Effects of poly-1,5-diaminoanthraquinone morphology on oxygen reduction in acidic solution

The poly-1,5-diaminoanthraquinone (P15DAAQ) modified Pt electrodes show electrocatalytic activity for oxygen reduction reaction (ORR) with oxygen reduction peak at about 0.39 V in 0.1 M H₂SO₄. The P15DAAQ with different thickness has different morphology. The effects of morphologies on the electrocatalytic behaviors of P15DAAQ for oxygen reduction reaction are investigated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) measurements. We propose two different O₂-transport processes on electrodes modified with thin P15DAAQ and thick P15DAAQ. Together with the quantitative analysis with O₂-transport dynamics, electron-transfer resistance, and catalytic reaction rate during ORR, thin P15DAAQ electrode performs better electrocatalysis for ORR, although thick P15DAAQ provides higher real surface area and more reactive sites which is beneficial for ORR within a short time.     

One-step chemical reduction of graphene oxide with oligothiophene for improved electrocatalytic oxygen reduction reactions

Oligothiophene (nTP, n = 1, 2, 3) has been used as the reductant for the first time in the preparation of graphene by the reduction of graphene oxide (GO). A simple single-step chemical approach has been developed to reduce and/or functionalize GO with nTP. The reaction takes place at room temperature under stirring of a suspension of GO and nTP in MeCN. The nTP has been grafted onto the surface of GO by reacting epoxy groups together with the reduced graphene oxide (rGO). It was observed that increasing the thiophene ring (hereafter, thiophene is referred to as TP; 2,2′ bithiophene as 2TP; and 2,2′:5′,2″ terthiophene as 3TP) can enhance the reduction reaction. All instrumental experiments have confirmed that nTP not only covalently bonded to the GO but also partly restored the conjugate structure of GO, as a reducing agent. The resultant rGO with 3TP (rGO3TP) has been demonstrated to show remarkable electrocatalytic activity toward oxygen reduction reaction (ORR) compared to typical rGO. The observed ORR electrocatalytic activity induced by the intermolecular charge-transfer provides a general approach to various carbon-based metal-free ORR catalysts.     

Impact of framework structure of ordered mesoporous carbons on the performance of supported Pt catalysts for oxygen reduction reaction

Ordered mesoporous carbons (OMCs) are investigated as support materials for Pt catalysts for oxygen reduction reaction (ORR). Three types of OMCs (CMK-3, CMK-3G, and CMK-5) are prepared by a nanocasting method using ordered mesoporous silica, SBA-15, as a template. These OMCs with the same hexagonal mesostructure have different carbon frameworks and graphiticity, which can affect their surface area and microporosity. Pt nanoparticles with an average size of 1 nm are uniformly supported on the three OMCs and Ketjenblack® and their electrochemical performance and durability are evaluated. Pt/CMK-3G exhibits the highest electrochemically active surface area, kinetic current density, mass activity, and half-wave potential, whereas Pt/CMK-3 shows the lowest values. Pt/CMK-3G also shows the highest ORR activity after an accelerated durability test, with a minimal shift in half-wave potential. The higher ORR activity of Pt/CMK-3G is attributed to the formation of highly crystalline Pt particles as well as its highly graphitic, crystalline carbon structure, which causes the weak adsorption of surface oxide and a strong interaction between the Pt particles and the support. Moreover, we can establish that the mass activity of the catalysts is nearly inversely proportional to the micropore volume of the carbon supports.     

Oxygen electroreduction on Ag(1 1 1): The pH effect

The rotating ring disk method (RRDE) is applied to investigate the pH effect on oxygen reduction reaction (ORR) on Ag(1 1 1) single crystal surface in 0.1 M KOH and 0.1 M HClO₄. In 0.1 M KOH, the ORR proceeds through 4e⁻ reaction pathway with a very small (0.5-2.5%) peroxide formation in the entire potential range. In 0.1 M HClO₄ the onset potential for the ORR is shifted for ca. 400 mV toward the higher overpotentials compared to the 0.1 M KOH solution. At the low overpotentials, in 0.1 M HClO₄ the ORR proceeds entirely as a 2e⁻ process, i.e, 100% H₂O₂ formation. At higher overpotentials, the initial mixed a 2e⁻ and 4e⁻ reduction is followed by the potential region where the ORR proceeds entirely as a 4e⁻ process, with H₂O formation as a final product. The pH dependent shift in the onset of the ORR as well as the reaction pathway has been explained based on both: a thermodynamic analysis of pH independent rate determining step, and on the pH dependent change in availability of surface active sites and adsorption energies of molecular oxygen and reaction intermediates.     

