Four-electron Oxygen Reduction Reaction Research Articles

Molecule-confined modification of graphitic C3N4 to design mesopore-dominated Fe-N-C hybrid electrocatalyst for oxygen reduction reaction

To design inexpensive carbon catalysts and enhance their oxygen reduction reaction (ORR) activity is critical for developing efficient energy-conversion systems. In this work, a novel Fe-N-C hybrid electrocatalyst with carbon nanolayers-encapsulated Fe3O4 nanoparticles is synthesized successfully by utilizing the molecular-level confinement of graphitic C3N4 structures via hemin biomaterial. Benefiting from the Fe-N structure prevalent on the carbon nanosheets and large mesopore-dominated specific surface area, the synthesized catalyst under optimized conditions shows excellent electrocatalytic performance for ORR with an EORR at 1.08 V versus reversible hydrogen electrode (RHE) and an E1/2 at 0.87 V vs. RHE, and outstanding long-term stability, which is superior to commercial Pt/C catalysts (EORR at 1.04 V versus RHE and E1/2 at 0.84 V versus RHE). Moreover, the low hydrogen peroxide yield (<11%) and average electron transfer number (~3.8) indicate a four-electron ORR pathway. Besides, the maximum power density of the home-made Zn-air battery using the obtained catalyst is 97.6 mW cm−2. This work provides a practical route for the synthesis of cheap and efficient ORR electrocatalysts in metal-air battery systems.

Enhanced Electrochemical H2O2 Production via Two-Electron Oxygen Reduction Enabled by Surface-Derived Amorphous Oxygen-Deficient TiO2-x.

The electrochemical oxygen reduction reaction (ORR) is regarded as an attractive alternative to the anthraquinone process for sustainable and on-site hydrogen peroxide (H2O2) production. It is however hindered by low selectivity due to strong competition from the four-electron ORR and needs efficient catalysts to drive the 2e- ORR. Here, an acid oxidation strategy is proposed as an effective strategy to boost the 2e- ORR activity of metallic TiC via in-site generation of a surface amorphous oxygen-deficient TiO2-x layer. The resulting a-TiO2-x/TiC exhibits a low overpotential and high H2O2 selectivity (94.1% at 0.5 V vs reversible hydrogen electrode (RHE)), and it also demonstrates robust stability with a remarkable productivity of 7.19 mol gcat.-1 h-1 at 0.30 V vs RHE. The electrocatalytic mechanism of a-TiO2-x/TiC is further revealed by density functional theory calculations.

Incorporation of Activated Biomasses in Fe-N-C Catalysts for Oxygen Reduction Reaction with Enhanced Stability in Acidic Media

Fe-N-C materials are promising oxygen reduction reaction (ORR) catalysts for replacing expensive platinum-based catalysts (Pt/C) in proton exchange membrane fuel cells. However, they still show low volumetric activity and stability compared to Pt/C catalysts, with carbon corrosion being one of the main factors for the loss of active Fe-Nx sites. Within this study, phosphoric acid-activated rye straw and coconut shells are revealed as a promising matrix for Fe-Nx sites with advanced stability against electrochemical carbon corrosion (5000 cycles, 1.0–1.5 VRHE, and 0.1 M HClO4) compared to a common Fe-N-C catalyst based on carbon black. Electrochemical characterization of the two biomass-based catalysts (Fe-N-CBio) shows on the one hand 50% higher stability in terms of mass activity as well as comparable activity and active site density but on the other hand a lower selectivity toward the four-electron ORR than the common Fe-N-C catalyst. Nitrite stripping experiments in acetate buffer as an electrolyte display a 1.5-fold stronger effect of carboxylic acid adsorption than on a common Fe-N-C catalyst, revealing differences to the electronic structure of the Fe-N-CBio catalyst. This difference is mainly attributed to the presence of phosphor species and higher amounts of nitrogen functionalities in the Fe-N-CBio catalysts. The presence of P is assumed to stabilize the carbon against carbon corrosion by inhibiting electron withdrawal from the C. This study points out the impact factors on Fe-N-C stability and further shows the promising application of activated biomasses in more stable and sustainable Fe-N-C catalysts for ORR.

