| RESEARCH HIGHLIGHTS |


to predict the growth of droplets around clusters of particles. These results demonstrate that it


is not possible to accurately simulate condensation based on a single factor, the team says. Indeed, it appears that there is a


competition between the particle and the substrate that determines how fast each one is covered in water as the droplet con- denses. “It points to significant implications for wetting at small scales,” says Leong. The team now hopes to model condensation


and liquid interactions at even smaller length scales.


1. Quang, T. S. B., Leong, F. Y., An, H., Tan, B. H. & Ohl, C-D. Growth and wetting of water droplet condensed between micron-sized particles and substrate. Scientific Reports 6, 30989 (2016).


Catalysis


A BREATH OF FRESH AIR


THREE-LAYER NANOPARTICLE CATALYSTS IMPROVE ZINC–AIR BATTERIES


electrolyte


Nanoparticles containing three different layers of material can help to boost the performance of a zinc–air battery, A*STAR researchers have found1. Zinc–air batteries are cheap, have a high


energy density, and last for a very long time. Their use of a water-based electrolyte makes them safer than other batteries, so they are often found in medical applications, such as hearing aids and heart-monitoring devices. The battery’s negative electrode contains


zinc metal, which gives up electrons when it reacts with hydroxide ions in the electrolyte. Those electrons generate a current as they flow to the positive electrode, where they react with oxygen from the air to produce more hydroxide ions. The sluggishness of the reaction with


oxygen limits the battery’s voltage output and its performance at high current. Finding a catalyst to speed up the reaction could yield higher power and energy densities, opening a wider range of potential applications. Yun Zong and Zhaolin Liu of the


A*STAR Institute of Materials Research and 40 A*STAR RESEARCH


Engineering and colleagues have developed a nanoparticle catalyst that could fit the bill. The particles are 20–50 nanometers across, with a cobalt core encased by an inner shell of cobalt oxide, which is surrounded by an outer shell of pyrolyzed polydopamine (PPD), a form of carbon ‘dotted’ with nitrogen atoms. These nanoparticles are coated on a porous carbon support that acts as an electrode. Their structure helps to prevent them from leaching cobalt or clumping together, and the protective outer shell also makes the nanoparticles more durable. These three-layer nanoparticles efficiently


transformed oxygen to hydroxide in a single step. The team suggests that nitrogen atoms in the PPD shell help to attract and make oxygen atoms more reactive on their way to catalytically active sites in the cobalt oxide and PPD. Meanwhile, the cobalt core and PPD shell help electrons to flow efficiently to the oxygen atoms. In contrast,


air cathode


similar particles containing only cobalt and cobalt oxide, or PPD alone, transformed oxygen in a two-step process that produced hydroperoxide, an undesirable and corrosive intermediate. The researchers tested their electrode in a


zinc–air battery (see image), and found that it could produce a current of 5 milliamps per square centimeter of electrode at 1.36 volts for 5 days, outperforming an electrode that relied on a conventional platinum catalyst. “The next stage of this research includes


the simplification of the synthetic route to facilitate large-scale synthesis of the nano- particles, and exploitation of other catalytic systems following the similar strategy,” says Zong.


1. Wang, Z., Li, B., Ge, X., Goh, F. W. T., Zhang, X. et al. Co@Co3


O4


composite as an efficient electrocatalyst for oxygen reduction reaction. Small 12, 2580–2587 (2016).


ISSUE 6 | JANUARY – MARCH 2017 @PPD core@bishell nanoparticle-based


A cathode containing nanoparticles made from cobalt, cobalt oxide and a carbon-based outer shell improved the performance of a zinc–air battery.


zinc anode


© 2016 A*STAR Institute of Materials Research and Engineering


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