Carbon-based Metal-free Materials Research Articles

Reactive species identification in KCl activated biochar/persulfate system under different pH condition: Theoretical calculation and column experiments

The challenges of remediating polluted water in mine areas are exacerbated by low emission dispersion and long-lasting pollution, particularly the significant fluctuations in pH value. Notably, the treatment of flotation reagent polluted water with variable pH scenarios remains a challenge. In this study, metal-free biochar-based materials (KBBC)/persulfate system was used for flotation reagent contaminated wastewater treatment with a wide pH range. Regarding the intrinsic nature of the biochar-based materials, ball milling enhanced the degree of graphitization, while introducing a hard template of KCl increased the proportion of carbonyl functional groups on their surface, thereby facilitating persulfate activation. Experimental findings revealed that the KBBC/PS system displayed efficient degradation of aniline aerofloat (AAF) as a representative flotation reagent across a wide pH range (pH = 3.6–10.4 with buffer). Investigations on the mechanism indicated that, under acidic and neutral conditions, superoxide radicals and direct electron transfer processes constituted primary pathways for pollutant degradation. Meanwhile, various active species contributed under alkaline conditions with increased involvement of free radicals (•OH and SO4•−) and singlet oxygen. Furthermore, continuous flow experiments were set up to prove the potential applicability of the KBBC/PS process. This study not only deepens the understanding of persulfate activation by metal-free carbon-based materials across a wide pH range but also expands their potential applications in mine-polluted water purification.

Dipole-Dipole Tuned Electronic Reconfiguration of Defective Carbon Sites for Efficient Oxygen Reduction into H2O2.

Metal-free carbon-based materials are one of the most promising electrocatalysts toward 2-electron oxygen reduction reaction (2e-ORR) for on-site production of hydrogen peroxide (H2O2), which however suffer from uncontrollable carbonizations and inferior 2e-ORR selectivity. To this end, a polydopamine (PDA)-modified carbon catalyst with a dipole-dipole enhancement is developed via a calcination-free method. The H2O2 yield rate outstandingly reaches 1.8mol gcat -1 h-1 with high faradaic efficiency of above 95% under a wide potential range of 0.4-0.7 VRHE, overwhelming most of carbon electrocatalysts. Meanwhile, within a lab-made flow cell, the synthesized ORR electrode features an exceptional stability for over 250h, achieved a pure H2O2 production efficacy of 306g kWh-1. By virtue of its industrial-level capabilities, the established flow cell manages to perform a rapid pulp bleaching within 30min. The superior performance and enhanced selectivity of 2e-ORR is experimentally revealed and attributed to the electronic reconfiguration on defective carbon sites induced by non-covalent dipole-dipole influence between PDA and carbon, thereby prohibiting the cleavage of O-O in OOH intermediates. This proposed strategy of dipole-dipole effects is universally applicable over 1D carbon nanotubes and 2D graphene, providing a practical route to design 2e-ORR catalysts.

Catalyst Stability in Electrolytic Production of Hydrogen Peroxide: Importance, Challenges, and Standardized Degradation Protocols

On-demand electrolytic hydrogen peroxide (H2O2) production is a significant technological advancement that offers a promising alternative to the traditional anthraquinone process.1,2 This method, which employs electrocatalysts to selectively reduce oxygen via two electrons transfers (ORR-2e–), has the potential to provide a sustainable and cost-effective method of producing H2O2. However, several challenges must be overcome in order to enable on-demand H2O2 electrosynthesis, including designing affordable and selective catalytic materials and preventing oxidative damage or degradation of catalytic materials.1,2 While catalyst development has traditionally focused on maximizing activity and selectivity, it is equally important to ensure that the catalysts maintain their performance over time in real-world applications. The stability of electrocatalysts is critical due to the harsh conditions they are subjected to during electrochemical reactions, such as high applied potential or currents, high temperatures, high pressures, and highly acidic or basic environments, as well as the oxidative power of H2O2 and reactive oxygen species that it can generate.3,4 If an electrocatalyst degrades or undergoes chemical changes during the reaction, it can result in reduced activity or selectivity, or even complete reaction failure. This not only leads to decreased yields and increased costs, but also poses potential safety hazards. The performance and degradation processes of catalysts can vary substantially depending on the reaction environment and the lineage of the catalyst used.3,4 In the last years, numerous classes of catalysts have emerged as highly active and selective for the electrocatalytic production of H2O2, especially transition metal-based materials and metal-free carbon-based materials.5 However, while numerous studies have focused on the activity and selectivity of catalysts for the ORR-2e–, research on their stability is still lacking. To effectively address the issue of catalyst material degradation under stress conditions and understand the underlying factors driving the degradation pathways, a unified and standardized approach is crucial. In this study, we focus on the deactivation processes in electrolytic H2O2 production and propose the use of specifically designed degradation protocols, employing a rotating ring-disk electrode setup. Our approach takes into account the most active and selective classes of catalyst for H2O2 production in acidic media, with the aim of enabling more researchers to tackle this crucial aspect and provide a comprehensive understanding and evaluation of the stability of electrocatalysts.