In-situ synthesis of Co3O4 nanocrystal clusters on graphene as high-performance oxygen reduction reaction electrocatalysts

ABSTRACT The energy problem is closely related to the survival and development of human beings. Nowadays, the research of fuel cells is making rapid progress, but the bottleneck of the development of fuel cells is slow oxygen reduction reaction (ORR) of cathode. At present, it is difficult to develop a kind of oxygen reduction catalyst with high performance, low energy consumption and low pollution. Here, we synthesised Co3O4 nanocrystals loaded on a single graphene oxide layer via a convenient one-step hydrothermal method. According to the transmission electron microscopy characterisation tests, the average size of Co3O4 nanocrystals is 5 nm. Compared with the commercially available Pt/C catalyst, the obtained Co3O4/rGO catalyst demonstrates the same four-electron (4e−) oxygen reduction reaction, and the catalytic activity of ORR is significantly higher. This is due to the large surface area provided by the middle part of the composite catalyst Co3O4/rGO, which provides an effective mass transfer and electron transfer path for the catalyst.

Boron and nitrogen co-doped carbon dots for boosting electrocatalytic oxygen reduction

Carbon dots (CDs) have become an emerging carbon nanomaterial for use in energy-conversion systems because of their large surface area and rapid electron transfer. Carbon dots (BN-CDs) doped with both boron and nitrogen were synthesized by a simple one-step electrochemical etching approach using low-cost petroleum coke as precursor. Compared with CDs doped with only B or N, BN-CDs showed an excellent four-electron oxygen reduction reaction (ORR) activity with a positive onset potential of 0.958 V and a large diffusion-limited current density of −4.32 mA cm−2. Furthermore, the long-term stability and methanol tolerance of BN-CDs were better than those of a commercial Pt/C catalyst. It was found by density functional theory (DFT) calculation that the co-doping of N and B promoted the adsorption of O2 molecules in the ORR process. This work provides new insight into the rational design of carbon nanomaterials and their use in energy conversion.

Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure.

The four-electron oxygen reduction reaction (4e--ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e--ORR. In this study, we investigated chemical and electrochemical 4e--ORR catalyzed by Co2(μ-O2) complexes bridged by xanthene (1) and anthracene (3) and by a Co2(OH)2 complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)2 as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOFinit = 6.8 × 102 s-1) and selectivity for 4e--ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of 1-3 have the O2 addition to [CoII2(OH2)2]4+ as an intermediate in common. In the only case that complex 1 was used as a catalyst, kcat depended on proton concentration because the reaction rate of the O2 addition to [CoII2(OH2)2]4+ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O2 and stabilizing the Co2(μ-O2) structure, thus increasing both the reaction rate and selectivity for 4e--ORR.

The Enhancement of Selectivity and Activity for Two-Electron Oxygen Reduction Reaction by Tuned Oxygen Defects on Amorphous Hydroxide Catalysts

Amorphous catalysts, thanks to their unique coordinately unsaturated properties and abundant of defect sites, incline to possess higher activity and selectivity than their crystalline counterparts....

Synergistic effect on the four-electron ORR of the electro-Fenton system to remove micropollutants using an MOF-derived catalyst with carbon black

The electro-Fenton (EF) process combines an oxygen reduction reaction (ORR) and Fenton-like reaction for application in water treatment. However, many electrode designs simply aim at H2O2 production or catalyze either the ORR or Fenton-like reaction. This paper explored the effect of pyrolysis on MIL-101(Fe) and found that a thermal treatment at 310 °C (310T) partly maintained the original crystal structure but exposed more Fe sites and oxygen vacancies (OVs). The fabricated 310T/CB@NF gas diffusion electrode had a 4e-ORR tendency, although it exhibited good performance even at low voltages (1.2 V) with the detected current below 10 mA. In the assessment of the H2O2 production, ability as a Fenton reagent and DCF removal, the EF process exhibited a synergy between 310T and carbon black (CB). 310T manifested good Fenton activity on the Fe sites and OVs and directly worked toward the DCF degradation. CB with a high ORR catalytic activity provided a basis for the high H2O2 yield, which is indirectly conducive to the DCF removal.

