English

• 1. Introduction

  Burning fossil fuels and increasing solid organic waste are significantly contributing adverse effects on both environment and public health [1]. Addressing the urgency to combat climate changes while meeting global energy demands, China has outlined its 14^th Five-Year Plan as an opportunity to align long-term climate goals with short-to-medium social and economic development plans [2]. In this context, converting waste biomass into high-value chemicals to take place of fossil chemicals is an important way to achieve the goals of "carbon neutrality" and "carbon peaking" [3]. Therein, anaerobic fermentation, a well-documented technique for waste management, shows great potential in energy recovery to produce high-value fuels or chemicals to reduce fossil fuel usage globally, ensuring a clean and environmentally-friendly disposal of organic wastes [4-6].

  Anaerobic fermentation is a complex biochemical process and consist of four sequential stages: hydrolysis, acidogenesis, acidogenesis, acetogenesis and methanogenesis [7]. It is supposed to produce valuable substances such as methane, hydrogen, short chain fatty acids (SCFAs) and medium chain fatty acids (MCFAs) [8-10]. Among them, MCFAs are identified as a more ideal platform compound and have higher economic value for producing various industrial chemicals, due to their special characteristics of strong hydrophobicity and high energy density [11]. Rather than produced from traditional petrochemical processes or plant and animal oils [12], MCFAs carried out by microbial chain elongation (CE) process through anaerobic fermentation biotechnology, are more environmentally-friendly and sustainable under the "two carbon" background.

  However, there are many inhibitory factors in the CE process. Especially when converting complex organic compounds to MCFAs, substrate (electron donors/acceptors) shortage, by-products formation, products/substrate toxicity, fermentation parameters, enzyme activities, products separation and extraction are all the main restriction factors [11, 13-15], which could intensively decrease the fermentation efficiencies of functional microbial communities with low yields and high costs. Therefore, using enhancing strategies is essential to improve fermentation efficiencies, like filler/carrier/supporting media addition as relevant researches revealed [16]. He and Luo et al. studied the effects of adding phenol to CE process can increase the abundance of genes in FAB pathway, adding Fe₃O₄ can remarkably shorten the reaction time, eliminate the toxic effect of undissolved carboxylic acid on microorganisms, and promote the reaction process of fermentation system [16].

  Among these technical measures, the production cost of additives above-mentioned is relatively high, making it difficult to implement in large-scale production applications. Carbon-based materials were produced by thermal decomposition of biomass, have become an efficiency enhancing strategy in the anaerobic fermentation system due to the superior characteristics of raw material diversification, low cost, rich surface functional groups, high carbon content, high surface area, and high adsorption capacity and immobilization effect [17-19]. Studies have demonstrated that carbon-based materials can be serve as additives in fermentation system to enhance carbon sequestration capacity, shorten the lag period, increase products yields, improve electron transfer efficiency and enrich functional microbiomes [6, 20]. For instance, biochar addition significantly increased the cumulative caproate concentration to 21.6 g/L and reduced the lag-phase of 2.3-fold compared to the CE fermentation system without biochar [21]. The biochar incorporation also drastically altered the microbial community structure during a short operation period and enriched the most predominant genera [22]. However, most recent reviews only focus on the microbial interaction mechanisms underlying biochar during anaerobic fermentation, there remains lack of a macro-level description regarding the current research status and also scarce of relevant literature reviews on the mechanism of carbon-based materials producing MCFAs via CE in waste fermentation more detailed.

