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Tailoring catalyst-ionomer and electrolyte interaction is crucial for the development of anion exchange membrane (AEM) water electrolysis. In this work, the interaction of Ni-MoO2 nanosheets with ionomers and electrolyte cations was investigated. The activity of Ni-MoO2 nanosheets for the hydrogen evolution reaction (HER) increased when tested in 1 M NaOH compared to 1 M KOH; however, it decreased when tested in 0.01 M KOH compared to 1 M KOH electrolyte. The capacitance minimum associated with the potential of zero free charge (pzfc) was shifted negatively from 0.5 to 0.4 V versus RHE when KOH concentration increased from 0.1 mM to 1 M KOH, suggesting a softening of the water in the double-layer to facilitate the OH- transport and faster kinetics of the Volmer step that lead to improved HER activity. The catalyst interaction with cationic moieties in the anion ionomer (or organic electrolytes) can also be rationalized based on the capacitance minimum, because the latter indicates a negatively charged catalyst during the HER, attracting the cationic moieties leading to the blocking of the catalytic sites and lower HER performance. The HER activity of Ni-MoO2 nanosheets is lower in benzyltrimethylammonium hydroxide (BTMAOH) than in tetramethylammonium hydroxide (TMAOH). Anion fumion ionomer and electrolytes with organic cations with benzyl group adsorption (such as BTMAOH) lead to decreased HER activity in comparison with TMAOH and Nafion. By utilizing Ni-MoO2 nanosheet electrodes as a cathode in a full non-platinum group metal (PGM) AEM electrolyzer, a current density of 1.15 A/cm2 at 2 V cell voltage in 1 M KOH at 50 °C was achieved. The electrolyzer showed exceptional stability in 0.1 M KOH for 65 h at 0.5 A/cm2.

Developing scalable synthetic routes for fabricating low‐cost CO‐tolerant and highly active electrocatalysts have significant relevance for fuel cell applications. Here, we report a novel and rapid single step synthesis of Ni enriched bimetallic core‐shell (Ni−Pt) rhombic dodecahedra nanoparticles (NPs) via the thermal decomposition of precursor in the presence of oleylamine. Electrochemical dealloying in acidic medium (0.1 M HClO4) subjecting NPs to different potential cycles is employed to vary the chemical composition and morphologies of parent bimetallic core‐shell NPs. As a result, rhombic dodecahedra NPs transform to different morphologies such as concave, nanocage (open structure), and squeezed nanocage. Our results reveal that concave rhombic dodecahedra shaped electrocatalysts show high resistance to CO poisoning compare to core‐shell, nanocage, and squeezed nanocage morphologies as well as commercial Pt/C electrocatalysts. The improved CO tolerant of concave rhombic dodecahedra bimetallic NPs results from the presence of optimal content of Ni along with Pt content at the surface. We also demonstrate that CO tolerance of concave bimetallic NPs can be changed by electrochemical parameters such as the scan rate and CO adsorption potential. Overall, the present study presents a conceptual strategy for achieving improved CO tolerance by change in the composition and morphologies of bimetallic core‐shell NPs. Marked Open Access

As the proton exchange membrane fuel cell (PEMFC) has improved its performance and power density, the efficiency has remained unchanged. With around half the reaction enthalpy released as heat, thermal gradients grow. To improve the understanding of such gradients, PEMFC component thermal conductivity has been increasingly investigated over the last ten years, and the catalyst layer (CL) is one of the components where thermal conductivity values are still scarce. CLs in PEMFC are where the electrochemical reactions occur and most of the heat is released. The thermal conductivity in this region affects the heat distribution significantly within a PEMFC. Thermal conductivities for a graphitized and a non-graphitized CL were measured for compaction pressures in the range of 3 and 23 bar. The graphitized CL has a thermal conductivity of 0.12 ± 0.05 WK–1m–1, whilst the non-graphitized CL conductivity is 0.061 ± 0.006 WK–1m–1, both at 10 bar compaction pressure. These results suggest that the graphitization of the catalyst material causes a doubling of the thermal conductivity of the CL. This important finding bridges the very few existing studies. Additionally, a 2D thermal model was constructed to represent the impact of the results on the temperature distribution inside a fuel cell.