N, S, and Cl tri-doped carbon boost the switching of radical to non-radical pathway in Fenton-like reactions: Synergism of N species and defects

Heteroatom doping represents a promising strategy for enhancing the generation of singlet oxygen (1O2) during the activation of peroxymonosulfate (PMS) using carbon-based catalysts; however, it remains a formidable challenge. In this study, we systematically controlled the structure of metal-free carbon-based materials by introducing different heteroatoms to investigate their efficacy in degrading organic pollutants in water via PMS activation. The results of reactive oxygen species detection showed that the dominant free radical in the four samples was different: CN (•SO4- and •OH), CNS (•O2-), CNCl (1O2), and CNClS (1O2). This led to the transformation of active species from free radicals to non-free radicals. The tri-doped carbons with nitrogen, sulfur, and chlorine (CNClS) exhibited exceptional performance in PMS activation and achieved a remarkable degradation efficiency of 95% within just 6 min for tetracycline. Moreover, a strong linear correlation was observed between the ratio of pyridine-N/graphite-N and ID/IG with the yield of 1O2, indicating that N species and defects play a crucial role in CNClS as they facilitate the transition from radical to non-radical pathways during PMS activation. These findings highlight the possibility that adjustable tri-heteroatom doping will expand the Fenton-like reaction for the treatment of actual wastewater.

Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90V versus RHE and a half-wave potential of 0.72V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

Jasmine waste derived biochar as green sulfate catalysts dominate non-free radical paths efficiently degraded tetracycline

Because of the potential environmental harm caused by the extensive application of tetracycline (TC), this study used jasmine waste rich in organic matter as a precursor and one-step carbonization into metal-free carbon-based materials to efficiently activate peroxymonosulfate (PMS) toward degrading TC. The jasmine waste biochar (JWB) with a heating rate of 10 °C min−1 and a heating temperature of 700 °C was selected as the most suitable material based on its catalytic performance. The effects of catalyst dose, PMS dose, initial pH value, coexisting inorganic anions and TC concentration on the JWB/PMS/TC system were thoroughly optimized. The results showed that the degradation efficiency of TC by JWB/PMS system was 90%. Meanwhile, the combination of electron paramagnetic resonance, masking experiments and X-ray photoelectron spectrometry confirmed that JWB degraded TC mainly through the non-radical radical pathway of 1O2 oxidation and mediated the electron transfer to PMS. In addition, some degradation products were analyzed by LC-MS and possible degradation pathways of the system were proposed. Therefore, this paper proposes a novel method for recycling jasmine waste and providing a low-cost catalyst for the oxidation treatment of refractory organic matter.