  Compared with traditional review methods, bibliometrics offers significant advantages in macro-research advantages to analyze development trends and hotspots using literatures as data base [23, 24]. Currently, the added carbon-based materials often refer to porous carbon materials (named as biochar, char, biocarbon, activated carbon, etc.), graphene and other modified carbon materials like iron-based biochar. To better reflect the use of carbon-based materials in anaerobic fermentation system, the publications numbers about these terms aforementioned from 2000 to 2023 in web of science core collection (WOS) are searched (Table S1 in Supporting information) and analyzed as the framework shown in Fig. S1 (Supporting information). During the data retrieval process, we employed both core and extended keyword screening methods. The data were comprehensively analyzed with the assistance of VOSviewer and CiteSpace software. The number variation of publications showed the trend of carbon-based materials in anaerobic fermentation becoming a research hotspot. Among them, applying carbon-based material to microbial CE system, indicating an increasing tendency in the past few years, which have made outstanding contributions to the scientific research of MCFAs synthesis.

  Along with the bibliometric analysis results, this study will start by introducing the current status of anaerobic fermentation, as well as the microbial CE technology. Then, the use of carbon-based material in anaerobic fermentation is summarized and discussed, especially the application and mechanism of adding carbon-based materials in CE-produced MCFAs. Finally, this review aims to present state-of-art knowledge of carbon-based materials addition in enhancing MCFAs biosynthesis efficiency, by pointing out shortcomings of current researches and providing suggestions for future research.

  2. Current status and application of carbon-based materials in anaerobic fermentation

  In light of the heightened consciousness towards environment conservation and the escalating energy requirements, anaerobic fermentation technology has experienced accelerated progress, witnessing widespread adoption across a multitude of sectors including industrial wastewater management, agricultural waste processing, and livestock and poultry manure treatment [25-27]. A multitude of factors influence the efficiency of anaerobic fermentation, encompassing but not limited to temperature, pH levels, carbon-to-nitrogen ratio, organic loading rate, retention time, essential nutrients, and sludge concentration [28-30]. Accordingly, enhancing the efficiency of anaerobic fermentation can be effectively achieved through methods such as co-digestion, substrate pretreatment, addition of supplements, and parameter optimization, all of which facilitate target substrates degradation and enhanced product formation [31, 32]. Among them, the incorporation of efficacious carbon-based material has emerged as a promising strategy for augmenting the overall fermentation efficiency.

  Fig. 1 shows the publication trends over the past two decades regarding anaerobic fermentation technology and the application of carbon-based materials within the context of anaerobic fermentation processes. From 2018 to 2023, research in this domain entered a phase of robust growth, with the annual number of publications rising from 166 to 396, and showing a continuing upward trend. Among these, articles and review papers constitute the principal types of scholarly output. The United States was the first country to initiate relevant studies [33], while China leads in terms of the total volume of published literature, accounting for 29.59% of the global share (Fig. S2 in Supporting information).

  Figure 1

  

  Figure 1.  Publication outputs and types analysis during 2000–2023.

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  Furthermore, by analyzing the literature within the WOS database that we established, it can be observed that the keyword co-occurrence network analysis results from the past decade can be categorized into three distinct stages (Fig. 2). From 2012 to 2014, the research focused on utilizing the high adsorption properties of carbon-based material, like biochar, to mitigate potential negative environmental impacts during the substance transformation in anaerobic fermentation [7, 34]. During 2015–2019, "hydrothermal carbonization - interspecific electron transfer - carrier properties - CH₄ production" have become the research hotspots. For example, Ipiales et al. concluded that anaerobic fermentation appeared as a potential solution allowing significant reduction of the organic load from hydrothermal carbonization treatment, while producing methane-rich biogas, thus contributing to energy recovery [35]. Carbon-based materials can also initiate diverse forms of direct interspecies electron transfer (DIET) through its inherent conductivity and surface redox functional groups, thereby bolstering the electron transfer efficiency among syntrophic oxidizing microorganisms [36-38]. Moreover, recent research (2020–2023) has substantiated that the innovative application of carbon-based materials is a promising and sustainable pathway for the efficient conversion of organic waste into renewable energy resources.

  Figure 2

  

  Figure 2.  Time zone view during 2012–2023.

