Modelling oxide formation and growth on platinum

Mechanistic Studies of Improving Pt Catalyst Stability at High Potential via Designing Hydrophobic Micro-Environment with Ionic Liquid in PEMFC

Recently, the focus of fuel cell technologies has shifted from light-duty automotive to heavy-duty vehicle applications, which require improving the stability of membrane electrode assemblies (MEAs) at high constant potential. The hydrophilicity of Pt makes it easy to combine with water molecules and then oxidize at high potential, resulting in poor durability of the catalyst. In this work, an ionic liquid [BMIM][NTF2] was used to modify the Pt catalyst (Pt/C + IL) to create a hydrophobic, antioxidant micro-environment in the catalyst layer (CL). The effect of [BMIM][NTF2] on the decay of the CL performance at high constant potential (0.85 V) for a long time was investigated. It was found that the performance attenuation of Pt/C + IL in the high-potential range (OCV 0.75 V) was less than that of commercial Pt/C after 10 h. The Pt-oxide coverage test showed that the hydrophobic micro-environment of the CL enhanced the stability by inhibiting Pt oxidation. In addition, the electrochemical recovery of Pt oxides showed that the content of recoverable oxides in Pt/C + IL was higher than that in commercial Pt/C. Overall, modifying the Pt catalyst with hydrophobic ionic liquid is an effective strategy to improve the catalyst stability and reduce the irreversible voltage loss caused by the oxide at high constant potential.

Micrometer-sized electrically programmable shape-memory actuators for low-power microrobotics

Shape-memory actuators allow machines ranging from robots to medical implants to hold their form without continuous power, a feature especially advantageous for situations where these devices are untethered and power is limited. Although previous work has demonstrated shape-memory actuators using polymers, alloys, and ceramics, the need for micrometer-scale electro-shape-memory actuators remains largely unmet, especially ones that can be driven by standard electronics (~1 volt). Here, we report on a new class of fast, high-curvature, low-voltage, reconfigurable, micrometer-scale shape-memory actuators. They function by the electrochemical oxidation/reduction of a platinum surface, creating a strain in the oxidized layer that causes bending. They bend to the smallest radius of curvature of any electrically controlled microactuator (~500 nanometers), are fast (<100-millisecond operation), and operate inside the electrochemical window of water, avoiding bubble generation associated with oxygen evolution. We demonstrate that these shape-memory actuators can be used to create basic electrically reconfigurable microscale robot elements including actuating surfaces, origami-based three-dimensional shapes, morphing metamaterials, and mechanical memory elements. Our shape-memory actuators have the potential to enable the realization of adaptive microscale structures, bio-implantable devices, and microscopic robots.

First-principles investigation of electrochemical dissolution of Pt nanoparticles and kinetic simulation

Dissolution is the primary route of Pt nanoparticle degradation in electrochemical devices, e.g., fuel cells. Investigation of potential-dependent dissolution kinetics of Pt nanoparticles is crucial to optimize the nanoparticle size and operating conditions for better performance. A mean-field kinetic theory under the steady-state approximation, combined with atomistic thermodynamics and Wulff construction, was developed to study the interplay between oxygen chemisorption, electrode potential, and particle size on the dissolution of Pt nanoparticles. We found that although oxygen chemisorption from electrode potential-induced water splitting can stabilize Pt nanoparticles through decreasing the surface energy and increasing the redox potential, the electrode potential plays a more decisive role in facilitating the dissolution of Pt nanoparticles. In comparison with the minor effect of oxygen chemisorption, an increase in the particle size, though reducing the dispersion, has a more significant effect on the suppression of the dissolution. These theoretical understandings on the effects of electrode potential and particle size on the dissolution are crucial for optimizing the nanoparticle size under oxidative operating conditions.