Water Condensation in the Nanoscale Pores of Pt/C Catalyst Particles and Its Impact on Catalyst Utilization: A Simulation Based on a Reconstructed Structure from Nanoimaging

In polymer electrolyte membrane fuel cells, carbon-supported platinum (Pt/C) catalyst particles require sufficient water condensation within the nanoscale pores to effectively utilize the interior Pt catalysts. Since experimental visualizations with nanoscale precision of this phenomenon are not yet possible, we utilized a Pt/C catalyst particle reconstructed from segmented nanoimaging of a catalyst powder, which served as the computational domain for lattice density functional theory (LDFT) simulation of water condensation. Paired with experimental water uptake data, LDFT successfully simulated high-resolution water condensation, capturing both thin-film and capillary water condensation phenomena. Using a simple proton movement method within the water network, we reproduced the Pt utilization data from a CO stripping experiment. Our findings highlight that at low relative humidity (RH), Pt utilization is influenced by thin water film formations, mainly dictated by the wettability properties of surfaces within primary pores and the Pt/C catalyst particle's exterior. Conversely, at high RH, Pt utilization is attributed to capillary water condensation in medium-to-large sized pores. This approach contributes a qualitative and quantitative discussion on hypotheses regarding the mechanism of Pt utilization, supporting recent studies (e.g., Girod, R.; Nat. Catal. 2023, 6, (5), 383-391).

Metal-Support Interaction between Titanium Oxynitride and Pt Nanoparticles Enables Efficient Low-Pt-Loaded High-Performance Electrodes at Relevant Oxygen Reduction Reaction Current Densities

In the present work, we report on a synergistic relationship between platinum nanoparticles and a titanium oxynitride support (TiOxNy/C) in the context of oxygen reduction reaction (ORR) catalysis. As demonstrated herein, this composite configuration results in significantly improved electrocatalytic activity toward the ORR relative to platinum dispersed on carbon support (Pt/C) at high overpotentials. Specifically, the ORR performance was assessed under an elevated mass transport regime using the modified floating electrode configuration, which enabled us to pursue the reaction closer to PEMFC-relevant current densities. A comprehensive investigation attributes the ORR performance increase to a strong interaction between platinum and the TiOxNy/C support. In particular, according to the generated strain maps obtained via scanning transmission electron microscopy (STEM), the Pt-TiOxNy/C analogue exhibits a more localized strain in Pt nanoparticles in comparison to that in the Pt/C sample. The altered Pt structure could explain the measured ORR activity trend via the d-band theory, which lowers the platinum surface coverage with ORR intermediates. In terms of the Pt particle size effect, our observation presents an anomaly as the Pt-TiOxNy/C analogue, despite having almost two times smaller nanoparticles (2.9 nm) compared to the Pt/C benchmark (4.8 nm), manifests higher specific activity. This provides a promising strategy to further lower the Pt loading and increase the ECSA without sacrificing the catalytic activity under fuel cell-relevant potentials. Apart from the ORR, the platinum-TiOxNy/C interaction is of a sufficient magnitude not to follow the typical particle size effect also in the context of other reactions such as CO stripping, hydrogen oxidation reaction, and water discharge. The trend for the latter is ascribed to the lower oxophilicity of Pt-based on electrochemical surface coverage analysis. Namely, a lower surface coverage with oxygenated species is found for the Pt-TiOxNy/C analogue. Further insights were provided by performing a detailed STEM characterization via the identical location mode (IL-STEM) in particular, via 4DSTEM acquisition. This disclosed that Pt particles are partially encapsulated within a thin layer of TiOxNy origin.

Superior Performance of an Iron-Platinum/Vulcan Carbon Fuel Cell Catalyst

This work reports on the synthesis of iron-platinum on Vulcan carbon (FePt/VC) as an effective catalyst for the electrooxidation of molecular hydrogen at the anode, and electroreduction of molecular oxygen at the cathode of a proton exchange membrane fuel cell. The catalyst was synthesized by using the simple polyol route and characterized by XRD and HRTEM along with EDS. The catalyst demonstrated superior electrocatalytic activity for the oxygen reduction reaction and the oxidation of hydrogen with a 2.4- and 1.2-fold increase compared to platinum on Vulcan carbon (Pt/VC), respectively. Successful application of FePt/VC catalyst in a self-breathing fuel cell also showed a 1.7-fold increase in maximum power density compared to Pt/VC. Further analysis by accelerated stress test demonstrated the superior stability of FePt on the VC substrate with a 4% performance degradation after 60,000 cycles. In comparison, a degradation of 6% after 10,000 cycles has been reported for Pt/Ketjenblack.

