Oxygen Reduction Reaction In Alkaline Media Research Articles

Iron-based graphene composite for oxygen reduction reaction in alkaline media: Electrocatalytic activity and lifetime evaluation

The development of electrocatalysts with high catalytic performance and low costs for oxygen reduction reaction (ORR) is still challenging. Herein, an overall solution for ORR in alkaline media, from the catalyst synthesis to catalyst regeneration and to the development of a rapid, reliable and easy approach for electrode surface evaluation, is presented. We focused on the development and characterization of an efficient material for ORR in alkaline media, α-Fe2O3 N-doped graphene (Fe-N-Gr). The associative pathway for the four electron transfer ORR mechanism is sustained by the Raman spectra recorded from the electrode surface. We highlighted the possibility of catalyst regeneration by a simple electrochemical method. After two regeneration rounds and 1500 cycles in O2-saturated 1 M NaOH, the catalyst still retains 40.8 % catalytic activity. Finally, as a part of the overall solution, we demonstrated that a methodology based on Raman spectroscopic measurements and machine learning algorithms can be applied for graphene-based catalysts lifetime investigation.

Enhanced Electrocatalytic Oxygen Reduction Reaction of TiO2 Nanotubes by Combining Surface Oxygen Vacancy Engineering and Zr Doping.

This work examines the cooperative effect between Zr doping and oxygen vacancy engineering in anodized TiO2 nanotubes (TNTs) for enhanced oxygen reduction reactions (ORRs). Zr dopant and annealing conditions significantly affected the electrocatalytic characteristics of grown TNTs. Zr doping results in Zr4+ substituted for Ti4+ species, which indirectly creates oxygen vacancy donors that enhance charge transfer kinetics and reduce carrier recombination in TNT bulk. Moreover, oxygen vacancies promote the creation of unsaturated Ti3+(Zr3+) sites at the surface, which also boosts the ORR interfacial process. Annealing at reductive atmospheres (e.g., H2, vacuum) resulted in a larger increase in oxygen vacancies, which greatly enhanced the ORR activity. In comparison to bare TNTs, Zr doping and vacuum treatment (Zr:TNT-Vac) significantly improved the conductivity and activity of ORRs in alkaline media. The finding also provides selective hydrogen peroxide production by the electrochemical reduction of oxygen.

Electrochemical Performance of Metal-Free Carbon-Based Catalysts from Different Hydrothermal Carbonization Treatments for Oxygen Reduction Reaction.

This research investigates the difference between products obtained through two hydrothermal carbonization treatments. Our aim is to synthesize metal-free, carbon-based catalysts for the oxygen reduction reaction (ORR) to serve as efficient and cost-effective alternatives to platinum-based catalysts. Catalysts synthesized using the traditional hydrothermal approach exhibit a higher electrocatalytic activity for ORR in alkaline media, despite their more energy-intensive production process. The superior performance is attributed to differences in the particle morphology and the chemical composition of the particle surfaces. The presence of functional groups on the surfaces of catalysts obtained via a traditional approach significantly enhances ORR activity by facilitating deprotonation reactions in an alkaline environment. Our research aims to provide a reference for future investigations, shifting the focus to the fine-tuning of surface chemical compositions and morphologies of metal-free catalysts to enhance ORR activity.

Laser Synthesis of Nonprecious Metal-Based Single-Atom Catalysts for Oxygen Reduction Reaction.

Development of nonprecious metal-based single-atom catalysts (SACs) has provided opportunities to substitute Pt group metals and offer maximum atom utilization and unique coordination environments. Among these catalysts, Fe-N-C catalysts with atomically dispersed Fe-N4 active sites have emerged as some of the most promising oxygen reduction reaction (ORR) catalysts. However, furnace synthesis of Fe-N-C catalysts with carbon substrate derived from metal-organic framework (MOF) involves a high-temperature procedure, in which nitrogen from the carbonized MOF tends to be removed, subsequently leading to a low density of active sites. In this work, we developed a rapid and simple solid-state route to fabricate SACs through laser-induced thermal activation (LITA) of carbonized zeolitic imidazolate framework-8 (ZIF-8) adsorbed with Fe precursors. The results demonstrate that the laser process effectively avoids the loss of nitrogen in the nitrogen-doped carbon substrate and achieves a loading of Fe single atoms of 2.3 wt %, in comparison with that of 1.2 wt % from the conventional furnace treatment. The Fe-N-C catalyst synthesized in the study presents a half-wave potential of 0.91 V for ORR in alkaline media, which is higher than that of commercial Pt/C (0.87 V). When used as a cathode catalyst in zinc-air batteries (ZABs), the battery exhibits excellent electrochemical performance. This work also demonstrates the versatility of the technique through the successful synthesis of Co-N-C and Ni-N-C single atoms on nitrogen-doped carbon substrates.

