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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, under such condition, O2-reduction reaction (ORR) often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Recent studies have demonstrated that the rational design of a catalytic system is based on nanostructures of platinum, or carbon-supported platinum (Pt/C), and metal oxide (MOx). By combining the catalytic properties of both components, such hybrid Pt-MOx catalysts are capable of tuning of the activity and durability of Pt. Due to strong interactions of MOx with Pt catalytic nanoparticles leading to the improved activity and durability during ORR, certain nanostructured metal oxides are considered as co-catalysts or active components of supports. The existence of specific interactions between MOx and noble metal (Pt) nanoparticles should improve the stability and activity of the metal catalytic sites due to modification of the Pt electronic structure and diminishing of the oxo (OH) species adsorption on Pt surface, thus promoting centers for the adsorption of oxygen and the cleavage of O=O bonds. The interactions mentioned above would also facilitate dispersion of Pt, inhibit their detachment and further aggregation, and, consequently, prevent or decrease their degradation during the fuel cell operation [1]. The catalytic activity of substoichiometric ceria, CeOx (where x < 2) additive results from its unique structure, presence of oxygen vacancies, and other features resulting from the 4f-electronic configuration of cerium. While the oxygen defects are likely to serve as active oxygen adsorption sites, the mixed-valent CeIII/CeIV redox sites should permit the electron shuffling within the lattice of oxygen vacancies and enhancing the ORR activity. The formation of oxygen-defects is accompanied by the localization of electrons left behind in Ce 4f states, thus leading to the formation of CeIII species capable of elongating and reducing the O–O bond strength of the adsorbed O2 molecule; consequently, by increasing the relative ratio of CeIII-to-CeIV, the ORR electrocatalytic activity can be improved [1]. Therefore, in the present study, we consider the intentionally reduced CeOx nanostructured components through subjecting them to high-temperature pretreatment in the presence of argon gas admixed with hydrogen. It is reasonable to expect that, the increased population of CeIII sites on the surfaces of the pretreated ceria particles would stabilize the neighboring active Pt-metal centers, exhibit reductive interactions toward Pt-oxo species, and to improve durability of the catalytic materials. Thus the boundary formed between the Pt-metal and CeOx-metal oxide should facilitate inhibition of the Pt oxide formation by the CeOx layer. Furthermore, our recent studies clearly show that, in the presence of ceria, the oxidative degradation of carbon carriers is also largely decreased. Finally, cerium oxide is known to act as the oxidative scavenger for free radicals such as hydroxyl (HO•) and hydroperoxyl (HOO•), which, once generated, would otherwise lead to the formation of undesirable hydrogen peroxide. Sub-stoichiometric CeOx is capable of rapidly switching between CeIII and CeIV oxidation states, thus inducing the decomposition of both radicals and peroxides. The above observation seems to be very helpful when it comes to designing highly active and durable ORR catalysts containing low platinum loadings. [1] A.Kostuch, I.A. Rutkowska, B. Dembinska, A. Wadas, E. Negro, K. Vezzù, V. Di Noto, P.J. Kulesza, Molecules 26 (2021) 5147. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).

