao | Seaweed-derived biochar with multiple active sites as a heterogeneous catalyst for converting macroalgae into acid-free biooil containing abundant ester and sugar substances[END_REF], cow dung [START_REF] Yao | Cow dung-derived biochars engineered as antibacterial agents for bacterial decontamination[END_REF], herbal medicine residues [START_REF] Wang | Adsorption of antibiotics from water by using Chinese herbal medicine residues derived biochar : preparation and properties studies[END_REF], brewer's spent grain (BSG) [START_REF] Yoo | Effects of brewer's spent grain biochar on the growth and quality of leaf lettuce (Lactuca sativa L. var. crispa.)[END_REF], freshwater macroalga [START_REF] Michalak | Biochar from a freshwater macroalga as a potential biosorbent for wastewater treatment[END_REF], plastics (PVC, PET, and PE) [START_REF] Singh | Pyrolysis of waste biomass and plastics for production of biochar and its use for removal of heavy metals from aqueous solution[END_REF], orange peel and albedo [START_REF] Adeniyi | Biochar from the thermochemical conversion of Orange (Citrus sinensis) peel and albedo: product quality and potential applications[END_REF], tobacco stalk [START_REF] Yu | From hazardous agriculture waste to hazardous metal scavenger : Tobacco stalk biochar-mediated sequestration of Cd leads to enhanced tobacco productivity[END_REF], oyster shell [START_REF] Lian | Influence of pyrolysis temperature on the cadmium and lead removal behavior of biochar derived from oyster shell waste[END_REF], cinnamon and cannabis [START_REF] Omidi | Biochar obtained from cinnamon and cannabis as effective adsorbents for removal of lead ions from water[END_REF], olive pit [START_REF] Campos | Assessing the effects of biochar on the immobilization of trace elements and plant development in a naturally contaminated soil[END_REF],

date stone [START_REF] Chakraborty | Explication of physically and chemically treated date stone biochar for sorptive remotion of ibuprofen from aqueous solution[END_REF], pig bone [START_REF] Deng | In situ N-, P-and Ca-codoped biochar derived from animal bones to boost the electrocatalytic hydrogen evolution reaction[END_REF], to cite but a few.

Fan et al. [START_REF] Fan | Using sewage sludge with high ash content for biochar production and Cu (II) sorption[END_REF] have reported that sewage sludge was subjected to pyrolysis in a muffle furnace at several temperatures such as 200, 300, 500, and 700 °C for four hours maintained in a nitrogen atmosphere. In another methodology, biochar precursors (lignocellulosic: canola straw, wheat straw, sawdust of white spruce, and non-lignocellulosic: manure pellet) were placed in a quartz reactor enclosed within a microwave-oven maintained in a nitrogen gas environment, where pyrolyzed at low (300 °C), medium (400 °C) and high temperature (500 °C) [START_REF] Nzediegwu | Fuel , thermal and surface properties of microwave-pyrolyzed biochars depend on feedstock type and pyrolysis temperature[END_REF]. Ayala-Cortés et al. [START_REF] Ayala-Cortés | Solar pyrolysis of agave and tomato pruning wastes: Insights of the effect of pyrolysis operation parameters on the physicochemical properties of biochar[END_REF] have reported a pyrolysis experiment utilizing a horizontal solar furnace. Dried agave leaves fibres, and tomato pruning were kept on an alumina crucible. In order to maintain non-stop exposure to concentrated solar energy, precursors were positioned inside the solar reactor, which has a continuous flow of argon gas and coupled temperature monitoring equipment (optical pyrometer and thermocouples).

Synthesis was carried out at various temperatures for agave (450, 600, 800, 900, 1100, 1400, and 1550 °C) and tomato pruning (450, 600, and 900 °C). Biochar preparation via concentrated solar thermal energy is exciting and can be applied to other wastes such as date palm waste [START_REF] Giwa | Pyrolysis of date palm waste to biochar using concentrated solar thermal energy: Economic and sustainability implications[END_REF]. Feng et al. [START_REF] Feng | Chemical-enzymatic fractionation to unlock the potential of biomass-derived carbon materials for sodium ion batteries[END_REF] have worked on a top-down approach where grape pomace, walnut shell, and Brewer's spent grain have been utilized as complex carbon sources. They have played with the components of raw biomass. Non-lignocellulosic components (minerals, proteins, soluble extractives, and fats), hemicellulose, and cellulose were removed at different steps before carbonization, leaving behind just lignin. Manolikaki et al. [START_REF] Manolikaki | Agronomic potential of biochar prepared from brewery byproducts[END_REF] have shown interesting biochar prepared out of Brewery by-products (BSG and surplus yeast). The experiment was carried out in a muffle furnace having a nitrogen supply, and the temperature was allowed to reach 300 °C.