Simultaneous sulfamethoxazole degradation with electricity generation by microbial fuel cells using Ni-MOF-74 as cathode catalysts and quantification of antibiotic resistance genes

Antibiotic wastewater presents serious challenges in water treatment. Metal-organic frameworks (MOFs) have received significant attention as promising precursors and sacrificial templates in the preparation of porous carbon-supported catalysts. Herein, we investigated the sulfamethoxazole (SMX) degradation and electrochemical performance of microbial fuel cells (MFCs) that applied as-prepared Ni-MOF-74 and Ni–N–C (Ni-MOF-74 underwent pyrolysis treatment at different temperatures) as air-cathode catalyst. Firstly, the electrocatalytic activity towards oxygen reduction reaction (ORR) of the catalyst was investigated by rotating disk electrode. The results showed that electron transfer number for Ni-MOF-74 was 2.12, while that of 800Ni–N–C was 3.44, which was close to four-electron reduction. Applying Ni-MOF-74 in MFCs, a maximum power density of 446 mW/m2 was obtained, which was close to that of 800Ni–N–C. Besides, using Ni-MOF-74 as cathode catalyst, a chemical oxygen demand removal rate of about 84% was obtained, and the degradation rate of 10 mg/L SMX was 61%. The degradation rate decreased with increasing antibiotic concentration, but the average degradation efficiency increased stepwise. Additionally, the relative abundance of resistant gene sul1 in the reactors of the new catalytic material was about 62% lower than that of sul1 in the control (Pt/C) reactors, and the relative abundance of sul2 was about 73% lower. Moreover, cost assessments related to the catalyst performance are presented. The findings of this study demonstrated that Ni-MOF-74 could be considered as a two-electron transfer ORR catalyst, and offers a promising technique for preparation of Ni–N–C for use as four-electron transfer ORR catalysts. In comparison, Ni-MOF-74 could be a promising ORR catalyst of MFCs for antibiotic degradation.

Morphology control of Ni doped rod like MIL-88A derived FeS2 embedded in nitrogen-rich carbon as an efficient electrocatalyst for the oxygen reduction reaction

The creation of cheap catalysts with an appropriate oxygen reduction reaction (ORR) is a prime concern. For the first time, series of Ni-doped iron sulfide nitrogen-rich carbon derived MOF/reduced graphene oxide composites were suggested as ORR catalysts with improved performance. The electrochemical efficiency of all specimens has been evaluated on the rotating disk electrode (RDE) using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The optimal sample showed an improvement in catalytic performance with an onset potential of 0.796 V due to the synergistic effect and the ratio of Ni-doping that offered a direct four-electron ORR pathway, similar to commercial carbon-supported Pt catalysts. The small Tafel slope (~63.1 mV/decade) and high durability that optained could be due to the recently launched efficient active sites by structure modification with Ni integration and presence of rGO. Finding shows that outcome of this research can be employed as an efficient ORR catalyst in metal/air batteries.

Thermally Controlled Construction of Fe–Nx Active Sites on the Edge of a Graphene Nanoribbon for an Electrocatalytic Oxygen Reduction Reaction

Pyrolytically prepared iron and nitrogen codoped carbon (Fe/N/C) catalysts are promising nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell applications. Fabrication of the Fe/N/C catalysts with Fe-Nx active sites having precise structures is now required. We developed a strategy for thermally controlled construction of the Fe-Nx structure in Fe/N/C catalysts by applying a bottom-up synthetic methodology based on a N-doped graphene nanoribbon (N-GNR). The preorganized aromatic rings within the precursors assist graphitization during generation of the N-GNR structure with iron-coordinating sites. The Fe/N/C catalyst prepared from the N-GNR precursor, iron ion, and the carbon support Vulcan XC-72R provides a high onset potential of 0.88 V (vs reversible hydrogen electrode (RHE)) and promotes efficient four-electron ORR. X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) studies reveal that the N-GNR precursor induces the formation of iron-coordinating nitrogen species during pyrolysis. The details of the graphitization process of the precursor were further investigated by analyzing the precursors pyrolyzed at various temperatures using MgO particles as a sacrificial template, with the results indicating that the graphitized structure was obtained at 700 °C. The preorganized N-GNR precursors and its pyrolysis conditions for graphitization are found to be important factors for generation of the Fe-Nx active sites along with the N-GNR structure in high-performance Fe/N/C catalysts for the ORR.

