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Posted: Oct 24, 2016
Improving OER performance with novel electrodeposited perovskite electrocatalysts
(Nanowerk Spotlight) Oxygen evolution reaction (OER) is the core process – but also the bottleneck – in many energy devices such as metal-air batteries and water-splitting techniques, calling for new insights in rational design of OER electrocatalysts.
"Development of super-active OER electrocatalysts has always been interesting and challenging," says Qiang Zhang, a faculty member at the Department of Chemical Engineering, Tsinghua University in Beijing, "and we have chosen perovskite oxides as the focus of our research."
He explains that the perovskite family exhibits superb OER reactivity, but its poor conductivity remains a big problem, not to mention that the morphology of perovskite oxides is hard to control.
"In situ hybridization of perovskite oxides with conductive frameworks is an efficient strategy to solve these problems, and we hope this design will work for perovskite," says Zhang
"If you go deep into the in situ design of perovskite oxides with conductive frameworks, you will find that it is not as easy as it seems to be," notes Bo-Quan Li, a graduate student in Zhang's group, tells Nanowerk. "Most perovskite oxides are achieved through high-temperature annealing under oxidative atmosphere, but conductive frameworks, either carbon or metal, cannot survive in such case. In chemical essence, there is an intrinsic contradiction of in situ hybridization of oxidative perovskite oxides and reductive conductive frameworks, and this is particularly common for many other OER catalytic systems."
"Since oxidative annealing is the problem, how about separating the oxidation process and annealing process?" Bo-Quan thought. "Aqueous solution is a mild condition suitable for preoxidation of perovskite oxide precursor and fabrication of the in situ hybridized structure. In this case, the preoxidized perovskite precursor will not need further oxidation during high-temperature annealing and the conductivity of the frameworks can be fully retained."
To verify this concept, the team introduced the aqueous preoxidation method to the LaCoFe perovskite oxide/nickel foam catalytic system.
"We selected H2O2 as the intermediate oxidant for preoxidation, and two electron oxygen evolution is an efficient method to in situ provide H2O2 from O2 in aqueous solution," said Zhang. "H2O2 then oxidized Co2+ into Co3+ described as cobalt Fenton process, which is the core reaction of the preoxidation strategy."
Meanwhile, the researchers introduced a third reaction – electrodeposition deposition reaction – to generate OH- and stabilize the preoxidized Co3+ by co-precipitation along with La3+ and Fe3+, resulting in perovskite precursor in situ hybridized with nickel foam.
"Such coupling of a serious of reactions illustrates the chemistry of preoxidation and rational design of synthetic methodology of OER electrocatalyst," Zhang points out.
Followed by inert annealing under Ar protection, the in situ hybrid composite of oxidative perovskite and reductive conductive frameworks was fabricated.
The as-synthesized electrocatalyst exhibits desirable morphology and electrocatalytic performance.
"Perovskite oxides are nano-sized and uniformly distributed on the surface of nickel foam, and that is exactly what we wanted," says Li. "The perovskite phase is well crystallized and fully oxidized with cobalt in its high oxidation state. The extraordinary OER performance of this electrocatalyst requires an ultralow overpotential of 350 mV for 10 mA cm-2 and is stable for more than 10,000 seconds with no obvious current density decrease at a constant potential. All the evidence indicates the success of our preoxidation strategy!"
These promising results demonstrate an effective method to integrate oxidative active phase with a reductive framework.
"To our knowledge, oxidative and reductive phases cannot coexist," concludes Zhang. "Yet we chemists are those who overcome such contradictions and make things which seem impossible happen. We believe that our preoxidation method not only is a rational strategy for development of OER electrocatalysts, but also opens our minds to redefine chemistry of the future."