The proton-exchange membrane fuel cell (PEMFC) technology is one of the most promising approaches for energy conversion in automotive applications. Despite the use of expensive noble metal electrocatalysts, the sluggish kinetics of the oxygen reduction reaction (ORR) in acid media, and formation of the undesirable hydrogen peroxide intermediate are still the main drawbacks, on top of the stability problems, when it comes to the widespread commercialization of PEMFC devices. The progress in this subject is greatly hindered by the high cost and scarcity of the state-of-the-art platinum-based materials which are regarded as the most effective cathode catalysts. Therefore, most of the current research studies have been devoted to the optimization of active centers and maximization of their utilization, which should allow the lowering of the cathode Pt loadings without loss of performance and durability. Unfortunately, the problem of electrochemical stability and the danger of generation of higher quantities of the undesirable hydrogen peroxide intermediate may become even more serious in the case of systems utilizing lower amounts of the Pt catalyst Among important strategies is hybridization, activation, and stabilization of carbon-supported Pt catalysts by functionalization through admixing with certain nanostructured and typically substoichiometric metal oxides. Special attention is paid to application of the bi- or multi-metallic Pt-based alloys in their various forms and structures. Additionally, doping of carbon carriers with heteroatoms would produce surface functional groups and active centers capable of not only improving the catalysts’ performance, but also affecting their stability. Among other important issues are such features as porosity, hydrophilicity, and degree of graphitization of carbon components, in addition to the existence of metal–support interactions, high electrochemical active surface area, electronic structure of interfacial Pt, and the feasibility of adsorptive or activating interactions with oxygen molecules. Voltammetric approaches, particularly the rotating ring-disk electrode (RRDE) methodology, are typically applied to diagnose mechanisms and dynamics of ORR. A tempting alternative for recording the electrochemical responses is to integrate the current and to report charge passed as a function of time. Chronocoulometry offers important advantages including good signal-to-noise ratio because the act of integration smooths random noise on the current transients. Furthermore, by integrating the responses, it is possible to separate surface phenomena (e.g. interfacial oxidation of carbon or platinum) more readily from bulk electrochemical responses (ORR). The double-potential-step chronocoulometry permits, in principle, estimation of the number of electrons (n) involved in the oxygen reduction and the percent of formation of the H2O2 intermediate. The problem lies in understanding the reaction kinetics and mechanisms in order to define properly experimental parameters (potential steps and pulse times). Comparative measurements are performed using RRDE voltammetry. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, or ZrO2) have been demonstrated to stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction of oxygen. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).

International audience; Novel member of polymetalloporphines, namely manganese polymetalloporphine of type I (pMnP-I) obtained by ion exchange from magnesium polyporphine of type I (pMgP-I) is reported for the first time and compared to its cobalt analogue (pCoP-I). Both polymer films have been obtained via two-step procedure: demetaladon of the pMgP-I electrode film via its exposure to trifluoroacetic acid solution, resulting in formation of the metal-free polyporphine of type I (pH(2)P-I) followed by electrochemically induced incorporation of Co or Mn ions from the acetonitrile solution of cobalt and manganese perchlorates. A further oxidative transformation of pCoP-I, polymer films has led to the corresponding polyporphines of type II, pCoP-II and pMnP-II, possessing such unique features as condensed polymer structure with a very high density of active sites and high electronic conductivity within a very broad potential range including the one corresponding to the neutral (uncharged) state of the polymer matrix. Both polymers of type II also exhibit interesting electrocatalytic activity toward oxygen electroreduction in aqueous neutral (pH 6.7) and alkaline (pH 13) media which was evaluated under cyclic voltammetric and steady-state conditions. The results demonstrate that the efficiency (regardless of the electrolyte) of both polymetalloporphines is comparable to bare platinum electrode. The effect of annealing of polymer-modified electrodes on their catalytic properties has also been considered.

(Co)polycondensation of resorcinol and heterocyclic aldehydes yields heteroaromatic porous polymers. Carbonization of such polymers results in N-doped, S-doped, N/S-co-doped or undoped carbon gels depending on the starting composition of reactants. If the (co)polycondensation is initiated with FeCl3, Fe-containing carbon gels are obtained. Depending on the nature of the heteroatom doping (solely N or S, both N and S or no heteroatom) the Fe-containing carbon gels exhibit distinct structural properties. While the presence of sulfur hinders Fe-driven graphitization, yielding carbons with high specific surface area (SSA), N-doping exhibits the contrary effect, resulting in highly graphitic structures with low SSAs. More importantly, increasing the Fe content in the N/S-co-doped carbons results in remarkable increases in microporosity, yielding materials with SSA of up to 1240 m2g-1. The Fe-containing carbon gels are considered non-precious metal catalysts for the oxygen reduction reaction. It is demonstrated that in the case of Fe/N/C/S type catalysts, the interaction between S, C and Fe (at the pyrolysis stage) results in the enhancement of microporosity, where S acts as a soft oxidant toward carbon, while Fe recaptures the sulfur- and carbon-containing pyrolytic gases. The incidental deposition of pyrolytic gases contributes significantly to the final structure of the catalysts.

