Palladium atomic layers coated on ultrafine gold nanowires boost oxygen reduction reaction.

Palladium-based nanocatalysts play an important role in catalyzing the cathode oxygen reduction reaction (ORR) for fuel cells working under alkaline conditions, but the performance still needs to be improved to meet the requirements for large-scale applications. Herein, Au@Pd core-shell nanowires have been developed by coating Pd atomic layers on ultrafine gold nanowires and display outstanding electrocatalytic performance towards alkaline ORR. It is found that Pd overlayers with atomic thickness can be coated on 3 nm Au nanowires under CO atmosphere and completely cover the surfaces. The obtained ultrafine Au@Pd nanowires exhibit an electrochemical active area (ECSA) of 68.5 m2/g and a mass activity of 0.91 A/mg (at 0.9 V vs. RHE), which is around 3.1 and 15.2 times higher than that of commercial Pd/C. The activity loss of the ultrafine Au@Pd nanowire after 10,000 cycles of accelerated degradation tests is only ∼20 %, demonstrating its much better stability compared to commercial Pd/C. Further characterizations combined with density functional theory (DFT) calculations demonstrate that the electronic interactions between Pd atomic layers and underlying Au can increase the electronic density of Pd and promote the efficient activation of oxygen, thus leading to the improved ORR performance.

Exploring the Effect of Pd on the Oxygen Reduction Performance of Pt by In Situ Raman Spectroscopy.

Directly monitoring the oxygen reduction reaction (ORR) process in situ is very important to deeply understand the reaction mechanism and is a critical guideline for the design of high-efficiency catalysts, but there is still lack of definite in situ evidence to clarify the effect between adsorbed intermediates and the strain/electronic effect for enhanced ORR performance. Herein, in situ surface-enhanced Raman spectroscopy (SERS) was employed to detect the intermediates during the ORR process on the Au@Pd@Pt core/shell heterogeneous nanoparticles (NPs). Direct spectroscopic evidence of the *OOH intermediate was obtained, and an obvious red shift of the *OOH frequency was identified with the controllable shell thickness of Pd. Detailed experimental characterizations and density functional theory (DFT) calculations demonstrated that such improved ORR activity after inducing Pd into Au@Pt NPs can be attributed to the optimized adsorbate-substrate interaction due to the strain and electronic effect, leading to a higher Pt-O binding energy and a lower O-O binding energy, which was conducive to O-O dissociation and promoted the subsequent reaction. Notably, this work illustrates a relationship between the performance and strain/electronic effect via the intermediate detected by SERS and paves the way for the construction of ORR electrocatalysts with high performance.

Unmasking the Critical Role of the Ordering Degree of Bimetallic Nanocatalysts on Oxygen Reduction Reaction by In Situ Raman Spectroscopy.

Precise control and accurate understanding of the ordering degree of bimetallic nanocatalysts (BNs) are challenging yet crucial to acquire advanced materials for the oxygen reduction reaction (ORR). AuCu BNs with various ordering degrees were synthesized to evaluate the influence of ordering degree on the ORR at a molecular level using in situ Raman spectroscopy. The activity of AuCu BNs was improved by over 2 times after a disorder-to-order transition, making the performance of highly ordered AuCu BNs exceed that of benchmark Pt/C. Direct Raman spectroscopic evidence of key intermediate (*OH) demonstrates that the active site is the combination site of Au and Cu. Moreover, two distinct *OH species are observed on the ordered and disordered structure, and the ordered site is more beneficial for ORR due to its lower affinity to *OH. This work deepens the understanding on the important role of ordering degree on BNs and enables the design of improved catalysts.

Reshaping the Cathodic Catalyst Layer for Anion Exchange Membrane Fuel Cells: From Heterogeneous Catalysis to Homogeneous Catalysis.

In anion exchange membrane fuel cells, catalytic reactions occur at a well-defined three-phase interface, wherein conventional heterogeneous catalyst layer structures exacerbate problems, such as low catalyst utilization and limited mass transfer. We developed a structural engineering strategy to immobilize a molecular catalyst tetrakis(4-methoxyphenyl)porphyrin cobalt(II) (TMPPCo) on the side chains of an ionomer (polyfluorene, PF) to obtain a composite material (PF-TMPPCo), thereby achieving a homogeneous catalysis environment inside ion-flow channels, with greatly improved mass transfer and turnover frequency as a result of 100 % utilization of the catalyst molecules. The unique structure of the homogeneous catalysis system comprising interconnected nanoreactors exhibits advantages of low overpotential and high fuel-cell power density. This strategy of reshaping of the catalyst layer structure may serve as a new platform for applications of many molecular catalysts in fuel cells.

