Kinetics and electrocatalytic activity of IrCo/C catalysts for oxygen reduction reaction in PEMFC

The purpose of this study is to develop a novel binary Iridium-Cobalt/C catalyst as a suitable substitute for platinum/C applied in proton exchange membrane fuel cells (PEMFCs). The carbon-supported IrCo catalysts were successfully synthesized using IrCl₃ and C₄H₆CoO₄ as the Ir and Co precursors respectively, in ethylene glycol (EG) refluxing at 120 °C. The nanostructured catalysts were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscope (TEM). Homogeneous catalyst particles supported on carbon showed a size of proximately 2 nm. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted for the characterization of the catalyst performances. With a cathodic loading of 0.4 mg_Ir cm⁻², 20%Ir-30%Co/C achieved a maximum power density of 501.6 mW cm⁻² at 0.418 V, with a 50 cm² H₂/O₂ single cell. Although such a performance is about 26% lower than commercial Pt/C catalyst, it is still helpful in terms of Pt replacement and cost reduction.     

A novel CuFe-based catalyst for the oxygen reduction reaction in alkaline media

The primary objective of this work is to develop alternative electrocatalysts to Pt-based materials for the oxygen reduction reaction (ORR) in alkaline fuel cells. We synthesized a bicore CuFe/C composite electrocatalyst by impregnation of iron and copper phthalocyanine-based complexes into a carbon support, followed by pyrolysis at 800-900 °C in an Ar atmosphere. This novel composite catalyst exhibits electrochemical performance for ORR in 0.1 M KOH similar to a commercial Pt/C (BASF Fuel Cell, 30%) catalyst at 6-fold lower CuFe loading. High resolution X-ray photoelectron spectroscopy (HR-XPS) results indicate that coordination bonding between Fe and N atoms still remains and show that a mixed Cu(I)/Cu(II) valency exists in the CuFe/C catalyst after high temperature heat treatment. The Cu(I)/Cu(II) redox mediator adjacent to Fe atoms is crucial to provide electrons to the N_xFe-O₂ adduct and maximize the overall rate of the reduction reaction. The results of this study may offer a new approach to development of efficient catalysts for oxygen reduction to water in alkaline media.     

To pursue FexCoy-PANI/CNT catalysts for oxygen reduction reaction in acid medium with controlled molecular self-assembly method

The mainstream of pyrolyzed transitional metal-nitrogen-carbon (M-N-C) catalysts for ORR still confront difficulty in PEMFC application. To pursue M-N-C structure from wet chemistry at ambient temperature, this paper prepares Fe_xCo_y-PANI/CNT porous structures composed of amorphous Fe and Co NPs into PANI layer on CNT surface, supported by the controlled molecular self-assembly mechanism (MS). For their ORR behaviors in acid medium, all Fe_xCo_y-PANI/CNT catalysts demonstrate similar features as Pt-based catalyst in low current density region, and 4e pathway and active sites in pore utilization in high current density region. Specifically, we disclosed nitrogen in PANI matrix dominates specific activity for ORR, and a little transitional metal attain mass activity at maximum. The active sites mounted into PANI matrix and 4e pathway help catalysts to achieve high durability. Thus, we extend a new type of platinum-free catalyst and develop a bottom-up approach for preparation-structure-activity, expecting to drive PEMFC remarkably.     

Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared from sodium ferric ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD) supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes (CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well as the pore volumes of catalysts were examined by nitrogen physisorption characterization. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using domestic wastewater as fuel. The maximum current density and power density recorded are 110 (mA m⁻²) and 127 ± 0.9 (mW m⁻²), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m⁻² and 226 ± 0.2 mW m⁻²), demonstrating that these catalysts can be used as substitutes for commercial Pt/C.     

