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Abstract: The synthesis of new “Pt‐free” electrocatalysts (ECs) for the oxygen reduction reaction (ORR) is reported. The ECs are characterized by a hierarchical “core‐shell” morphology; the “core” is made of graphene, that is covered by a cratered, microporous carbon nitride (CN) “shell”. The latter supports nanoparticles of M1 and Sn metals (M1=Co; Ni) in “coordination nests”. The latter are holes in the CN matrix, whose walls consist of N‐ and C‐ligands. Two groups of ECs are studied: (i) “pristine” ECs; and (ii) “activated” ECs, that are obtained from the “pristine” ECs by means of a suitable activation process (A) aimed at improving the performance in the ORR. Here, the interplay existent between (i) the chemical composition, morphology, structure and A; and (ii) the ORR performance and mechanism is investigated. The resulting insights improve the fundamental understanding of this family of ECs and open the door to the devising of new preparations of “Pt‐free” ECs for the ORR, which: (i) are stabilized by a CN matrix and; (ii) exhibit an improved performance. [ABSTRACT FROM AUTHOR]

The article focuses on carbon nitride-based electrocatalysts for application in low-temperature fuel cells (FCs). Topics discussed include low-temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs) and anion-exchange membrane fuel cells (AEMFCs), requirement of oxygen reduction reaction (ORR) electrocatalysts (ECs) to minimize cathode polarization losses and three dimensional (3D)-crosslinked hybrid macromolecular precursors.

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O2-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, Ta2O5, Nb2O5, and ZrO2), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O2-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional. [ABSTRACT FROM AUTHOR]

Hybrid systems composed of the reduced graphene oxide-supported platinum and multiwalled carbon nanotube-supported iridium (both noble metals utilized at low loadings on the level of 15 and ≤2 µg cm−2, respectively) were considered as catalytic materials for the reduction of oxygen in acid media (0.5-mol dm−3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate are tested using rotating ring–disk electrode (RRDE) voltammetric experiments. The efficiency of the proposed catalytic systems was also addressed by performing galvanodynamic measurements with gas diffusion electrode (GDE) half-cell at 80 °C. The role of carbon nanotubes is to improve charge distribution at the electrocatalytic interface and facilitate the transport of oxygen and electrolyte in the catalytic systems by lowering the extent of reduced graphene oxide restacking during solvent evaporation. The diagnostic electrochemical experiments revealed that—in iridium-containing systems—not only higher disk currents, but also somehow smaller ring currents are produced (when compared to the Ir-free reduced graphene oxide-supported platinum), clearly implying formation of lower amounts of the undesirable hydrogen peroxide intermediate. The enhancement effect originating from the addition of traces of iridium (supported onto carbon nanotubes) to platinum, utilized at low loading, may originate from high ability of iridium to induce decomposition of the undesirable hydrogen peroxide intermediate. [ABSTRACT FROM AUTHOR]

This work reports the synthesis, the physicochemical characterization and the electrochemical studies of new electrocatalysts (ECs) for the oxygen reduction reaction (ORR) that: (i) exhibit a hierarchical architecture; and (ii) do not comprise platinum. The active sites of the ECs consist of Fe and Sn species stabilized in “ coordination nests ” of a carbon nitride (CN) matrix. The latter exhibits a rough, microporous morphology and acts as a “ shell ” covering a graphene “ core ”. This paper: (i) discusses the role played by Fe as the “ active metal ” in this family of ECs; and (ii) examines in detail how the physicochemical properties and, correspondingly, the electrochemical performance are affected by a suitable activation procedure A meant to boost the ORR kinetics. The results lead to an improved fundamental understanding on the features of the active sites, including the impact of both A and the pH of the environment in their performance and ORR mechanism. These insights clarify the most desirable features to be included in high-performing ECs belonging to this family, paving the way to the synthesis of next-generation, efficient ECs for the ORR that do not comprise platinum. [ABSTRACT FROM AUTHOR]