Structure of Biochar and Biochar Cycle

The biochar structure can be described using the formulae:

where m denotes "m" rows in the biochar structure, and n denotes "n" columns (Figure 7a) [START_REF] Xiao | H/C atomic ratio as a smart linkage between pyrolytic temperatures, aromatic clusters and sorption properties of biochars derived from diverse precursory materials[END_REF]. Besides the aromatic chemical structure, it is also essential to consider the H/C atomic ratio resulting from the pyrolysis temperature. Figure 7b displays a plot of H/C versus T obtained by Xiao et al. [START_REF] Xiao | H/C atomic ratio as a smart linkage between pyrolytic temperatures, aromatic clusters and sorption properties of biochars derived from diverse precursory materials[END_REF] for several biomass sources. An empirical correlation is shown in the inset; it permits confidently estimating the H/C ratio expected from the pyrolysis temperature (heating rate -5°C/min and holding time 6h).

C(2mn+2n+2m)H(2+2m+2n) (5) When biochar is in the 2D form, it becomes even more interesting as it acts as a sheetshaped substrate for varieties of structures. For example, Zhao et al. [START_REF] Zhao | Layered NiPS3 nanoparticles anchored on two-dimensional nitrogen-doped biochar nanosheets for ultra-high rate sodium-ion storage[END_REF] have used this surface as a platform to grow NiPS3 nanoparticles. It is found that high-temperature pyrolysis is usually required to make graphitized biochar structure [START_REF] Tam | Synthesis of Porous Biochar Containing Graphitic Carbon Derived From Lignin Content of Forestry Biomass and Its Application for the Removal of Diclofenac Sodium From Aqueous Solution[END_REF]. In recent years, there has been increased attention toward 3D interconnected biochar due to its superiority over 2D in terms of short ion diffusion paths and continuous e -pathways. This topic has been comprehensively discussed by Norouzi et al. [START_REF] Norouzi | Biochar-based composites as electrode active materials in hybrid supercapacitors with particular focus on surface topography and morphology[END_REF].

Modification of Biochar

Usually, biochar contains limited functionalities, small surface area as well as small pore volume. Pores developed in the course of carbonization get blocked by tarred materials during dry pyrolysis. This creates strong need for chemical/physical treatment for better specific surface area, pore structure and volume, in order to exploit its real properties [START_REF] Enaime | Biochar for wastewater treatmentconversion technologies and applications[END_REF][START_REF] Hagemann | Activated carbon , biochar and charcoal : linkages and synergies across pyrogenic carbon's ABCs[END_REF][START_REF] Anto | Activation strategies for biochar to use as an efficient catalyst in various applications[END_REF].

Biological [START_REF] Tao | Enhanced Cd removal from aqueous solution by biologically modified biochar derived from digestion residue of corn straw silage[END_REF] (digestion), physical (magnetic, steam/gas/CO2, grinding/ball mill, and microwave) and chemical (composite, surfactant, oxidizing agents, alkali and acid) processes are three currently investigated ways of biochar modifications [START_REF] Faheem | Application of biochar in advanced oxidation processes: supportive, adsorptive, and catalytic role[END_REF][START_REF] Wang | Preparation , modification and environmental application of biochar : A review[END_REF]. For examplemicropores formation occurs when pretreated with H3PO4, leading to an increase in biochar specific surface area [START_REF] Chu | Phosphoric acid pretreatment enhances the specific surface areas of biochars by generation of micropores *[END_REF], carbon retention and sorption ability [START_REF] Zhao | Roles of phosphoric acid in biochar formation: synchronously improving carbon retention and sorption capacity[END_REF]. However, we have also noted development of biochar porous structure upon maceration of the initial biomass in wateralcohol medium [START_REF] Snoussi | Green, zero-waste pathway to fabricate supported nanocatalysts and anti-kinetoplastid agents from sugarcane bagasse[END_REF], or when catalyst precursors are impregnated on the biomass powder prior to pyrolysis [START_REF] Tang | Unusual, hierarchically structured composite of sugarcane pulp bagasse biochar loaded with Cu/Ni bimetallic nanoparticles[END_REF]. These routes should further be exploited in future developments.

