Eliminating dissolution of platinum-based electrocatalysts at the atomic scale

Pt-Based Intermetallic Nanocrystals in Cathode Catalysts for Proton Exchange Membrane Fuel Cells: From Precise Synthesis to Oxygen Reduction Reaction Strategy

Although oxygen reduction reaction (ORR) catalysts have been extensively investigated and developed, there is a lack of clarity on catalysts that can balance high performance and low cost. Pt-based intermetallic nanocrystals are of special interest in the commercialization of proton exchange membrane fuel cells (PEMFCs) due to their excellent ORR activity and stability. This review summarizes the wide range of applications of Pt-based intermetallic nanocrystals in cathode catalysts for PEMFCs and their unique advantages in the field of ORR. Firstly, we introduce the fundamental understanding of Pt-based intermetallic nanocrystals, and highlight the difficulties and countermeasures in their synthesis. Then, the progress of theoretical and experimental studies related to the ORR activity and stability of Pt-based intermetallic nanocrystals in recent years are reviewed, especially the integrated strategies for enhancing the stability of ORR. Finally, the challenges faced by Pt-based intermetallic nanocrystals are summarized and future research directions are proposed. In addition, numerous design ideas of Pt-based intermetallic nanocrystals as ORR catalysts are summarized, aiming to promote further development of commercialization of PEMFC catalysts while fully understanding Pt-based intermetallic nanocrystals.

Direct Integration of Strained-Pt Catalysts into Proton-Exchange-Membrane Fuel Cells with Atomic Layer Deposition

The design and fabrication of lattice-strained platinum catalysts achieved by removing a soluble core from a platinum shell synthesized via atomic layer deposition, is reported. The remarkable catalytic performance for the oxygen reduction reaction (ORR), measured in both half-cell and full-cell configurations, is attributed to the observed lattice strain. By further optimizing the nanoparticle geometry and ionomer/carbon interactions, mass activity close to 0.8 A mg_Pt ^-1 @0.9 V iR-free is achievable in the membrane electrode assembly. Nevertheless, active catalysts with high ORR activity do not necessarily lead to high performance in the high-current-density (HCD) region. More attention shall be directed toward HCD performance for enabling high-power-density hydrogen fuel cells.

In Silico High-Throughput Screening of Ag-Based Electrocatalysts for Anion-Exchange Membrane Fuel Cells

The alkaline environment in anion-exchange membrane fuel cells allows the use of Pt-free electrocatalysts, thus reducing the system cost. We performed a theoretical high-throughput study of various low-cost Ag-based oxygen reduction reaction anode electrocatalysts and assessed their catalytic performance using density functional theory. From the Materials Project database, a total of 106 binary Ag alloys were investigated by estimating their heat of formation, dissolution potential, and overpotential on low-index surfaces. We confirmed that EuAg₅, BaAg₅, and SrAg₅ have higher catalytic activities and durabilities than pure Ag. By following the chemical trend of the results, we further proposed LaAg₅ and PrAg₅, which were not included in the database, as promising candidates. All candidates are in the space group P6/mmm and contain alkaline earth metal or lanthanide elements.

Stabilizing Pt-Based Electrocatalysts for Oxygen Reduction Reaction: Fundamental Understanding and Design Strategies

Proton exchange membrane fuel cells (PEMFCs) with high efficiency and nonpollution characteristics have attracted massive attention from both academic and industrial communities due to their irreplaceable roles in building the future sustainable energy system. However, the stability issue of Pt-based catalysts for oxygen reduction reaction (ORR) has become a central constraint to the widespread deployment of the devices relative to the catalytic activity. This review aims to provide comprehensive insights into how to improve the stability of Pt-based catalysts for ORR. First, the basic physical chemistry behind the catalyst degradation, including the fundamental understandings of carbon corrosion, catalyst dissolution, and particle sintering, is highlighted. After a discussion of advanced characterization techniques for the catalyst degradation, the design strategies for improving the stability of Pt-based catalysts are summarized. Finally, further insights into the remaining challenges and future research directions are also provided.

Designing Atomically Dispersed Au on Tensile-Strained Pd for Efficient CO2 Electroreduction to Formate

Pd is one of the most effective catalysts for the electrochemical reduction of CO₂ to formate, a valuable liquid product, at low overpotential. However, the intrinsically high CO affinity of Pd makes the surface vulnerable to CO poisoning, resulting in rapid catalyst deactivation during CO₂ electroreduction. Herein, we utilize the interaction between metals and metal-organic frameworks to synthesize atomically dispersed Au on tensile-strained Pd nanoparticles showing significantly improved formate production activity, selectivity, and stability with high CO tolerance. We found that the tensile strain stabilizes all reaction intermediates on the Pd surface, whereas the atomically dispersed Au selectively destabilizes CO* without affecting other adsorbates. As a result, the conventional COOH* versus CO* scaling relation is broken, and our catalyst exhibits 26- and 31-fold enhancement in partial current density and mass activity toward electrocatalytic formate production with over 99% faradaic efficiency, compared to Pd/C at -0.25 V versus RHE.