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  Based on the literature in WOS database that we established, we also found that frontier research since 2010, especially focused on exploring how carbon-based materials play positive roles through various mechanisms, such as suppressing excessive accumulation of acids/ammonia to optimize the environmental friendliness of the conversion process [39, 40]; enhancing the overall energy recovery efficiency to ensure maximum capture and utilization of the chemical energy embedded within the waste during the transformation process [41-43] investigating the adsorption, stabilization, and even degradation effects of these materials on hard-to-degrade pollutants like heavy metal ions, polycyclic aromatic hydrocarbons, and other hazardous chemicals, as well as biological risk factors like antibiotic resistance genes [44-46], aiming to achieve deep purification and safe disposal in the resource utilization process of waste treatment systems (Fig. S3 in Supporting information). For instance, Hao et al. discovered that a novel three-dimensional mesh-like magnetic biochar can enrich and support dominant species associated with polycyclic aromatic hydrocarbons (PAHs) metabolism, significantly enhancing the degradative function of PAHs and the methane production capability of the anaerobic fermentation system [46]. While the application of carbon-based materials in anaerobic fermentation has indeed seen some progress, its purpose extends beyond merely addressing technical bottlenecks in the energy conversion process of organic waste. It also aims at reinforcing the environmental compatibility and energy utilization efficiency of the entire system, as well as tackling challenges posed by complex pollutants.

  In addition to H₂ and CH₄ production, microbial CE synthesizing MCFAs, is another resource utilization strategy to promote organic waste fermentation to produce valuable acids products. Researches indicate that the unique physical properties and high redox activity of carbon-based materials can facilitate electron transfer during CE process, thereby enhancing substrate utilization and increasing the production of MCFAs [11, 14]. In the next section, the microbial CE, as well as the applications and working mechanisms of carbon-based materials in the production of in MCFAs production will be discussed in detail.

  3. Carbon-based materials in CE

  3.1 Introduction of CE technology

  Microbial CE harnesses electron acceptors (EAs, exemplified by acetate and propanoate, etc.) and electron donors (EDs, hydrogen and organics, etc.) as the fundamental components of microbial metabolism drive substrate transformation into high-value chemicals through diverse biochemical pathways [11, 47]. These pathways mainly encompass, but are not restricted to, the oxidation of EDs, the reverse β-oxidation pathway (RBO), the fatty acid biosynthesis (FAB) pathways (Fig. 3, Tables S2 and S3 in Supporting information), and other reductive branches like the Wood-Ljungdahl pathway [48-50]. EDs serve as precursor materials for the CE process [50, 51]. Oxidation of EDs generates energy, reducing equivalents in the form of NADH and NADPH, and the crucial intermediate acetyl-CoA, which feed into the subsequent RBO and FAB cycles. Then acetyl-CoA, by virtue of its 2-carbon unit, adds two carbons to the EA molecule with every cycle, playing a central role in systematically extending EA molecules such as acetic acid into longer-chain fatty acids, e.g., butyrate and caproate, catalyzed by concerted action of multiple enzymes [51, 52].

  Figure 3

  

  Figure 3.  Carbon chain elongation pathway mechanism.

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  MCFAs and their derivatives, emerging as key intermediate chemicals from the conversion of waste biomass valorization, present a broad spectrum of potential applications. Despite their promise, studies reveal that the production yield of MCFAs can be influenced by a multitude of inhibitory factors, including but not limited to temperature, pH, the electron donor-to-acceptor ratio, fermentation techniques, and operational parameters such as hydraulic retention time (HRT), organic loading rate (OLR), and hydrogen partial pressure (pH₂). For instance, elevating the OLR can result in the accumulation of volatile fatty acids (VFAs) and hydrogen, which consequently lowers the system pH to levels that hinder microbial synthesis activities, thereby diminishing caproate output and productivity [53]. Grootscholten et al. documented that shifts in microbial community structure and growth patterns occur at temperatures exceeding 40 ℃, concurrent with the heightened activity of methanogenic bacteria, which leads to a significant conversion of short-chain fatty acids (SCFAs) to methane, indirectly suppressing the production of MCFAs [54]. Furthermore, elevated hydrogen partial pressures, particularly above 0.03 atm, are known to impede the anaerobic oxidation step of MCFAs, a critical component of the carbon chain elongation process, thus necessitating a careful balance to prevent the over-oxidation of ethanol to acetate [55].