Application of Biochar in Hydrogen Production

Literature has revealed that biochar itself is catalytically active [START_REF] Afolabi | Kinetic features of ethanol steam reforming and decomposition using a biochar-supported Ni catalyst[END_REF]. Currently, the field of biochar has progressed at a remarkable pace for the purpose of hydrogen evolution. Norouzi et al. [START_REF] Norouzi | Superior visible-light photocatalytic activity of biocarbon derived from sewage sludge in the absence of active phase for hydrogen production[END_REF] have made sewage sludge biochar and shown hydrogen production from H2O under artificial solar-light irradiation (visible light) driven. This metal-free photocatalyst has exhibited remarkable photoactivity of 3845 µmol/g of hydrogen yield. This value is more significant than their traditional carbon nanoallotropes such as CNTs and graphene, and carbon nitride. This superior activity is due to the high surface area, electron-conductive nature, inorganic materials, hydrophilic surface functional groups, and aromatic carbon structure. This work is an excellent example of a stable, eco-friendly, low-cost, and sustainable technique [START_REF] Norouzi | Superior visible-light photocatalytic activity of biocarbon derived from sewage sludge in the absence of active phase for hydrogen production[END_REF][START_REF] Norouzi | Superior activity of metal oxide biochar composite in hydrogen evolution under artificial solar irradiation : A promising alternative to conventional metal-based photocatalysts[END_REF] contributing toward sustainable development goals 7:affordable and clean energy (SDG 7) [START_REF] Gupta | Sustainable development goals and inclusive development[END_REF]. Biochar manifests excellent potential as an alternative to commercial active materials in energy storage systems: supercapacitors and Li-ion batteries [START_REF] Pourhosseini | Synthesis of a novel interconnected 3D pore network algal biochar constituting iron nano particles derived from a harmful marine biomass as high performance asymmetric supercapacitor electrodes[END_REF].

Catalytic decomposition of methane over biochar can also produce hydrogen. This concept can be well understood by the reaction given below [START_REF] Patel | Production of hydrogen by catalytic methane decomposition using biochar and activated char produced from biosolids pyrolysis[END_REF][START_REF] Harun | Hydrogen production via thermocatalytic decomposition of methane using carbon-based catalysts[END_REF]:

CH4 C + 2H2 (6)
A comparative study of 12 varieties of biochar manufactured at three temperatures (300, 500, and 700 °C) and obtained from 4 sources (sawdust, wheat bran, peanut shell, and sewage sludge.) was carried out. Experiments with biochar produced at higher temperatures have shown increased hydrogen production compared to the biochar formed at low temperatures [START_REF] Wang | Impacts of different biochar types on hydrogen production promotion during fermentative co-digestion of food wastes and dewatered sewage sludge[END_REF].

Minerals Containing Biochar

In the case of anaerobic digestion (mesophilic) of food waste to increase hydrogen generation, the incorporation of biochar has enhanced the YH (cumulative hydrogen yield) and RH (max. H2 generation rate) by 107 % and 54 %, respectively. Incidentally, Fe, K, and Ca are the key component in biochar which have played a significant role in hydrogen-generating bacteria Clostridium butyricum by enhancing their butyric acid, acetic acid, and prevalence. In this regard, comparatively leached biochar was less productive [START_REF] Sugiarto | Effect of biochar in enhancing hydrogen production by mesophilic anaerobic digestion of food wastes: The role of minerals[END_REF].

Biochar from pig bone powder (prepared by thermal treatment at 700, 800, and 900 °C), in situ co-doped with N, P, and Ca was utilized for electrocatalytic HER. The factors such as better conductivity = 0.053 ± 0.004 Scm -1 , low charge-transfer resistance = 23.9 Ω, high electrochemically active surface area (EASA =18.9 m 2 g -1 ), presence of minerals (N, P, and Ca), and high specific area (~280 m 2 g -1 ) are responsible for remarkable activity [START_REF] Deng | In situ N-, P-and Ca-codoped biochar derived from animal bones to boost the electrocatalytic hydrogen evolution reaction[END_REF].

