Nanostructured Niobium and Titanium Carbonitrides as Electrocatalyst Supports.

Nanostructured niobium-titanium carbonitrides, (Nb,Ti)C1-xNx, with the cubic-rock salt structure are prepared without the use of reactive gases via thermal treatment (700-1200 °C) under nitrogen of mixtures of guanidine carbonate and ammonium niobate (V) oxalate hydrate, with addition of ammonium titanyl oxalate monohydrate as a titanium source. The bulk structure and chemical composition of the materials are characterized using powder X-ray diffraction (XRD) and powder neutron diffraction, elemental homogeneity is studied using energy dispersive spectroscopy (EDS) mapping using transmission electron microscopy (TEM), and surface chemical analysis is examined using X-ray photoelectron spectroscopy (XPS). Nanoscale crystallites of between 10 and 50 nm are observed by TEM, where EDS reveals the homogeneity of metal distribution for the mixed-metal materials. Titanium carbonitrides are found to be air sensitive, reacting with air under ambient conditions, while titanium-niobium carbonitrides are found to degrade in aqueous sulfuric acid. The niobium carbonitrides, however, show some stability toward acidic solutions. Materials are produced with composition NbC1-xNx with x between 0.35 and 0.45, and more carbon-rich materials (x ≈ 0.35) are found as the synthesis temperature is increased, as proven by Rietveld refinement of crystal structure against powder neutron diffraction data. Despite phase purity seen by diffraction and negligible bulk carbon content, XPS shows a complex surface chemistry for the NbC1-xNx materials, with evidence for Nb2O5-like oxide species in a carbon-rich environment. The NbC1-xNx prepared at 900 °C has a surface area around 50 m2 g-1, making it suitable as a catalyst support. Loading with iridium provides a material active for the oxygen evolution reaction in 0.1 M sulfuric acid, with minimal leaching of either Nb or Ir after 1000 cycles.

Importance of Chemical Activation and the Effect of Low Operation Voltage on the Performance of Pt-Alloy Fuel Cell Electrocatalysts.

Pt-alloy (Pt-M) nanoparticles (NPs) with less-expensive 3d transition metals (M = Ni, Cu, Co) supported on high-surface-area carbon supports are currently the state-of-the-art (SoA) solution to reach the production phase in proton exchange membrane fuel cells (PEMFCs). However, while Pt-M electrocatalysts show promise in terms of increased activity for oxygen reduction reaction (ORR) and, thus, cost reductions from the significantly lower use of expensive and rare Pt, key challenges in terms of synthesis, activation, and stability remain to unlock their true potential. This work systematically tackles them with a combination of electrocatalyst synthesis and characterization methodologies including thin-film rotating disc electrodes (TF-RDEs), an electrochemical flow cell linked to an inductively coupled plasma mass spectrometer (EFC-ICP-MS), and testing in 50 cm2 membrane electrode assemblies (MEAs). In the first part of the present work, we highlight the crucial importance of the chemical activation (dealloying) step on the performance of Pt-M electrocatalysts in the MEA at high current densities (HCDs). In addition, we provide the scientific community with a preliminary and facile method of distinguishing between a "poorly" and "adequately" dealloyed (activated) Pt-alloy electrocatalyst using a much simpler and affordable TF-RDE methodology using the well-known CO-stripping process. Since the transition-metal cations can also be introduced in a PEMFC due to the degradation of the Pt-M NPs, the second part of the work focuses on presenting clear evidence on the direct impact of the lower voltage limit (LVL) on the stability of Pt-M electrocatalysts. The data suggests that in addition to intrinsic improvements in stability, significant improvements in the PEMFC lifetime can also be obtained via the correct MEA design and applied limits of operation, namely, restricting not just the upper but equally important also the lower operation voltage.

Dense Pt Nanowire Electrocatalyst for Improved Fuel Cell Performance Using a Graphitic Carbon Nitride-Decorated Hierarchical Nanocarbon Support.

An innovative strategy is presented to engineer supported-Pt nanowire (NW) electrocatalysts with a high Pt content for the cathode of hydrogen fuel cells. This involves deposition of graphitic carbon nitride (g-CN) onto 3D multimodal porous carbon (MPC) (denoted as g-CN@MPC) and using the g-CN@MPC as an electrocatalyst support. The protective coating of g-CN on the MPC provides good stability for the electrocatalyst support against electrochemical oxidation, and also enhances oxygen adsorption and provides additional active sites for the oxygen reduction reaction. Compared with commercial carbon black Vulcan XC-72R (denoted as VC) support material, the larger hydrophobic surface area of the g-CN@MPC enables the supported high-content Pt NWs to disperse uniformly on the support. In addition, the unique 3D interconnected pore networks facilitate improved mass transport within the g-CN@MPC support material. As a result, the g-CN@MPC-supported high-content Pt catalysts show improved performance with respect to their counterparts, namely, MPC, VC, and g-CN@VC-supported Pt NW catalysts and the conventional Pt nanoparticle (NP) catalyst (i.e., Pt(20 wt%)NPs/VC (Johnson Matthey)) used as the benchmark. More importantly, the g-CN-tailored high-content Pt NW (≈60 wt%) electrocatalyst demonstrates high PEM fuel cell power/performance at a very low cathode catalyst loading (≈0.1 mgPt  cm-2 ).