Density functional theory study of the sulfur/oxygen doped CoN4-graphene electrocatalyst for oxygen reduction reaction

It has been found that heteroatoms doped transition metal-nitrogen embedded carbon materials are potential candidates for high-efficient oxygen reduction reaction (ORR) catalysts. Herein, the four-electron ORR mechanism and activity of CoN4, CoN4Sx, and CoN4Ox (x = 1−4) have been investigated theoretically. The results indicate that sulfur (S) doping can effectively enhance ORR activity of CoN4, but oxygen (O) doping cannot. Specifically, due to the weakened *OH binding, the overpotential of CoN4Sx (x = 1−3) is decreased by about 100 mV compared with that of CoN4. Especially, the ORR overpotential of CoN4S1 is as small as 0.25 V. However, for almost all CoN4Ox, the O doping could make the binding strength of *OH be strengthened, leading to high ORR overpotential. The electronic structure analysis of CoN4S1 reveals that due to the activation of nitrogen atoms after S doping and the tuned energy gap of pristine CoN4, its ORR activity is enhanced.

Significantly boosted oxygen electrocatalysis with cooperation between cobalt and iron porphyrins.

Developing electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great importance. Herein, Co tetrakis(pentafluorophenyl)porphyrin (Co-P) and Fe chloride tetrakis(pentafluorophenyl)porphyrin (Fe-P) were loaded on carbon nanotubes (CNTs) for combining the electrocatalytic advantages of both Co-P and Fe-P. The resultant (Co-P)0.5(Fe-P)0.5@CNT composite displayed significantly boosted activity for the selective four-electron ORR with a half-wave potential of 0.80 V versus reversible hydrogen electrode (RHE) and for the OER with a potential of 1.65 V versus RHE to obtain 10 mA cm-2 current density in 0.1 M KOH. A Zn-air battery assembled from (Co-P)0.5(Fe-P)0.5@CNT exhibited a small charge-discharge voltage gap of 0.74 V at 2 mA cm-2, a high power density of 174.5 mW cm-2 and a good rechargeable stability (>120 cycles).

FeN@N-doped graphitic biochars derived from hydrothermal-microwave pyrolysis of cellulose biomass for fuel cell catalysts

Iron-nitrogen moieties encapsulated in the N-doped graphitic and hierarchical biochars are derived from the hydrothermal pyrolysis and microwave NH3 activation of cellulose and lignin. The physicochemical properties of these biochars are identified by using various analytical instruments and their application as electrocatalysts in oxygen reduction reaction (ORR) is also evaluated in alkaline solution. As compared to the lignin-derived biochars (named as L-y), the cellulose-derived biochars (named as C-x) have superior activities toward ORR due to the formation of highly graphitic carbon shells (as proofed by using X-ray diffraction and high-resolution transmission electron microscope) in the biochars. Moreover, the C-1% (i.e., 1% of iron in the cellulose-derived biochars) exhibits the best electrocatalytic performance (onset potential = −0.05 V vs. Ag/AgCl) and follows a near four-electron ORR route, which could be ascribed to the more active iron-nitrogen sites (as evidenced by X-ray photoelectron and X-ray absorption spectroscopy) covered with hierarchical carbon shells (as identified by N2 adsorption-desorption isotherms). The cellulose-derived biochars with distinctive iron-nitrogen retained in the shell of N-doped graphitic and hierarchical carbons by employing a time-saving and energy-efficient microwave activation procedure may render a potential application in the ORR.

Elucidating the Role of Oxide-Oxide/Carbon Interfaces of CuOx-CeO2/C in Boosting Electrocatalytic Performance.