The doping of nitrogen in biochar directly affects its specific surface area and electrical conductivity. For example, N-doping in biochar derived from corncob has decreased specific surface area and increased electrical conductivity by ~14 times. The value of specific surface area before and after doping are 944 and 831 m 2 g -1 , respectively [START_REF] Zhang | Biohydrogen production amended with nitrogen-doped biochar[END_REF].

Biomass-derived carbide

With 1 M KOH electrolyte, HER electrocatalyst was manufactured from treating watermelon rinds powder with hexaammonium heptamolybdate tetrahydrate (NH4)6Mo7O24⋅4H2O) and fumaric acid (C4H4O4). Pyrolysis was conducted at 800, 850 and 900 °C under nitrogen gas and the resulting biochars exhibited Tafel slope 71, 75 and 78 mVdec -1 with electrocatalytic performance (η/10mAcm -2 ) 133, 147, and 177mV, respectively. The remarkable HER performance is due to the facile mass transfer of electrons, unique pore size, and large surface area [START_REF] Fakayode | Simplistic two-step fabrication of porous carbon-based biomass-derived electrocatalyst for efficient hydrogen evolution reaction[END_REF].

Another study reported that N, K Co-activated biochar (feedstock-pomelo peel) was exploited as a source for molybdenum carbide. Effective HER is due to populated active sites, large specific surface area (611 m 2 g -1 ), facile e -transfer, and enhanced intrinsic activity [START_REF] Guo | K Co-activated biochar-derived molybdenum carbide as efficient electrocatalysts for hydrogen evolution[END_REF].

Metallic Catalysts Loaded on Biochar

The combination of iron nanoparticles in the metallic state (zero valent) and biochar has synergistically improved grass fermentation induced by bacteria, therefore, leading to biohydrogen generation (89.8% increment) [START_REF] Yang | Synergistic enhancement of biohydrogen production from grass fermentation using biochar combined with zero-valent iron nanoparticles[END_REF]. As a matter of fact, biochar constitutes a nutrient for microbial growth whilst Fe(0) removes oxygen from the medium of fermentation medium (Equation ( 6)) thus improving the activity of oxygen-sensitive hydrogenase. In addition, the production of Fe 2+ from Fe(0) by electrolysis produces H2 and harnesses hydrogenase activity in the system (Equation ( 7)).

Fe(0) + O2 + 2H2O → 2Fe 2+ + 4OH - (6)
Fe(0) + 2H2O → Fe 2+ + 2OH -+H2

In the case of steam reforming of CH3COOH, metals like Ni, when supported on biochar activated with alkali KOH and HNO3, give liquid hourly space velocity (LHSV)=10h -1 at 700 °C with a H2 yield of 71.2% [START_REF] Chen | Hydrogen production via steam reforming of acetic acid over biochar-supported nickel catalysts[END_REF].

Metal Oxide Catalysts Loaded on Biochar

A microwave hydrothermal technique was employed to prepare mesoporous biochar/ZnFe2O4 composite. Biochar plays a key role as an e -mediator, which has promoted vis-light photocatalytic H2 production [START_REF] Chen | Facile fabrication of mesoporous biochar/ZnFe2O4 composite with enhanced visible-light photocatalytic hydrogen evolution[END_REF]. Figure 10 compares the hydrogen evolution rate using photocatalyst and biochar-supported catalyst (Figure 8b) with the highest performance for BFZ-5 (biochar/ZnFe2O4 mass ratio=5/1). The photocatalyst could be employed at least three times without significant hydrogen evolution loss. Figure 8c depicts possible mechanisms of photocatalyzed dihydrogen production. BZF composite is shown in (c). Reproduced from ref. [START_REF] Chen | Facile fabrication of mesoporous biochar/ZnFe2O4 composite with enhanced visible-light photocatalytic hydrogen evolution[END_REF] with permission of Elsevier.

Liu et al. have reported that biochar can be a good substrate. Its surface can be utilized to create several morphological structures. They have demonstrated this by synthesizing biochar (elmmoney feedstock) and using the one-pot solvothermal method to create a MnFe2O4 sphere (hollow and porous in characteristics). With current density= 10 mAcm -2 , little overpotential (156 mV) is observed. They claim that the current technology could be regarded as a potentially general approach for bifunctional electrocatalysts, as well as various other transition metal oxides that can be utilized in the field of water splitting with the concept of saving energy [START_REF] Liu | Hollow porous MnFe2O4 sphere grown on elm-money-derived biochar towards energy-saving full water electrolysis[END_REF].