Influence of Nitrogen Doping into Carbon on the Activation Barrier of ORR in Alkaline Medium: An Investigation Based on Eyring Analysis

The oxygen reduction reaction (ORR) is investigated on metal-free carbon (Vulcan XC-72) and nitrogen-doped (∼≤1%) carbon (N/C-900) in 0.1 M KOH. The product distribution (O2 to OH- and HO2-) as a function of overpotential (η) in the temperature range of 293-323 K is analyzed using a rotating ring-disk electrode (RRDE) assembly. The kinetic current due to reduction of O2 to HO2- is estimated and used in the Eyring analysis to determine the change in enthalpy of activation (ΔH#). It is shown that doping of carbon with nitrogen (even with ≤1 wt %) causes substantial increase in the number of active sites (almost 2-fold) and reduction in ΔH# at any η. Moreover, ΔH# is a stronger function of η on N/C-900 as compared to that on the carbon surface.

Ultralong nitrogen/sulfur Co‐doped carbon nano‐hollow‐sphere chains with encapsulated cobalt nanoparticles for highly efficient oxygen electrocatalysis

AbstractThe development of simple and effective strategies to prepare electrocatalysts, which possess unique and stable structures comprised of metal/nonmetallic atoms for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), is currently an urgent issue. Herein, an efficient bifunctional electrocatalyst featured by ultralong N, S‐doped carbon nano‐hollow‐sphere chains about 1300 nm with encapsulated Co nanoparticles (Co‐CNHSCs) is developed. The multifunctional catalytic properties of Co together with the heteroatom‐induced charge redistribution (i.e., modulating the electronic structure of the active site) result in superior catalytic activities toward OER and ORR in alkaline media. The optimized catalyst Co‐CNHSC‐3 displays an outstanding electrocatalytic ability for ORR and OER, a high specific capacity of 1023.6 mAh gZn−1, and excellent reversibility after 80 h at 10 mA cm−2 in a Zn‐air battery system. This work presents a new strategy for the design and synthesis of efficient multifunctional carbon‐based catalysts for energy storage and conversion devices.

Growth of polyoxomolybdate with a porous pyramidal structure on carbon xerogel nanodiamond as an efficient electro-catalyst for oxygen reduction reaction.

The slow kinetics of the oxygen reduction reaction (ORR) limits the large-scale usage of the fuel cells. Thus, it is crucial to develop an efficient and stable electrocatalyst for the ORR. Herein, facile synthesis of three-dimensional nitrogen-doped carbon xerogel diamond nanoparticles, CDNPs support is reported. The as-prepared CDNPs support was functionalized with a Keggin-type polyoxomolybdate via the hydrothermal process (POM@CDNPs). As the characterization techniques revealed, this nanocomposite possesses a three-dimensional structure, high density of nitrogen doping, and well-dispersed porous pyramidal morphology of POM, making it a promising catalyst for ORR in alkaline medium. The POM@CDNPs nanocomposite exhibits an outstanding activity for ORR with a limiting current density that reaches -7.30 mA cm-2 at 0.17 V vs. RHE. Moreover, a half-wave potential of 0.773 V is delivered with a stability of about 99.9% after the 100th repetitive cycle as this catalyst forces the ORR to the direct-four-electron pathway. This work spots the advantages of hybridizing the sp3 of the nanodiamond with the sp2 of the carbon xerogels to increase the conductivity of the support material. In addition, the role of the porous pyramidal morphology of the POM on the activity of the nanocomposite was evaluated. This study suggests using advanced carbon-based electro-catalysts with outstanding activity and stability.