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights have been gained. Nevertheless, transfer of new knowledge to real-world fuel cell systems is still a significant challenge. To close this gap, we present a novel in situ method combining a gas diffusion electrode (GDE) half-cell with inductively coupled plasma mass spectrometry (ICP-MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes were evaluated directly. We observed that 1) specific Pt dissolution increased significantly with decreasing Pt loading, 2) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments was considerably lower, and 3) by adding a membrane onto the catalyst layer, Pt dissolution was reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re-deposition and equilibrium potential.

Resolving the nanoparticles' structure-property relationships at the atomic level: a study of Pt-based electrocatalysts

Achieving highly active and stable oxygen reduction reaction performance at low platinum-group-metal loadings remains one of the grand challenges in the proton-exchange membrane fuel cells community. Currently, state-of-the-art electrocatalysts are high-surface-area-carbon-supported nanoalloys of platinum with different transition metals (Cu, Ni, Fe, and Co). Despite years of focused research, the established structure-property relationships are not able to explain and predict the electrochemical performance and behavior of the real nanoparticulate systems. In the first part of this work, we reveal the complexity of commercially available platinum-based electrocatalysts and their electrochemical behavior. In the second part, we introduce a bottom-up approach where atomically resolved properties, structural changes, and strain analysis are recorded as well as analyzed on an individual nanoparticle before and after electrochemical conditions (e.g. high current density). Our methodology offers a new level of understanding of structure-stability relationships of practically viable nanoparticulate systems.

Structural transformations of solid electrocatalysts and photocatalysts

Heterogeneous catalysts often undergo structural transformations when they operate under thermal reaction conditions. These transformations are reflected in their evolving catalytic activity, and a fundamental understanding of the changing nature of active sites is vital for the rational design of solid materials for applications. Beyond thermal catalysis, both photocatalysis and electrocatalysis are topical because they can harness renewable energy to drive uphill reactions that afford commodity chemicals and fuels. Although structural transformations of photocatalysts and electrocatalysts have been observed in operando, the resulting implications for catalytic behaviour are not fully understood. In this Review, we summarize and compare the structural evolution of solid thermal catalysts, electrocatalysts and photocatalysts. We suggest that well-established knowledge of thermal catalysis offers a good basis to understand emerging photocatalysis and electrocatalysis research.

Self-driven microstructural evolution of Au@Pd core-shell nanoparticles for greatly enhanced catalytic performance during methanol electrooxidation

The lack of direct insight into the microstructural evolution of catalytic materials under electrochemical polarization has inhibited the development of heterogeneous catalysts. By investigating a typical Au@Pd core-shell nanostructure, the present study discloses the microstructural evolution of heterogeneous catalytic materials during the methanol electrooxidation reaction (MOR). The electrocatalytic activity of the as-prepared Au@Pd_core-shell nanoparticles continuously increased during the first 100 successive voltammetry cycles of the MOR. Microstructural characterization studies revealed that during the MOR, an Au/Pd mixed bimetallic shell was formed by the self-driven microstructural evolution of the Au@Pd_core-shell nanoparticles. Both the experimental and calculation results indicated that the Au/Pd mixed bimetallic shell reduced the binding strength of OH^- and CO on the catalyst surface. The exposed Au atoms in the shell region also produced large-scale reactive ˙OH radicals that facilitated the oxidative removal of the adsorbed carbonaceous species from the adjacent Pd active sites.

Challenges in applying highly active Pt-based nanostructured catalysts for oxygen reduction reactions to fuel cell vehicles

The past 30 years have seen progress in the development of Pt-based nanocatalysts for the oxygen reduction reaction, and some are now in production on a commercial basis and used for polymer electrolyte fuel cells (PEFCs) for automotives and other applications. Further improvements in catalytic activity are required for wider uptake of PEFCs, however. In laboratories, researchers have developed various catalysts that have much higher activities than commercial ones, and these state-of-the-art catalysts have potential to improve energy conversion efficiencies and reduce the usage of platinum in PEFCs. There are several technical issues that must be solved before they can be applied in fuel cell vehicles, which require a high power density and practical durability, as well as high efficiency. In this Review, the development history of Pt-based nanocatalysts and recent analytical studies are summarized to identify the origin of these technical issues. Promising strategies for overcoming those issues are also discussed.