Biochar Composite with other Carbon Materials

Due to potentiality of biochar, currently there has been lots of efforts to make nanocomposite of biochar with other carbon materials such as growth of CNTs on biochar [START_REF] Zhang | Production of carbon nanotubes on bio-char at low temperature via microwave-assisted CVD using Ni catalyst[END_REF]. Another example: a study has been conducted by Zhou et al. [START_REF] Zhou | 3D g-C3N4/WO3/biochar/Cu2+-doped carbon spheres composites: Synthesis and visible-light-driven photocatalytic hydrogen production[END_REF] on a composite consisting of g-C3N4, biochar, WO3 and Cu 2+ -doped carbon sphere (Figure 9) for visible light driven H2 evolution, they demonstrated that biochar was acting as a platform on which stable Z-scheme heterojunction was made.

Figure 9. Hydrogen production efficiency of 3D g-C3N4/WO3/biochar/Cu 2+ -doped carbon spheres (CWB-CuCs). Reprinted from the reference [START_REF] Zhou | 3D g-C3N4/WO3/biochar/Cu2+-doped carbon spheres composites: Synthesis and visible-light-driven photocatalytic hydrogen production[END_REF],with permission from Elsevier.

To conclude, biochar in supported photocatalytic systems, apart from playing an obvious role as support for nanoparticles [START_REF] Lopes | Biochar as a support for nanocatalysts and other reagents: Recent advances and applications[END_REF], enhances the active sites and surface area by welldistributing nanoparticles on its surface. In addition, biochar acts as an electron reservoir, reduces band-gap energy, serves for shuttling electrons, and promotes charge separation [START_REF] Mian | Recent progress in biochar-supported photocatalysts: Synthesis, role of biochar, and applications[END_REF].

No doubt that biochar and related materials help in overcoming the problem of high overpotential and small H2 yield in water splitting, a green and efficient way to get hydrogen [START_REF] Yang | Biochar nanocomposite derived from watermelon peels for electrocatalytic hydrogen production[END_REF][START_REF] Amikam | Biochar-assisted iron-mediated water electrolysis process for hydrogen production[END_REF]. It is important to know that biochar, depending on their synthesis methodology, they mimic graphene [START_REF] Tang | π-π stacking derived from graphene-like biochar/g-C3N4 with tunable band structure for photocatalytic antibiotics degradation via peroxymonosulfate activation[END_REF][START_REF] Wang | Preparation of novel biochar containing graphene from waste bamboo with high methylene blue adsorption capacity[END_REF]. So, it may be seen as an interesting surface, like graphene, to catalyze several reactions directly, or as a platform on which different photocatalysts could be immobilized and dispersed. There are different models reported in the literature to describe the structure of coal and activated carbon [START_REF] Smith | Structural analysis of char by Raman spectroscopy : Improving band assignments through computational calculations from first principles[END_REF] as shown in Figure 10. According to Rosalind

Franklin, carbon obtained by pyrolysis of organic materials can be categorized into two categories: graphitizing or non-graphitizing (small, randomly oriented graphene like sheets), as shown in Figure 10b and Figure 10c, respectively [START_REF] Franklin | Crystallite growth in graphitizing and non-graphitizing carbons[END_REF]. It is also interesting to discuss that there is structural resemblance of non-graphitizing carbon with fullerene (presence of fullerene like sheets) as shown in Figure 10a [START_REF] Harris | Imaging the atomic structure of activated carbon[END_REF]. It shows the curved nature of carbon fragments containing cyclopentane, cyclohexane and cycloheptane-ring systems. Figure 10d, showing the 2D coal structure, points to possible carbon matrix with defects and complex oxygenated groups [START_REF] Shinn | From coal to single-stage and two-stage products : a reactive model of coal structure[END_REF]. These models are excellent proof of connection between biochar and other nanocarbon, more specifically, graphene and fullerene. These are the reason that makes scientist to also think about making composite of biochar with nanocarbons. For example, a report by Zhang et al. [START_REF] Zhang | Microwave-assisted synthesis of biochar-carbon-nanotube-NiO composite as high-performance anode materials for lithium-ion batteries[END_REF], have developed biochar-CNT-nickel oxide composite for energy applications. by Shinn [START_REF] Shinn | From coal to single-stage and two-stage products : a reactive model of coal structure[END_REF], reproduced with permission from Elsevier.

