Effect of the chemical order on the electrocatalytic activity of model PtCo electrodes in the oxygen reduction reaction

Review on Magnetism in Catalysis: From Theory to PEMFC Applications of 3d Metal Pt-Based Alloys

The relationship between magnetism and catalysis has been an important topic since the mid-20th century. At present time, the scientific community is well aware that a full comprehension of this relationship is required to face modern challenges, such as the need for clean energy technology. The successful use of (para-)magnetic materials has already been corroborated in catalytic processes, such as hydrogenation, Fenton reaction and ammonia synthesis. These catalysts typically contain transition metals from the first to the third row and are affected by the presence of an external magnetic field. Nowadays, it appears that the most promising approach to reach the goal of a more sustainable future is via ferromagnetic conducting catalysts containing open-shell metals (i.e., Fe, Co and Ni) with extra stabilization coming from the presence of an external magnetic field. However, understanding how intrinsic and extrinsic magnetic features are related to catalysis is still a complex task, especially when catalytic performances are improved by these magnetic phenomena. In the present review, we introduce the relationship between magnetism and catalysis and outline its importance in the production of clean energy, by describing the representative case of 3d metal Pt-based alloys, which are extensively investigated and exploited in PEM fuel cells.

Intermetallic PtFe Electrocatalysts for the Oxygen Reduction Reaction: Ordering Degree-Dependent Performance

Platinum-based atomically ordered alloys (i.e., intermetallic compounds) have distinct advantages over disordered solid solution counterparts in boosting the cathodic oxygen-reduction reaction (ORR) in proton-exchange-membrane fuel cells. Nevertheless, the pivotal role of ordering degree of intermetallic catalysts in promoting ORR performance has been ignored heavily so far, probably owing to the lack of synthetic routes for controlling the ordering degree, especially for preparing highly ordered intermetallic catalysts. Herein, a family of intermetallic PtFe catalysts with similar particle size of 3-4 nm but varied ordering degree in a wide range of 10-70% are prepared. After constructing the PtFe/Pt core/shell structure with around 3 Pt-layer skin, a positive correlation between the ordering degree of the intermetallic catalysts and their ORR activity and durability is identified. Notably, the highly ordered PtFe/Pt catalyst exhibits a high mass activity of 0.92 A mg_Pt ^-1 at 0.9 V_iR-corrected as cathode catalyst in H₂ -O₂ fuel cell, with only 24% loss after accelerated durability tests. The ordering degree-dependent performance can be ascribed to the compressive strain effect induced by the intermetallic PtFe core with smaller lattice parameters, and the more thermodynamically stable intermetallic structure compared to disordered alloys.

Engineering the atomic arrangement of bimetallic catalysts for electrochemical CO2 reduction

The electrochemical CO2 reduction reaction (CO2RR) to form highly valued chemicals is a sustainable solution to address the environmental issues caused by excessive CO2 emissions. Generally, it is challenging to achieve high efficiency and selectivity simultaneously in the CO2RR due to multi-proton/electron transfer processes and complex reaction intermediates. Among the studied formulations, bimetallic catalysts have attracted significant attention with promising activity, selectivity, and stability. Engineering the atomic arrangement of bimetallic nanocatalysts is a promising strategy for the rational design of structures (intermetallic, core/shell, and phase-separated structures) to improve catalytic performance. This review summarizes the recent advances, challenges, and opportunities in developing bimetallic catalysts for the CO2RR. In particular, we firstly introduce the possible reaction pathways on bimetallic catalysts concerning the geometric and electronic properties of intermetallic, core/shell, and phase-separated structures at the atomic level. Then, we critically examine recent advances in crystalline structure engineering for bimetallic catalysts, aiming to establish the correlations between structures and catalytic properties. Finally, we provide a perspective on future research directions, emphasizing current challenges and opportunities.

Random alloy and intermetallic nanocatalysts in fuel cell reactions

Fuel cells that use small organic molecules or hydrogen as the anode fuel can power clean electric vehicles. From an experimental perspective, the possible fuel cells' electrocatalytic reaction mechanisms are obtained through in situ electrochemical spectroscopy techniques and density functional theory calculations, providing theoretical guidance for further development of novel nanocatalysts. As advanced nanocatalysts for fuel cells' electrochemical reactions, alloy nanomaterials have greatly improved electrocatalytic activity and stability and have attracted widespread attention. Enhanced electrocatalytic performance of alloy nanocatalysts could be closely related to the synergistic effects, such as electronic and strain effects. Depending on the arrangement of atoms, alloys can be classified into random alloy and intermetallic compounds (ordered structure). Intermetallic compounds generally have lower heats of formation and stronger heteroatomic bonding strength relative to the random alloy, resulting in high chemical and structural stability in either full pH solutions or electrochemical tests. Here, we summarize the latest advances and the structure-function relationship of noble metal alloy nanocatalysts, among which Pt-based catalysts are the main ones, as well as comprehensively understand why they significantly affect the electrocatalytic performance of fuel cells. Novel alloy nanocatalysts with a robust three-phase interface to achieve efficient charge and mass transfer can obtain desirable activity and stability in the electrochemical workstation tests, and is expected to acquire a higher power density on fuel cell test systems with harsh test conditions.

Ordered Pt3Co Intermetallic Nanoparticles Derived from Metal-Organic Frameworks for Oxygen Reduction

Highly ordered Pt alloy structures are proven effective to improve their catalytic activity and stability for the oxygen reduction reaction (ORR) for proton exchange membrane fuel cells. Here, we report a new approach to preparing ordered Pt₃Co intermetallic nanoparticles through a facile thermal treatment of Pt nanoparticles supported on Co-doped metal-organic-framework (MOF)-derived carbon. In particular, the atomically dispersed Co sites, which are originally embedded into MOF-derived carbon, diffuse into Pt nanocrystals and form ordered Pt₃Co structures. It is very crucial for the formation of the ordered Pt₃Co to carefully control the doping content of Co into the MOFs and the heating temperatures for Co diffusion. The optimal Pt₃Co nanoparticle catalyst has achieved significantly enhanced activity and stability, exhibiting a half-wave potential up to 0.92 V vs reversible hydrogen electrode (RHE) and only losing 12 mV after 30 000 potential cycling between 0.6 and 1.0 V. The highly ordered intermetallic structure was retained after the accelerated stress tests made evident by atomic-scale elemental mapping. Fuel cell tests further verified the high intrinsic activity of the ordered Pt₃Co catalysts. Unlike the direct use of MOF-derived carbon supports for depositing Pt, we utilized MOF-derived carbon containing atomically dispersed Co sites as Co sources to prepare ordered Pt₃Co intermetallic catalysts. The new synthesis approach provides an effective strategy to develop active and stable Pt alloy catalysts by leveraging the unique properties of MOFs such as 3D structures, high surface areas, and controlled nitrogen and transition metal dopings.

How the hydrogen sorption properties of palladium are modified through interaction with iridium

The instability of hydrogen sorption properties in Ir (positive sorption energies) leads to the destabilization of hydrogen sorption in a Pd layer interacting with Ir.