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 H2SO4 (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.

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 &le, 2 &micro, g cm&minus, 2, respectively) were considered as catalytic materials for the reduction of oxygen in acid media (0.5-mol dm&minus, 3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate are tested using rotating ring&ndash, 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 &deg, 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&mdash, in iridium-containing systems&mdash, 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.

Studies concentrated on the development of carbon supported PdxPty/C electrocatalysts for the oxygen reduction reaction (ORR). The application of the reverse ‘water-in-oil’ microemulsion method allowed to obtain a number of electrocatalysts with metal nanoparticles of ca. 5–6 nm in size. Catalysts with the constant metal loading of 10 wt% and various Pd:Pt atomic ratios were investigated. The textural features of the PdPt/C catalysts (surface area, morphology) as well as the PdPt phase (metal particle size, bulk composition) were characterized by a number of techniques. A particular attention was paid to the characterization of the surface properties of the PdPt particles using cyclic voltammetry technique. For all Pt contents (5 to 70 at%) a segregation of metal components (Pt, Pd) occurred leading to the Pt enrichment in the top layers. The observed Pt enrichment manifested especially strongly for PdPt catalysts with bulk Pt content below 20 at%. The surface fraction of Pt as high as 0.8 to 0.9 was attained regardless of nominal Pt content, starting from 30 at%. Among the catalysts, those with Pt content of 50 and 70 at% exhibited the highest activity towards ORR in terms of positively shifted half-wave potential, even surpassing that of Pt/C. Moreover, the obtained specific activity and turnover frequency values also suggested similar activities of catalysts with Pt content of 50 and 70 at%. A negligible formation of hydrogen peroxide on these two catalysts, indicated almost exclusively a desired four-electron process leading to water formation. The observed effect of Pt content on ORR activity is discussed in terms of surface properties of the PdPt nanoparticles.

We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) process based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated with nanoparticles of metal/alloy/oxide but are instead based on Fe and Sn subnanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art.

Graphene is, in principle, a promising material for consideration as component (support, active site) of electrocatalytic materials, particularly with respect to the reduction of oxygen, an electrode reaction of importance to low-temperature fuel cell technology. Different concepts of utilization, including nanostructuring, doping, admixing, preconditioning, modification or functionalization of various graphene-based systems for catalytic electroreduction of oxygen are elucidated, as well as important strategies to enhance the systems’ overall activity and stability are discussed.

Carbon-supported selenium-modified iridium nanoparticles have been synthesized using IrCl 3 and selenourea serving as a precursor of selenium and nitrogen atoms. Here nanostructured iridium is chosen as model base metal for fundamental catalytic considerations because it exhibits interfacial properties resembling both platinum and ruthenium. The systems' electrocatalytic properties have been studied in sulfuric acid electrolyte toward reduction of oxygen and formation of hydrogen peroxide intermediate in comparison to bare iridium and platinum catalysts. To get insight into the reaction dynamics and mechanisms, such electrochemical diagnostic techniques as cyclic voltammetry and rotating ring-disk electrode voltammetry have been considered. To mimic operation of catalysts in real fuel cells, additional experiments utilizing gas diffusion electrode have also been performed. Materials are subjected to surface analytical, structural and microscopic characterization using X-ray photoelectron (XPS), fluorescence (EDX), and diffraction (XRD) methods as well as transmission and scanning electron microscopies. At low (optimum) coverages of selenium on surfaces of iridium nanoparticles, the reduction of oxygen tends to proceed at more positive potentials in comparison to bare iridium under analogous conditions. Apparently, strong affinity of bare iridium to form oxo groups on its surface (known as inhibiting oxygen reduction) is largely suppressed in presence of selenium or nitrogen atoms. But the optimum selenium-modified iridium system produces somewhat higher amounts of the hydrogen peroxide intermediate presumably due to partial physical blocking of iridium (metal and metal oxo) sites (that would otherwise be active toward the reduction of the H 2 O 2 intermediate). High tolerance (during reduction of oxygen) of the optimum Ir-based catalyst (functionalized using selenourea) to the parasitic (e.g. in polymer membrane fuel cells) simultaneous oxidation of organic fuels (e.g. methanol or ethanol) should be mentioned as well.