Design and Preparation of Fe-N5 Catalytic Sites in Single-Atom Catalysts for Enhancing the Oxygen Reduction Reaction in Fuel Cells.

There is an urgent need for developing nonprecious metal catalysts to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Atomically dispersed M-Nx/C catalysts have shown promising ORR activity; however, enhancing their performance through modulating their active site structure is still a challenge. In this study, a simple approach was proposed for preparing atomically dispersed iron catalysts embedded in nitrogen- and fluorine-doped porous carbon materials with five-coordinated Fe-N5 sites. The C@PVI-(DFTPP)Fe-800 catalyst, obtained through pyrolysis of a bio-inspired iron porphyrin precursor coordinated with an axial imidazole from the surface of polyvinylimidazole-grafted carbon black at 800 °C under an Ar atmosphere, exhibited a high electrocatalytic activity with a half-wave potential of 0.88 V versus the reversible hydrogen electrode for ORR through a four-electron reduction pathway in alkaline media. In addition, an anion-exchange membrane electrode assembly (MEA) with C@PVI-(DFTPP)Fe-800 as the cathode electrocatalyst generated a maximum power density of 0.104 W cm-2 and a current density of 0.317 mA cm-2. X-ray absorption spectroscopy demonstrated that a single-atom catalyst (Fe-Nx/C) with an Fe-N5 active site can selectively be obtained; furthermore, the catalyst ORR activity can be tuned using fluorine atom doping through appropriate pre-assembling of the molecular catalyst on a carbon support followed by pyrolysis. This provides an effective strategy to prepare structure-performance-correlated electrocatalysts at the molecular level with a large number of M-Nx active sites for ORR. This method can also be utilized for designing other catalysts.

Engineering Energy Level of Metal Center: Ru Single-Atom Site for Efficient and Durable Oxygen Reduction Catalysis.

Emerging as a new frontier in heterogeneous catalysis, single-atom site catalysts (SSCs) have sparked enormous attention and bring about new opportunities to oxygen reduction electrocatalysis. Despite considerable progress achieved recently, most of the reported SSCs suffer from either insufficient activity or unsatisfactory stability, which severely retards their practical application. Here, we demonstrate a novel Ru-SSC with appropriate adsorption free energy of OH* (ΔGOH*) to confer excellent activity and low Fenton reactivity to maintain long-term stability. The as-developed Ru-SSC exhibits encouraging oxygen reduction reaction turnover frequency of 4.99 e- s-1 sites-1, far exceeding the state-of-the-art Fe-SSC counterpart (0.816 e- s-1 sites-1), as a result of Ru energy level regulation via spontaneous OH binding. Furthermore, Ru-SSC exhibits greatly suppressed Fenton reactivity, with restrained generation of reactive oxygen species directly observed, thus endowing the Ru-SSC with much more superior stability (only 17 mV negative shift after 20 000 cycles) than the Fe-SSC counterpart (31 mV). The practical application of Ru-SSC is further validated by its excellent activity and stability in a real fuel cell device.

Heat treated carbon supported iron(ii)phthalocyanine oxygen reduction catalysts: elucidation of the structure-activity relationship using X-ray absorption spectroscopy.

This paper focuses on studying the influence of the heat treatment on the structure and activity of carbon supported Fe(ii)phthalocyanine (FePc/C) oxygen reduction reaction (ORR) catalysts under alkaline conditions. The FePc macrocycle was deposited onto ketjen black carbon and heated treated for 2 hours under inert atmosphere (Ar) at different temperatures (400, 500, 600, 700, 800, 900 and 1000 °C). The atomic structure of Fe in each sample has been determined by XAS and correlated to the activity and ORR mechanisms determined in electrochemical half cells and in a complete H2/O2 anion exchange membrane fuel cells (AEM-FC). The results show that the samples prepared at 600 and 700 °C have the highest electrochemical catalytic activity for the ORR, consistent with the findings that the FeN4 active sites are thermally stable up to 700 °C, confirmed by both XANES linear combination fittings and EXAFS fittings. Upon annealing at temperatures above 800 °C, the FeN4 structure partially decomposes to small iron nanoparticles. The transition from the FeN4 structure to metallic Fe results in a significant loss in ORR activity and an increase in the production of undesirable HO2- during catalysis.