  Current methodologies for MCFAs synthesis are beset by these inhibitory constraints, often displaying suboptimal yields and efficiency. To overcome these limitations and augment the economic feasibility and efficacy of MCFAs production, various strategies must be explored, such as employing additives, optimizing process parameters, and refining microbial strains. For instance, Wang et al. found that the production and selectivity of MCFAs were effectively promoted when zerovalent iron (ZVI) was added at concentrations ranging from 1−20 g/L in the reactor. They achieved a maximal MCFAs yield of 15.4 g COD/L and selectivity of 71.7% at 20 g/L ZVI, corresponding to a 5.3-fold and 4.8-fold increase over non-ZVI conditions [56]. The results showed that ZVI declined the oxidation−reduction potential, fostering a more conducive anaerobic fermentation environment and increasing electron transfer efficiency from EDs to EAs [56]. Similarly, Ren et al. successfully raised caproate production by 21% through the implementation of a 0.05 Ⅴ electric field in a continuously operated CE bioreactor for 150 days [57]. The weak electric field improved substrate utilization efficiency, and it was observed that the abundance of key functional genes associated with CE pathway enzymes and the functional-level population of microbes increased, evidenced by enhanced transmembrane transport and energy metabolic activities [57]. Thus, continuous exploration and implementation of these and other innovative strategies hold the promise of dramatically improving substrate conversion efficiency and, subsequently, significantly boosting the performance of MCFAs biosynthesis processes.

  3.2 Application of carbon-based materials in CE

  Previous literatures have highlighted the benefits of incorporating carbon-based materials like biochar, activated carbon, and graphene into microbial CE systems to abbreviate the lag phase and boost the content of MCFAs in the products. Based on the analysis of literature from 2000 to 2023 in the WOS database we constructed, we further identified several applications of carbon-based materials in CE systems. Additionally, new studies published in 2024 are also discovered relevant to this research, with specific timeline shown in Fig. 4 and Table S4 (Supporting information).

  Figure 4

  

  Figure 4.  Timeline overview of carbon-based materials in CE (detailed information is in Table S4).

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  Researchers reported that the appropriate amount of carbon-based material indeed has a positive effect on CE. For instance, Liu et al. were among the first to introduce biochar into the CE system using anaerobic sludge from paper mill, discovering that a dosage of 20 g/L biochar reduced the lag phase by 2.3-fold and significantly increased the caproate concentration to 21.1 g/L [21]. Ghysels et al. also reported a substantial improvement in the selectivity for caproate yield upon the introduction of activated carbon in the fermentation system, especially at high dose, could potentially increase the selectivity towards caproate to 92%, a marked increase from the 84% observed under control conditions [58]. Besides, Xiang et al. in their study elucidated that a concentration of 15 g/L of powder activated carbon in the anaerobic acidogenic reactor led to a 4-fold enhancement in the daily output of MCFAs in the system relative to the un-augmented control [59].

  The researchers found that adding carbon-based materials with different particle sizes exerts differential impacts on the performance of CE fermentation systems. Liu et al. undertook comparative assessments on biochar with different particle sizes, uncovering that after a 6-day fermentation period, the biochar fraction with particles less than 5 mm yielded a caproate concentration of 32.81 mmol/L, significantly surpassing the 1.11–2.34 mmol/L range observed in other fractions [22]. By day 31, the selectivity for caproate, a principal objective product in CE, reached a remarkable 93.56%, which underscored the substantial enhancement in caproate yield facilitated by smaller particle-sized biochar [22]. Complementary to this, Wu et al. conducted experiments examining biochar with two different particle size ranges (2000–5000 and 75–150 µm), observing that a decrease in particle size from 2000−5000 µm to 75−150 µm led to a notable rise in MCFAs production increased by 0.9 ± 0.2 g COD/L, representing a 12.2% ± 0.4% increment over the control [59]. Compared with large-particle size of biochar, the unique properties of small-particle size biochar were mainly determined by the significantly higher specific surface area, carbon content, true density, and K⁺ content in aqueous solution [22]. Unique properties and structure were the critical factor for the removal of toxic substances [22], which in turn increases the yield of MCFAs.