Application of graphene-type materials in electrocatalysis is a topic of growing scientific and technological interest. A tremendous amount of research has been carried out in the field of oxygen electroreduction, particularly with respect to potential applications in the fuel cell research also with use of graphene-type catalytic components. This work addresses fundamental aspects and potential applications of graphene structures in the oxygen reduction electrocatalysis. Special attention will be paid to creation of catalytically active sites by using nonmetallic heteroatoms as dopants, formation of hierarchical nanostructured electrocatalysts, their long-term stability, and application as supports for dispersed metals (activating interactions).

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of high-surface-area zeolite-type acidic salt of cesium phosphododecatungstate (Cs2.5H0.5PW12O40) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2–5 μg cm−2) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs2.5H0.5PW12O40-free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1–2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in acid and alkaline electrolytes. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when carbon nanostructures materials are combined with transition metal hexacyanometallates. An electrocatalytic system, that utilizes transition metal hexacyanometallates nanoparticles modified carbon nanostructures materials (e.g. reduced graphene oxide, carbon nanotubes), is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

Electrocatalytic activity of carbon (Vulcan XC-72) supported selenium-modified ruthenium, RuSe x /C, nanoparticles for reduction of oxygen was enhanced through intentional decoration with iridium nanostructures (dimensions, 2–3 nm). The catalytic materials were characterized in oxygenated 0.5 mol dm −3 H 2 SO 4 using cyclic and rotating ring disk voltammetric techniques as well as using transmission electron microscopy and scanning electron microscopy equipped with X-ray dispersive analyzer. Experiments utilizing gas diffusion electrode aimed at mimicking conditions existing in the low-temperature fuel cell. Upon application of our composite catalytic system, the reduction of oxygen proceeded at more positive potentials, and higher current densities were observed when compared to the behavior of the simple iridium-free system (RuSe x /C) investigated under the analogous conditions. The enhancement effect was more pronounced than that one would expect from simple superposition of voltammetric responses for the oxygen reduction at RuSe x /C and iridium nanostructures studied separately. Nanostructured iridium acted here as an example of a powerful catalyst for the reduction of H 2 O 2 (rather than O 2 ) and, when combined with such a moderate catalyst as ruthenium–selenium (for O 2 reduction), it produced an integrated system of increased electrocatalytic activity in the oxygen reduction process. The proposed system retained its activity in the presence of methanol that could appear in a cathode compartment of alcohol fuel cell.

Present studies concentrated on the preparation, characterization, and electroactivity of palladium–polypyrrole (Pd/PPY) catalysts for oxygen reduction reaction. In particular, the effect of Nafion ionomer on their electroactivity was evaluated. In all catalysts prepared by “water-in-oil” microemulsion method, the Pd nanoparticles of ca. 7 nm in size appeared regardless of the Pd content (ranging from 2 to 20 wt.%). For comparison, carbon black-supported (Vulcan XC-72) catalyst (20 wt.% Pd) was also synthesized. Coating of the Pd/PPY samples with Nafion ionomer reduced their surface area and porosity. Chemical interaction due to Nafion acid functionalities affected the N-state of pyrrole as well as electron state of Pd in the Pd/PPY catalysts. As a result, the contribution of more oxidized palladium (Pdδ+) increased. These interactions played an essential role in the electroactivity of Pd/PPY for oxygen reduction reaction. The increased amount of Nafion relative to that of PPY reduced limiting current density whereas the half-wave potential shifted to a more positive value and the fraction of hydrogen peroxide remarkably decreased.