Co2P encapsulated in N, O co-doped carbons as bifunctional electrocatalysts for oxygen evolution and reduction reactions

Exploring highly-efficient and low-cost non-precious metal electrocatalysts for electrochemical reactions such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is of crucial importance for clean and renewable energy technologies. Herein, we report the synthesis of Co2P encapsulated in N- and O- co-doped carbons (Co2P@NOCs), which can serve as excellent noble metal-free bifunctional electrocatalysts for OER and ORR in alkaline media. Especially, the Co2P@NOC-2 electrocatalyst exhibits a low overpotential of 330 mV to deliver the current density of 10 mA cm−2 for OER in 1.0 M KOH solution, with a small Tafel slope of 97 mV dec−1. With respect to ORR activity, Co2P@NOC-2 gives positive onset (Eonset) and half-wave (E1/2) potentials of 0.90 and 0.75 V (vs. RHE) in 0.1 M KOH electrolyte, respectively. Additionally, it presents better methanol tolerance and long-term durability as compared to the commercial Pt/C (20 wt·%) catalyst. The high electrocatalytic activity and good long-term stability toward OER and ORR can be attributed to the conductive heteroatoms-doped carbon layer, the embedded Co2P active sites and the synergistic effects between them. The strong synergism can significantly promote the electron transfer during the reaction and enhance the OER and ORR performance.

The preparation of bifunctional Co-N co-doped carbon with bamboo-like hollow tubular as an efficient electrocatalyst for oxygen reduction and methanol oxidation reaction

• Bamboo-like hollow tubular Co-N-C was prepared by direct pyrolysis method. • The Co-N-C catalyst exhibits a high specific surface area. • The Pt@Co-N-C has a high electrochemical surface area. • The Co-N-C and Pt@Co-N-C exhibit excellent ORR and MOR activity and stability. Catalysts with distinguished properties and stability are very important for the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) of fuel cells. Herein, we describe a convenient and efficient preparation means to synthesize a bamboo-like hollow tubulous structure Co-N co-doped carbon material (Co-N-C) via a late-model of Co-Imace coordinated compound as the precursor by direct pyrolysis. Additionally, Pt nanoparticles supported by Co-N-C catalysts (Pt@Co-N-C) were prepared using the ethylene glycol reduction procedure. The obtained catalyst exhibited an excellent electro-catalytic property for ORR in alkaline media, which possessed a more positive half-wave potential (0.868V) than commercial Pt/C (0.852V). Moreover, the Co-N-C maintained preeminent long-term stability (91.6 %) after the 21000s and also owned a stronger methanol resistance than Pt/C. The obtained Pt@Co-N-C electrocatalyst exhibited higher MOR activity (96.3 mA cm -2 ), which is 1.6 times greater than that of Pt/C, benefiting from the high electrochemical surface area of Pt@Co-N-C (68 m 2 g -1 ). Furthermore, the Pt@Co-N-C exhibited lower onset potentials, higher I f /I b ratios, and well long-term stability than Pt/C.

Oxygen Reduction Reaction Catalyzed By Silicon and Nitrogen Co-Doped Carbon

Finding an alternative catalysts for the oxygen reduction reaction (ORR) has been a key issue in modernelectrocatalysis. Traditional platinum-based catalysts are not viable in the long term and thus much efforthas been focused on non-precious metal catalysts. Heteroatom doped and metal-heteroatom co-dopedcarbon materials have emerged as the main candidate for replacing the platinum-based catalysts.Different MN4 macrocycles, such as metal phthalocyanines and porphyrins, have been studied aspromising materials to prepare electrocatalysts for ORR in alkaline media [1]. While the development ofnon-precious metal and non-metal carbon catalysts has gained a lot of attention recently, non-metalphthalocyanines are a less studied topic [2].In this work a novel catalyst material including multi-walled carbon nanotubes (MWCNT) and siliconphthalocyanine dichloride (SiPcCl2) was synthesized and the electrochemical activity towards the oxygenreduction reaction (ORR) in alkaline media was studied. Different ratios of phthalocyanine-to-nanotubeswere pyrolyzed at 800 °C. After optimizing the ratio of phthalocyanine-to-nanotubes, the pyrolysis processwas also carried out at different temperatures, but the results showed that the best temperature forpyrolysis is still 800 °C. ORR activity on SiPc/MWCNT catalyst was studied in 0.1 M KOH by employingrotating disc electrode method. Physical characterization was carried out with X-ray photoelectronspectroscopy and transmission electron microscopy. The results showed successful incorporation ofsilicon and nitrogen into the carbon framework. We show that doping MWCNTs with a non-metalphthalocyanine creates a highly active ORR catalyst rivaling the activity of metal-based catalysts inalkaline conditions. This work clearly demonstrates that non-metal macrocyclic compound modifiedcarbons, similar to the the SiPc/MWCNT can be used as alternative cathode catalysts for the metal-airbatteries and alkaline membrane fuel cells.[1] Y. Liu, X. Yue, K. Li, J. Qiao, D.P. Wilkinson, J. Zhang, PEM fuel cell electrocatalysts based ontransition metal macrocyclic compounds, Coord Chem Rev. 315 (2016) 153-177.[2] K.-K. Türk, K. Kaare, I. Kruusenberg, M. Merisalu, U. Joost, L. Matisen, V. Sammelselg, J.H. Zagal, K.Tammeveski, Oxygen electroreduction on zinc and dilithium phthalocyanine modified multiwalled carbonnanotubes in alkaline media, J. Electrochem. Soc. 164 (2017) H338-H344.