  Researchers also discovered that diversified carbon-based materials show significant differences in their capacity to facilitate the CE process, a phenomenon attributed to the inherent variations in physicochemical characteristics, which encompass differences in pore structure, distribution of surface functional group, specific surface area, stability and so on. Wu et al. added pyrochar into an anaerobic fermentation system and found that at an optimal mass ratio of pyrochar to sub-strate of 2 g/g, the yield and selectivity of MCFAs peaked at 13.67 COD/L and 56.8%, marking a 115% and 128% increase over the control group, respectively [60]. The results of the present study were attributed to the ability of pyrochar, with its porous structure and high electrical conductivity (42.42 µS/cm), to mediate biological reactions [22]. The enhanced conductivity of the microbial system, resulting from the addition of pyrochar, was a critical factor in promoting the CE process. Luo et al. investigated the impact of biochar sourced from different sources (coconut shell-derived biochar, corn stalk-derived biochar rice stalk-derived biochar) during sludge anaerobic fermentation [39]. Their findings revealed all biochar-amended treatments outperformed the control in terms of accumulated MCFAs, with the biochar derived from coconut shell yielding the highest accumulation at 254.15 mmol C/L, translating to a 30.37% increase as compared with the control [39]. Research by Li and Xie et al. found that coconut shell-derived biochar had the most ultra-micropores with sizes smaller than 5 µm, large specific surface area (147.8 m²/g) and total pore volume (0.122 cm³/g) [39]. Abundant pore structures and large specific surface area were conducive to functional microorganism colonization [39], enabling coconut shell-derived biochar to be more advantageous for promoting MCFAs production.

  The addition of carbon-based materials alongside varied reinforcement strategies have different effects on the MCFAs output within CE systems. Ma et al. introduced granular activated carbon (GAC) at different filling ratio into a fluidized cathode electro-fermentation setup, discovering that a GAC filling ratio of 8% maximized the electrochemical activity of the biofilm [61]. Under these conditions, the caproate yield, carbon recovery rate and electron recovery rate surpassed those of the GAC-absent reactors by 2.1, 1.8 and 1.6 folds, respectively [61]. With the appropriate amount of granular activated carbon injection, the reactor biofilm has high electrochemical activity and accelerates electron transfer. Additionally, the raised specific surface area of the cathode allows microorganisms to sufficiently contact the substrate in the solution, significantly enhancing the efficiency of the CE process [66]. In another study, activated carbon (AC) pretreated with hydrochloric acid at pH 5.0 was introduced into a CE fermentation reactor, which led to the revelation that electron transfer efficiency, soared to 63.13%, culminating the highest caproate concentration at 78.92 mmol/L [62]. This result was due to the chemical surface treatment with hydrochloric acid, which significantly influenced the surface functional groups on the AC, altering its surface acidity while preserving its adsorption capacity and promoting the caproate production. Du et al. deployed liquid metabolites as substrates for MCFAs synthesis [63]. During dark fermentation, they incorporated biochar derived from cornstalk residues after 1.7% (v/v) sulfuric acid pretreatment and enzymatic hydrolysis obtaining the maximal MCFAs concentration of 1740 mg/L, 61% higher than that of the control group [63]. These findings underscored the necessity of considering diverse reinforcement strategies in conjunction with carbon-based material supplementation to optimize the stimulatory impact on CE processes, which might further help with developing tailored modifications and process enhancements to elevate product yields and efficiency.