Enhanced electrocatalytic activity due to additional phosphorous doping in nitrogen and sulfur-doped graphene: A comprehensive study

Effect on oxygen reduction reaction (ORR) of ternary-doped reduced graphene oxide (RGO) as an electrocatalyst is evaluated by employing thiourea as a single source of nitrogen (N) and sulfur (S), and triphenylphosphine for phosphorous (P) as precursors for heteroatom doping. The topographical studies show that by doping the RGO, disruption in surface charge and spin asymmetry is introduced into the carbon matrix due to the difference in the bond length and electronegativity between carbon and heteroatoms, which makes carbon lattice ORR active. Ternary (N, S and P)-doped RGO shows excellent ORR activity, which is ∼2 times better than that of binary (N and S)-doped RGO, and ∼5 times better than that of single (P)-doped RGO. The catalytic activity of the ternary-doped carbon even exceeds the commercial Pt in alkaline medium. Additional P doping causes remarkable synergistic effect on binary N and S-doped RGO by generating active P–N species, improving graphitic order and increasing surface area as well as mesopore volume, which in turn enhances the ORR activity.     

Methanol Tolerance of CN x Oxygen Reduction Catalysts

Nitrogen-containing carbon nanofiber (CN _x ) catalysts grown by acetonitrile pyrolysis over 2 wt% Fe, Co, and Ni impregnated supports of alumina, silica and magnesia were tested for methanol tolerance during the oxygen reduction reaction (ORR) by rotating ring disk electrode (RRDE) method. Catalysts tested in acidic electrolytes containing 1–3 M methanol showed no methanol poisoning during ORR and were inactive for methanol oxidation.     

Oxidation Mechanisms and Kinetics of 1D-SiC/C/SiC Composite Materials: I, An Experimental Approach

The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 μm) treated at T > 1000°C the pores are rapidly scaled by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self-healing character. Conversely, long exposures (300 h) at 900°C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO₂ conversion, fly, the self-healing character is not observed in composites with a thick interphase (e.g., 1 μm) since carbon is totally consumed before silica can seal the pores.     

Modification of SO4 2--ZrO2 and Pt/SO4 2--ZrO2 properties during n-hexane isomerization

The modification of the textural properties and crystalline structure of SO₄ ^2--ZrO₂ and Pt/SO₄ ^2--ZrO₂ during n-hexane reaction at 473 K and 6 kg cm^-2 has been studied in the presence of either hydrogen or nitrogen. Sulfur content before and after reaction and the amount of coke at the end of the reaction were measured. The coke deposited on the catalysts blocks the pores of small size and decreases the surface area of the used catalysts. After regeneration, surface area is not completely restored. The loss of sulfur during reaction, probably associated to the reaction medium, also produces a decrease in surface area by the collapse of the smallest pores which generates larger ones. The transformation of tetragonal to monoclinic crystalline structure of zirconia begins to occur when sulfur content drops below a critical value. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling of the desulfation process in NOx storage and reduction catalysts

One of the major challenges for the NO_x Storage and Reduction Catalysts technology used in automotive exhaust remains the sulfur susceptibility, which calls for efficient desulfation strategies. The sulfation and desulfation processes are systematically studied via measurements and mathematical modeling of the physicochemical processes. The role of oxygen storage which influences the reducing agents availability for desulfation is explained and a respective reaction model is presented. The bulk oxygen storage component appears to be involved in sulfur storage, which further emphasizes the importance of oxygen–sulfur storage interactions. Next, the observed release of sulfur species under lean mode is discussed along with a proposed reaction mechanism which involves SO₂ formation via O₂ reaction with elemental sulfur on the surface. The parameters of the complex reaction model are calibrated in order to reproduce the observed trends at least in a qualitative manner. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2117–2127, 2017     