CeO2 overlapped with nitrogen-doped carbon layer anchoring Pt nanoparticles as an efficient electrocatalyst towards oxygen reduction reaction

To engineering high-efficient, sustainable and novel Pt-based composite system, a newly “Pt-oxide” based composites electrocatalyst of “CeO₂ overlapped with nitrogen-doped carbon layer anchoring Pt nanoparticles” (PtCeO₂@CN) has been fabricated. In comparison with Pt/C, the results exhibit that PtCeO₂@CN possesses a preferable methanol tolerance ability, superior stability (30000 s degradation: 35% for PtCeO₂@CN vs. 50% for Pt/C), and more positively the onset potential (16 mV) as well as half-wave potential (29 mV) towards oxygen reduction reaction. Further, the investigation shows that PtCeO₂@CN has a certain selectivity with quasi-four electron pathway (n = 3.2–3.3 e^?). This is attributed to the establishment of “nitrogen-doped carbon layer” structure, which heightens the conductivity of CeO₂, further promotes electron transfer between Pt and CeO₂, as well as strengthens the anchoring effect for Pt nanoparticles. Overall, this study would shed bright light to develop some effective Pt-oxide based composite electrocatalysts.     

Nanocrystaline tungsten carbide supported Au–Pd electrocatalyst for oxygen reduction

Au–Pd nanobimetallic particles supported on nanocrystaline tungsten carbide as electrocatalysts for oxygen reduction were prepared by an intermittent microwave heating (IMH) method. XRD measurement revealed that AuPd alloy formed during the IMH process. We showed these novel electrocatalysts could offer the activities that surpass that of the state-of-the-art Pt-based electrocatalysts for oxygen reduction reaction. The AuPd–WC/C electrode showed an over 70 mV shift towards more positive potentials compared to Pt/C electrode for ORR. The advantage seemed to come from the novel support of tungsten carbide which itself has the catalytic activity to enhance the catalytic activity of the metal electrocatalysts.     

Influence of preparation process on non-noble metal-based composite electrocatalysts for oxygen reduction reaction

Composite electrocatalysts based on the transition metal cobalt are synthesized for the oxygen reduction reaction (ORR) by mechanically mixing three basic precursors containing carbon black, cobalt acetate and tetramethoxy-phenylporphyrin (TMPP). The influence of various mixing processes, solvents, pyrolysis temperature and precursor ratios on the ORR activity of the electrocatalysts is investigated by means of their electrochemical characteristics. Levich–Koutecky plots show that the number of transferred electrons during ORR on these catalysts varies between 2 and 4. A pyrolyzed mixture synthesized by ultrasonication exhibits better activity for ORR than that prepared by ball milling. The solvent is found to have a significant effect on the performance of the catalysts in acidic media. The catalyst synthesized in water, a poor solvent of TMPP, has better activity than that synthesized in a good solvent of TMPP, such as N,N-dimethylformamide and acetic acid. The optimum pyrolysis temperature is 600 °C. Various mole ratios and Co/carbon weight ratios are examined. Maximum activity is found at a 1:1 TMPP/Co mole ratio and a 5% Co/carbon weight ratio.     

Co3O4@g-C3N4 supported on N-doped graphene as effective electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) is a key component of numerous energy conversion equipment, including metal-air batteries and fuel cells. Reasonably designing high-efficiency non-noble materials as ORR electrocatalysts is crucial for large-scale practical applications. In this work, a calcination-hydrothermal method is used to prepare Co₃O₄@g-C₃N₄ (g-C₃N₄ wrapped Co₃O₄ nanoparticle) supported on nitrogen doped graphene (NG). The electrochemical activity of composites is estimated by cyclic voltammograms and linear sweep voltammetry in 0.1 M KOH medium. Owing to the positive synergistic role stemming from the Co₃O_4, g-C₃N₄, Co-N_x effective sites and N modified graphene in the composite material, the Co₃O₄@g-C₃N₄/NG owns positive onset potential of 0.920 V (vs. RHE) and half-wave potential of 0.846 V (vs. RHE), which are superior to onset potential of 0.917 V and half-wave potential of 0.824 V for commercial Pt/C, respectively. Additionally, it also exhibits longer-term stability and stronger methanol resistance comparing with Pt/C. The nonprecious metal catalyst could be used as a hopeful catalyst to substitute commercial Pt/C for ORR.     