Application of a catalyst preparation method known as ‘water-in-oil’ microemulsion allowed to obtain a series of palladium/polypyrrole (Pd/PPY) electrocatalysts with various Pd contents (2–20 wt%) and Pd particle size within a narrow and similar range of 6–8 nm. The textural features of the samples (surface area, porosity) as well as the palladium phase (metal particle size, shape) were characterized by a number of techniques. Electrocatalytic activity of the prepared materials towards oxygen reduction reaction (ORR) was studied in 0.5 M H2SO4 using rotating ring disc electrode voltammetry (RRDE) and cyclic voltammetry techniques. All prepared Pd/PPY samples exhibited electrocatalytic activity towards ORR. At a constant palladium loading on the electrode, the polymer to Pd weight ratio was found to play a decisive role in the electrocatalytic performance of the samples. Their catalytic performance in terms of the increased reduction current and positively shifted half-wave potential (RRDE measurements) was improved when Pd content in polymer was grown from 2 to 12 wt% Pd. At the optimum Pd content, 10–12 wt%, the analysis of the RRDE data confirmed that molecular oxygen underwent mainly a desired four-electron reduction. By further increasing the Pd content and thus decreasing the PPY to Pd ratio, the selectivity to water continuously decreased while the fraction of undesired H2O2 product significantly increased. Therefore, a role of PPY to Pd weight ratio in the electrocatalytic performance of the investigated catalysts is discussed here.

A new group of carbon-supported ruthenium- and iridium-based electrocatalysts for oxygen reduction reaction (ORR) was synthesized, using RuCl3 or IrCl3 and selenourea (as precursor of both Se and N). The heat treated catalysts (later abbreviated as RuSeN/C and IrSeN/C) were subjected to physical characterization, like X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Studies revealed changes in surface properties of Ru- and Ir- based catalysts. Catalytic performances of obtained catalysts were evaluated using Rotating Ring Disk Electrode (RRDE) in acid solution. The ORR activity and hydrogen peroxide generation on designed selenides were dependent on the ratio of metal and selenourea used in precursor blend and were separately characteristic for ruthenium and iridium. While RuSeN/C exhibited high ethanol tolerance, the ORR activity of IrSeN/C in the presence of ethanol depended on the amount of selenourea. CoSeN/C and FeSeN/C were also prepared and characterized as low-cost alternative catalysts.

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures especially graphene/graphene oxide materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue, namely with ultra-thin Co, and Ni hexacyanoferrate layers and trace amount of platinum for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in acid and alkaline electrolytes. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined with analogue of polynuclear Prussian Blue and trace amount of Pt nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased. An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified graphene and graphene related materials, is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

Hybrid systems composed of the reduced-graphene-oxide supported gold, iridium or bimetallic (AuIr) nanoparticles (at loadings typically below 5 µg cm-2) have been considered as active matrices for platinum catalysts utilized at low loadings (-2) during the reduction of oxygen in mostly acid but also alkaline media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared both by the electrochemical and chemical reductionapproaches in which the pre-reduced Keggin-type phosphomolybdate heteropolyblue species act as the reducing agent for the HAuCl4 precursor. Polyoxmetallate (PMo12O40 3-) adsorbates or “capping ligands” tend to stabilize gold (iridium or bimetallic) 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 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of gold, irridium 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 the electrochemical experiments performed in 0.1 mol dm-3 KOH) the phosphomolybdate adsorbates are removed from the interface because they undergo dissolution and decomposition in alkaline medium. High electrocatalytic activity of the reduced-graphene oxide-supported catalytic systems toward the reduction of oxygen is demonstrated using chronoamperometry and gas-diffusion electrode, in addition to the conventional and rotating ring-disk electrode (RDE) voltammetry. Among important issues is the presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). When using the reduced graphene oxide carriers for Au (Ir or AuIr) nanostructures together with Pt nanoparticles, the resulting catalytic systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with the lower formation of the hydrogen peroxide intermediate (