MOF-Derived Fe-Zn-N-C Catalysts for Precious Metal Free Cathodes Showing High Performance in Anion-Exchange Membrane Fuel Cells

Hydrogen technologies such as low-temperature fuel cells are, besides batteries, the most promising technologies for mobility and transport applications. Currently, Proton Exchange Membrane Fuel Cells (PEMFCs) are, in terms of low-temperature fuel cells, the state-of-the-art technology achieving high power densities and reasonable stabilities. With the recent development and increasing availability of stable and performant hydroxide conductive ionomers, Anion Exchange Membrane Fuel Cells (AEMFCs) have gained increasing interest in recent years as they combine the advantages of PEMFCs, like low-temperature operation and high-power density with the low component costs of alkaline fuel cells.1,2 Especially the possibility of replacing the expensive Pt-based electrocatalysts used in PEMFCs with cheaper electrocatalysts like nickel-based materials for the anode and iron-based materials for the cathode could significantly decrease the fuel cell costs.1,3,4 Although the oxygen reduction reaction (ORR) at the cathode is favored in alkaline media compared to acidic media, the ORR is still a challenging reaction in AEMFCs.5,6 Over the last decades, various materials were investigated in order to replace the expensive Pt electrocatalysts at the cathode. Among these materials, iron- and nitrogen-doped carbons (Fe-N-C) with molecular iron sites (Fe-Nx) show comparable catalytic activities to Pt and decent stabilities.7–9 Fe-N-C catalysts can be obtained by co-pyrolysis of Fe- N-, and C-sources and subsequential acid leaching. One way to achieve Fe-N-C catalysts with high dispersion of Fe-Nx sites is by using Fe-doped metal-organic frameworks (Fe-MOFs) as precursors. These combine the presence of pre-coordinated Fe-Nx motives as well as high porosity and high specific surface area.10 To produce Fe-N-C catalysts, we synthesized Fe-, and Zn-doped MOFs (Fe-Zn-MOF) as multicomponent Fe-, Zn-, N-, and C-precursors and pyrolyzed them in the presence of additional nitrogen sources at high temperatures. The resulting Fe-Zn-N-C catalysts revealed high dispersion of Fe and Zn, high specific surface areas (400-600 m2/g), and high porosity as revealed by XRD, EDX, HAADF STEM, and N2 physisorption. Depending on the pre-treatment of the Fe-Zn-MOF, the Fe content of the resulting Fe-N-C catalyst could be varied. Benefiting from the high Fe-dispersion and the high specific surface areas, the best performing Fe-Zn-N-C catalyst with high Fe content shows high activity towards the ORR in alkaline media (0.1 mol/L KOH) as demonstrated by RDE measurements featuring a high half-wave potential (0.87 V vs. RHE) and high mass activity (47 mA/mgcat) and thereby outperforming a commercial 50 wt.% Pt/C catalyst in terms of half-wave potential (0.83 V vs. RHE).To investigate the performance of the Fe-Zn-N-C catalysts in AEMFCs, membrane electrode assemblies (MEAs) were prepared with the synthesized Fe-Zn-N-C catalysts at the cathode, a commercial PtRu/C catalyst (40 wt.% Pt, 20 wt.% Ru on carbon black, AlfaAesar) at the anode, and a commercial ionomer as membrane and catalyst layer ionomer. The AEMFC with the best performing Fe-Zn-N-C catalyst revealed a high peak power density of 850 mW/cm², which is among the highest reported peak power densities for non-precious metal cathode catalysts in combination with a commercially available anion exchange ionomer.1 D. R. Dekel, J. Power Sources, 2018, 375, 158–169.2 R. O’Hayre, S.-W. Cha, W. Colella and F. B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons, Inc, Hoboken, NJ, USA, 2016.3 X. Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, T. S. A. Hor, Y. Zong and Z. Liu, ACS Catal., 2015, 5, 4643–4667.4 E. S. Davydova, S. Mukerjee, F. Jaouen and D. R. Dekel, ACS Catal., 2018, 8, 6665–6690.5 M. Hren, M. Božič, D. Fakin, K. S. Kleinschek and S. Gorgieva, Sustain. Energy Fuels, 2021, 5, 604–637.6 N. Ramaswamy and S. Mukerjee, Adv. Phys. Chem., 2012, 2012, 1–17.7 X. Ren, B. Liu, X. Liang, Y. Wang, Q. Lv and A. Liu, J. Electrochem. Soc., 2021, 168, 044521.8 L. Osmieri, L. Pezzolato and S. Specchia, Curr. Opin. Electrochem., 2018, 9, 240–256.9 M. M. Hossen, K. Artyushkova, P. Atanassov and A. Serov, J. Power Sources, 2018, 375, 214–221.10 C. Li, D. H. Zhao, H. L. Long and M. Li, Rare Met., 2021, 40, 2657–2689.