  3.3 Mechanism of carbon-based materials in CE

  3.3.1   Modulation of microbial community structure

  The porous structure of carbon-based materials, coupled with their large specific surface area and total pore volume, could facilitate microbial attachment and supports the development of cell networks and microbial proliferation [64], which therefore provide CE microorganisms with ample colonization sites. For example, Luo et al. found that supplementation with biochar optimally enriched the Clostridium_sensu_stricto genus within CE-functional microbiomes, which, which are commonly implicated in MCFAs production [65]. Research by Yang et al. also found that upon addition of GAC, the microbial community in the anaerobic reactor gradually coalesced, transitioning from an initial three-cluster configuration to a single cluster after a given incubation period, leading to significant restructuring of the microbial composition [66]. Therefore, caproate-producing Clostridium (at 75% abundance) and proteolytic Proteobacteria emerged as dominant members of this altered community in this research [66]. Generally, the porosity and rough surface of GAC fostered the formation of stable microbial aggregates and supported a robust, stable microbial community structure.

  3.3.2   Potential establishment of microbial DIET

  Previous studies have proved that DIET mechanism leverages microbial cellular architectures competent in electron conveyance, complemented by external facilitators such as carbon-based materials, to alleviate reliance on enzymatic intermediation, thereby augmenting inter-species electron transfer efficiency [67]. Lin et al. demonstrated that the integration of graphene, a carbon-based material, into an anaerobic fermentation system significantly modifies the synergistic interactions between methanogenic archaea and acid-producing microorganisms, leading to a 28% boost in methane production, notably in Methanobacterium (71.1%) and Methanosarcina (11.3%) through DIET [68]. This work emphasized that the alteration of trophic relationships and the enhancement of methane productivity through DIET mechanisms. Whether for hydrogen or methane production, carbon-based materials have been shown to augment microbial electron transfer mechanism, facilitating electron exchange between syntrophic bacteria and methanogens in anaerobic fermentation systems, which potentiate DIET process [38].

  The CE biosynthesis process of MCFAs production also falls within the realm of anaerobic fermentation processes. Tao and Chen et al. highlighted the mutualistic interaction between CE microorganisms and methanogenic bacteria, illustrating that species such as Geobacter metallireducens and Geobacter sulfurreducens can engage in DIET via biochar, which underscored the role of conductive carbon-based materials in mediating interspecies interactions [69, 70]. Contrera et al. in their investigation of CE fermentation and DIET-driven biological processes, verified the electrochemical properties of conductive carbon-based materials [71]. It should be pointed out that although thermodynamics alone could not elucidate the significant advantages of diet-driven CE, it may also be an alternative to DIET to promote the CE process [71]. Moreover, Ma et al. observed in an electrofermentation setup using a cathode as the electron donor that an increase in biochar filling ratio escalated the charge transfer resistance at the electrode-electrolyte interface from 5.583 Ω to 20.45 Ω, heightening material transfer resistance, which in turn diminished electron generation and transfer capabilities, negatively affecting DIET [61]. Conversely, at an activated carbon filling rate of 8%, the GAC-CE system achieved peak carbon and electron recovery rates, with approximately 61% of carbon and 53% of electrons being recuperated in MCFAs, respectively [28]. Therefore, it could be speculated that, carbon-based materials have possibility to optimize electrical conductivity and foster electron transfer efficiency to establish potential DIET mechanism between the located CE microorganisms and their syntrophic partners.

