Enhanced ORR activities in the presence of NH-CP. a) Number of electrons transferred from NH-CP to diatomic oxygen (). b to d) ORR polarization curves of cobalt-based electrocatalysts in the presence of NH-CP. Operation frequencies (TOF) of the four-electron ORR on three different electrocatalysts (CoO, Co3O4 and LiCoO2) in the presence of NH-CP.
Oxygen Reduction Reaction
The metal-air batteries typically include zinc-air (Zn-Air) and lithium-air (Li-Air) batteries. The theoretical working potentials for fuel cells and metal-air batteries are given below (Table 2).
Kinetics of Oxygen Reduction Reaction
Electrocatalysts for Oxygen Reduction Reaction
The catalytic activity for the ORR of N-doped carbon arises from the enhancement of oxygen absorption due to the change in the charge density of carbon atoms adjacent to N (Scheme 2a). Recently, carbon atoms adjacent to the pyridinic N were suggested as active sites under acidic conditions (Scheme 2b). a) Charge density distribution in N.4 doped carbon nanotubes (b) Schematic pathway for ORR in N.5 doped carbon.
Linear Scaling Relationship
In terms of complexity, metal-only systems, such as metal nanoparticles and metal alloys, are at the lowest level. At the second level of complexity there are heteroatom doped systems such as single metal atoms in other metals, high entropy alloys and doped p-block catalysts.
Dual Electrocatalysis
The physicochemical nature of the original active sites was altered by the introduction of secondary elements that ensured that the surface species were still bound to a single active site. Inorganic/organic hybrid double catalyst systems were presented in this work as examples of the associative activation electrocatalysis.
Conductivity-dependent Completion of Oxygen Reduction on Oxide Catalysts
- Introduction
- Results and Discussion
- Experimental
- Conclusions
ORR polarization curves of the CoO electrocatalyst without (blue) or with pPy (red) at the 10th cycle (dashed lines) and the 500th cycle (solid lines). Volcano plots were used to show the dependence of the ORR activity on the catalyst descriptor. The binding energy of monatomic oxygen (*O, resulting from the dissociative adsorption of diatomic oxygen) or hydroxide (*OH, resulting from the protonation of *O) to the active site of the catalysts (ΔEO or ΔEOH)6, 131 as well as the center of the d band of the catalysts132 were used as a catalyst descriptor.
Polypyrrole-assisted Oxygen Electrocatalysis on Perovskite Oxides
Introduction
Oxygen reduction reaction (ORR) is used as the cathodic process of fuel cells and metal-air batteries for generating electricity.71-75 Its reverse reaction, oxygen evolution reaction (OER), is the anodic process for splitting water and charging process of metal-air batteries. 76, 77 High reversibility between ORR and OER must be guaranteed in rechargeable metal air batteries72-75 while fuel cells and water splitting are based on either forward or reverse reaction of the oxygen-to-water conversion. Platinum is known as the best ORR catalyst while the oxide layer formed on its surface under oxidative conditions seriously weakens the catalytic activity for OER.78-80 Iridium or ruthenium oxides have been considered the best OER catalysts.81 Their electrocatalytic activities however, of ORR is not as high as that of OER, significantly inferior to other catalysts. Iridium alloys with transition metals, as another form of iridium-containing catalysts (not the oxide form), efficiently catalyzed ORR while any forms of ruthenium did not work as the ORR catalysts.82 It is challenging to find a catalyst with high electroactivities for both to develop. ORR and OER.
Simple perovskite oxides (ABO3; A = alkaline and/or rare earth metals, B = transition metals) were proposed as monofunctional catalysts for OER or ORR40, 41, 49 and bifunctional catalysts46-48. Improvement of the electroactivity of ORR is necessary to ensure bifunctionality and reduce the potential gap between ORR and OER. In this work, two different perovskite oxides were used as bifunctional catalysts for both ORR and OER in alkaline media at room temperature: BSCF as a simple perovskite and NBSC as a double perovskite (NdBa0.25Sr0.75Co2O5.9 in Figure 10).83 The particle sizes of both oxides were controlled similarly or identically at 400 to 500 nm (compare Ketjenblack at ~30 nm).
Results and Discussion
It should be noted that the Eonset of pPy/C (0.7 V) is less positive than that of perovskite and pPy/C composites. In addition, BSCF achieved a more complete ORR in the presence of pPy: n increased from 3.5 without pPy to 3.7 (Figure 13c). On the contrary, the overpotentials of ORR and OER polarizations decreased significantly with increasing amount of pPy/C.
Electrodeposition of pPy on NBSC+C. ORR on pPy-coated perovskite oxide. pPy was electrochemically deposited on the NBSC. It appears to be in contrast to the improvement in the onset potentials and n values promoted by the increasing amount of pPy/C (Figure 25). In the cases of pPy/C, its amount is directly related to the effective surface area of pPy.
