Lowering metal loadings onto Pt–Pd–Cu/graphene nanoribbon nanocomposites affects electrode collection efficiency and oxygen reduction reaction performance

Molybdenum Microalloying L12-Pt3Co Intermetallic Compounds via Bimodal Ordering Process by Mo Surface Termination and PDA-Coated Particle Confinement for Stable PEMFC

Ordered platinum (Pt) intermetallic compounds with high performance expression under proton exchange membrane fuel cell (PEMFC) operating conditions are a prerequisite for practical application. However, the heat treatment required for forming an ordered structure can lead to severe agglomeration and uneven distribution of nanoparticles, posing a significant challenge to efficient synthesis. Here, a molybdenum (Mo) assisted structural evolution strategy to controllably synthesize sub-4 nm Pt3Co intermetallic compounds is proposed. The results of experiments combined with density functional theory calculations demonstrate that the participation of trace Mo (0.2 wt.%) not only triggers the "growth site locking effect" and effectively suppresses the growth of nanoparticles, but also effectively adjusts the electronic structure, thereby optimizing the adsorption/desorption of oxygen intermediates. The preeminent intrinsic activity on an optimized catalyst reaches a mass activity as high as 1.22 A mgPt -1 and has extraordinary stability with only a 5.7% decrease after 30 k cycles. This study paves a new path for the practical application of low Pt catalysts in PEMFCs in the future.

Trace Tungsten Microalloying PtCuCo Medium Entropy Alloys: Substructure Reconstruction-Triggered High-Performance for PEMFC

Refractory metals (W, Nb, or Mo) microalloying Pt-based alloys with unprecedented performance may serve as advanced electrocatalysts for proton exchange membrane fuel cells (PEMFCs). These alloys are endowed with unique stabilizing substructures or lattice defects through the microalloying effect. Herein, trace W microalloying PtCuCo medium entropy alloys (W-PtCuCo) are reported via a stepwise synthesis strategy, starting with home-made Cu nanowires as sacrificial templates by anhydrous solid-phase milling route, and then followed by galvanic replacement-assisted solvothermal in ethylene glycol (EG). In PEMFC tests, the obtained W-PtCuCo exhibits an ultrahigh peak power density and mass power density (relative to cathode) reaching 2.09 W cm-2 and 20.9 W mgPt -1 , respectively. During the accelerated degradation test (ADT), the mass activity just lost only 3% after 30 k cycles, much better than the above benchmark catalyst. The microalloying-dependent performances shall be attributed to the presence of abundant stepped surfaces, twisted edges, and other lattice defects terminated by W via substructure reconstruction that indeed alters the electronic structure and strain level of the alloys. This work first provides an atomic-level insight into the microalloying-dependent electrocatalytic performance of Pt-based alloys, which is of great significance for developing next-generation efficient catalysts for PEMFC.

Exploring the Potential of Heteroatom-Doped Graphene Nanoribbons as a Catalyst for Oxygen Reduction

In this study, we created a series of N, S, and P-doped and co-doped carbon catalysts using a single graphene nanoribbon (GNR) matrix and thoroughly evaluated the impact of doping on ORR activity and selectivity in acidic, neutral, and alkaline conditions. The results obtained showed no significant changes in the GNR structure after the doping process, though changes were observed in the surface chemistry in view of the heteroatom insertion and oxygen depletion. Of all the dopants investigated, nitrogen (mainly in the form of pyrrolic-N and graphitic-N) was the most easily inserted and detected in the carbon matrix. The electrochemical analyses conducted showed that doping impacted the performance of the catalyst in ORR through changes in the chemical composition of the catalyst, as well as in the double-layer capacitance and electrochemically accessible surface area. In terms of selectivity, GNR doped with phosphorus and sulfur favored the 2e- ORR pathway, while nitrogen favored the 4e- ORR pathway. These findings can provide useful insights into the design of more efficient and versatile catalytic materials for ORR in different electrolyte solutions, based on functionalized carbon.

Passivating the pH-Responsive Sites to Configure a Widely pH-Stable Emulsifier for High-Efficiency Benzyl Alcohol Oxidation

Widely pH-stable emulsions configured by solid emulsifiers with high chemical stabilities and anti-corrosion properties under strong acid or alkaline conditions are highly sought after for practical and wide application of Pickering interfacial catalysis. Herein, a unique strategy was reported for synthesis of a widely pH-stable and novel emulsifier by passivating the pH-responsive sites on graphene oxide nanoribbon (GONR) surface using ionic liquid (IL). The suitable wettability of GONR-IL was derived from the positive binding energy between IL and water, which ensured the stability of the emulsion in a wide pH range. Benefiting from the passivated surface chemistry of GONR, the emulsion microreactor stabilized by GONR-IL exhibited a remarkable stability over a wide range of pH values. A GONR-IL-supported Pd catalyst stabilized at the toluene-water interface achieved an excellent emulsion catalytic activity for benzyl alcohol oxidation (conversion of 92 %), which was exceedingly higher than that of Pd/GONR (<1 %), Pd/CNTs-IL (51 %), or Pd/GO-IL (8 %).