Highly efficient metal-free phosphorus-doped platelet ordered mesoporous carbon for electrocatalytic oxygen reduction

Platinum-free electrocatalysts especially, various heteroatom-doped carbon nanostructures have attracted particular attraction as plausible solution for commercializing fuel cell technology. In this direction, novel phosphorus-doped platelet ordered mesoporous carbon (P-pOMC) is developed for the first time as metal-free electrocatalyst for alkaline oxygen reduction reaction. The P-pOMC is synthesized by nanocasting method using platelet ordered mesoporous silica as template. Various characterizations reveal that the P-pOMC materials have covalently bound P atoms with carbon framework for facilitation of oxygen reduction reaction (ORR) and also have very high surface area with uniform distribution of short mesoporous channels for unhindered mass transfer. Combination of P doping and excellent surface properties empowers the newly-developed P-pOMC catalyst to show high ORR activity nearly equal to that of state of the art Pt catalyst along with superior long-term stability and excellent methanol tolerance.     

Synthesis, characterization and electrocatalytic behaviour of non-alloyed PtCr methanol tolerant nanoelectrocatalysts for the oxygen reduction reaction (ORR)

In order to point out the effect of the second metal in platinum-based catalysts, a synthesis method by colloidal route derived from that of Bönnemann was used to prepare non-alloyed Pt_1−xCr_x/C electrocatalysts active towards the oxygen reduction reaction (ORR). The non-alloyed character of the catalysts was showed by XRD analysis. The Pt/Cr electrocatalyst having an nominal atomic ratio, as determined by EDX then corresponding to bulk composition and not surface composition, close to (0.8:0.2) showed higher activity for ORR in methanol-free oxygen saturated electrolyte, whereas the catalyst having an atomic ratio of (0.7:0.3) displayed higher activity for ORR at low overpotentials in saturated oxygen electrolyte containing 0.1 M methanol.Correlation of XRD and electrochemical results allows us to point out the effect of electronic interactions in catalyst activity towards ORR. It was also shown that adding chromium to platinum does not alter the reaction mechanism of oxygen reduction, and that in presence of low methanol concentration, the ORR occurs via the four-electron process according to the same mechanism as in methanol-free solution.     

Synthesis of Pt Nanowires Inside Aerosol Derived Spherical Mesoporous Silica Particles

Spherical mesoporous silica particles prepared by evaporation induced self assembly (EISA) were used as templates to form Pt nanowires. Transmission electron microscope (TEM) images of these aerosol-derived silica particles reveal hexagonally ordered pores coiled within each particle, with no obvious termination of the pores on the external surface. Near the particle surface the pores are seen to run parallel to the surface, consistent with the external constraint of spherical geometry. For MCM-41 type mesoporous materials, the pores are straight and accessible at either end for pore filling, but for spherical silica particles prepared by EISA, the pores are not open to the external surface. Hence it is remarkable that Pt nanowires can be formed within the closed pores inside these spherical silica particles, where conventional mechanisms of pore filling would not be expected to be operative. These results suggest that the silica walls in these mesoporous silica allow transport of volatile Pt complexes during wet reduction in H₂. The permeability to gases makes these spherical silica particles especially suitable for gas phase catalytic reactions, while at the same time confining metallic particles within the silica pores.     

Poly(methyl methacrylate) nanocomposites based on graphene oxide: a comparative investigation of the effects of surface chemistry on properties and foaming behavior

