Hydrogen Electrooxidation Research Articles

New insights into sulfur poisoning behavior of Ni-YSZ anode from long-term operation of anode-supported SOFCs

The performance characteristics of high-performance Ni-YSZ (yttria-stabilized zirconia) anode-supported solid oxide fuel cells (SOFCs) are examined under typical fuel cell operating conditions over a period of ∼3000 h when hydrogen contaminated with different concentrations of H2S was used as the fuel. Analyses reveal some new insights into the sulfur poisoning behavior of a Ni-YSZ anode. First, an initial drop in power output upon exposure to H2S-contaminated fuels is more dramatic in an anode-supported cell (with small cell resistance) than in electrolyte-supported cell (with large cell resistance), creating an illusion that a high-performance cell appears to be less sulfur tolerant than a low-performance cell. Second, the transition time for this rapid performance drop to cease is much longer for an anode-supported cell (with a thicker anode) than an electrolyte-supported cell (with a thinner anode). Third, the degree of sulfur poisoning (or the cell resistance increase due to sulfur poisoning) diminishes with operating cell current density, suggesting that the water produced at the active sites on anode surface from electro-oxidation of hydrogen (or oxygen ions) may promote the oxidation and removal of adsorbed sulfur. This effect becomes less pronounced at higher concentration of H2S, but still significant at ∼1 ppm of H2S. Fourth, the subsequent slow degradation in performance after the rapid performance drop upon initial exposure to H2S can be avoided, indicating that the previously reported slow degradation is unlikely the inherent behavior of a Ni-YSZ anode, but associated with other complications. This further implies that sulfur poisoning may be reversible and the performance drop could be fully recovered when the fuel is switched back to clean hydrogen.

Peculiarities of hydrogen-oxygen mixture ionization at platinum electrodes

Polarization curves at platinum electrodes are measured in acid and alkaline solutions saturated with hydrogen, oxygen, and their mixture, under the condition of the controlled solution agitation. It is shown that the current of the gaseous mixture ionization differs markedly of that expected according to the principle of the currents’ additivity. The non-fulfillment of the currents’ additivity is observed even in the region of the limiting currents of hydrogen electrooxidation and oxygen electroreduction measured in the gases’ individual solutions. Possible reasons for the described behavior of the oxygen-hydrogen mixture at platinum electrode are analyzed. It was concluded that the oxygen reduction is hindered when hydrogen is present in the solution.

Effect of pore diameter of wormholelike mesoporous carbon supports on the activity of Pt nanoparticles towards hydrogen electrooxidation

Pt nanoparticles are successfully deposited on wormholelike mesoporous carbons (WMCs) using a pulse microwave-assisted polyol method. WMCs with two different pore diameters are used. The particle size of Pt on both supports is identical, about 3 nm. It has been found that Pt utilization efficiency is very low when the pore diameter of WMCs ( D p) is equal to the diameter of Pt nanoparticles ( D Pt). However, in the case that D p is more than twice D Pt, the electrochemical surface area and Pt utilization efficiency are dramatically enhanced, and in turn, hydrogen electrooxidation activity is greatly improved.

Electrocatalytic Activity and CO Tolerance Properties of Mesostructured Pt/WO3 Composite as an Anode Catalyst for PEMFCs

A mesostructured Pt/WO3 electrochemical catalyst has been prepared by a loading small amount of Pt (7.5 wt %) on mesoporous tungsten oxide, which was synthesized by a simple one-step casting method using mesoporous silica (KIT-6) as a hard template. The resultant mesostructured Pt/WO3 catalyst showed high electrocatalytic activity for hydrogen electrooxidation, and the mass activity (mA·mg-Pt−1, per mass of Pt) of it for hydrogen electrooxidation was more than three times of that of the commercial 20 wt % Pt/C catalyst (E-TEK). In addition, it exhibited much improved resistance to CO poisoning relative to the 20 wt % Pt/C catalyst. Since the catalyst is also stable in an electrochemical environment, it could serve as an alternative electrocatalyst for proton exchange membrane fuel cells with high electrocatalytic activity and CO tolerance.

Underpotential deposition of hydrogen on Pt(111): a combined direct molecular dynamics/density functional theory study

A combined direct molecular dynamics/density functional theoretical study of the electro-oxidation of molecular hydrogen at the Pt(111)/water interface has been carried out to provide insights into the adsorbed underpotential deposition (UPD) states of hydrogen at submonolayer coverages of bridging hydrogen atoms Hads(bridge). As in our previous study, H2 oxidation proceeds via the Heyrovsky process, forming a hydrated H+ ion and an UPD hydrogen atom, which is strongly adsorbed and is lying nearly flat in a bridging position on the Pt(111) surface. At submonolayer coverage, the UPD Hads(bridge) atoms spontaneously self-assemble to form a hexagonal 2D honeycomb network on the Pt(111) surface, a signature that could be considered to be characteristic of UPD states of hydrogen.