There has been growing interest in the development of highly active catalysts for electroreduction of oxygen that could be effectively utilized in low-temperature fuel cells. The important practical issue is related to the need of lowering costs of catalysts by substituting the noble metals with inexpensive abundant elements. In this respect materials containing metallic moieties coordinated by nitrogen atoms embedded in the carbon matrix (M-N-C) are recognized as most suitable candidates to replace platinum catalysts. More recently, the approach of using such biomimetic centers has been considered for the carbon dioxide reduction, the reaction which permits generation of carbon-based chemicals, fuels or syngas. Since both reactions (O2 and CO2 reductions) are characterized by slow kinetics and may proceed through different pathways producing various products, their efficiency and selectivity still remain fundamental problems for practical applications. It is well established that the best performing M-N-C catalysts (with high specific surface area and large fraction of micropores) are produced from nitrogen-containing polymeric precursors. In these study we present a new type of self-supporting M-N-C materials, namely nitrogen and sulfur co-doped iron containing highly porous carbon gels (obtained through pyrolysis of organic gels) as efficient non-precious metal catalysts for both the oxygen and carbon dioxide electroreductions. The precursors of the catalysts were prepared via the sol-gel polycondensation (induced with iron(III) cations) of resorcinol together with the mixture of two heterocyclic aldehydes containing nitrogen and sulfur. The role of S is to produce carbon gels with high specific surface areas (e.g. when compared to carbon gels containing solely N heteroatom). It is commonly accepted that rotating ring(platinum)-disk(glassy carbon) electrode voltammetry can be effective diagnostic tool for elucidation of the performance and selectivity (towards water or hydrogen peroxide) of the oxygen electroreduction. We are going to demonstrate that a combination of the rotating ring-disk electrode methodology and stripping type voltammetry (at platinum ring) can be also used for the determination of the reaction mechanism, competitive formation (evolution) of hydrogen, and determination of CO-type adsorbates generated at the catalyst during the CO2 electroreduction.

Reduced graphene-oxide-supported the various metals analogue of polynuclear Prussian Blue are considered here together with traces of platinum as well as low loadings of platinum (10-15 mg cm-2) as catalytic materials for reduction of oxygen in acid media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of oxygen in acid solution (the highest currents and the most positive electroreduction potentials) using cyclic and rotating ring-disk electrode (RDE) voltammetric experiments. Among important issues are possible activating interactions between analogue of Prussian Blue, platinum nanoparticles 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). An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified reduced graphene-oxide-supported is also characterized here by additional diagnostic experiments, namely galvanodynamic measurements with use of so called gas diffusion electrode (GDE) operating under conditions resembling those characteristic of the cathode in a real fuel cell.

Combination of multi-walled carbon nanotubes, cobalt porphyrin and tungsten oxide in the film (deposited onto glassy carbon electrode substrate) produces an electrocatalytic system capable of effective reduction of oxygen in such acid medium as 0.5 mol dm −3 H 2 SO 4 . Co-existence of cobalt porphyrin and tungsten oxide, together with dispersed carbon nanotubes, leads to the enhancement effect evident from some positive shift in the oxygen reduction voltammetric potential and the significant increase of voltammetric currents (relative to those characteristic of the system free of carbon nanotubes and WO 3 ). The multi-component electrocatalytic film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at the cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the tungsten oxide support. An important function of carbon nanotubes is to improve transport of electrons within the electrocatalytic multi-component film.

Combination of multiwalled carbon nanotubes, cobalt porphyrin and tungsten oxide produces a multifunctional electrocatalytic system capable of the effective reduction of oxygen in acid medium. The resulting inks have been utilized during sequential deposition of components. Co-existence of Co-porphyrin, tungsten oxide together with dispersed carbon nanotubes leads to synergistic effect that is evident from positive shift of the oxygen reduction potentials, significant increase of voltammetric currents (relative to those observed at a system not containing carbon nanotube-supported tungsten oxide) and decrease of hydrogen peroxide produced during oxygen reduction. It is reasonable to expect that the reduction of oxygen is initiated at metalloporphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at partially reduced tungsten oxide. In addition to the methanol tolerance other important issue is the tolerance of the proposed system to ethanol that may appear due to crossover in the cathode compartments of ethanol fuel cells.