Nitrogen doped amorphous carbon as metal free electrocatalyst for oxygen reduction reaction

A non metal catalyst for the oxygen reduction reaction is prepared by simply pyrolyzing ion exchange resin D113 in NH₃. The product is nitrogen doped amorphous carbon. The pyrolysis of D113 exchanged with iron ion results in nitrogen doped graphitic carbon. The amorphous carbon is easier to be doped by NH₃ with higher nitrogen content. The nitrogen doped amorphous carbon is more active than graphitized carbon, together with much improved stability. The higher activity is explained by the higher total nitrogen content and higher pyridinic/graphitic nitrogen percentage. The higher stability is because there is no loss or dissolution of the active sites. The results of this work prove metal element and graphitization of carbon are not necessary factors for nitrogen doped carbon as non noble metal catalyst for the oxygen reduction reaction.     

MoO2 nanocrystals down to 5 nm as Pt electrocatalyst promoter for stable oxygen reduction reaction

The single molybdenum oxide (MoO₂) crystals down to 5 nm in diameter on carbon (denoted as C-MoO₂) are synthesized based on ion-exchange principle for the first time. The structures, morphologies, chemical and electrocatalytic performances of as-synthesized nanomaterials are characterized by physical, chemical and electrochemical methods. The results indicate that electrocatalysts made with Pt nanoparticles supporting on C-MoO₂ (denoted as Pt/C-MoO₂) are highly active and stable for oxygen reduction reaction (ORR) in fuel cells. A mass activity of 187.4 mA mg⁻¹_Pt at 0.9 V is obtained for ORR, which is much higher than that on commercial Pt/C (TKK) electrocatalyst (98.4 mA mg⁻¹_Pt). Furthermore, the electrochemical stability of Pt/C-MoO₂ is more excellent than that of Pt/C (TKK). The origin of the improvement in catalytic activity can be attributed to the synergistic or promotion effect of MoO₂ on Pt. The improvement in electrochemical stability is due to the strong interaction force between Pt and MoO₂.     

Cu2S/Ni3S2 ultrathin nanosheets on Ni foam as a highly efficient electrocatalyst for oxygen evolution reaction

It is an inevitable choice to find efficient and economically-friendly electrocatalysts to reduce the high overpotential of oxygen evolution reaction (OER), which is the key to improve the energy conversion efficiency of water splitting. Herein, we synthesized Cu₂S/Ni₃S₂ catalysts on nickel foam (NF) with different molar ratios of Ni/Cu by a simple two-step hydrothermal method. Cu₂S/Ni₃S₂-0.5@NF (CS/NS-0.5@NF) effectively reduces the overpotential of OER, displaying small overpotentials (237 mV@100 mA cm^?2 and 280 mV@500 mA cm^?2) in an alkaline solution, along with a low Tafel slope of 44 mV dec^?1. CS/NS-0.5@NF also presents an excellent durability at a relatively high current density of 100 mA cm^?2 for 100 h. The excellent performance is benefited by the prominent structural advantages and desirable compositions. The nanosheet has a high electrochemical active surface area and the porous structure is conducive to electrolyte penetration and product release. This work provides an economically-friendly Cu-based sulfide catalyst for effective electrosynthesis of OER.     

Polyoxomolybdate anchored graphite oxide: Noble metal-free electrocatalyst for oxygen reduction reaction

We present the synthesis of a noble metal-free electrocatalyst, polyoxomolybdate/reduced graphite oxide (PMA/rGO) composite, which showed enhancement in the kinetics for oxygen reduction reaction (ORR). The composite material was prepared by simple and cost effective method. Mere heating of the precursors at low temperature (200 °C) resulted in molecular assembly of PMA on GO in the form of clusters which behaved as active centers for efficient ORR. The electrochemical study of PMA/rGO-2 (PMA to GO weight ratio of 1:2) catalyst carried out by rotating disk electrode (RDE) method, showed considerable electrocatalytic activity with E_onset of 1.0 V vs. RHE and current density of 4.0 mA/cm² at 1600 rpm in alkaline condition. Additionally, as-prepared PMA/rGO-2 catalyst showed a single step ~ 4 electron transfer pathway similar to commercial Pt/C catalyst; confirmed through rotating ring disk electrode (RRDE) study. Interestingly, PMA/rGO-2 electrocatalyst exhibited substantially higher stability than Pt/C catalyst even after 20K potential cycles (though the current density of former catalyst is inferior to later). Further, in a methanol cross-over test, PMA/rGO-2 was found to be inactive towards methanol oxidation reactions, which could nullify the issues due to the fuel cross-over effect, if employed as cathode in direct methanol fuel cells. The enhanced ORR activity and significant stability is attributable to the anchoring and homogenous distribution of polyoxomolybdate clusters on graphite oxide.     