Quantifying Competitive Degradation Processes in Supported Nanocatalyst Systems

The stability of supported metal nanoparticles determines the activity and lifetime of heterogeneous catalysts. Catalysts can destabilize through several thermodynamic and kinetic pathways, and the competition between these mechanisms complicates efforts to quantify and predict the overall evolution of supported nanoparticles in reactive environments. Pairing in situ transmission electron microscopy with unsupervised machine learning, we quantify the destabilization of hundreds of supported Au nanoparticles in real-time to develop a model describing the observed particle evolution as a competition between evaporation and surface diffusion. Data mining of particle evolution statistics allows us to determine physically reasonable values for the model parameters, quantify the particle size at which the Gibbs-Thomson pressure accelerates the evaporation process, and explore how individual particle interactions deviate from the mean-field model. This approach can be applied to a wide range of supported nanoparticle systems, allowing quantitative insight into the mechanisms that control their evolution in reactive environments.

Solvent-Dependent Growth and Stabilization Mechanisms of Surfactant-Free Colloidal Pt Nanoparticles

Understanding the formation of nanoparticles (NPs) is key to develop materials by sustainable routes. The Co4Cat^TM process is a new synthesis of precious metal NPs in alkaline mono-alcohols well-suited to develop active nanocatalysts. The synthesis is 'facile', surfactant-free and performed under mild conditions like low temperature. The reducing properties of the solvent are here shown to strongly influence the formation of Pt NPs. Based on the in situ formation of CO adsorbed on the NP surface by solvent oxidation, a model is proposed that accounts for the different growth and stabilization mechanisms as well as re-dispersion properties of the surfactant-free NPs in different solvents. Using in situ and ex situ characterizations, it is established that in methanol, a slow nucleation with a limited NP growth is achieved. In ethanol, a fast nucleation followed by continuous and pronounced particle sintering occurs.

Particle Size Effect on Platinum Dissolution: Practical Considerations for Fuel Cells

The high costs of polymer membrane electrolyte fuel cells (PEMFCs) remain a roadblock for a competitive market with combustion engine vehicles. The PEMFC costs can be reduced by decreasing the size of Pt nanoparticles in the catalyst layer, thereby increasing the Pt dispersion and utilization. Furthermore, high-power performance loss due to O₂ transport resistance is alleviated by decreasing the particle size and increasing dispersion. However, firm conclusions on how Pt particle size impacts durability remain elusive due to synthetic difficulties in exclusively varying single parameters (e.g., particle size and loading). Therefore, here the particle size of Pt nanoparticles was varied from 2.0 to 2.8 and 3.7 nm while keeping the loading constant (30 wt %) on a Vulcan support using the two-step surfactant-free toolbox method. By studying the electrochemical dissolution in situ using online inductively coupled plasma mass spectrometry (online ICP-MS), mass-specific dissolution trends are revealed and are attributed to particle-size-dependent changes in electrochemically active surface area. Such degradation trends are critical for the start/stop of PEMFCs and currently require the implementation of potential control systems in consumer vehicles. Additionally, shifts in the onset of anodic dissolution and also oxidation to more negative potentials with decreasing particle size were observed. These results indicate a similar mechanism of anodic dissolution related to place-exchange when moving from extended polycrystalline Pt to nanoparticle scales. The negative shifts in the onset as the particle size decreases highlight a practical limitation for PEMFCs during load/idle conditions: without further material improvements, which inhibit Pt dissolution, reduction in costs and improvement in high-power performance via increased Pt utilization and dispersion will not be possible by decreasing particle sizes further.

Potential-Dependent Volcano Plot for Oxygen Reduction: Mathematical Origin and Implications for Catalyst Design

Climbing up the volcano peak stands as a challenging problem for oxygen reduction. Repeated efforts have been made to fine-tune the binding energy of oxygen reaction intermediates within a narrow region of 0.2 eV by adjusting the catalyst electronic structure. Herein, we address ourselves to two different, oft-neglected but nontrivial questions: (a) Does a superior oxygen reduction reaction catalyst in rotating disk electrode experiments still work well in practical fuel cells (usually at a different potential)? (b) For a given catalyst, can we place it on the volcano peak by adjusting the electrode potential (ϕ^M), which can be easily varied within 0.5 V in experiments, and the potential at the reaction plane in solution (ϕ^OHP), which is modulated by double-layer electrostatic effects? To answer these two questions, we articulate the mathematical origin of the volcano plot and reveal its dependence on ϕ^M and ϕ^OHP by combining a microkinetic model for the oxygen reduction reaction and a mean-field model for the double layer. Furthermore, we explore possible approaches of adjusting ϕ^OHP, for instance, by varying electrolyte concentration and particularly by tuning the electrostatic properties of the support material in a supported catalyst system. The investigation of how electrostatic properties of the support material affect the volcano plot of a supported catalyst opens an additional channel of catalyst-support interactions.

