Oxygen‐Defect‐Rich Cobalt Ferrite Nanoparticles for Practical Water Electrolysis with High Activity and Durability

Abstract

The scope of any metal oxide as a catalyst for driving electrocatalytic reactions depends on its electronic structure, which is correlated to its oxygen‐defect density. Likewise, to transform a spinel oxide, such as cobalt ferrite (CoFe2O4), into a worthy universal‐pH, bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions (HER and OER, respectively), oxygen defects need to be regulated. Prepared by coprecipitation and inert calcination at 650 °C, CoFe2O4 nanoparticles (NPs) require 253 and 300 mV OER overpotentials to reach current densities of 10 and 100 mA cm−2, respectively, if nickel foam is used as a substrate. With cost‐effective carbon fiber paper, the OER overpotential increases to 372 mV at 10 mA cm−2 at pH 14. The NPs prepared at 550 °C require HER overpotentials of 218, 245, and 314 mV at −10 mA cm−2 in alkaline, acidic, and neutral pH, respectively. The intrinsic activity is reflected from turnover frequencies of >3 O2 s−1 and >5 H2 s−1 at overpotentials of 398 and 259 mV, respectively. If coupled for overall water splitting, the extremely durable two‐electrode electrolyzer requires a cell potential of only 1.63 V to reach 10 mA cm−2 at pH 14. The homologous couple also splits seawater at impressively low cell voltages of 1.72 and 1.47 V at room temperature and 80 °C, respectively.

Microwave-Assisted Auto-Combustion Synthesis of Binary/Ternary CoxNi1−xFerrite for Electrochemical Hydrogen and Oxygen Evolution

“…The huge consumption of unsustainable carbon-emitting fossil fuels is the primary culprit for the current energy crunch and environmental concerns. , Hydrogen, a carbon-free energy source is a favorable response to confront the energy crisis and environmental issues such as global warming. , Discovering a completely new, environmentally benign, cheap, and efficacious strategy to generate hydrogen is a persistent challenge. Electrochemical water splitting stands out among other systems regarding its benefits including hydrogen production with high purity and leaving no carbon footprints. − This process consists of two half-reactions, namely, hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode. , So far, precious-metal-based electrocatalysts such as Pt/C and RuO 2 have been identified as state-of-the-art materials to drive the reaction and lower the overpotential higher than the theoretical one, 1.23 V. , …”

“…5−7 This process consists of two half-reactions, namely, hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode. 6,8 So far, preciousmetal-based electrocatalysts such as Pt/C and RuO 2 have been identified as state-of-the-art materials to drive the reaction and lower the overpotential higher than the theoretical one, 1.23 V. 9,10 Over the past decades, plenty of attempts have been made to design and develop catalysts that alloys, 11−13 carbides, 14,15 nitrides, 16,17 oxides, 18−20 phosphides, 21−23 borides, 24,25 and chalcogenides 26,27 are some of the materials that have been widely explored. Amidst a wide spectrum of electrocatalytic chemicals, the well-known metal compounds of Fe, Co, and Nilocated in the same group with Ru, Ir, and Pt, individually and hence expected to show relatively identical catalytic traits 5 and different compositions of their mixed oxides are believed to have outstanding catalytic activity for electrolysis of water.…”

Nickel Boride/Boron Carbide Particles Embedded in Boron‐Doped Phenolic Resin‐Derived Carbon Coating on Nickel Foam for Oxygen Evolution Catalysis in Water and Seawater Splitting

“…The peaks at 189, 190, and 191.8 eV are assigned to BC 3 , BC 2 O, and BCO 2 ‐type bonds, respectively, suggesting that the carbon is doped with B atoms [21a,24] . Moreover, the O 1s spectrum for the Ni x B/B 4 C/B−C PR /NF−B5P1 (Figure 4e) exhibits two peaks at 529.7 and 531.1 eV, which are attributed to Ni−O and adsorbed surface oxygen [4a,10a,16a,25] …”

Preferential Adsorption of Hydroxide Ions onto Partially Crystalline NiFe-Layered Double Hydroxides Leads to Efficient and Selective OER in Alkaline Seawater

“…The overall O 1s spectra can be deconvoluted into three different types of O-containing species based on the binding energy. The red peaks correspond to adsorbed water molecules and possibly OH – at the amorphous phase of NiFe-LDH (collectively referred to as the adsorbed oxygen), ,− the green peaks can be attributed to OH – bound to metal cations at the defects (referred to as the defect oxygen), ,, and the blue peaks are given rise by the M–O bonds in the lattice of the brucite-like layer (referred to as the lattice oxygen). , At the partially crystalline NiFe-LDH, 49.2 ± 7.3% of the total oxygen signal originate from the adsorbed oxygen, 40.5 ± 3.9% from the defect oxygen, and 10.3 ± 3.7% from the lattice oxygen. By contrast, at the highly crystalline NiFe-LDH, the percentage of the adsorbed oxygen is exceedingly low (2.1 ± 1.2%), while the defect oxygen and lattice oxygen take up 38.8 ± 2.4% and 59.1 ± 1.5% of the total signal, respectively.…”