Fe- and Co-Containing Nitrogen-Doped Nanocarbon Catalysts from 5-Methylresorcinol for Anion Exchange Membrane Fuel Cells

Replacing fossil fuels with alternative energy sources has a growing scientific and societal interest because of the the environmental concerns related with their utilization. Low-temperature polymer electrolyte fuel cells have become a subject of intense research as the future cornerstone of renewable energy-based economy.Platinum nanoparticles supported on high-area carbon materials (Pt/C) are commonly used as the cathode catalyst in low-temperature fuel cells. However, the high cost of precious metal-based catalysts limits the commercial viability of such fuel cells. Considerable effort has been directed to design efficient and cheap electrocatalysts through the last decades and transition metal-containing nitrogen-doped carbon nanomaterials have shown the most promising results. A simple strategy to prepare these catalyst materials is high-temperature pyrolysis of organic carbon precursors in the presence of nitrogen and metal sources, where carbonization and N-doping occur simultaneously.1 In this work, highly active oxygen reduction reaction (ORR) electrocatalysts from 5-methylresorcinol, dicyandiamide (DCDA) and Fe and/or Co salts were prepared by a simple one-pot synthesis procedure.2 It has been proposed that DCDA polymerizes into graphitic carbon nitride at around 550 °C, which acts as a reactive template during carbonization of organic precursors, decomposing above 750 °C to produce the micro- and macropores necessary for efficient mass transfer and yielding N-doped graphene-like carbon structures.3 The SEM and TEM analysis revealed that the materials are composed of wrinkled carbon structures doped with nitrogen and metals, and also contain metal nanoparticles encapsulated in N-doped carbon layers. According to the XRD analysis, these nanoparticles are mainly composed of Co, Fe/Fe3O4 and FeCo alloy for the CoNC, FeNC and FeCoNC catalysts, respectively. The specific surface area of the materials was between 290 and 410 m2 g−1. The XPS analysis revealed that the catalysts contain metal-coordinated N centers, while pyridinic nitrogen is the most abundant N species in all materials. Electrocatalytic activity for ORR in alkaline media was evaluated via the rotating disk electrode (RDE) method. The ratio of the precursors was optimized and bimetallic catalysts were found to be more active toward ORR than their monometallic counterparts. Acid treatment slightly increased the catalysts’ ORR activity. The acid-treated bimetallic catalyst (FeCoNC-at) showed exceptional ORR performance, comparable to that of commercial Pt/C (20 wt%) (Figure 1a). This can be attributed to the high surface metal and nitrogen contents, which at least partly are present as nitrogen-coordinated metal (M-Nx) centers, as confirmed by inhibition of ORR in the presence of cyanide ions. The catalyst also demonstrated a rather high stability in short-time tests (15000 potential cycles) and good tolerance to methanol. The FeCoNC-at catalyst was further tested in an anion exchange membrane fuel cell (AEMFC) with hexamethyl-p-terphenyl poly(benzimidazolium) (HMP-PMBI) membrane,4 where a high value of peak power density (P max= 415 mW cm–2) was achieved (Figure 1b).2 References Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells, J. Mater. Chem. A 6, 776–804 (2018).Kisand, A. Sarapuu, D. Danilian, A. Kikas, V. Kisand, M. Rähn, A. Treshchalov, M. Käärik, M. Merisalu, P. Paiste, J. Aruväli, J. Leis, V. Sammelselg, S. Holdcroft, K. Tammeveski, Transition metal-containing nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol for anion exchange membrane fuel cell application, J. Colloid Interface Sci. 584, 263–274 (2021).-T. Ren, Z.-Y. Yuan, A universal route to N-coordinated metals anchored on porous carbon nanosheets for highly efficient oxygen electrochemistry, J. Mater. Chem. A 7, 13591–13601 (2019).G. Wright, J. Fan, B. Britton, T. Weissbach, H.-F. Lee, E. A. Kitching, T. J. Peckham, S. Holdcroft, Hexamethyl-p-terphenyl poly(benzimidazolium): a universal hydroxide-conducting polymer for energy conversion devices, Energy Environ. Sci. 9, 2130–2142 (2016). Figure 1