Chemically-reduced graphene-oxide-supported gold nanoparticles and traces of iridium are considered here together with loadings of platinum as catalytic materials for reduction of oxygen in alkaline and acid media, respectively. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared by the chemical reduction method, in which the NaBH4-prereduced Keggin-type phosphomolybdate heteropolyblue acts as the reducing agent for the precursor (HAuCl4). Polyoxmetallate (PMo12O40 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 H2SO4) 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) the phosphomolybdate adsorbates are removed from the interface as they undergo dissolution in alkaline medium; and 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 electrode (RDE) voltammetric experiments. 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). When using the silica or titania functionalized reduced graphene oxide as carriers for Au (or Ir) and Pt nanoparticles, the resulting systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with even lower formation of hydrogen peroxide. Furthermore, the durability of this family of catalysts was outstanding. Finally, by doping the reduced graphene oxide supported Pt with traces of Ir (-2), decreases amounts of produced H2O2 (

The present study refers to a novel and unique approach of fabrication and deposition of gold nanoparticles on the surfaces of reduced graphene oxide and multi-walled carbon nanotubes. Among important issues is application of inorganic (rather than organic) capping ligand, heteropolymolybdate, to modify and stabilize (as well as probably also to link with the oxygen or hydroxyl groups on graphene surfaces) gold nanoparticles. During operation in alkaline medium or neutral media, polyoxometallates disappear but catalytically highly active gold remains and it exhibits excellent stability. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of carbon dioxide in neutral medium and oxygen in alkaline solution. What is even more important is that both carbon nanotubes and graphene have occurred to act effectively as carriers for gold nanostructures. Mutual activating interactions are feasible. The conclusions are reached on the basis of diagnostic electrochemical (e.g. rotating ring disk voltammetry), spectroscopic (FTIR) and microscopic (SEM, TEM) experiments. A series of comparative experiments with different carbon carriers and model catalytic materials (e.g. Vulcan-supported platinum) have also been performed. With respect to oxygen reduction, our diagnostic experiments at different concentrations of H2O2, support a view that the effect of the fast following chemical (H2O2-reductive-decomposition) reaction could be the dominating factor in explaining the observed positive potential shift observed during the oxygen reduction. The fact, that the optimum graphene-based catalytic system produced the oxygen reduction peak current comparable to that observed at the model platinum containing catalyst, would imply the efficient four-electron-type reduction mechanism. Graphene and other distinct carbon nanostructures are explored as supports (carriers) for dispersed metallic silver nanocenters which exhibit electrocatalytic activity toward reduction of oxygen in alkaline media. To facilitate fabrication, immobilization and distribution of Ag(0) nanoparticles, various types of graphene (e.g. reduced graphene oxide, RGO, and CFx graphene with carbon black) have been modified with the silver analogue of polynuclear Prussian Blue, namely with ultra-thin silver(I) hexacyanoferrate(II,III) layers. Following the heat-treatment step at temperatures as high as 400-600oC, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic silver sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in KOH electrolyte. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined or intermixed with Vulcan XC-72R nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased.

The design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures especially graphene/graphene oxide materials with superior structural and functional properties for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. To facilitate fabrication, immobilization and distribution of non-precious metal nanoparticles, various types of graphene (e.g. reduced graphene oxide, RGO, and CFx graphene with carbon black) have been modified with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue, namely with ultra-thin Co, and Ni hexacyanoferrate layers. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in KOH electrolyte. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined or intermixed with Vulcan XC-72R nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased. An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles modified graphene and graphene related materials, is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

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