Electrocatalyst Performance at the Gas/Electrolyte Interface under High-Mass-Transport Conditions: Optimization of the "Floating Electrode" Method.

The thin-film rotating disk electrode (TF-RDE) is a well-developed, conventional ex situ electrochemical method that is limited by poor mass transport in the dissolved phase and hence can only measure the kinetic response for Pt-based catalysts in a narrow overpotential range. Thus, the applicability of TF-RDE results in assessing how catalysts perform in fuel cells has been questioned. To address this problem, we use the floating electrode (FE) technique, which can facilitate high-mass transport to a catalyst layer composed of an ultralow loading of catalyst (1-15 µgPt cmgeo-2) at the gas/electrolyte interface. In this paper, the aspects that have critical effects on the performance of the FE system are measured and parametrized. We find that, in order to obtain reproducible results with high performance, the following factors need to be taken into account: system cleanliness, break-in procedure, hydrophobic agent, ionomer type, and the measurements of catalyst surface area and loading. For some of these parameters, we examined a range of different approaches/materials and determined the optimum configuration. We find that the gas permeability of the hydrophobic agent is an important factor for improving the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) performance. We provide evidence that the suppression of the HOR and ORR introduced by the Nafion ionomers is more than a local mass transport barrier but that a mechanism involving the adsorption of the sulfonate on Pt also plays a significant role. The work provides intriguing insights into how to manufacture and optimize electrocatalyst systems that must function at the gas/electrolyte interface.

In situ XAFS of acid-resilient iridate pyrochlore oxygen evolution electrocatalysts under operating conditions.

Pyrochlore iridates (Na,Ca)2-xIr2O6·H2O are acid-stable electrocatalysts that are candidates for use in electrolysers and fuel cells. Ir LIII-edge X-ray absorption fine structure spectroscopy in 1 M H2SO4 at oxygen evolution conditions suggests the involvement of the electrons from the conduction band of the metallic particles, rather than just surface iridium reacting.

Structures of mixed manganese ruthenium oxides (Mn1-xRux)O2 crystallised under acidic hydrothermal conditions.

A synthesis method for the preparation of mixed manganese-ruthenium oxides is presented along with a detailed characterisation of the solids produced. The use of 1 M aqueous sulfuric acid mediates the redox reaction between KRuO4, KMnO4 and Mn2+ to form ternary oxides. At reaction temperature of 100 °C the products are mixtures of α-MnO2 (hollandite-type) and ß-MnO2 (rutile-type), with some evidence of Ru incorporation in each from their expanded unit cell volumes. At reaction temperature of 200 °C solid-solutions ß-Mn1-xRuxO2 are formed and materials with x ≤ 0.6 have been studied. The amount of Ru included in the oxide is greater than expected from the ratio of metals used in the synthesis, as determined by elemental analysis, implying that some Mn remains unreacted in solution. Powder X-ray diffraction (XRD) shows that while the unit cell volume expands in a linear manner, following Vegard's law, the tetragonal lattice parameters, and the a/c ratio, do not follow the extrapolated trends: this anisotropic behaviour is consistent with the different local coordination of the metals in the end members. Powder XRD patterns show increased peak broadening with increasing ruthenium content, which is corroborated by electron microscopy that shows nanocrystalline material. X-ray absorption near-edge spectra show that the average oxidation state of Mn in the solid solutions is reduced below +4 while that of Ru is increased above +4, suggesting some redistribution of charge. Analysis of the extended X-ray absorption fine structure provides complementary local structural information, confirming the formation of a solid solution, while X-ray photoelectron spectroscopy shows that the surface oxidation states of both Ru and Mn are on average lower than +4, suggesting a disordered surface layer may be present in the materials.

Invariance of experimental observables with respect to coarse-graining in standard and many-body dissipative particle dynamics.