A Pentagonal Defect-Rich Metal-Free Carbon Electrocatalyst for Boosting Acidic O2 Reduction to H2O2 Production

Metal-free carbon-based materials are considered one of the most promising two-electron oxygen reduction reaction (2e- ORR) electrocatalysts for the green synthesis of hydrogen peroxide (H2O2). However, most reported carbon electrocatalysts perform much more effectively in alkalis than in acids. Herein, by creatively using fullerene (C60) as the precursor subject to ammonia treatment, we designed and synthesized a pentagonal defect-rich nitrogen-doped carbon nanomaterial (PD/N-C). It achieves outstanding ORR activity, 2e- selectivity, and stability in acidic electrolytes, surpassing the benchmark PtHg4 alloy catalyst. Impressively, the flow cell based on the PD/N-C catalyst achieves nearly 100% Faraday efficiency with a remarkable H2O2 yield, representing the best improvement among all the metal-free catalysts. Experimental and theoretical results reveal that such superb 2e- ORR performance of PD/N-C originates from the synergism between pentagonal defects and nitrogen dopants. This work presents an effective strategy for the design and construction of highly efficient acid-resistant carbon electrocatalysts for H2O2 production and beyond.

N, O-diatomic dopants activate catalytic activity of 3D self-standing graphene carbon aerogel for long-cycle and high-efficiency Li-CO2 batteries

Carbon-based metal-free materials are regarded as viable cathode catalysts for Li-CO2 batteries due to their low costs and lightweight. And heteroatom doping (such as N atom) has great potential to improve the catalytic activity of carbon-based catalysts. However, the underlying catalytic mechanism is yet unclear, which hinders the construction of high-efficiency catalysts and further improvements in electrochemical performance. Especially, the role of oxygen-containing groups prevalent in carbon-based catalysts has never been explored. In this work, guided by theoretical simulation, a self-standing N-doped graphene carbon aerogel with certain oxygenic groups was well-designed and synthesized by a straightforward, one-step thermal approach as the cathode catalyst. N dopant can effectively regulate the electronic structure of graphene and thus lower the free energy change of reactants/intermediate species. The intrinsic oxygen-containing functional groups still presented in graphene aerogel can further stabilize CO2-related intermediate species and improve the catalytic activity through a synergistic coupling effect with N dopant. This effect was originally discovered and clarified in the Li-CO2 battery system. Additionally, intriguing 3D hierarchical pores of as-obtained graphene carbon aerogel not only guarantee good conductivity but also offer a vast surface area to expose numerous accessible active sites. The resulting Li-CO2 batteries showed a significantly enhanced initial energy efficiency of approximately 78.46% and remarkable cyclic stability of more than 1500 h at 20 μA cm−2. This fundamental understanding of the structure-performance relationship gives new ideas for creating extremely effective carbon-based metal-free catalysts for Li-CO2 batteries.

Carbonyl and defect of metal-free char trigger electron transfer and O2•− in persulfate activation for Aniline aerofloat degradation

Residual flotation reagents in mineral processing wastewater can trigger severe ecological threats to the local groundwater if they are discharged without treatment. Metal-free biochar-induced persulfate-advanced oxidation processes (KCBC/PS) were used in this study to elucidate the degradation of aniline aerofloat (AAF) – a typical flotation reagent. In KCBC/PS system, AAF can be removed at low doses of catalyst (KCBC, 0.05 g/L) and oxidant (PS, 0.3 mM) additions with high efficiency. The analysis revealed the dominance of O2•− among the identified reactive oxygen species (ROS), which achieved deeper mineralization for the AAF degradation in the KCBC/PS system. The role of the electron transfer mechanism was equally important; the importance was corroborated by the chemical quenching experiments, electron spin resonance (ESR) detection, probe experiments, and electrochemical analysis. It benefited from the electron transfer mechanism in the KCBC/PS system and exhibited a wide pH adaptation (3.5–11) and high resistance to inorganic anions for real mining wastewater treatment. Combined with theoretical calculations and other analyses, the carbonyl group was deemed to be the active site of the non-radical pathway of biochar, while the site of the conversion of SO4•− to superoxide radicals by biochar activation represented a defect. These findings revealed a synergistic effect of multiple active sites on PS activation in biochar-based materials. Moreover, the intermediate degradation products of AAF from mass spectrometry indicated a possible pathway through the density functional theory (DFT) method, which was effective in reducing the environmental toxicity of pollutants for the first time according to the T.E.S.T software and seed germination experiments. Overall, our study proposed a novel modification strategy for cost-effective and environmentally friendly biochar-based catalysts, while also deepening our understanding of the mechanism of activation of persulfate by metal-free carbon-based materials.