(Keynote) Interpretation of Oxygen Reduction Activity Volcano Correlations for MN4 Molecular Catalysts Compared to Those for Metallic Electrodes

A classical well-known reactivity index in electrocatalysis is the binding energy of key intermediates to the active sites. For example, this is well documented for the catalytic activity of metal electrodes for the oxygen reduction reaction (ORR)[1], alloys and metal oxides and less studied for molecular catalysts [2]. The activity at constant potential (log(i)E) plotted versus the M-O2 binding energy has the shape of a volcano. This is well documented in several papers, especially by the group of Norksov for metallic electrodes [1] and can be applied to many electrochemical reactions. This has been extended to MN4 molecular catalysts. In contrast to metallic electrodes MN4 molecular catalysts have discrete energy levels and the active site is the central metal, surrounded by an organic ligand with an MN4 central moiety. Essentially one of the key intermediates is the binding of the reacting O2 molecule to the active sites at the rate determining step as: [MN4]ad + O2(aq) + e- ↔ [RMN4O2 -]ad where MN4 is a surface confined macrocyclic complex or a pyrolyzed catalysts bearing a MN4 active moiety embedded in a graphene or graphitic structure. As the ORR reaction involves the transfer of several electrons ( 2, 2+2 or 4 electrons), several adsorbed intermediates can be involved. If the controlling step is the first step, it is expected that for the binding step when DGad = 0 a maximum activity should be observed and the partial coverage of adsorbed intermediate is = 0.5. On thermodynamic ground this corresponds to a thermoneutral condition for the best catalyst. In this work we have tested this hypothesis by studying ORR in alkaline media using several iron porphyrins and iron phthalocyanines as catalysts immobilized on graphite and carbon nanotubes. The trends in reactivity versus the Fe(III)/(II) redox potential for ORR describe a typical volcano correlation. However if the currents are divided by the θFe(II), the fraction of active sites calculated at E=-0.24 V vs SCE using the Nernst equation, log(i/ θFe(II),)E vs. E°Fe(III)/(II) gives a straight line of slope -0.120V/decade. The maximum corresponds to θFe(II) = 0.5 and occurs at a potential close to that used for comparing the activities. A similar behaviour is observed for the oxidation of hydrazine but the continuation of the straight line is observed at potentials more positive than that of the maximum. Again the maximum is observed at the potential chosen for comparison. Those catalysts having Eº’ Fe(III)/(II) >> θFe(II) higher than -0.56V (for hydrazine oxidation) are in the Fe(III) state as predicted by the Nernst equation. That oxidation state Fe(III) is inactive for both reactions studied as OH- ions are strongly bound to Fe(III), especially in alkaline media. So the falling of the activities in the strong binding side of the volcano can be attributed preferentially to a gradual decline in the number of Fe(II) active sites and not to gradual decrease of the fraction (1-θ ) of empty or available active sites due to occupancy by intermediates. Schmickler and Santos [3] have shown that some volcano correlations for HER on metals that are in an oxidized form, and then inactive for this reason.We conclude that even though there are similarities between the volcano correlations for metallic and molecular catalysts, there some differences in the interpretation of the activity maximum that might not correspond to a thermoneutral condition. Acknowledgements: This work was supported by ANID, Anillo Project ACT 192175 and Fondecyt, Project 1181037

Graphene featuring imidazolium rings and electrostatically immobilized polyacrylate chains as metal-free electrocatalyst for selective oxygen reduction to hydrogen peroxide