  Like conventional anaerobic fermentation, the potential DIET in CE systems might also mainly occurred on the surface of carbon-based materials (Fig. 5). Carbon-based materials can provide a more efficient microbial electron transport path by enabling electron donors to transfer electrons to electron acceptors more efficiently, which enhanced the electron exchange between CE-related bacteria and methanogens, and realizing the potential enhancement of DIET [38, 62]. Carbon-based materials also give full play to its electrical conductivity advantage, and delivers DIET for distant microorganisms, which assists indirect electron transfer through redox groups on particle surface [37]. Moreover, the porous architecture of biochar enables the adsorption and immobilization of CE microorganisms, fostering the formation of robust microbial aggregates and biofilm structures [37]. Thus, in CE fermentation systems, these structures also have possibilities to reinforce intra-community linkages and expedite the facilitation of the CE process, then capitalizing on the structural and electrochemical properties of biochar to enhance CE efficiency. It should be pointed out that academic investigations are still insufficient for the key characteristics of carbon-based materials and potential DIET mechanism, especially for MCFAs production system via CE bioprocess. There is still a long way to go to confirm the corresponding achievements.

  Figure 5

  

  Figure 5.  Potential DIET in CE.

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  3.3.3   Promotion of CE metabolic pathways

  Recent researches have revealed that incorporation of carbon-based materials into fermentation systems fosters heightened microbial metabolic activity, with a consequent elevation in relevant enzyme activities and enrichment of functional genes that positively stimulate the RBO and FAB pathways [59, 72]. Wu et al. found a significant abundance of genes correlated with RBO and FAB pathways in biochar-amended experimental setups, specifically noting a 57.36% and 72.19% escalation in genes associated with fatty acid biosynthesis [65]. Additionally, the abundance of the functional enzymes that promote acetyl-CoA production alcohol dehydrogenase and aldehyde dehydrogenase were both increased in the biochar-amended fermentation system [65]. This implicates biochar's facilitation of ethanol conversion to acetyl-CoA, thereby enhancing energy capture by microorganisms and bolstering the CE progression. Xiang et al. also documented a substantial rise in the abundance of genes encoding for pivotal enzymes in the RBO pathway, specifically acetyl-CoA C-acetyltransferase, acetoacetyl-CoA, and butyryl-CoA dehydrogenase, with increments of 58.9%, 20%, and 105%, respectively, in the presence of PAC [59]. Meanwhile, genes coding for ethanol-oxidizing enzymes such as adh, aldh, phosphate acetyltransferase, and acetate kinase also displayed marked increases, reinforcing the notion that PAC positively modulates ethanol oxidation and RBO pathway functionality [59]. As evidenced by these studies, the introduction of diverse carbon-based materials has proven efficacious in stimulating both RBO and FAB pathways. This stimulation is attributed to the proliferation and enrichment of acid-producing bacteria under the dual influence of carbon-based materials and the synergistic action of redox-active functional groups on their surfaces. Therefore, carbon-based materials could promote the CE process and ensure the yield of MCFAs by enhance the key metabolic pathways.

  4. Prospects

  Based on the bibliometric analysis, this study documented current status and application of carbon-based materials in anaerobic fermentation technology in the nearly past 20 years. The mechanism and practical implications of MCFAs synthesized from carbon-based materials during CE are discussed. Hotspots of research on the synthesis of MCFAs using carbon-based materials, the applications and innovations of carbon-based materials in different particle sizes, types, and modification through different reinforcement strategies are summarized and analyzed. To further enhance MCFAs production, future work aims to employ advanced techniques, such as high-resolution microscopy, spectral analysis, electrochemical testing, micro-X-ray computed tomography, and in situ monitoring. These approaches aim to investigate the microstructural changes and activity characterization of carbon-based materials during the CE process and elucidate how these materials interact with MCFAs synthesis enzymes, substrates, and intermediates.

  For future in-depth investigations into the application and mechanisms of carbon-based materials for enhanced MCFAs production, potential areas of focus include (Fig. 6):

  Figure 6

  

  Figure 6.  Perspectives on carbon-based materials in CE.

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  (1) Innovation in carbon-based materials and valorization of biomass-derived resources: The focus lies in engineering carbon-based materials with tailored structures and functionalities, such as carbon nanotubes, graphene, and heavy metal-doped carbons, aimed at amplifying their catalytic, adsorptive, and stabilizing properties in MCFAs synthesis. Concurrently, the exploitation of biomass resources, including agricultural and forest residues as well as food waste, is pursued with a view to harnessing their environmental advantages in MCFA production and broadening the spectrum of available carbon-based materials [73].