Unveiling the inter-molecular charge transfer mechanism of N-doped graphene/carbon nanotubes heterostructure toward oxygen reduction process for Zn-air battery

Heteroatom-doped carbon-based materials have been deemed as highly efficient electrocatalysts for cathodic oxygen reduction reaction (ORR) in the sustainable energy conversion technologies. However, the limited active sites caused by the aggregation of carbon layers ineluctably hinder the mass transfer, thereby generating insufficient ORR performance. Herein, we offer a rational and facile method to manufacture a hybrid heterostructure of N-doped graphene and carbon nanotubes at the pyrolysis temperature of 900 °C (N-G/CNTs-900), which exhibits a better ORR activity than most of reported carbon-based ORR electrocatalysts, and comparable to that of Pt/C. Thus-fabricated N-G/CNTs-900 features hierarchically porous architecture, high specific surface area, abundant defects, and high active N content, and what’s more, it also affords active sites for oxygen activation and subsequent 4e− reduction processes due to the hetero-interlayer charge transfer and the electron accumulations of the carbon atoms on the surface layer by theoretical calculations. Compared with commercial Pt/C, N-G/CNTs-900-based Zn-air battery presents a higher peak power density (133.60 mW/cm2) and a larger specific capacity (707 mAh/gZn). This work provides a novel idea to design the high-efficiency and long-term durability ORR catalyst, which is benefit for boosting the commercial applications of metal-free carbon-based materials in the renewable energy conversion devices.

Facile template-free synthesis of 3D cluster-like nitrogen-doped mesoporous carbon as metal-free catalyst for selective oxidation of H2S

Construction of metal-free carbon-based materials for efficient sulfur resource recovery in pollutant H2S is very critical for both applied environmental catalysis and fundamental material exploration. Herein, we report a template-free hydrothermal method to prepare hierarchical nitrogen-doped mesoporous carbon (HNMCs) materials with abundant nitrogen sites and large BET surface area. The representative HNMC-800 achieves a H2S conversion of ∼100% and sulfur selectivity of ∼95% at 150 °C in H2S selective oxidation. Moreover, HNMCs catalysts are resistant to sulfur poisoning, displaying good catalytic durability, with no significant loss of sulfur yield in the run of 80 h, outperforming the pure active carbon, g-C3N4, and commercial Fe2O3. This work provides the possibility of utilizing hierarchical nitrogen-doped carbon materials as metal-free catalysts for the selective oxidation of H2S.

Etching-directed nitrogen doping strategy to construct efficient defective sites in carbon for electrocatalytic oxygen reduction

Construction of synergistic coupling sites of intrinsic carbon defects and N dopants in N-doped carbon is of great significance to improve the catalytic activities of metal-free carbon-based materials toward oxygen reduction reaction (ORR). However, the carbon-defect construction and N doping by existing strategies are usually independent processes, resulting in a random arrangement of the intrinsic carbon defects and N-dopants (i.e., hardly forming the coupling sites) in the resultant carbon materials. Herein, an etching-directed nitrogen doping strategy for constructing carbon defect/N dopant coupling sites is developed, in which nitrogen is directionally doped at the freshly created intrinsic carbon defects during the synthesis of N-doped carbon. The as-prepared N-doped carbon catalyst enriched with such coupling sites renders a remarkable ORR catalytic activity with an ORR half-wave potential of 0.891 V vs. RHE and an excellent long-term durability (3% current loss after 15,000 s at 0.6 V vs. RHE). Experimental characterizations and theoretical calculations reveal the formation mechanism of the coupling sites as well as the origin of their high catalytic activities toward ORR. This etching-directed nitrogen doping strategy shows great universality for the controllable construction of active sites in carbon-based materials derived from different precursors for electrochemical energy storage and conversion applications.

Metal-Free Boron/Phosphorus Co-Doped Nanoporous Carbon for Highly Efficient Benzyl Alcohol Oxidation.