The effects of oxygen functional groups and alkyl chains at the surface of graphene oxide (GO) on the thermal stability, mechanical properties and foaming behavior of poly(methyl methacrylate) (PMMA) nanocomposites were investigated. Alkyl‐functionalized GO (GO‐ODA) was prepared by grafting octadecylamine (ODA) on the surface of GO. PMMA/GO and PMMA/GO‐ODA nanocomposite were obtained by solution blending and were foamed using supercritical carbon dioxide (scCO₂). GO‐ODA, with the presence of alkyl chains, showed a better dispersion capability in PMMA matrix than GO with a large amount of oxygen functional groups. In addition, the good dispersion capability increased thermal stability and mechanical strength. In comparison with PMMA/GO samples foamed at 70 °C, PMMA/GO‐ODA nanocomposite foams displayed improved cell structures with higher cell density, smaller cell size and more homogeneous cell size distribution, which results from the strong heterogeneous nucleation due to alkyl chains on the GO surface. The foaming behaviors became more complicated at 80 °C as the GO might be intercalated and exfoliated with the aid of scCO₂, thus further enhancing the heterogeneous nucleation during the foaming process. The results indicated that the surface chemistry of GO was closely related to the properties and foaming behavior of the nanocomposites. © 2016 Society of Chemical Industry     

Activated carbons with metal containing bentonite binders as adsorbents of hydrogen sulfide

Wood-based activated carbon was ground and mixed with 10% bentonite binders containing either iron, zinc or copper cations adsorbed within the interlayer space and/or on the external surface of bentonite flakes. To better understand the role of transition metals, carbon was also impregnated with iron, zinc and copper salts. The structure of materials after modification was determined using nitrogen adsorption. The modification resulted in a decrease in porosity, especially in micropore volume, as a result of combined mass dilution effect and adsorption/re-adsorption of metals in small pores. Introduction of bentonite binders containing adsorbed metal increased the capacity of carbon for hydrogen sulfide only in the case of material containing copper. Copper also significantly increases the performance of carbon as an H₂S adsorbent when impregnation is applied whereas the effects of other metals used in this study are much less pronounced. It is likely that copper present in the small pores acts as a catalyst for oxygen activation causing hydrogen sulfide oxidation. As a result of this process, elemental sulfur is formed which, when present in small pores, is oxidized to weakly adsorbed SO₂. The SO₂ is removed from the surface when continuous reaction with hydrogen sulfide occurs. Thus, even though binding carbon with spent bentonites after copper adsorption increases the capacity of carbon toward H₂S removal, the formation of SO₂, another undesirable pollutant, does detract somewhat from the procedure.     

Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part II. O2 reduction

In order to mimic the operation of the air-cathode in a direct borohydride alkaline fuel cell, we studied the oxygen reduction reaction (ORR) in sodium hydroxide solution containing traces of borohydride. The activity of several ORR electrocatalysts, namely carbon-supported platinum, gold, silver and manganese oxide, has been investigated using slow-scan linear voltammetry. Whereas platinum is one of the best electrocatalyst in pure sodium hydroxide, none of the classical electrocatalysts: gold, silver and platinum, exhibit sufficient selectivity towards the ORR. When BH₄⁻ is present in solution, the potential taken by electrodes using such materials is a mixed potential, following the competition between the ORR and the NaBH₄ hydrolysis and/or oxidation. Conversely, manganese oxide-based electrocatalysts exhibit very interesting behaviour towards the ORR in alkaline medium; while their intrinsic ORR activity in pure sodium hydroxide is quite as good as that for platinum, they still display a remarkable selectivity for this reaction when the electrolyte contains traces of sodium borohydride.As a result, carbon-supported manganese oxide-based nanoparticles seem very interesting materials to be used in direct borohydride fuel cell.     

Surface layer of Pt-O-Ce bonds on CeOx nanowire with high ORR activity converted by proton beam irradiation

To develop the state-of-the-art polymer membrane fuel cells. Both maximization of oxygen reduction reaction (ORR) activity on Pt cathode and minimization of Pt content in the cathode are required. For this challenge, the defect interface on oxide support was modified by proton beam irradiation method. Pt-CeO_x nanowire/C (Pt/C = 0.02) was fabricated using the proton beam irradiation method. Since the radical density generated by proton beam irradiation is two orders of magnitude greater than that of electron beam irradiation, the CeO_x nanowire surface was fully converted to a thin layer of Pt-O-Ce bonds under proton beam irradiation. The ORR activity observed for fabricated sample with above active surface layer was higher than that of conventional Pt/C (Pt/C = 0.2) and comparable to that of Pt-CeO_x nanowire/C (Pt/C = 0.2) fabricated by conventional methods. From the combination of microanalysis characterization and surface atomistic simulation, we concluded that the Pt-O-Ce bond was formed on defect-rich regions of the CeO_x nanowire and this leads to a maximized ORR activity on the fabricated sample. Based on all experimental data, it is concluded that the surface modification of CeO_x nanowire support using proton beam irradiation is useful for a lowering the Pt content of the cathode with high ORR activity.     