Effects of stabilizers on the synthesis of Pt3Cox/C electrocatalysts for oxygen reduction

Pt₃Co_x/C electrocatalysts for use as cathodes in proton exchange membrane fuel cells are fabricated using various stabilizers to control the different reduction speeds between Pt and Co ions. Four different types of stabilizers—sodium acetate, oleylamine, tetraoctylammonium bromide (TOAB), and hexadecyltrimethylammonium bromide (CTAB)—differing in molecular structures and ionic states are tested. Primarily, Pt₃Co_x/C alloy nanoparticles are synthesized with 0.6 < x < 0.8 after heat treatment to remove the residual stabilizers. A significant improvement in the activity for oxygen reduction reaction is observed in the case of TOAB- and CTAB-mediated Pt₃Co_x/C catalysts. In particular, CTAB-mediated catalysts exhibit the best activity, which is about 2-times higher mass activity than commercial Pt/C catalyst. The higher mass activity is believed to result from not only the alloying effects with small atomic size Co but also better dispersion and smaller particle size after heat treatment at relatively low temperature.     

Bamboo-like carbonitride nanotubes with multi-type active sites for oxygen reduction reaction in both alkaline and acid mediums

To omni-directionally utilize carbon-based material in nanoscale and improve its catalytic activity for oxygen reduction reaction (ORR), bamboo-like carbonitride nanotubes (bCNTs) with high-density multi-type active sites (CoO@Co–N-bCNT) are facilely synthesized via a multi-step method referring to thermal pyrolysis of Co²⁺ complexed melamine, acid leaching and second annealing. Multi-type active sites including encapsulated Co nanoparticles, intercalated Co/CoO species, Co-N_x coordinated sites and defect-rich surface are present in the as-prepared CoO@Co–N-bCNT electrocatalyst. The types and densities of these active sites are easily tuned via the ratio of melamine to Co²⁺ in precursors and subsequent treatment. Due to the high-density multi-type active sites and bamboo-like tubular structure, CoO@Co–N-bCNT electrocatalyst exhibits high catalytic activity for ORR with high stability in both alkaline and acid electrolytes. Quasi-solid-state zinc air battery (ZAB) assembled with the CoO@Co–N-bCNT as the cathode exhibits open circle voltage of 1.39 V and peak power density of 14.9 mW cm^?2. Two-cell series of quasi-solid-state ZABs can light LED indicator for 7 h, suggesting its promising practical application. The studies provide facile strategy to design the carbon-based electrocatalysts with high performance and tune their active sites.     

A novel non-noble electrocatalyst for oxygen reduction in proton exchange membrane fuel cells

Tungsten nitride supported on carbon black was prepared by temperature-programmed reaction (TPR) process and is proposed as a catalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The as-prepared catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The ORR activities of the catalyst were studied by electrochemical measurements and single cell tests, respectively. The results indicated that the tungsten nitride electrocatalyst exhibited attractive catalytic activity and stability for the ORR in PEMFCs. It is expected to be a promising cathode electrocatalyst for PEMFCs, especially for the comparatively high temperature proton exchange membrane fuel cells.     

Optimization of RuxSey electrocatalyst loading for oxygen reduction in a PEMFC

The synthesis, characterization and optimization of Ru_xSe_y catalyst loading as a cathode electrode for a single polymer electrolyte membrane fuel cell, PEMFC were investigated. Ru_xSe_y catalyst was synthesized via a decarbonylation of Ru₃(CO)₁₂ and elemental selenium in 1,6-hexanediol under refluxing conditions for 2 h. The powder electrocatalyst was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and examined for the oxygen reduction reaction (ORR) in 0.5M H₂SO₄ by rotating disk electrode (RDE) and in membrane-electrode assemblies, MEAs for a single PEMFC. Results indicate the formation of agglomerates of crystalline particles with nanometric size embedded in an amorphous phase. The catalyst exhibited high current density and lower overpotential for the ORR compared to that of Ru_x cluster catalyst. Dispersed Ru_xSe_y catalyst loading on Vulcan carbon was optimized as a cathode electrode by performance testing in a single H₂–O₂ fuel cell.     