Comparison of hydrogen production efficiency of Biochar and other Carbon Allotropes

The hydrogen production efficiency of a catalyst is calculated based on three factors: the amount of hydrogen produced, the amount of catalyst taken, and the time duration. The efficiency of a cell can be shown using the following equation [START_REF] Kim | Toward practical solar hydrogen productionan artificial photosynthetic leaf-to-farm challenge[END_REF]: Depending on the type of biomass used, biochar's inherent chemical composition varies. It should thus be stressed that the cost-effectiveness of biochar makes it highly interesting for its commercial use. In contrast, fullerene, MWCNTs, and graphene do not exhibit any hydrogen production activity; they are catalytically inactive toward hydrogen production reaction for that particular condition. 0 µmol [START_REF] Peng | Hydrothermal preparation of multiwalled carbon nanotubes (MWCNTs)/CdS nanocomposite and its efficient photocatalytic hydrogen production under visible light irradiation[END_REF] a calculated considering the mass of catalyst and catalyzed reaction time.

STH = [ 𝑂𝑢𝑡𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑠 𝐻2 𝐸𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑠𝑜𝑙𝑎𝑟 𝑙𝑖𝑔ℎ𝑡 ] (6)

Future Prospects

The future of carbon nanoallotropes is bright by taking advantage of the synergy between two or more nanocarbons. For example, when C60 fullerene (high e -attracting ability) and graphene (conductivity) collaborate, it enhances the hydrogen production ability of pure g-C3N4 by ~540 times. They enhance the photogenerated charge separation and increase the usage efficacy and migration ability of photogenerated electrons [START_REF] Song | C60/graphene/g-C3N4 composite photocatalyst and mutually-reinforcing synergy to improve hydrogen production in splitting water under visible light radiation[END_REF]. Focusing on this can enable the expansion of green energy areas for sustainable development. Carbon dots have the potential to dominate the zero-dimensional materials [START_REF] Liang | Carbon dots: zero-dimensional carbon allotrope with unique photoinduced redox characteristics[END_REF]. Interestingly, due to their response to light stimulation, they have high potential as photocathodes for hydrogen evolution reactions from water in a metal-free photo-electrochemical set-up [START_REF] Paul | Stimuli responsive multicolour fluorescence emission in carbon nanodots and application in metal free hydrogen evolution from water †[END_REF]. Variation in the structure of graphene can create a novel carbon allotrope, such as a biphenylene network [START_REF] Fan | Biphenylene network: A nonbenzenoid carbon allotrope[END_REF].

Also, enriching the biochar with nitrogen before doping with metal and/or non-metal catalysts are expected to improve biochar application in the green generation of hydrogen. In addition, compositing the biochar with metal and non-metal-doped carbon nitride can boost the application of the biochar in hydrogen generation due to the thermal and optical properties of g-C3N4 [START_REF] Sadjadi | Biochar-Based Graphitic Carbon Nitride Adorned with Ionic Liquid Containing Acidic Polymer : A Versatile , Non-Metallic Catalyst for Acid Catalyzed Reaction[END_REF][START_REF] Zhu | Fabrication of the metal-free biochar-based graphitic carbon nitride for improved 2-Mercaptobenzothiazole degradation activity[END_REF]. A new approach that will emerge soon is the biochar-based boron-carbon nitride composited with fullerene. [START_REF] Norouzi | Superior visible-light photocatalytic activity of biocarbon derived from sewage sludge in the absence of active phase for hydrogen production[END_REF][START_REF] Norouzi | Superior activity of metal oxide biochar composite in hydrogen evolution under artificial solar irradiation : A promising alternative to conventional metal-based photocatalysts[END_REF] reactions [START_REF] Ahsan | Tuning the Intermolecular Electron Transfer of Low-Dimensional and Metal-Free BCN/C60 Electrocatalysts via Interfacial Defects for Efficient Hydrogen and Oxygen Electrochemistry[END_REF]. Preparing BCN from biochar will be an additional application to change waste to fortune.

To the best of our knowledge, the chemical attachment of biochar-carbon nanomaterials for hydrogen generation is either absent or, if present, rarely reported in the literature. Despite its potentiality for different applications, availability, and simplicity, it has not been fully exploited, more specifically in the field of green energy. This gap can be a good boost for flourishing the biochar field in H2 production. This field can be a unique area to work.

