The
world’s shift to the production of energy from sustainable
sources requires the development of large energy storage. One of the
best methods to store surplus energy produced from environmentally
friendly methods is as elemental hydrogen, using electrolysis in alkaline
electrolyzers. Currently, this technology is hampered by the sluggish
oxygen evolution reaction (OER), which limits its overall efficiency
and durability. One of the most popular directions is to develop cheap,
durable, and active platinum-group-metal-free (PGM-free) catalysts.
In this category, the benchmark catalyst is NiFeOOH. Here, synthetic,
electrochemical, spectroscopic, and theoretical methods were used
to design, synthesize, and investigate novel PGM-free catalysts with
enhanced durability and activity. Using an easy and cheap one-step
synthetic precipitation method, titanium atoms in various amounts
were introduced in the NiFeOOH structure, forming Ni
x
Fe
y
Ti
z
OOH. One of these compounds (Ni:Fe:Ti = 85.75:7.70:6.55) shows a
very low overpotential on GC (400 mV, at a current density of 10 mA/cm2) and high current density (27.9 mA cm–2) at a potential of 1.8 V vs RHE. This is a higher activity toward
the OER in comparison to the benchmark catalyst; in addition, the
compound has higher stability at prolonged exposure to high potentials.
Trimetallic double hydroxide NiFeCo−OH is prepared by coprecipitation, from which three different catalysts are fabricated by different heat treatments, all at 350 °C maximum temperature. Among the prepared catalysts, the one prepared at a heating and cooling rate of 2 °C min−1 in N2 atmosphere (designated NiFeCo−N2‐2 °C) displays the best catalytic properties after stability testing, exhibiting a high current density (9.06 mA cm−2 at 320 mV), low Tafel slope (72.9 mV dec−1), good stability (over 20 h), high turnover frequency (0.304 s−1), and high mass activity (46.52 A g−1 at 320 mV). Stability tests reveal that the hydroxide phase is less suitable for long‐term use than catalysts with an oxide phase. Two causes are identified for the loss of stability in the hydroxide phase: a) Modeling of the distribution function of relaxation times (DFRT) reveals the increase in resistance contributed by various relaxation processes; b) density functional theory (DFT) surface energy calculations reveal that the higher surface energy of the hydroxide‐phase catalyst impairs the stability. These findings represent a new strategy to optimize catalysts for water splitting.
Alkaline
electrolyte membrane electrolyzers are a promising technology
to efficiently produce clean hydrogen without the use of critical
raw materials. At the heart of these electrolyzers are the electrocatalysts,
which facilitate the cathodic and anodic reactions, with the latter
oxygen evolution reaction (OER) being the most sluggish. In recent
years, aerogels have become a very well-studied class of materials
due to their unique properties, including very high surface area.
Until now, aerogels have not been used to catalyze the OER by themselves
but were mainly considered catalyst supports. Here, mixed-metal nickel–iron
oxide aerogels were synthesized with a modified epoxide route synthesis
and tested as OER catalysts. Depending on the Ni/Fe ratio, they show
very high catalytic activity and low overpotential to reach 10 mA
cm–2 (at η = 380 mV). This activity is beyond
that of the existing state-of-the-art platinum group metal-free OER
catalysts.
In order to solely rely on renewable and efficient energy sources, reliable energy storage and production systems are required. Hydrogen is considered an ideal solution since it can be produced...
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