So
far, many studies on the oxygen-evolution reaction (OER) by
Mn oxides have been focused on activity; however, the identification
of the best performing active site and corresponding catalytic cycles
is also of critical importance. Herein, the real intrinsic activity
of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the
first time. At pH ≈ 14, the onset of OER for layered Mn oxide
in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible
hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease
in the overpotential of OER was recorded for layered Mn oxide as well
as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel
slope. Furthermore, we find that both Ni and Fe ions increase OER
remarkably in the presence of layered Mn oxide, but that pure layered
Mn oxide is not an efficient catalyst for OER without Ni and Fe under
alkaline conditions. Thus, pure layered Mn oxide and electrolytes
are critical factors in finding the real intrinsic activity of layered
Mn oxide for OER. Our results call into question the high efficiency
of layered Mn oxides toward OER under alkaline conditions and also
elucidate the significant role of Ni and Fe impurities in the electrolyte
in the presence of layered Mn oxide toward OER under alkaline conditions.
Overall, a computational model supports the conclusions from the experimental
structural and electrochemical characterizations. In particular, substitutional
doping with Fe decreases the thermodynamic OER overpotential up to
310 mV. Besides, the thermodynamic OER onset potential calculated
for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus,
not in the stability range of Mn oxides.
Ni/Fe oxides are among the most widely used catalysts for water splitting. This paper outlines a new approach to synthesize Ni−Fe layered double hydroxides (Ni−Fe LDHs) for oxygen-evolution reaction (OER). Herein, we show that a dendrimer with carboxylate surface groups (generation 3.5) could react with Ni(II) ions to form a precatalyst for OER. During electrochemical OER, this precatalyst converted to Ni−Fe LDH, which is an efficient catalyst toward OER in the presence of Fe(III) ions. The catalyst was characterized by a number of methods and applied for OER using fluorinedoped tin oxide (FTO), Au, Pt, Ni foam, and glassy carbon electrodes. The catalyst shows a current density of 100 mA/cm 2 on the surface of the Ni foam, using only 297 mV overpotential and with the Tafel slope of 60.8 mV/decade. A current density of 50 mA/cm 2 on the surface of Au or Pt requires 333 and 317 mV overpotentials, respectively. The slopes of the Tafel plots for the catalyst on Au, GC, and Pt are 52.5, 47.1, and 37.4 mV/decade, respectively. The dendrimer resulted in a large dispersibility and an increase in active sites of Ni−Fe LDH, as well as the formation of Ni−Fe LDH.
Metal–organic
frameworks (MOFs) are extensively investigated
as catalysts in the oxygen-evolution reaction (OER). A Ni–Fe
MOF with 2,5-dihydroxy terephthalate as a linker has been claimed
to be among the most efficient catalysts for the oxygen-evolution
reaction (OER) under alkaline conditions. Herein, the MOF stability
under the OER was reinvestigated by electrochemical methods, X-ray
diffraction, X-ray absorption spectroscopy, energy-dispersive spectroscopy,
scanning electron microscopy (SEM), transmission electron microscopy,
nuclear magnetic resonance, operando visible spectroscopy, electrospray
ionization mass spectroscopy, and Raman spectroscopy. The peaks corresponding
to the carboxylate group are observed at 1420 and 1520 cm–1 using Raman spectroscopy. The peaks disappear after the reaction,
suggesting the removal of the carboxylate group. A drop in carbon
content but growth in oxygen content after the OER was detected by
energy-dispersive spectra. This shows that after the OER, the surface
of MOF is oxidized. SEM images also show deep restructures in the
surface morphology of this Ni–Fe MOF after the OER. Nuclear
magnetic resonance and electrospray ionization mass spectrometry show
the decomposition of the linker in alkaline conditions and even in
the absence of potential. These experimental data indicate that during
the OER, the synthesized MOF transforms to a Fe–Ni-layered
double hydroxide, and the formed metal oxide is a candidate for the
OER catalysis. Generalization is not true; however, taken together,
these findings suggest that the stability of Ni–Fe MOFs under
harsh oxidation conditions should be reconsidered.
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