One of the advantages working in the field of biochar is the natural abundance of its sources.

There are plenty of precursors that can be used which is not yet explored or rarely utilized for biochar production. Due to the constant increase in overall population growth in the upcoming era, the cost of materials can be a crucial factor for their commercialization. In this regard, biochar can be one of the first candidates on the materials list.

Conclusions

Biochar is a unique carbonaceous material that is a hot topic for research in the current world.

To improve the circular economy, biochar is an ideal material. Various fascinating precursors can be used to synthesize biochar, such as brewer's spent grain or sugarcane bagasse. One of the exciting facts about biochar is that its extent of graphitization can be easily controlled by playing with temperature for their manufacturing. Sometimes, biochar may mimic the structure of graphene can be very interesting for application involving low-cost ecofriendly biocarbon catalyst. It is interesting to say that carbon fragments in biochar may resemble fullerene. This resemblance with previously known nanocarbons brings an important outlook for biochar future. Biochar composite is an extraordinary and promising alternative candidate to carbon nanoallotropes for the application of hydrogen production. It is also a material that can play a significant role in metal-free catalysis. Its properties are depended on several parameters, including the type of feedstock, pyrolysis process (temperature, methodology, residence time, atmosphere, heating rate, etc.), and mineral composition. Hitherto, much work was devoted to wastewater treatment, and, comparatively, energy applications were less investigated. From what has been stated above, one could expect dramatic progress in the coming years in the utilization of biochar for energy and particularly hydrogen production.

Figure 1 :

 1 Figure 1: Different carbon allotropes. Reprinted from the reference [4] under ACS Author Choice.

Figure 2 :

 2 Figure 2: Plot of publication year of papers published in the field of biochar and carbon materials for hydrogen production vs number of publication. For biochar data, it is multiplied by ten times.

Figure 3 .

 3 Figure 3. Photocatalytic H2 production. (a) Xe lamp irradiated various carbon dots samples for H2 generation, (b) 5 cycles of H2 generation through CD2, (c) effect of pH, and (d) effect of hole scavenger (CH3OH). Reprinted from reference [81] , © The Author(s) 2017 , which is under a Creative Commons Attribution 4.0 International License.

Figure 4 (

 4 Figure 4(A). Ternary composite (Pt-TiO2/fCNTs) reaction mechanism, reprinted from reference [87], under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence and Figure 4(B) Si-ZnO and (b) CNT-ZnO photoanodes in PEC water splitting, reprinted from reference [88] , under Creative Commons Attribution License.

Figure 5 .

 5 Figure 5. (a) Charge transfer and separation in RGO-Zn0.8Cd0.2S system, (b) Mechanism of hydrogen generation, and (c) Comparison of photocatalytic activity of different samples.

Figure 6 .

 6 Figure 6. (a) hydrogen production using CdS, GD and different CdS/GD samples, (b) dependence on time, and (c) mechanism of photocatalytic process at CdS/GD composite.

Figure 7 .

 7 Figure 7. Chemical composition of biochar: (a) aromatic clusters and their expected H/C ratios; (b) experimental H/C ratio versus pyrolysis temperature. Empirical formulae to determine H/C versus temperature is shown in the inset. Reproduced from [144], licensed under commons attribution 4.0 International License.

Figure 8 .

 8 Figure 8. H2 evolution rate for ZnFe2O4 reference sample and BZF composites (a), and for BZF-5 vs time and run number. The proposed mechanism of catalyzed H2 production over

Figure 10 .

 10 Figure 10. Various models to explain the structure of coal and activated carbon [181]. (a) Shown by Harris et al. [183], reproduction permission obtained from IOP publishing, (b,c) Hypothesized by Franklin [182], reproduced with permission from Royal Society and (d) given

  hydrogen conversion efficiency, AQE = Apparent quantum efficiency, AQY = Apparent quantum yield, n = no. of e -s required for the reaction ( ̴ 2 for photocatalytic hydrogen generation), I = rate of incident photons, R = rate of hydrogen generation.

Table 1 .

 1 Hand-picked published reviews having the concept of biochar and carbon nanomaterials for energy storage and production.