Combined in situ EXAFS and electrochemical investigation of the oxygen reduction reaction on unmodified and Se-modified Ru/C

In this work we study the kinetics of the oxygen reduction reaction on carbon-supported Ru nanoparticles modified with various amounts of Se. Rotating disk electrode is used to determine kinetic currents for the ORR in 0.1 M H₂SO₄ at 298 K and O₂ partial pressures from 1 to 0.01 atm. The dependence of the ORR activity on Se/Ru ratio shows volcano-type behavior with ca. 10 fold increase of the mass activity at 0.1 < Se/Ru < 0.3. The reaction order in O₂ is close to 1 in the interval of overpotentials from 0.4 to 0.7 V, and is independent of the presence of Se. Regardless the amount of Se, the Tafel slope demonstrates continuous increase from ca. 70 mV/dec at 0.4 V to ca. 140 mV/dec at 0.6 V overpotential. In situ EXAFS spectra are measured at Ru K-edge (in the transmission mode) and Se K-edge (in the fluorescence mode) in argon and oxygen saturated 0.1 M H₂SO₄ solutions in the interval of electrode potentials from 0.050 to 0.750 V RHE. The data are used to explore the surface state changes of Ru and Ru_xSe_y particles and clarify the promoting role of Se during the ORR.     

In situ preparation of inorganic nanoparticles in amino-functionalized porous cellulose particles

Amino-functionalized cellulose particles with a porous structure were prepared by a solvent-releasing method with 1-butyl-3-methylimdazolium chloride ([Bmim]Cl) followed by an amination reaction, and were incorporated with silica. The amino groups provided the base catalyst for the formation of silica on the porous structure during the in situ sol–gel reaction of tetraethyl orthosilicate. After the reaction, no free silica remained in the reaction medium. The microscopic morphologies (specific surface area, pore diameter, and pore volume) of the composite particles were affected by the silica content in the composites, which was controllable by the sol–gel reaction time. When silver nanoparticles were introduced to the pores of the cellulose particles by reduction of AgNO₃ with amino groups, the composite exhibited catalytic ability and recyclability.     

Carbon-supported manganese oxide nanoparticles as electrocatalysts for oxygen reduction reaction (orr) in neutral solution

Manganese oxides (MnO _x ) catalysts were chemically deposited onto various high specific surface area carbons. The MnO _x /C electrocatalysts were characterised using a rotating disk electrode and found to be promising as alternative, non-platinised, catalysts for the oxygen reduction reaction (ORR) in neutral pH solution. As such they were considered suitable as cathode materials for microbial fuel cells (MFCs). Metal [Ni, Mg] ion doped MnO _x /C, exhibited greater activity towards the ORR than the un-doped MnO _x /C. Divalent metals favour oxygen bond splitting and thus orientate the ORR mechanism towards the 4-electron reduction, yielding less peroxide as an intermediate.     

Measurement of oxygen reduction activities via the rotating disc electrode method: From Pt model surfaces to carbon-supported high surface area catalysts

The aim of this report is to scrutinize the thin-film rotating disc electrode (TF-RDE) method for investigating the electrocatalytic activity of high surface area catalysts. Special emphasis is given to the oxygen reduction reaction (ORR) on carbon-supported platinum catalysts. On the basis of measurements on four different Pt catalyst samples with various average particle sizes, it is demonstrated in detail how the intrinsic properties of the catalyst, i.e., the mass activity (A/g_Pt) and the specific activity (A/m²_Pt), are evaluated. The potential sources of error are critically discussed and guidelines for the measurements are given. Furthermore, the specific ORR activities determined for the different catalyst samples are analyzed and compared to polycrystalline Pt. The previously reported effect of the particle size on the specific activity for the ORR is interpreted on the basis of the shift in the potential of zero total charge and the concomitant alteration of the adsorption properties.