A First-Principles Simulation of the Electrochemical Interface: Electrooxidation of Molecular Hydrogen and UPD States of Hydrogen at Pt-Electrode/Water Interface

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Combined Deconvolution and CNLS Fitting Approach Applied on the Impedance Response of Technical Ni ∕ 8YSZ Cermet Electrodes

A high-resolution impedance study of the hydrogen oxidation in (yttria-stabilized zirconia) cermet anodes has been realized in consideration of a broad range of operating conditions (temperature and partial pressure of fuel gas components , , , He). A major problem in this respect concerns the origin and physical interpretation of empirical equivalent circuits used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: (i) at first, by the deconvolution of a distribution function of relaxation times (DRT), four different processes and their characteristic relaxation times have been identified. Two processes at frequencies represent a gas-conversion process or, respectively, a gas diffusion, whereas two processes at higher frequencies are associated with the electro-oxidation of hydrogen at active sites, including the charge transfer reaction and the ionic transport. (ii) Subsequently, the last mentioned processes were fitted to a “transmission line” model describing the electronic and ionic transport properties of the cermet. The high resolution of the DRT combined with the numeric accuracy of the complex nonlinear least square (CNLS) fit enabled us to determine (i) the effective ionic conductivity of the cermet, (ii) the spatial extension of the electrochemically active area adjacent to the electrolyte/electrode interface, and (iii) the charge transfer resistance and its thermal activation energy.

Cobalt molybdenum carbides as anode electrocatalyst for proton exchange membrane fuel cell

Cobalt molybdenum (Co-Mo) carbides were prepared by the carburization of Co-Mo oxides at temperatures of 723–973 K in a stream of CH 4/H 2 gas. The carburized catalysts were evaluated using a single-stack fuel cell and three-electrode cell. The results showed high activities for the anodic electrooxidation of hydrogen over the Co-Mo catalysts carburized at 873 and 923 K. The 873 K carburized Co-Mo catalyst had the highest activity and achieved 10.9% of the performance of a commercial Pt/C catalyst in a single-stack fuel cell. The XRD, TPC, TPR and XPS results showed that the Co-Mo oxycarbide in the bulk and on the surface are the active species for the hydrogen oxidation reaction.

Investigation of composite Ni-doped perovskite anode catalyst for electrooxidation of hydrogen in solid oxide fuel cell

Ni-doped perovskite anodes have been investigated for oxidation of hydrogen in IT-SOFC. Perovskite materials are characterized by mixed electronic–ionic conductivity that combined to the high intrinsic catalytic activity of Ni allows to achieve high reaction rates. Suitable power densities of about 420mW/cm2 for the direct oxidation of hydrogen were achieved at 800°C. Temperature programmed reduction analysis carried out in hydrogen reveals that the catalyst at 800°C has totally lost the perovskite structure showing formation of substructures such as NiOOH and NiLa2O4.

Mixed-mode oscillations and cluster patterns in an electrochemical relaxation oscillator under galvanostatic control

We studied the dynamics of a prototypical electrochemical model, the electro-oxidation of hydrogen in the presence of poisons, under galvanostatic conditions. The lumped system exhibits relaxation oscillations, which develop mixed-mode oscillations (MMOs) for low preset currents. A fast-slow analysis of the homogeneous dynamics reveals that the MMOs arise from a fast oscillating subsystem and a one-dimensional slow manifold. In the spatially extended system, the galvanostatic constraint imposes a synchronizing global coupling that drives the system into cluster patterns. The properties of the cluster patterns (CPs) result from an intricate interplay of the nature of the local oscillators, the global constraint, and a nonlocal coupling through the electrolyte. In particular, we find that the global constraint suppresses small-amplitude oscillations of MMOs and prevents domains oscillating out of phase from occupying equal regions in phase space. The nonlocal coupling causes each individual clustered region to oscillate on a different limit cycle. Typically multistability of CPs is found. Coexisting patterns possess different oscillation periods and a different total fraction in space that occupies the in-phase or out-of-phase state, respectively.