Herein, we report the synthesis and bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of a CuOx-CeO2/C electrocatalyst (EC) with rich oxide-oxide and oxide-carbon interfaces. It not only demonstrates a smaller Tafel slope (65 mV dec-1) and higher limiting current density (-5.03 mA cm-2) but also exhibits an onset potential (-0.10 V vs Ag/AgCl) comparable to that of benchmark Pt/C. Besides undergoing the favorable direct four-electron ORR pathway, it unveils a loss of 23% of its initial current after 6 h of a stability test and a negative shift of 4 mV in the half-wave potential after the accelerated durability test compared to the corresponding current loss of 28% and negative shift of 20 mV for Pt/C. It also reveals remarkable OER activity in an alkaline medium with a low onset potential (0.20 V) and a smaller Tafel slope (177 mV dec-1). The bifunctional ORR/OER activity of CuOx-CeO2/C EC can be ascribed to the synergistic effects, its unique structure with enriched oxygen vacancies owing to the presence of Ce4+/Ce3+, robust oxide-oxide and oxide-carbon heterointerfaces, and homogeneous dispersion of oxides over the carbon bed, which facilitates faster electronic conduction.

Oxygen Reduction Reaction Activity of Iron (II) Phthalocyanine Monolayers in Acidic Media Studied Using a Rotating Disk Electrode Technique

Metal complexes such as metalloporphyrins and metallophthalocyanines are widely studied in various research fields due to their unique physical and chemical properties. Especially, their catalytic activities for oxygen reduction reaction (ORR) are of great interest because this reaction plays an important role in energy conversion or biological redox processes. Among various metal complexes, Iron (II) phthalocyanine (FePc) is expected as a candidate for four-electron ORR catalysts. However, most of the kinetic studies for ORR have been carried out using heat-treated catalysts of FePc. Unfortunately, their chemical structures become ill-defined after the heat treatment, and thus, the nature of active sites is still unclear. Actually, there is a possibility that four-electron ORR activity is induced by dimeric FePc. The intrinsic activity of monomeric FePc, especially in acidic media, is still under discussion. In this study, well-defined monolayers of monomeric FePc complexes were fabricated on chemically modified Au surfaces using two-step assembly [1]. Their ORR activity was quantitatively measured in acidic media using a rotating disk electrode (RDE) technique. The catalytic behavior of FePc was compared with that of Cobalt (II) tetraphenylporphyrins (CoTPP), which is known as two-electron ORR catalyst.The monolayers of FePc or CoTPP were fabricated on Au surfaces as follows. First, an Au disk electrode with diameter of 10 mm was modified with self-assembled monolayers of pyridine-4-thiol (PySH) or 1,6-diisocyanohexane (C6DI). Next, metal complexes of FePc or CoTPP were immobilized on top of these molecular monolayers through axial ligation with their different terminal groups. The ORR activity of these samples was measured in O2-saturated 0.05 M H2SO4 aqueous solution using the RDE method.Figure shows rotation rate dependences of RDE polarization curves for CoTPP and FePc on C6DI-modified Au. According to the Koutecky-Levich analysis of these curves at 0 V vs. RHE, the electron transfer number was estimated to be 1.9 for CoTPP, indicating that CoTPP indeed catalyzed only two-electron ORR. On the other hand, the estimated transfer number for FePc was 2.4, suggesting that this complex was indeed able to catalyze the four-electron ORR in the acidic condition. Interestingly, the catalytic behavior of FePc on C6DI-modified Au was apparently different from that on PySH-modified Au. In the poster, we will present details about the ORR activities of FePc measured on various monolayer-modified Au electrodes.Reference[1] Toyama T, Sato S, Motobayashi K, Uosaki K, Ikeda K. J Solid State Electrochem. DOI: 10.1007/s10008-019-04461-9.Figure. Rotation rate dependence of RDE polarization curves for FePc/C6DI/Au and CoTPP/C6DI/Au measured in 0.05 M H2SO4 solution, where FePc/C6DI/Au and CoTPP/C6DI/Au denote FePc and CoTPP monolayers immobilized on C6DI-modified Au, respectively. Figure 1

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Stabilization of bilirubin oxidase in a biogel matrix for high-performance gas diffusion electrodes