Dissipative particle dynamics (DPD) is a well-established mesoscale simulation method. However, there have been long-standing ambiguities regarding the dependence of its (purely repulsive) force field parameter on temperature as well as the variation of the resulting experimental observables, such as diffusivity or surface tension, with coarse-graining (CG) degree. Here, we rederive the temperature dependence of DPD interaction parameter and revisit the role of the CG degree in standard DPD simulations. Consequently, we derive a scaling of the input variables that renders the system properties invariant with respect to CG degree and illustrate the versatility of the method by computing the surface tensions of binary solvent mixtures. We then extend this procedure to many-body dissipative particle dynamics and, by computing surface tensions of the same mixtures at a range of CG degrees, demonstrate that this newer method, which has not been widely applied so far, is also capable of simulating complex fluids of practical interest.

Water-splitting electrocatalysis in acid conditions using ruthenate-iridate pyrochlores.

The pyrochlore solid solution (Na(0.33)Ce(0.67))2(Ir(1-x)Ru(x))2O7 (0≤x≤1), containing B-site Ru(IV) and Ir(IV) is prepared by hydrothermal synthesis and used as a catalyst layer for electrochemical oxygen evolution from water at pH<7. The materials have atomically mixed Ru and Ir and their nanocrystalline form allows effective fabrication of electrode coatings with improved charge densities over a typical (Ru,Ir)O2 catalyst. An in situ study of the catalyst layers using XANES spectroscopy at the Ir L(III) and Ru K edges shows that both Ru and Ir participate in redox chemistry at oxygen evolution conditions and that Ru is more active than Ir, being oxidized by almost one oxidation state at maximum applied potential, with no evidence for ruthenate or iridate in +6 or higher oxidation states.

The origin of high activity but low CO(2) selectivity on binary PtSn in the direct ethanol fuel cell.

The most active binary PtSn catalyst for direct ethanol fuel cell applications has been studied at 20 °C and 60 °C, using variable temperature electrochemical in situ FTIR. In comparison with Pt, binary PtSn inhibits ethanol dissociation to CO(a), but promotes partial oxidation to acetaldehyde and acetic acid. Increasing the temperature from 20 °C to 60 °C facilitates both ethanol dissociation to CO(a) and then further oxidation to CO2, leading to an increased selectivity towards CO2; however, acetaldehyde and acetic acid are still the main products. Potential-dependent phase diagrams for surface oxidants of OH(a) formation on Pt(111), Pt(211) and Sn modified Pt(111) and Pt(211) surfaces have been determined using density functional theory (DFT) calculations. It is shown that Sn promotes the formation of OH(a) with a lower onset potential on the Pt(111) surface, whereas an increase in the onset potential is found upon modification of the (211) surface. In addition, Sn inhibits the Pt(211) step edge with respect to ethanol C-C bond breaking compared with that found on the pure Pt, which reduces the formation of CO(a). Sn was also found to facilitate ethanol dehydrogenation and partial oxidation to acetaldehyde and acetic acid which, combined with the more facile OH(a) formation on the Pt(111) surface, gives us a clear understanding of the experimentally determined results. This combined electrochemical in situ FTIR and DFT study provides, for the first time, an insight into the long-term puzzling features of the high activity but low CO2 production found on binary PtSn ethanol fuel cell catalysts.

Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes.

The voltammetry of Pt{111}, Pt{100}, Pt{110} and Pt{311} single crystal electrodes as a function of perchloric acid concentration (0.05-2.00 M) has been studied in order to test the assertion made in recent reports by Watanabe et al. that perchlorate anions specifically adsorb on polycrystalline platinum. Such an assertion would have significant ramifications for our understanding of electrocatalytic processes at platinum surfaces since perchlorate anions at low pH have classically been assumed not to specifically adsorb. For Pt{111}, it is found that OHad and electrochemical oxide states are both perturbed significantly as perchloric acid concentration is increased. We suggest that this is due to specific adsorption of perchlorate anions competing with OHad for adsorption sites. The hydrogen underpotential deposition (H UPD) region of Pt{111} however remains unchanged although evidence for perchlorate anion decomposition to chloride on Pt{111} is reported. In contrast, for Pt{100} no variation in the onset of electrochemical oxide formation is found nor any shift in the potential of the OHad state which normally results from the action of specifically adsorbing anions. This suggests that perchlorate anions are non-specifically adsorbed on this plane although strong changes in all H UPD states are observed as perchloric acid concentration is increased. This manifests itself as a redistribution of charge from the H UPD state situated at more positive potential to the one at more negative potential. For Pt{110} and Pt{311}, marginal changes in the onset of electrochemical oxide formation are recorded, associated with specific adsorption of perchlorate. Specific adsorption of perchlorate anions on Pt{111} is deleterious to electrocatalytic activity in relation to the oxygen reduction reaction (ORR) as measured using a rotating disc electrode (RDE) in a hanging meniscus configuration. This study supports previous work suggesting that a large component of the ORR activity on platinum is governed by simple site blocking by specifically adsorbed anions and/or electrosorbed oxide.