Electrodeposition of Highly Porous Pt Nanoparticles Studied by Quantitative 3D Electron Tomography: Influence of Growth Mechanisms and Potential Cycling on the Active Surface Area

Recent Progress on Revealing 3D Structure of Electrocatalysts Using Advanced 3D Electron Tomography: A Mini Review

Electrocatalysis plays a key role in clean energy innovation. In order to design more efficient, durable and selective electrocatalysts, a thorough understanding of the unique link between 3D structures and properties is essential yet challenging. Advanced 3D electron tomography offers an effective approach to reveal 3D structures by transmission electron microscopy. This mini-review summarizes recent progress on revealing 3D structures of electrocatalysts using 3D electron tomography. 3D electron tomography at nanoscale and atomic scale are discussed, respectively, where morphology, composition, porous structure, surface crystallography and atomic distribution can be revealed and correlated to the performance of electrocatalysts. (Quasi) in-situ 3D electron tomography is further discussed with particular focus on its impact on electrocatalysts’ durability investigation and post-treatment. Finally, perspectives on future developments of 3D electron tomography for eletrocatalysis is discussed.

Effect of Pt Deposits on TiO2 Electrocatalytic Activity Highlighted by Electron Tomography

Characterizing materials at small scales presents major challenges in the engineering of nanocomposite materials having a high specific surface area. Here, we show the application of electron tomography to describe the three-dimensional structure of highly ordered TiO₂ nanotube arrays modified with Pt nanoparticles. The titanium oxide nanotubes were prepared by the electrochemical anodization of a Ti substrate after which Pt was deposited by magnetron sputtering. Such a composite shows high electrochemical activity that depends on the amount of the metal and the morphological parameters of the microstructure. However, a TiO₂ structure modified with metallic nanoparticles has never been visualized in 3D, making it very difficult to understand the relationship between electrocatalytic activity and morphology. In this paper, TiO₂ nanotubes of different sizes and different amounts of Pt were analyzed using the electron microscopy technique. Electrocatalytic activity was studied using the cyclic voltammetry (CV) method. For selected samples, electron tomography 3D structure reconstruction was performed to describe their fine microstructure. The highest activity was detected in the sample having bigger nanotubes (25 V) where the porosity of the structure was high and the Pt content was 0.1 mg cm^-2. 3D imaging using electron tomography opens up new possibilities in the design of electrocatalytic materials.

Fine-Tuning Porosity and Time-Resolved Observation of the Nucleation and Growth of Single Platinum Nanoparticles

Porous metal nanoparticles (NPs) are important to a variety of applications; however, robust control over NP porosity is difficult to achieve. Here, we demonstrate control over NP porosity using nanodroplet-mediated electrodeposition by introducing glycerol into water droplets. Porosity approached 0 under viscous conditions (>6 cP), and intermediate viscosities allowed the fine-tuning of NP porosity between 0 and 15%. This method also allowed for control over average pore radius (1 to 5 nm) and pore density (2 to 6 × 10¹⁵ pores per square meter). Reduced mass transfer within water droplets was validated by studying single chloroplatinate-filled water droplet (droplet radius of ∼450 nm) collisions on a platinum ultramicroelectrode (UME, r_UME = 5 μm). Collision transient lifetimes in the i- t response increased with increasing viscosity, and the total charge per event was conserved. The change in shape was consistent with the nucleation and growth of a platinum NP within the droplet, which was confirmed by fitting transients to classical nucleation and growth theory for single centers as a function of over-potential. This analysis allowed electrokinetic growth and diffusion-controlled growth to be distinguished and semi-quantified at the single NP level.

Highly porous palladium nanodendrites: wet-chemical synthesis, electron tomography and catalytic activity

A simple procedure to obtain highly porous hydrophilic palladium nanodendrites in one step is described.

Triple phase boundary and power density enhancement in PEMFCs of a Pt/C electrode with double catalyst layers

Exploring efficient approaches to design electrodes for proton exchange membrane fuel cells (PEMFCs) is of great advantage to overcome the current limitations of the standard platinum supported carbon (Pt/C) catalyst. Herein, a Pt/C electrode consisting of double catalyst layers (DCL) with low Pt loading of around 0.130 mg_Pt cm⁻² is prepared using spray and electrophoresis (EPD) methods. The DCL electrode demonstrated a higher electrochemical surface area (ECSA-52.5 m² g_Pt⁻¹) and smaller internal resistance (133 Ω) as compared to single catalyst layer (SCL) sprayed (37.1 m² g_Pt⁻¹ and 184 Ω) or EPD (42.4 m² g_Pt⁻¹ and 170 Ω) electrodes. In addition, the corresponding DCL membrane electrode assembly (MEA), which consists of a Pt/C DCL electrode at the anode side and a Pt/C sprayed electrode at the cathode side, also showed improved PEMFC performance as compared to others. Specifically, the DCL MEA generated the highest power density of 4.9 W mg_Pt⁻¹, whereas, the SCL MEAs only produced 3.1 and 3.8 W mg_Pt⁻¹, respectively. The superior utilization of the Pt catalysts into the DCL MEA can originate from the enrichment of the triple phase boundary (TPB) presented on the Pt/C DCL electrode, which can strongly promote the adsorbed hydrogen intermediates' removal from the anode side, thus improving the overall PEMFC performance.

Pt/C double catalyst layers (DCL), serving as an anodic electrode, have been utilized in a PEMFC application for the first time.

Porous PtPd alloy nanotubes: towards high performance electrocatalysts with low Pt-loading

Porous PtPd alloy nanotubes with Pt contents down to 5 at% are powerful, Pt-lean electrocatalysts.

Electron Tomography of Plasmonic Au Nanoparticles Dispersed in a TiO2 Dielectric Matrix

Plasmonic Au nanoparticles (AuNPs) embedded into a TiO₂ dielectric matrix were analyzed by combining two-dimensional and three-dimensional electron microscopy techniques. The preparation method was reactive magnetron sputtering, followed by thermal annealing treatments at 400 and 600 °C. The goal was to assess the nanostructural characteristics and correlate them with the optical properties of the AuNPs, particularly the localized surface plasmon resonance (LSPR) behavior. High-angle annular dark field-scanning transmission electron microscopy results showed the presence of small-sized AuNPs (quantum size regime) in the as-deposited Au-TiO₂ film, resulting in a negligible LSPR response. The in-vacuum thermal annealing at 400 °C induced the formation of intermediate-sized nanoparticles (NPs), in the range of 10-40 nm, which led to the appearance of a well-defined LSPR band, positioned at 636 nm. Electron tomography revealed that most of the NPs are small-sized and are embedded into the TiO₂ matrix, whereas the larger NPs are located at the surface. Annealing at 600 °C promotes a bimodal size distribution with intermediate-sized NPs embedded in the matrix and big-sized NPs, up to 100 nm, appearing at the surface. The latter are responsible for a broadening and a redshift, to 645 nm, in the LSPR band because of increase of scattering-to-absorption ratio. Beyond differentiating and quantifying the surface and embedded NPs, electron tomography also provided the identification of "hot-spots". The presence of NPs at the surface, individual or in dimers, permits adsorption sites for LSPR sensing and for surface-enhanced spectroscopies, such as surface-enhanced Raman scattering.