Using Palladium and Gold Palladium Nanoparticles Decorated with Molybdenum Oxide for Versatile Hydrogen Peroxide Electroproduction on Graphene Nanoribbons

Electrocatalytic production of H₂O₂ via a two-electron oxygen reduction reaction (ORR-2e^-) is regarded as a highly promising decentralized and environmentally friendly mechanism for the production of this important chemical commodity. However, the underlying challenges related to the development of catalytic materials that contain zero or low content of noble metals and that are relatively more active, selective, and resistant for long-term use have become a huge obstacle for the electroproduction of H₂O₂ on commercial and industrial scales. The present study reports the synthesis and characterization of low metal-loaded (≤6.4 wt %) catalysts and their efficiency in H₂O₂ electroproduction. The catalysts were constructed using gold palladium molybdenum oxide (AuPdMoO_x) and palladium molybdenum oxide (PdMoO_x) nanoparticles supported on graphene nanoribbons. Based on the application of a rotating ring-disk electrode, we conducted a thorough comparative analysis of the electrocatalytic performance of the catalysts in the ORR under acidic and alkaline media. The proposed catalysts exhibited high catalytic activity (ca. 0.08 mA g_noble metal^-1 in an acidic medium and ca. 6.6 mA g_noble metal^-1 in an alkaline medium), good selectivity (over 80%), and improved long-term stability toward ORR-2e^-. The results obtained showed that the enhanced ORR activity presented by the catalysts, which occurred preferentially via the two-electron pathway, was promoted by a combination of factors including geometry, Pd content, interparticle distance, and site-blocking effects, while the electrochemical stability of the catalysts may have been enhanced by the presence of MoO_x.

Crystal Orientation in Pt-Based Alloys Induced by W(CO)6: Driving Oxygen Electroreduction Catalysis

Integrating crystal orientation as well as structural and compositional advantages into one catalyst might be a promising strategy for high-performance Pt-based catalysts for proton-exchange membrane fuel cells. Herein, by introducing W(CO)₆ as a structure-oriented template, Pt-based alloys with a well-defined crystal orientation along the (111) facet were obtained. The oxygen reduction reaction mass and specific activities of the crystal-facet-tuned alloys reach a new level. Moreover, the outstanding durability stems from the combination of their exposed crystal facets and incorporated W. The density functional theory calculation results reveal that the formation of the preferred (111) alloys can be attributed to the lower free energy of (111) facets and the weaker adsorption of CO released by W(CO)₆. This proposed synthesis strategy of using transition-metal carbonyl compounds as additives to synthesize alloys with strong crystal orientation may open a door to the design of various alloy catalysts with ultrahigh activity.

Subnanoscale Platinum by Repeated UV Irradiation: From One and Few Atoms to Clusters for the Automotive PEMFC

The unaffordable costs of the automotive proton exchange membrane fuel cell (PEMFC), remaining a roadblock for commercial applications as an alternative to combustion engine vehicles, can be overcome partially by remarkably increasing the utilization of irreplaceable platinum (Pt). Herein, atomically precise Pt with scalable atoms ranging from 1 to 43 atoms, stabilized by a homemade carbon from white radish without any ligands, is prepared by a repeated UV irradiation method that is industrially scalable. Compared with the isolated Pt₁ in the form of Pt-N₄, octahedral Pt₆, and icosahedron Pt₁₃, the ordered Pt₄₃ cluster (∼0.75 nm) with higher metal coordination number displays much higher oxygen reduction reaction performance with a mass activity, which is about 1036% higher than that obtained by state-of-the-art Pt/C, an increase by a factor of ∼3.3 as compared with the DOE 2020 target (0.44 A mg_Pt^-1). The utilization rate of Pt atoms reaches up to 94.7%, much higher than that of Pt (2 nm, 56%), capable of further reducing the amount of platinum that is required for PEMFCs. Moreover, the cluster exhibits an outstanding stability due to the improved Pt vacancy formation energy raised by stronger atom interaction in the close-packed cluster. The cluster exhibits a unique finite size effect from self-tuned energy band and strain levels. A clear strain effect on the d-band center is first presented for pure Pt without distortion from ligands like a second metal. Therefore, the assembly of subnanometer Pt with atom alteration opens up new horizons in designing efficient platinum group metal (PGM) catalysts by reducing the size to subnanometer scale.

Oxygen Reduction Reaction Catalyzed by Pt3M (M = 3d Transition Metals) Supported on O-doped Graphene

Pt3M (M = 3d transition metals) supported on oxygen-doped graphene as an electrocatalyst for oxygen reduction was investigated using the periodic density functional theory-based computational method. The results show that oxygen prefers to adsorb on supported Pt3M in a bridging di-oxygen configuration. Upon reduction, the O–O bond breaks spontaneously and the oxygen adatom next to the metal–graphene interface is hydrogenated, resulting in co-adsorbed O* and OH* species. Water formation was found to be the potential-limiting step on all catalysts. The activity for the oxygen reduction reaction was evaluated against the difference of the oxygen adsorption energy on the Pt site and the M site of Pt3M and the results indicate that the oxygen adsorption energy difference offers an improved prediction of the oxygen reduction activity on these catalysts. Based on the analysis, Pt3Ni supported on oxygen-doped graphene exhibits an enhanced catalytic performance for oxygen reduction over Pt4.