While graphene-based materials hold a strong potential as electrocatalysts, owing to their astonishing properties and functionalization capability, their intrinsic catalytic activity is more often masked under the umbrella of the precious and industrial metals that are used as counterparts for boosting the electrocatalytic performance. In the present work, we envisioned and realized a simple, stepwise surface engineering path for the modification of graphene oxide, aiming at elevating its intrinsic electrocatalytic activity, excluding any metals, towards the oxygen reduction reaction (ORR). More specifically, oxidized graphene was initially covalently decorated with amphoteric imidazole rings, which were then converted to the corresponding imidazolium counterparts. At the last step, anion exchange of the grafted imidazolium species was performed to immobilize via electrostatic interactions polyacrylate chains. The electrocatalytic performance of the so-formed graphene-based ensembles towards the ORR in alkaline media was evaluated, unveiling the 2e– reduction of oxygen, selectively producing hydrogen peroxide. The results indicate that both covalent and non-covalent surface engineering of graphene with organic chains matters towards electrocatalysis and should not be considered negligible. Our findings and insights provided are a stepping stone on the quest of graphene-based ensembles as metal-free electrocatalysts.

Molecular MnN4-Complex immobilized on carbon black as efficient electrocatalyst for oxygen reduction reaction

Owing to rich-redox chemistry, metal-N4-molecular catalysts have been found as promising electrocatalysts for oxygen reduction reaction (ORR), however, de-metalation of the metal atom from N4-cavity in the electrolyte limits their activity and endurance. Herein, a MnN4-macrocyclic complex was prepared and characterized via various techniques. The results of analytical and theoretical studies suggest that the MnN4-macrocyclic complex has saddle-shaped geometry. Further, this complex was incorporated with black carbon (MnN4@CB) and studied for electrocatalytic ORR in alkaline media. The MnN4@CB composite displayed comparable ORR formal potential (−0.14 V vs Ag/AgCl) to the traditional 20% Pt/C catalyst (−0.13 V vs Ag/AgCl) and remarkable methanol tolerance. Theoretical studies revealed that the highest occupied molecular orbital (HOMO) of MnN4 has almost the same energy as the π∗ orbital of an O2 molecule, resulting in optimal coupling between HOMO and π∗ orbital, leading to efficient ORR. The charge distribution and 2D Fourier optimization of the MnN4-complex suggested that the N-donor atoms increased the electron density on Mn-atom, and transferred into the π∗ orbital of O2, weakening the O–O bond during ORR. In addition to developing low-cost ORR electrocatalysts, this work will contribute to defining their activity descriptors.

Oxygen plasma induced interfacial CoOx/Phthalocyanine Cobalt as bifunctional electrocatalyst towards oxygen-involving reactions

Performance of electrocatalysts towards oxygen-involving reactions is strongly associated with the surface and interface properties. Cobalt phthalocyanine (CoPc) materials have been widely explored as oxygen-involving electrocatalysts but their intrinsic catalytic activity is always poor. We present here a surface oxygen plasma approach to directly treat CoPc for producing interfacial CoO x nanodots anchored on CoPc (CoO x /CoPc), which exhibits high activity towards both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). The optimized CoO x /CoPc demonstrates a much better catalytic activity with a half-wave potential of 0.63 V and an average electron transfer number of 3.64 than the CoPc (0.59 V and n = 2.37) towards ORR in alkaline media. Moreover, OER performance of the CoO x /CoPc is also significantly enhanced, showing a current density of 22.2 mA cm −2 that is 36 times higher than that of CoPc (0.6 mA cm −2 ) at an overpotential of 0.49 V. It is found that the enhanced performance of the CoO x /CoPc is attributed to the high electrochemical active surface area, highly active CoO x , as well as desired interfacial structure. • Oxygen plasma inducing strategy has been firstly employed to modify bulk CoPc. • Highly active CoO x nanodots are in-siut generated on the surface of CoPc. • The desired interfacial structure between CoO x nanodots and CoPc is formed. • ORR and OER performance of the CoO x /CoPc were significantly enhanced. • The plasma inducing strategy could be extended to fabricate other heterogeneous catalysts.