An in‐depth understanding of the electronic structures of catalytically active centers and their surrounding vicinity is key to clarifying the structure–activity relationship, and thus enabling the design and development of novel metal‐free carbon‐based materials with desired catalytic performance. In this study, boron atoms are introduced into phosphorus‐doped nanoporous carbon via an efficient strategy, so that the resulting material delivers better catalytic performance. The doped B atoms alter the electronic structures of active sites and cause the adjacent C atoms to act as additional active sites that catalyze the reaction. The B/P co‐doped nanoporous carbon shows remarkable catalytic performance for benzyl alcohol oxidation, achieving high yield (over 91% within 2 h) and selectivity (95%), as well as low activation energy (32.2 kJ mol−1). Moreover, both the conversion and selectivity remain above 90% after five reaction cycles. Density functional theory calculations indicate that the introduction of B to P‐doped nanoporous carbon significantly increases the electron density at the Fermi level and that the oxidation of benzyl alcohol occurs via a different reaction pathway with a very low energy barrier. These findings provide important insights into the relationship between catalytic performance and electronic structure for the design of dual‐doped metal‐free carbon catalysts.

Activating γ-graphyne nanoribbons as bifunctional electrocatalysts toward oxygen reduction and hydrogen evolution reactions by edge termination and nitrogen doping

Carbon-based metal free materials (CMFCs) as electrocatalysts have been a hot issue and are receiving growing attention. In this paper, we applied intensive density functional theory (DFT) calculations to study the electrocatalytic activities of γ-graphyne nanoribbons (γGyNRs) toward the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Our results revealed γGyNRs to be excellent bifunctional CMFCs after functionalization by termination or N-doping at the edge to introduce proper spin densities. The scaling relationship analysis enhanced our earlier understanding that the binding strength of H atom (ΔEH) is a general descriptor for designing bifunctional electrocatalysts for HER and ORR. Further study revealed that the light gradient boosting machine is a promising machine learning algorithm to accurately predict the electrocatalytic activities of γGyNRs. Feature importance analysis suggested that the surrounding environment of the active site plays a more significant role than we previously anticipated. Overall, we suggest promising γGyNRs as HER and ORR bifunctional electrocatalysts for the first time and provide deeper insight into the reaction mechanism, which is beneficial for the rational design of novel CMFCs.

Recent advances in electrocatalytic oxygen reduction for on-site hydrogen peroxide synthesis in acidic media

Electrocatalytic oxygen reduction reaction (ORR) via two-electron pathway is a promising approach to decentralized and on-site hydrogen peroxide (H2O2) production beyond the traditional anthraquinone process. In recent years, electrochemical H2O2 production in acidic media has attracted increasing attention owing to its stronger oxidizing capacity, superior stability, and higher compatibility with various applications. Here, recent advances of H2O2 electrosynthesis in acidic media are summarized. Specifically, fundamental aspects of two-electron ORR mechanism are firstly presented with an emphasis on the pH effect on catalytic performance. Major categories of promising electrocatalysts are then reviewed, including noble-metal-based materials, non-noble-metal single-atom catalysts, non-noble-metal compounds, and metal-free carbon-based materials. The innovative development of electrochemical devices and in situ/on-site application of electrogenerated H2O2 are also highlighted to bridge the gap between laboratory-scale fundamental research and practically relevant H2O2 electrosynthesis. Finally, critical perspectives on present challenges and promising opportunities for future research are provided.

Natural DNA-derived highly-graphitic N, P, S-tridoped carbon nanosheets for multiple electrocatalytic applications

In the electrocatalytic oxygen reduction and hydrogen evolution reactions (ORR/HER), metal-free carbon-based materials presented great potentials as alternatives to Pt because of their flexible adjustability of electronic and geometric properties. The pursuit of high-performance carbons demand for high-level doping, large surface area and high graphitization, but the improvement of the former two aspects would inevitably impair the third one, which seems a contradictive proposition. Herein, by a facile “mix-and-pyrolyze” method, natural DNA was first developed to synthesize N, P, S-tridoped carbons (NPSC) with porous sheet structures, high graphitization, large surface areas and abundant high-quality dopants simultaneously. Thus, the newly-prepared NPSC displays superior alkaline ORR performance comparable to Pt/C together with excellent acidic activity and better stability. Meanwhile, NPSC is one of most active acidic HER catalysts and the best alkaline HER catalyst among carbon counterparts. Density function theory computation reveled the origins of superior performance are attributed to positively charged C sites neighboring to S for ORR and P sites for HER because of DNA-derived advantaged structural features. The as-fabricated rechargeable zinc–air battery based on current tridoped carbon has closed peak power density and specific capacity but much better charge–discharge stability compared with Pt/C.