Enzyme immobilization on solid conducting surfaces faces challenges for practical applications in technologies such as biosensors and biofuel cells. Short-term stability, poor electrochemical performance, and enzyme inhibition are some unsolved issues. Here, we show a simple methodology for bilirubin oxidase (BOD) immobilization on carbon-based gas diffusion electrode for four-electron electrochemical oxygen reduction reaction. BOD is incorporated into a Nafion® matrix and crosslinked with glutaraldehyde by a one-pot reaction in buffered solution, producing a stable BOD-based biogel. The biogel provides stable electrode performance and allows the direct electron-transfer mechanism of multicopper centers buried in the enzyme. A biocatalytic reduction current of −1.52 ± 0.24 mA cm−2 at 0.19 ± 0.06 V was observed under gas diffusion conditions. Additionally, the bioelectrode showed unprecedented long-term stability under continuous operation combined with satisfactory catalytic current without redox mediator. The BOD-based biogel layer thickness and the entrapment of BOD into Nafion network are crucial for the biocathode stability, and BOD crosslinking by glutaraldehyde contributes to enhance the catalytic currents. Further, the BOD-based biogel provides a suitable microenvironment for long-term enzymatic activity involving three-phase interfacial reaction. The present study provides new insights into enzyme immobilization to overcome the critical short-term stability of enzyme-based electrochemical devices for practical applications.

Single-Atom Co-N4 Electrocatalyst Enabling Four-Electron Oxygen Reduction with Enhanced Hydrogen Peroxide Tolerance for Selective Sensing.

Electrocatalysis of the four-electron oxygen reduction reaction (ORR) provides a promising approach for energy conversion, storage, and oxygen monitoring. However, it is always accompanied by the reduction of hydrogen peroxide (H2O2) on most employed catalysts, which brings down the electrocatalytic selectivity. Here, we report a single-atom Co-N4 electrocatalyst for the four-electron ORR at an onset potential of 0.68 V (vs RHE) in neutral media while with high H2O2 tolerance, outperforming commercial Pt electrocatalysts. Electrochemical kinetic analysis confirms that the Co-N4 catalytic sites dominantly promote the direct four-electron pathway of the ORR rather than the two sequential two-electron reduction pathways with H2O2 as the intermediate. Density functional theory calculations reveal that H2O2 reduction is hampered by the weak adsorption of H2O2 on the porphyrin-like Co centers. This endows the electrocatalyst with improved resistance to current interference from H2O2, enabling highly selective O2 sensing as validated by the reliable sensing performance in vivo. Our study demonstrates the intriguing advantage of single-atom catalysts with high capacity for tailoring metal-adsorbate interactions, broadening their applications in environmental and life monitoring.

Biochar cathode: Reinforcing electro-Fenton pathway against four-electron reduction by controlled carbonization and surface chemistry

Porous biochars have attracted tremendous interests in electrochemical applications. In this study, a family of biochars were prepared from cellulose subject to different carbonization temperatures ranging from 400 to 700 °C, and the biochars were in-situ activated by a molten salt (ZnCl2) to construct a hierarchically porous architecture. The activated porous biochars (ZnBC) were used as a carbocatalyst for electro-Fenton (EF) oxidation of organic contaminants. Results showed that high-temperature carbonization improved the activity of biochar for four-electron oxygen reduction reaction (ORR) due to the rich carbon defects, while the mild-temperature treatment regulated the species and distribution of oxygen functional groups to increase the production of hydrogen peroxide (H2O2) via a selective two-electron ORR pathway. ZnBC-550 was the best cathode material with a high ORR activity without compromise in H2O2 selectivity; a high production rate of H2O2 (796.1 mg/g/h) was attained at −0.25 V vs RHE at pH of 1. Furthermore, Fe(II) addition induced an electro-Fenton system to attain fast decomposition of various organic pollutants at −0.25 V vs RHE (reversible hydrogen electrode) and pH of 3 with a satisfactory mineralization efficiency toward phenolic pollutants. The EF system maintains its excellent stability for 10 cycles. Hydroxyl radicals were identified as the dominant reactive oxygen species based on in situ electron paramagnetic resonance (EPR) analysis and radical quenching tests. This study gains new insights into electrocatalytic H2O2 production over porous biochars and provides a low-cost, robust and high-performance electro-Fenton cathode for wastewater purification.