  (2) Advancing analytical techniques and mechanistic insights: To clarify the connection between the microstructural attributes of carbon-based materials and their catalytic efficacy and stability in MCFA synthesis, advanced analytical methodologies are to be employed. High-resolution microscopy, spectroscopic analyses, electrochemical assays, and micro-computed tomographic imaging will be I nstrumental in elucidating microstructural transformations and activity profiles during CE processes [74]. Complementarily, in situ monitoring techniques, isotope labeling methodologies, and other sophisticated tools will be utilized to delve into the intricate interactions between carbon-based materials and the enzymes, substrates, and intermediates involved in MCFA biosynthesis [75].

  (3) Strategies for enhanced MCFA synthesis efficiency: Efforts are directed towards refining specific methodologies that leverage carbon-based materials to manipulate microbial community structures and metabolic pathways. By harnessing the synergies between these materials and microorganisms, bio-stimulation techniques, such as enzyme immobilization, biofilm-based reactors, and photocatalytic systems, can be developed to amplify the biosynthetic efficiency of MCFAs [76, 77].

  (4) Integration and optimization of biochar-enabled reaction systems: A holistic approach is proposed to streamline the diverse reaction stages involving carbon-based materials in CE systems, entailing the consolidation of material preparation, pretreatment, biotransformation, and product extraction. This integration aims to establish a greener and more sustainable MCFAs production paradigm, ensuring efficient process coupling.

  (5) Modeling and intelligent control system development: Combining experimental findings with theoretical frameworks, mathematical models grounded in the unique features of carbon-based materials will be constructed for MCFAs production. Leveraging artificial intelligence and big data analytics, this endeavor encompasses process parameter optimization, fault detection, error control, and real-time monitoring, thereby facilitating the transition to an intelligently managed production process.

  (6) Evaluation of environmental and economic viability: A life cycle assessment will evaluate the environmental footprint of carbon-based materials from procurement to disposal, encompassing raw material sourcing, manufacturing, energy usage, greenhouse gas emissions, and resource depletion. This ensures the environmental sustainability of MCFAs production is enhanced by carbon-based materials. An economic analysis will account for the costs associated with material preparation, application, and MCFAs synthesis, juxtaposing these expenses against traditional methodologies to ascertain economic viability and market competitiveness, thereby informing strategies for industrial-scale implementation.

  By employing the aforementioned multifaceted research methodologies, it is expected to reveal the deep mechanism of carbon-based materials in strengthening the production of MCFAs in the future. This endeavor aspires to innovate and advance novel carbon-based materials alongside their application technologies. It could be characterized by heightened efficiency, environmental sustainability, and economic benefits. Thus, it is anticipated to foster a greener and more efficient pathway for industrial MCFAs production, thereby propelling its development in harmony with ecological and economic objectives.

  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.

  CRediT authorship contribution statement

  Bo-Ran Chang: Writing – original draft, Methodology, Conceptualization. Lin Deng: Writing – original draft, Methodology, Investigation, Conceptualization. Qing-Lian Wu: Writing – review & editing. Wan-Qian Guo: Supervision. Hui-Ying Xue: Supervision.

  Acknowledgments

  This work was financially supported by the National Key R & D Program of China (No. 2019YFC1906600) and the National Natural Science Foundation of China (No. 52000132).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110411.

  
  
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  Figure 1  Publication outputs and types analysis during 2000–2023.

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  Figure 2  Time zone view during 2012–2023.

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  Figure 3  Carbon chain elongation pathway mechanism.

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  Figure 4  Timeline overview of carbon-based materials in CE (detailed information is in Table S4).

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  Figure 5  Potential DIET in CE.

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  Figure 6  Perspectives on carbon-based materials in CE.

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