Alloyed Pt3M (M = Co, Ni) nanoparticles supported on S- and N-doped carbon nanotubes for the oxygen reduction reaction

Sulfur- (S-CNT) and nitrogen-doped (N-CNT) carbon nanotubes have been produced by catalytic chemical vapor deposition (c-CVD) and were subject to an annealing treatment. These CNTs were used as supports for small (≈2 nm) Pt₃M (M = Co or Ni) alloyed nanoparticles that have a very homogeneous size distribution (in spite of the high metal loading of ≈40 wt % Pt), using an ionic liquid as a stabilizer. The electrochemical surface area, the activity for the oxygen reduction reaction and the amount of H₂O₂ generated during the oxygen reduction reaction (ORR) have been evaluated in a rotating ring disk electrode experiment. The Pt₃M/N-CNT catalysts revealed excellent electrochemical properties compared to a commercial Pt₃Co/Vulcan XC-72 catalyst. The nature of the carbon support plays a key role in determining the properties of the metal nanoparticles, on the preparation of the catalytic layer, and on the electrocatalytic performance in the ORR. On N-CNT supports, the specific activity followed the expected order Pt₃Co > Pt₃Ni, whereas on the annealed N-CNT support, the order was reversed.

Controlled Synthesis of Surfactant-Free Water-Dispersible Colloidal Platinum Nanoparticles by the Co4Cat Process

The recently reported Co4Cat process is a synthesis method bearing ecological and economic benefits to prepare precious-metal nanoparticles (NPs) with optimized catalytic properties. In the Co4Cat process, a metal precursor (e.g., H₂ PtCl₆ ) is dissolved in an alkaline solution of a low-boiling-point solvent (methanol) and reduced to NPs at low temperature (<80 °C) without the use of surfactants. Here, the Co4Cat process to prepare Pt NPs is described in detail. The advantages of this new synthesis method for research and development but also industrial production are highlighted in a comparison with the popular "polyol" synthesis. The reduction of H₂ PtCl₆ from Pt^IV to Pt^II and further to Pt⁰ is followed by UV/Vis and XANES/EXAFS measurements. It is demonstrated how the synthesis can be accelerated, how size control is achieved, and how the colloidal dispersions can be stabilized without the use of surfactants. Despite being surfactant-free, the Pt NPs exhibit surprisingly long-term (up to 16 months) stability in water over a wide pH range (4-12) and in aqueous buffer solutions. The Co4Cat process is thus relevant to produce NPs for heterogeneous catalysis, electro-catalysis, or bio/medical applications.

Facile deposition of Pt nanoparticles on Sb-doped SnO2 support with outstanding active surface area for the oxygen reduction reaction

Synthesis of Sb–SnO₂ supported Pt nanoparticles with an outstanding ECSA for the oxygen reduction reaction.

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

Oxygen reduction is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in fuel cells and metal-air batteries. In this review, the prospect of perovskite oxides for the oxygen reduction reaction (ORR) in alkaline media is reviewed with respect to fundamental insight into activity and possible mechanisms. For gaining these insights, special emphasis is placed on highly crystalline perovskite films that have only recently become available for electrochemical interrogation. The prospects for applications are evaluated based on recent progress in the synthesis of perovskite nanoparticles. The review concludes with the current understanding of oxygen reduction on perovskite oxides and a perspective on opportunities for future fundamental and applied research.

PdZn nanoparticle electrocatalysts synthesized by solution combustion for methanol oxidation reaction in an alkaline medium

Herein, we report the synthesis of PdZn nanoparticle (NP) electrocatalysts for the methanol oxidation reaction (MOR).

Platinum nanocatalysts on metal oxide based supports for low temperature fuel cell applications

In this manuscript a survey of the contemporary research related to platinum nanocatalysts on metal oxide based supports for low temperature fuel cell applications is presented.

Structural disordering of de-alloyed Pt bimetallic nanocatalysts: the effect on oxygen reduction reaction activity and stability

We show how alloying and excessive de-alloying affect ORR activity and the structural integrity of PEMFC nanocatalysts.

Study of the formation and quick growth of thick oxide films using platinum nanoelectrodes as a model electrocatalyst

We report a study of the formation and quick growth of thick films of platinum oxide on platinum nanoelectrodes at low anodic potentials. Here, structurally well-defined platinum nanoelectrodes are used as a model platform for nanoscale platinum electrocatalysts. Platinum films are formed on the surface of the nanoelectrode upon application of a constant anodic potential in an acidic environment for an extended time period. A current spike is initially observed, which is attributed to capacitance charging, the oxidation of water, and the initial oxidation of the platinum surface. A finite residual current follows the initial current spike, which is composed of both water oxidation and the oxidation of platinum metal concealed beneath the growing oxide layer. These films are observed to be structurally irreversible, grow to be relatively thick, and protrude out of the glass insulating material encasing the nanoelectrode due to the added volume of the oxygen incorporated into the growing platinum oxide film. Once reduced, the platinum metal remains protruding out of the glass, and its presence is confirmed by both SEM imaging and cyclic voltammetry. Steady-state voltammetric data shows a finite increase in the diffusion-limited faradaic current of the nanoelectrode, relative to the initial steady-state current, after the oxidation/reduction of the platinum which is due to an increased area of the protruding platinum metal. A minimum apparent rate of ∼1.2 nm/min can be calculated for the growth of the platinum oxide film. The use of platinum nanoelectrodes has shown several distinct advantages in this study, including better control of the size and morphology of the individual electrocatalysts, the ability to image using electron microscopy, and the ability to use voltammetry to evaluate the geometry of the electrode quickly.