Promising nature-based nitrogen-doped porous carbon nanomaterial derived from borassus flabellifer male inflorescence as superior metal-free electrocatalyst for oxygen reduction reaction

In this present study, novel hierarchical nitrogen-doped porous carbon for use as a metal-free oxygen reduction reaction (ORR) electrocatalyst is derived from borassus flabellifer male inflorescences by calcining at 1000 °C in an inert atmosphere using metal hydroxides as activating agent and melamine as nitrogen doping agent. The BET surface areas of the lithium-ion (Li-ion), potassium-ion (K-ion) and calcium-ion (Ca-ion) activated carbon are observed to be 824.02, 810.88 and 602.88 m² g^-1 respectively. Another interesting fact is that the total surface energy calculated by wicking method (73.2 mJ/m²), is found to be higher for Li-ion activated carbons. Among the prepared nitrogen-doped porous carbon, Li-ion activated system, showed an outstanding performance in ORR reaction in alkaline medium, thanks to its high surface area and notable surface activity. An incontrovertible of note that ORR half-wave potential of Li-ion activated nitrogen-doped carbon (0.90 V) is relatively higher in comparison to the commercial 20 wt % Pt/C catalyst (0.86 V). Inspite of overwhelming performance, the ORR reaction followed the preferred 4- electron transfer mechanism involving in the direct reduction pathway in all activated carbons. The ORR performance is also noticeably better and comparable to the best results in the literature based on biomass derived carbon catalysts.     

Agaricus bisporus residue-derived Fe/N co-doped carbon materials as an efficient electrocatalyst for oxygen reduction reaction

Biomass-derived multielement-co-doped carbon materials with ultrahigh active-sites density and unique physicochemical properties hold great promise for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Agaricus bisporus residue as a type of biomass waste is produced after microbial growth on biomass substrates, contributing to its natural multidimensional framework and nutrient elements residual. Based on this advantage, this paper further combined with (NH₄)₃PO₄ and FeCl₃·6H₂O to provide N, P, and Fe. Finally, the Fe/N co-doped carbon catalyst with hierarchical porous structure (SN-Fe-ZA) was fabricated by a facile hydrothermal-pyrolysis synthesis route. The characteristic of SN-Fe-ZA exhibited an obvious honeycomb porous structure, high nitrogen doping content of 2.36 at%, and its specific surface area was up to 1646.4 m²·g⁻¹ with abundant micro-/mesoporous. Electrochemical measurements further indicated that SN-Fe-ZA possessed a distinct ORR electrocatalytic activity in alkaline solution. Compared with the electrochemical parameters of commercial Pt/C electrocatalyst, SN-Fe-ZA had the equivalent onset potential (0.968 V) and half-wave potential (0.820 V). Besides, it showed a more excellent electrochemical stability and stronger methanol-tolerant. This research proposed a promising approach to prepare hierarchical porous and multielement-co-doped catalyst from renewable biomass waste as effective cathode electrocatalytic materials for alkaline fuel cells.     

Stability of hemin/C electrocatalyst for oxygen reduction reaction

Hemin has been reported to be an effective electrocatalyst for mediating the oxygen reduction reaction. In this work, the stability of hemin/C is extensively investigated in both acid and alkaline media by the electrochemical methods. It is found that the pristine hemin/C yields significant change in the composition and the electrochemical features when it undergoes the potential cycling in acid media. In comparison, the catalyst shows superior stability in alkaline media. The pyrolysis can improve the stability of the hemin/C catalyst by removing the organic groups in hemin; however, the heat treatment cannot prevent the metal ion loss in acid media. Finally, the acid-leaching experiment reveals that the active center for the 4-electron reaction tends to get lost in acid, indicating that the iron metal ion should be involved in catalyzing the 4-electron reduction reaction. Furthermore, the XPS result indicates that the element N is also involved in the active center. Therefore, it can be concluded that the Fe–N contributes to the active center for the complete reduction of oxygen in alkaline media.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.