Constructing Graphitic-Nitrogen-Bonded Pentagons in Interlayer-Expanded Graphene Matrix toward Carbon-Based Electrocatalysts for Acidic Oxygen Reduction Reaction.

Metal-free carbon-based materials with high electrocatalytic activity are promising catalysts for the oxygen reduction reaction (ORR) in several renewable energy systems. However, the performance of carbon-based materials is far inferior to that of Pt-based catalysts in acid electrolytes. Here, a novel carbon-based electrocatalyst is reported toward ORR in 0.1 m HClO4 with half-wave potential of 0.81V and better durability (100 h reaction time)than commercial 20 wt% Pt/C. It is achieved by constructing graphitic-nitrogen (GN)-bonded pentagons in graphitic carbon to improve the intrinsic activity of the carbon sites and increasing the amount of active sites via expanding the interlayer spacing. X-ray absorption spectroscopy and aberration-corrected electron microscopy characterizations confirm the formation of GN-bonded pentagons in this carbon material. Raman and X-ray photoelectron spectroscopy reveal that the activity is linearly associated with the amounts of both pentagons and adjacent GN atoms. Density function theory further demonstrates that adjacent GN atoms significantly increase the charge density at the carbon atom of a GN-bonded pentagon, which is the activity origin for the ORR.

Effect of KOH on the Energy Storage Performance of Molasses-Based Phosphorus and Nitrogen Co-Doped Carbon

In this study, we have evaluated the effect of potassium hydroxide (KOH) on the energy storage performance of metal-free carbon-based materials prepared from molasses. Molasses are a renewable-resource biomass and economical by-product of sugar refinement, used here as a carbon precursor. Two co-doped carbon materials using molasses were synthesized via a time and cost-efficient microwave carbonization process, with ammonium polyphosphate as a phosphorus and nitrogen doping agent. The phosphorus and nitrogen co-doped carbon (PNDC) samples were prepared in the presence and absence of a chemical activating agent (KOH), to study the role of chemical activation on PNDCs. Physical characterizations were performed to gain insight into the composition, pore size and topographical data of each material. Electrochemical characterization via cyclic voltammetry in 1 M sulfuric acid (H2SO4) as well as in 6 M KOH as electrolytes, revealed high current density and specific capacitance for the chemically activated material (PNDC2) compared to one without chemical activation (PNDC1). The capacitance value of 244 F/g in KOH electrolyte was obtained with PNDC2. It is concluded that addition of KOH prior to carbonization increases the surface functionality, which significantly enhances the electrochemical properties of the PNDC material such as current density, stability, and specific capacitance.

Recent progress of carbon-based metal-free materials in thermal-driven catalysis

The carbon-based metal-free materials as catalysts (named as carbocatalysts) have been attracting tremendous attentions in electric-, solar- and thermal-driven reactions nowadays. Compared to electrocatalysis and photocatalysis, the thermal-driven catalysis (thermocatalysis) including liquid phase and gas phase reactions involves wider scope and is relatively easy to realize practical large-scale applications. Over the past several years, some striking achievements on the design of new carbon-based metal-free materials with well-defined structures and heteroatom groups as well as the revelation of new reaction mechanisms and active sites in thermocatalysis have been obtained. However, comparative discussions regarding these recent achievements have been rarely highlighted. In this review, we systematically summarize and discuss six kinds of carbocatalysts and their applications in thermocatalysis. These materials include typical oxygen-attached carbon, surface modified carbon (graft with certain organic compounds), mono-doped carbon, co-doped carbon, carbon nitride and materials with carbon as dopant. Some new reaction processes as well as the related reaction mechanisms, active sites and intermediates are reviewed critically. Moreover, an outlook on the in-depth investigation of the metal-free carbocatalysis in the future is provided.

Platinum Group Metal-Free Catalysts for Oxygen Reduction Reaction: Applications in Microbial Fuel Cells

Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.