Chemically-reduced graphene-oxide-supported gold nanoparticles with a diameter, of 40–60 nm are considered here as catalytic materials for the reduction of oxygen in alkaline medium in comparison to analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared by the chemical reduction method, in which the NaBH 4 -prereduced Keggin-type phosphomolybdate heteropolyblue acts as the reducing agent for the precursor (HAuCl 4 ). Polyoxometallate (PMo 12 O 40 3− ) capping ligands stabilize gold nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm −3  H 2 SO 4 ) that heteropolymolybdates form readily stable adsorbates on nanostructures of both gold and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of electrochemical diagnostic experiments (performed in 0.1 mol dm −3 KOH): ( i ) the phosphomolybdate adsorbates are removed from the interface as they undergo dissolution in alkaline medium; and ( ii ) the Au nanoparticles (Au loading, 30 μg cm −2 ) remain well-dispersed on the carbon as evident from transmission electron microscopy. High electrocatalytic activity of the reduced-graphene oxide-supported Au nanoparticles toward reduction of oxygen in alkaline medium is demonstrated using cyclic and rotating ring-disk voltammetric experiments. The latter system could also act as the active support for Pt nanoparticles during the reduction of oxygen. Among important issues are possible activating interactions between gold and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). [ABSTRACT FROM AUTHOR]

In this report, the preparation of Fe-carbon nitride (CN)-based electrocatalysts (ECs) with a “core-shell” morphology for the oxygen reduction reaction (ORR) is described. The ECs consist of spherical XC-72R carbon nanoparticles, the “cores” , that are covered by a CN matrix, the “shell” , that embeds Fe species in “coordination nests” . The latter consist of hollow cavities in the CN matrix, whose internal surface is covered by N- and C-ligands able to stabilize alloy nanoparticles or active sites. Two families of CN-based ECs are prepared, which are grouped on the basis of the concentration of N atoms in the CN “shell” . Each group comprises of both a “pristine” and an “activated” EC; the latter is obtained from the “pristine” EC by a suitable series of treatments ( A ) devised to improve the ORR performance. The chemical composition of the CN-based ECs is determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and microanalysis. The thermal stability under both inert and oxidizing atmospheres is gauged by High-Resolution Thermogravimetric Analysis (HR-TGA). The structure is probed by powder X-ray diffraction, and the morphology is inspected by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM). The surface area of the CN-based ECs is determined by nitrogen physisorption techniques, and the surface composition is probed by X-ray Photoelectron Spectroscopy (XPS). The electrochemical performance and reaction mechanism of the CN-based ECs in the ORR is investigated in both acid and alkaline environments by cyclic voltammetry with the Thin-Film Rotating Ring-Disk Electrode setup (CV-TF-RRDE). The influence of the preparation parameters and of the treatments on the physicochemical properties, the ORR performance, and reaction mechanism is studied in detail. In the alkaline environment the FeFe 2 -CN l 900/C A “core-shell” EC shows a remarkable ORR onset potential of 0.908 V vs. RHE which, with respect to the value of 0.946 V vs. RHE of the Pt/C ref., classifies the proposed materials as very promising “Platinum Group Metal-free” ECs for the ORR. [ABSTRACT FROM AUTHOR]

This report presents the interplay between the synthesis parameters, physicochemical properties, and electrochemical performance of core-shell low-Pt electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on Pt x Ni active sites stabilized on a carbon nitride shell. The impact of the pyrolysis temperature (T f), of the support core (H) composition and of an electrochemical dealloying-activation step on the EC morphology and on the accessibility and stability of the active sites are studied in detail. Three supports are employed based on carbon nanoparticles and/or graphene platelets. The ORR performance of activated ECs measured by cyclic voltammetry with the thin-film rotating ring-disk electrode approach is strongly affected by T f and H. The best performing ECs are tested in single proton exchange membrane fuel cells under operating conditions. The simultaneous presence of graphene and carbon in H improves the dispersion of active sites, resulting in a vastly improved mass activity and durability in comparison with a benchmark state-of-the-art Pt/C EC. [Display omitted] • Hierarchical core-shell Pt–Ni ORR electrocatalysts (ECs) are obtained. • Interplay between synthesis, properties, and performance of ECs is discussed. • Hierarchical Pt–Ni ECs exhibit mass activities surpassing DOE targets. • Hierarchical Pt–Ni ECs are more durable than Pt/C reference ECs. [ABSTRACT FROM AUTHOR]