Spray-Pyrolytic Tunable Structures of Mn Oxides-Based Composites for Electrocatalytic Activity Improvement in Oxygen Reduction

Hybrid nanomaterials based on manganese, cobalt, and lanthanum oxides of different morphology and phase compositions were prepared using a facile single-step ultrasonic spray pyrolysis (USP) process and tested as electrocatalysts for oxygen reduction reaction (ORR). The structural and morphological characterizations were completed by XRD and SEM-EDS. Electrochemical performance was characterized by cyclic voltammetry and linear sweep voltammetry in a rotating disk electrode assembly. All synthesized materials were found electrocatalytically active for ORR in alkaline media. Two different manganese oxide states were incorporated into a Co3O4 matrix, δ-MnO2 at 500 and 600 °C and manganese (II,III) oxide-Mn3O4 at 800 °C. The difference in crystalline structure revealed flower-like nanosheets for birnessite-MnO2 and well-defined spherical nanoparticles for material based on Mn3O4. Electrochemical responses indicate that the ORR mechanism follows a preceding step of MnO2 reduction to MnOOH. The calculated number of electrons exchanged for the hybrid materials demonstrate a four-electron oxygen reduction pathway and high electrocatalytic activity towards ORR. The comparison of molar catalytic activities points out the importance of the composition and that the synergy of Co and Mn is superior to Co3O4/La2O3 and pristine Mn oxide. The results reveal that synthesized hybrid materials are promising electrocatalysts for ORR.

ZnS anchored on porous N, S-codoped carbon as superior oxygen reduction reaction electrocatalysts for Al-air batteries

The development of non-precious based oxygen reduction reaction (ORR) catalysts with outstanding catalytic performance is desirable but still a grand challenge for practical Al-air battery. Herein, we report a vulcanization-assisted pyrolysis strategy for creating zeolitic imidazolate framework-derived catalysts with a N, S co-doped carbon support and highly exposed ZnS and Zn-Nx sites. The trithiocyanuric acid (TCA) is found not only to introduce S into the carbon derived from ZIF-8 and ZnS to adjust the electronic structure of carbon matrix during the pyrolysis, but also result in a shrinkage of carbon framework with a hierarchical porous structure. Such an architecture boosts abundant active sites exposed and accelerates remote mass transportation. As a result, the optimized 3.5ZnS/NSC-NaCl-900 delivers an impressive enhanced performance toward ORR in alkaline medium with a high half-wave potential of 0.905 V (vs. reversible hydrogen electrode), which is superior to most of non-precious metal-based catalysts. Density functional theory calculations unveil that the ZnS in 3.5ZnS/NSC-NaCl-900 can effectively lower the Gibbs energy barrier of crucial steps and therefore promotes the reaction kinetics. Furthermore, 3.5ZnS/NSC-NaCl-900 also displays greater power density and specific capacity than Pt/C in Al-air batteries.

Mechanochemical Synthesis as a Greener Way to Produce Iron‐based Oxygen Reduction Catalysts

AbstractIron‐based catalysts have been reported manifold and studied as platinum group metal (PGM) free alternatives for the catalysis of the oxygen reduction reaction (ORR). However, their sustainable preparation by greener synthesis approaches is usually not discussed. In this work, we propose a new method for the sustainable preparation of such catalysts by using a mechanochemical approach, with no solvents and non‐toxic chemicals. The materials obtained from low temperature carbonization (700 °C) exhibit considerable and stable catalytic performance for ORR in alkaline medium. A catalyst obtained from iron hydroxide, tryptophan, dicyandiamide, and ammonium nitrate shows the best electrocatalytic performance with an overpotential of 921 mV vs. RHE at 0.1 mA/cm2 and an electron transfer number of 3.4.

Harnessing selective and durable electrosynthesis of H2O2 over dual-defective yolk-shell carbon nanosphere toward on-site pollutant degradation

Electrocatalytic 2e− oxygen reduction reaction (ORR) strategy for decentralized and on-demand production of H2O2 has emerges as an appealing alternative to prevailing anthraquinone process. Nevertheless, the sustainable development of high-performance and low-cost electrocatalysts with high selectivity and durability remains elusive. Herein, we report a metal-free electrocatalyst affording B/N co-doped yolk-shell carbon nanosphere with oxygen-vacancy-decorated graphene armor (B/N-HCNS@VO-G) toward promoted 2e− ORR in alkaline media. Such a dual-defective electrocatalyst harvests excellent H2O2 electrosynthesis performance with a high selectivity of 91 % and a stable operation of 24 h, which is markedly superior to its counterparts and compares favorably with the state-of-the-arts. Density functional theory calculations further reveal the essential roles of defective graphene coating played in advancing the 2e− ORR selectivity of B/N-HCNS@VO-G. More impressively, thus-produced H2O2 (realizing a high yield of 56 ppm at 0.7 V) satisfies an in-situ antibiotic and dye degradation, holding practical promise for on-site wastewater remediation.