Pt/WC as an anode catalyst for PEMFC: Activity and CO tolerance

A mesoporous tungsten carbide of WC-phase was synthesized by using ammonium meta tungstate as tungsten precursor and resorcinol–formaldehyde polymer as carbon source in the presence of a surfactant. The platinum supported on this material with a low loading (7.5 wt%) served as an effective CO tolerant electro anode catalyst. The Pt/WC catalyst showed two times higher activity per mass of Pt for hydrogen electro-oxidation compared to a commercial Pt/C catalyst (E-Teck). In addition, it exhibited much improved resistance to CO poisoning relative to the Pt/C catalyst. Since the catalyst is also stable in electrochemical environment, it could become an alternative anode catalyst for PEMFC.

On the enhanced electrocatalytic activity of Pd overlayers on carbon-supported gold particles in hydrogen electrooxidation

Palladium-gold particles with varied composition were prepared by Pd electrochemical deposition on Au nanoparticles immobilized on model carbon support. Pd-Au/C catalysts were characterized ex situ by transmission electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy, and in situ, by underpotential deposition of hydrogen and copper adatoms, and CO stripping. Hydrogen oxidation reaction on pristine and CO-poisoned Pd-Au/C particles was studied using rotating disk electrode (RDE) technique. It was found that the decrease of the effective Pd overlayer thickness below ca. two monolayers resulted in a two-fold increase of the exchange current density of the hydrogen oxidation reaction and in significant increase of CO tolerance.

Electrochemical impedance spectroscopy of mixed conductors under a chemical potential gradient: a case study of Pt|SDC|BSCF

The AC impedance response of mixed ionic and electronic conductors (MIECs) exposed to a chemical potential gradient is derived from first principles. In such a system, the chemical potential gradient induces a gradient in the carrier concentration. For the particular system considered, 15% samarium doped ceria (SDC15) with Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) (BSCF) and Pt electrodes, the oxygen vacancy concentration is a constant under the experimental conditions and it is the electron concentration that varies. The resulting equations are mapped to an equivalent circuit that bears some resemblance to recently discussed equivalent circuit models for MIECs under uniform chemical potential conditions, but differs in that active elements, specifically, voltage-controlled current sources, occur. It is shown that from a combination of open circuit voltage measurements and AC impedance spectroscopy, it is possible to use this model to determine the oxygen partial pressure drop that occurs between the gas phase in the electrode chambers and the electrode|electrolyte interface, as well as the interfacial polarization resistance. As discussed in detail, this resistance corresponds to the slope of the interfacial polarization curve. Measurements were carried out at temperatures between 550 and 650 degrees C and oxygen partial pressure at the Pt anode ranging from 10(-29) to 10(-24) atm (attained using H(2)/H(2)O/Ar mixtures), while the cathode was exposed to either synthetic air or neat oxygen. The oxygen partial pressure drop at the anode was typically about five orders of magnitude, whereas that at the cathode was about 0.1 atm for measurements using air. Accordingly, the poor activity of the anode is responsible for a loss in open circuit voltage of about 0.22 V, whereas the cathode is responsible for only about 0.01 V, reflecting the high activity of BSCF for oxygen electro-reduction. The interfacial polarization resistance at the anode displayed dependences on oxygen partial pressure and on temperature that mimic those of the electronic resistivity of SDC15. This behavior is consistent with hydrogen electro-oxidation occurring directly on the ceria surface and electron migration being the rate-limiting step. However, the equivalent resistance implied by the oxygen partial pressure drop across the anode displayed slightly different behavior, possibly indicative of a more complex reaction pathway.

Electronic interactions decreasing the activation barrier for the hydrogen electro-oxidation reaction

A unified model for electrochemical electron transfer reactions which explicitly accounts for the electronic structure of the electrode recently proposed by us is applied to the hydrogen oxidation reaction at different metal electrocatalysts. We focus on the changes produced in the transition state (saddle point) as a consequence of the interactions with d-bands. We discuss different empirical correlations between properties of the metal and catalytic activity proposed in the past. We show which role is played by the band structure of the different metals and its interaction with the molecule for decreasing the activation barrier. Finally, we demonstrate why some metals are better electrocatalysts for the hydrogen electro-oxidation reaction than others.

Hydrogen Gas Diffusion Electrode Prepared from Porous Carbon Spheres Dispersed with Pd–Ag Alloy Nanoparticles

Abstract Hydrogen gas diffusion electrodes were prepared from porous carbon spheres dispersed with Pd–Ag alloy nanoparticles with various Pd/Ag ratios. The performance of the electrodes evaluated by observing the polarization characteristics in the electro-oxidation of hydrogen markedly depended on the ratio of Pd and Ag in the alloy.