Five bimetallic electrocatalysts (ECs) including a carbon nitride (CN) support are synthesised through the pyrolysis of a solid precursor obtained through sol-gel and gel-plastic processes. The resulting ECs are characterised through ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) and XPS (X-ray Photoelectron spectroscopy); their performance and reaction mechanism in the oxygen reduction reaction (ORR) are evaluated with the CV-TF-RRDE method (Cyclic Voltammetry Thin-Film Rotating Ring-Disk Electrode). Special attention is given to XPS results with the aim to carry out a thorough investigation of the surface concentration and the chemical environments of the different elements, as well as providing information on the structure of the metal components of the ECs and their interactions with the carbon nitride support. The correlation of the results obtained from the chemical analyses, XPS and the electrochemical studies allows to improve the fundamental understanding of: (i) the interplay between the preparation parameters and the surface and bulk chemical composition of the CN-supported ECs; and (ii) the factors controlling the ORR kinetics and reaction pathway in bimetallic CN-supported ECs, identifying the roles played by the various metal species. In this regard, the VI-period metal included in each EC has the highest impact on the ORR performance. Indeed, the ORR onset potential rises from ca. 300 to ca. 780 to ca. 900 mV vs. RHE for ECs based on Au, Ir and Pt, respectively. On the other hand, the second metal ( i.e. , Ni, Rh, and also Ir in the case of an EC including both Pt and Ir) plays a secondary role. With respect to the Pt/C ref., the CN-supported EC including Pt and Ni exhibits a higher onset potential (900 and 911 mV vs. RHE, respectively); finally, it is shown that Ir and Rh promote the selectivity of the ORR in the 4-electron mechanism. [ABSTRACT FROM AUTHOR]

Platinum nanoparticles have been deposited onto hybrid carriers composed of iron hexacyanoferrate (Prussian Blue) pretreated or functionalized reduced graphene oxide (rGO), and the resulting system's electrocatalytic activity has been investigated during oxygen reduction reaction in acid solution. Physicochemical properties of the Pt nanoparticle-containing materials have been obtained using transmission electron microscopy, X-ray diffraction and various electrochemical diagnostic techniques including cyclic voltammetry and rotating ring-disk electrode (RRDE) voltammetry. Although functionalization of rGO carriers with Prussian Blue, followed by the system's thermal treatment at 300°C, does not change activity of Pt catalytic centers in terms of the electroreduction potential of oxygen in 0.5 mol dm−3 H 2 SO 4 (under rotating disk voltammetric conditions), the presence of cyanide-bridged iron ionic sites tends to decrease formation of the undesirable hydrogen peroxide intermediate, as demonstrated by monitoring ring currents during RRDE measurements. The results imply usefulness of PB-modified rGO supports for dispersed Pt nanoparticles at the low loading of 10 μg cm−2 in the oxygen reduction electrocatalysis. In addition to the possible metal-support activating interactions, the mechanism, in which oxygen reduction is initiated at Pt centers and the hydrogen peroxide intermediate is reductively decomposed at reactive PB-modified rGO supports, is feasible. • Modification of reduced graphene oxide with Prussian Blue produces active supports for Pt nanoparticles. • Oxygen reduction proceeds mostly to water in acid medium according to 4-electron mechanism. • Lower amounts of hydrogen peroxide intermediate are observed during ORR in acid medium. [ABSTRACT FROM AUTHOR]