	Running title	Scope and main lines of the review	Year
			[ref.]
	Recent advancement on	Photocatalytic applications of biochar based semiconducting	2022
	the sustainable biochar	materials.	[26]
	Hydrogen production	Carbonaceous photocatalyst (graphene, graphite, carbon nanotubes,	2022
	from water splitting	activated carbon), non-carbonaceous photocatalyst, co-catalyst,	[27]
		sacrificial agents, and the factors affecting photocatalytic reactivity.
	Electrochemical energy	Rice biomass waste valorization (biochar) for electrochemical	2022
	storage electrodes	energy storage.	[28]
	Graphdiyne based	GDY electronic structures and fundamental lattice. Graphdiyne-	2021
	catalysts for energy	based energy storage materials and electrocatalyst.	[29]
	Biochar-based	Chemically and physically treated biochar-based hybrids and	2020
	composites as electrode	comparision of their electrochemical performance specifically in the	[30]
		energy storage system.
	Recent advances in	Metal (noble metals, non-noble metals & alloys), non-noble metal	2020
	electrocatalytic	compounds (transition metal oxides, nitrides, dichalcogenides,	[31]
	hydrogen evolution	phosphides, carbides, and borides), and metal-free hydrogen
		evolution reaction electrocatalyst.
	Low dimensional	Graphene, GO/rGO, graphitic carbon nitride, and graphene quantum	2020
	carbon-based catalysts	sheets or dots for H2O splitting through photo/electrochemical and	[32]
		photo-catalytically.
	Emerging applications	Energy conversion and storage (fuel cell, batteries, H2 generation	2019
	of biochar-based	and storage, supercapacitors, and oxygen electrocatalysts.) in	[33]
	materials	biochar related materials.
	Carbon-based	Synthesis, properties, and application (biomedical and	2018
	nanomaterials/allotropes:	electrochemical energy storage) of different carbon materials. It	[34]
		includes the discussion about carbon derived from biomass also.

Table 2

 2 compares the hydrogen production activity of different carbon-based catalysts (biochar and other allotropes of carbon). The values reported in the literature were given in different units; we have thus considered the time or the amount of catalyst used to compare the various activities. Surprisingly, biochar shows remarkable catalytic activity, up to 80% of the synthetic graphitized nanodiamond, but two-fold that of graphdiyne. The reason behind this peculiar hydrogen generation performance of biochar can be attributed to the inherent mix of mineral

composition (in terms of mg/Kg dry wt, As (<0.72, Cd (12.36), Cr (292.72), Co (12.68, Cu (296.63), Pb (195.93), Hg (<0.23), Mo (4.67), Ni (66.25), Se (<1.40) and Zn (896.21)) [159].

Table 2 .

 2 Comparison of hydrogen production activity of biochar with other carbon nanoalltropes.

	Materials	Reagent		Light	Hydrogen	Reference
				source	production
	Fullerene	Aqueous soln.	Visible	0 µmol/h/g	[67]
	(C60)	with		light
		triethanolamine	(λ>420
		and eosin Y.	nm), 300W
				Xe lamp.
	Pure	Aqueous soln.	Visible
	MWCNTs	of 0.25 M	light,(λ≥
		Na2SO3	and	420	nm)
		0.35 M Na2S

  Boron carbon nitride (BCN)/C60 has proven to have an outstanding electrocatalytic H2 evolution/oxidation reactions and O2 evolution or reduction

		(scarifical	(300 W Xe
		reagent).		lamp
	Graphene	Aqueous soln.	λ≥ 420 nm,	0 µmol/h	[188]
		of	0.05M	200 W Xe
		Na2SO3	and	lamp
		0.1M Na2S
	Graphdiyne	15%	v/v	5-W light	29.4µmol/5h	[104]
		triethanolamine	emitting	(588 µmol/h/g) a
		solution (ph=9)	diode lamp
	Graphitized	Water		532	nm,	32 µmol/h	[189]
	Nanodiamond			Nd:YAG	(1600
				lase pulse	µmol/h/g) a
	Biochar	Water	with	300W		3845µmol/g
		10vol%		Xenon arc	(1282
		triethylamine	lamp,		µmol/h/g) a
				420nm

Acknowledgments Conflict of Interest References

Acknowledgments: Arvind K. Bhakta is thankful to bourse WBI Excellence World (N° Imputation-101386, and Article Budgétaire-33.01.00.07).

Conflicts of Interest:

The authors declare no conflict of interest.