Towards Understanding the Impedance Response of Ni/YSZ Anodes

A comprehensive impedance study on the hydrogen oxidation in Ni/8YSZ Cermet anodes has been realized. A major problem in this respect concerns the physical interpretation of empirical 'equivalent circuits' used to fit the experimental data. We applied a two-stage approach for the evaluation of the impedance data: I) at first by the deconvolution of a distribution function of relaxation times (DRT) four different processes and their characteristic relaxation times have been identified. Two parts at frequencies <1 kHz represent a gas-conversion process or respectively a gas diffusion, whereas two parts at higher frequencies (2-30 kHz) are associated with the electro-oxidation of hydrogen at active sites. II) Subsequently the last mentioned processes were fitted to a transmission line model describing the electronic and ionic transport properties of the Ni/8YSZ Cermet. This approach resulted in the determination of 1) the total ionic conductivity of the Ni/8YSZ Cermet and 2) the spatial extension of the electrochemical active zone into the electrode

Synthesis and characterization of high-surface area tungsten carbides and application to electrocatalytic hydrogen oxidation

In analogy with W 2 N and WO 3 , WS 2 and WP were found to undergo the CH 4 carburization to produce α-WC. α-WC obtained from WS 2 and W 2 N exhibited a high specific surface area compared with other routes. The introduction of a small amount of Pt into the WCs prepared in these ways contributed to a remarkable activity enhancement for the hydrogen electro-oxidation reaction. Tungsten carbides (WC) with different crystalline phases prepared from WO 3 as the starting material showed electrocatalytic activity in the order of α-WC > β-WC 1− x > β-W 2 C for the hydrogen oxidation reaction. α-WC obtained by methane carburizing of tungsten nitride (W 2 N), which was prepared from nitridation of WO 3 , produced much higher surface area than that from the direct carburization of WO 3 . We first found capable of carburize tungsten sulfide (WS 2 ) and tungsten phosphide (WP) to produce α-WC though they require severe carburization reaction. In particular, α-WC obtained from WS 2 exhibited a high specific surface area as well as α-WC by way of W 2 N. The XPS analysis revealed that the surface properties of α-WC are significantly dependent on the preparative methods. A large amount of graphite carbon was deposited on the surface of α-WC from direct carburization, whereas, there was far less on α-WC by way of W 2 N and WS 2 .The electrocatalytic activities of WCs obtained in these ways were greatly improved. The binary catalysts of WC promoted with Pt were evaluated in comparison with the current Pt catalyst. The introduction of a small amount of Pt onto the WC contributed to a remarkable anodic current enhancement. It turned out that the specific activity (activity based on the surface area of Pt metal) was higher than that of the commercial catalyst. It is concluded that the interaction Pt with α-WC enhance the utilization of Pt for the hydrogen electro-oxidation reaction.

CTAB assisted microwave synthesis of ordered mesoporous carbon supported Pt nanoparticles for hydrogen electro-oxidation

Mesoporous carbon with ordered hexagonal structure derived from the co-assembly of triblock copolymer F127 and resol was employed as the carbon support of Pt catalysts for hydrogen electro-oxidation. Structural characterizations revealed that the mesoporous carbon exhibited large surface area and uniform mesopores. The Pt nanoparticles supported on the novel mesoporous carbon were fabricated by a facile CTAB assisted microwave synthesis process, wherein CTAB was expected to improve the wettability of carbon support as well as the dispersion of Pt nanoparticles. X-ray diffraction and transmission electron microscopy were applied to characterize the Pt catalysts. It was found that the Pt nanoparticles were uniform in size and highly dispersed on the mesoporous carbon supports. The cyclic voltammograms in sulfuric acid demonstrated that the electrochemical active surface area of Pt catalysts prepared with CTAB was two times than that without CTAB.

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200-300 degrees C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H2O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 degrees C, with the conductivity rising to a value of 2.2 x 10(-2) S cm(-1) at 240 degrees C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH2PO4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH2PO4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm(-2). Thus, for fuel cells in which the supported electrolyte membrane was only 25 microm in thickness and in which a peak power density of 415 mW cm(-2) was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH2PO4, a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

Preparation of Porous Carbon Spheres Dispersed with Pd–Ag Alloy Nanoparticles

Abstract Porous carbon spheres dispersed with Pd–Ag alloy nano-particles have been prepared by simultaneous complexation of PdII and AgI onto the chelating resin and subsequent thermal treatment, and the obtained alloy–carbon composites were examined as the electrode material in electrooxidation of hydrogen.