Experimental
Ring disk electrodes (RRDE) of glassy carbon disk and platinum ring were used as the working electrode (disk area = 0.1256 cm2) while a platinum wire and a Hg/HgO electrode were used as the counter and reference electrodes, respectively. ORR polarization curves were obtained on the disk electrode from a cathodic sweep from +0.1 V to -0.7 V (vs. Hg/HgO) at 10 mV s-1 after five cycles of CVs. The anodic sweep from +0.35V to +0.9V (versus Hg/HgO) after nine cycles is presented as OER polarization curves.
The potential values were converted from versus Hg/HgO to versus the reversible hydrogen electrode (RHE) by: Hg/HgO + 0.929 V = RHE. For the correction, the potential difference between Hg/HgO and RHE was measured in a cell using platinum wires as working and counter electrodes in a hydrogen-saturated aqueous electrolyte of 0.1 M KOH with Hg/HgO as reference electrodes. The same catalyst ink as described above was used to fill the NBSC for the working electrode.
Conclusions
Secondary-amine-conjugated Polymer-assisted Oxygen Reduction Reaction on Cobalt-based
Introduction
Dual-catalyst system, called synergistic catalysis or dual catalysis, where two catalysts work simultaneously to reduce the activation energy of the reaction has generally been well described in chemical synthesis.8-10 Similar concept was calculated proposed in electrocatalysis for ORR and oxygen evolution reaction ( OER) as a way to circumvent these scaling relationships between adsorption energies for different reaction intermediates, which fundamentally limit catalytic activities.15 In this work, we demonstrate that the ORR activities of heterogeneous catalysts are significantly enhanced by simply mixing catalyst particles with secondary catalysts. -amine conjugated polymers (HN-CPs). No sophisticated material development of catalysts was involved in the development of the intimate interaction between the active sites of catalysts and the HN-CPs. The HN-CP-assisted enhancement in the catalytic activity was understood as oxygen activation of HN-CPs during the reduction of the oxygen species at catalytically active sites.
A series of NH-CPs were tested as the second component of the double catalysis catalyst to donate electrons to oxygen to generate polarized oxygen or O2δ-, including polypyrrole (pPy), polyaniline (pAni), polyindole (pInd), polycarbazole (pCbz), and polyethyleneimine ( pEI). The number of electrons donated from NH-CP to oxygen (δ) successfully characterized the effects of NH-CP on ORR activity as a single descriptor, providing a measure of the enhancement of the catalytic activity of Co-based catalysts with NH-CP. In addition to oxygen activation, from a mechanistic point of view, we propose the possibility that the NH-CP proton is transferred to the surface oxygen species at the active sites of the catalysts.
Results and Discussion
NH-CP-assisted ORR activity was proportional to the δ of NH-CP for all considered cobalt-based catalysts (Figure 35). Protonation of the diatomic oxygen intermediate on the catalyst (formation of *OOH) was the rate-determining step (RDS) of the entire ORR process in the absence of pPy (Figure 39 and Figure 40). The proton transfer step (Figure 41a) is a very feasible RDS in the applied potential situation where the surface singlet oxygen (*O) is protonated by a proton from the secondary amine NH-CP.
A measure of ORR activity was plotted against δ in the additional catalyst version of the Volcano plot (Figure 35). Both dependence on NH-CP and dependence of ORR electrocatalytic activities on the catalyst were found on the volcano plot (left part of Fig. 35). NH-CPs having δ < 0.2 did not affect the catalytic activity of the catalysts (NH-CP-ineffective zone).
At a fixed NH-CP (catalyst dependence by vertical comparison in the volcano plot of Figure 35), the η1/2 representing the ORR activity at higher overpotentials (relatively higher than the onset potential) was determined by the ORR activity of the catalysts in the absence of the NH-KPs. However, the catalytic activity of the weakest catalyst Co3O4 in the presence of pPy exceeded that of CoO in the absence of NH-CPs.
Experimental
Five microliters of the catalyst ink was dropped onto the glassy carbon disk electrode of the rotating ring disk electrodes. The ring disk electrode of the glassy carbon disk and platinum ring was used as the working electrode (disk area = 0.1256 cm2). The amount of charge transfer (δ) of the NH-CPs was obtained by a Mulliken population analysis.136.
The antiferromagnetic distribution of moments in CoO was used to calculate the mech. Using the optimized unit cell, a symmetric slab model of the (001) surface, consisting of 4 atomic layers, with a vacuum slab of 20 Å was constructed (Figure 52b). With this result, we designed the following reaction pathway by considering proton transfer.
Conclusions
Active sites of nitrogen-doped carbon materials for the oxygen reduction reaction elucidated using model catalysts Science. Design principles for oxygen reduction activity on perovskite oxide catalysts for fuel cells and metal–. Identification of catalytic sites for oxygen reduction in iron and nitrogen doped graphene materials Nat.
Fe@N-Graphene Nanoplatelet-embedded Carbon Nanofibers as Efficient Electrocatalysts for Oxygen Reduction Reaction Advanced Science. Nitrogen-doped graphene as an efficient metal-free electrocatalyst for oxygen reduction in ACS Nano fuel cells. 3D nitrogen-doped graphene airgel-supported Fe3O4 nanoparticles as efficient electrocatalysts for oxygen reduction reaction J.
