The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions

Abstract

Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn0.5Fe0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn0.5Fe0.5 exhibits high stability for both reactions.

Electrocatalytic Conversion of Glycerol to Oxalate on Ni Oxide Nanoparticles-Modified Oxidized Multiwalled Carbon Nanotubes

“…In addition to this, NiO x /MWCNTs-Ox displays a higher dispersion of oxide nanoparticles, as well as a similar particle size (∼5 nm) and distribution of particles located inside and outside the channels of the MWCNTs-Ox (Table S1), which may contribute to the considerably larger catalytic activity of the Ni-containing sample as compared to MO x /MWCNTs-Ox with M = Mn, Fe, or Co. Yet another important difference is the type of bulk phase, which according to XRD consisted of NaCl-type Ni (oxidation state 2+) and spinel-type oxide in the cases of Fe, Mn, or Co. , The higher content of M 2+ in NiO x /MWCNTs-Ox could facilitate substrate adsorption in which the glycerol molecules are adsorbed onto NiO and/or Ni­(OH) 2 , rather than onto NiOOH species . This becomes obvious from the linear sweep voltammograms in the absence and in the presence of either glycerol or ethanol, corresponding to the OER, the GOR, and the ethanol oxidation reaction (EOR), respectively (Figure ).…”

“…Corresponding high-resolution TEM micrographs are shown in Figure S1. Synthesis and in-depth characterization of these materials were reported previously − and are summarized in Table S1. Additionally, XRD patterns and XPS spectra are shown in Figures S2 and S3, respectively.…”

“…Subsequently, the dry powder was subjected to incipient wet impregnation using as precursors aqueous solutions of either Ni­(II), Co­(II), Fe­(III), or Mn­(II) nitrates, followed by drying at 110 °C for 4 h, and annealing at 350 °C for 4 h. Metal oxide nanoparticles of the corresponding metal precursor embedded in MWCNTs-Ox, with a total metal loading of ∼14 wt % were obtained. Characterization of the different catalysts was reported previously ,, and is summarized in Table S1. Corresponding TEM micrographs, XRD patterns, and XPS spectra are shown in Figures S1–S3, respectively.…”

Nitrogen‐Rich Carbonaceous Materials for Advanced Oxygen Electrocatalysis: Synthesis, Characterization, and Activity of Nitrogen Sites

“…Application and commercialization of rechargeable metal–air batteries remain challenged by a dearth of suitable bifunctional oxygen electrodes, exhibiting not only high activity toward the two reactions, but also having long‐term stability under the operating conditions of both the ORR and the OER. [ 141 ] The efficiency of oxygen conversion in rechargeable metal–air batteries is also limited by the inherent slow kinetics of the two reactions, resulting in a large energy barrier. [ 142 ] The development of economical and robust non‐noble metal and even metal‐free electrocatalysts used in rechargeable metal–air batteries has attracted wide attention.…”

Nitrogen and Oxygen Functionalization of Multi‐Walled Carbon Nanotubes for Tuning the Bifunctional Oxygen Reduction/Oxygen Evolution Performance of Supported FeCo Oxide Nanoparticles

“…The ΔE values obtained at the end of the alternating ORR/OER tests were 0.88, 0.91, and 0.96 V for FeCoOx(nitrate)/N‐MWCNT‐Ox, FeCoOx(carbonyl)/N‐MWCNT‐Ox and FeCoOx(nitrates)/MWCNT‐Ox, respectively. Even though these results represent a major limitation in the applicability of these catalysts in regenerative technologies, such as rechargeable metal‐air batteries or reversible fuel cells, which is an obstacle encountered often with bifunctional ORR/OER catalysts, [43] it is clear that the functionalization with nitrogen played a beneficial role for nanoparticle stability, and that the textural characteristics resulting from the selected synthesis routes effectively led not only to an increase in activity, likely due to an increase of the electrical conductivity and selectivity, but also to an enhancement of the stability at both OER and ORR conditions. Further optimization of the structural properties together with a two‐layer bifunctional oxygen electrode approach, that is, the use of independent ORR and OER catalytic films in reversible systems, [5e] could increase further the applicability of the investigated FeCoOx catalysts.…”