Water electrolyzers that use a membrane electrolyte between the electrodes are a promising technology towards mass production of renewable hydrogen. High power setups produce a lot of heat which has to be transported through the cell, making heat management essential. Knowing thermal conductivity values of the employed materials is crucial when modeling the temperature distribution inside an electrolyzer. The thermal conductivity was measured for different titanium-based porous transport layers (PTL) and a partially methylated Hexamethyl-p-Terphenyl Polybenzimidazolium (HMT-PMBI-Cl- membrane. The four titanium-based sintered transport layers materials have thermal conductivities between 1.0 and 2.5 0.2 WK−1m−1 at 10 bar compaction pressure. The HMT-PMBI-Cl- membrane has a thermal conductivity of 0.19 0.04 WK−1m−1 at 0% relative humidity at 10 bar compaction pressure and 0.21 0.03 WK−1m−1 at 100% relative humidity ( water molecules per ion exchange site at room temperature) at 10 bar compaction pressure. Combining the determined thermal conductivity values with data from the literature, 2D thermal models of a proton exchange membrane water electrolyzer (PEMWE) and an anion exchange membrane water electrolyzer (AEMWE) were built to evaluate the temperature distribution in the through-plane direction. A temperature difference of 7–17 K was shown to arise between the center of the membrane electrode assembly and bipolar plates for the PEMWE and more than 18 K for the AEMWE. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Fuel cells capable of synthetic glucose catalysis have revolved around the implementation of abiotic catalysts that require extreme acid and alkaline environments. These are not compatible with implantable medical sensor systems, and hence, there is a need to develop abiotic catalysts that operate at neutral pH. This paper presents structural and electrochemical characteristics of a nanoporous electrode designed for abiotic glucose oxidation in the presence of oxygen in neutral physiological media. The electrode was fabricated by annealing e-beam deposited thin films of platinum (Pt) and nickel (Ni) into a Pt–Ni alloy on a silicon substrate. The porous nature of the alloy enhances electrochemical properties by increasing the real surface area ~ 500 times compared to the geometric surface area of as-prepared multilayer thin films. This was reflected in the exchange current density of the electrode annealed at 800 °C being twice that of the electrode annealed at 650 °C. The cell voltage increase, due to the addition of dissolved physiological oxygen of 2 ppm, was about 100 ± 8 mV under a load current density of 2 µA cm−2. After running for 72 h in a physiological saline solution with 5 mM glucose, the increase in the electrode potential was only 23 µV h−1. These results suggest that the nanoporous Pt–Ni alloy anode offers an improved catalytic stability with time and should be a viable candidate for use in abiotic catalyzed glucose fuel cell systems operating under physiological conditions.

The effect of applying sandblasting during pretreatment while preparing titanium based IrO 2 − Ta 2 O 5 anodes by the conventional method was investigated. It was observed that sandblasting influences the surface morphology both before and after coating as deeper and smaller etching pits are obtained on the substrate before coating process and rougher surface is obtained after coating. As a result larger outer electrochemical active surface area (ECSA) is obtained on the anodes with sandblasting, which was determined based on cyclic voltammetry, whereas the inner ECSA is independent of the pretreatment. The polarization measurements in a quasi-steady state reveal that sandblasting has slight influence on the catalytic activity. Accelerated lifetime tests were carried out in acidic 0.9 mol L−1 Na2SO4 solution (pH = 2) at a current density of 5 × 107 A cm−2 under galvanostatic conditions. It shows that sandblasting would shorten the lifetime of the anode due to oxidation of the titanium substrate. This is suggested to be due to the shorter distance between the lowest spot of the outer coating surface and the highest spot of the outer substrate of the anode after applying sandblasting in titanium substrate pretreatment.

Iridium and ruthenium oxide are active electrocatalysts for oxygen evolution. The relation between preparation method, structure, and behavior of mixed oxides of iridium and ruthenium are of interest in order to obtain active and stable catalysts. In this work the structure of mixed Ru–Ir oxides synthesized by the polymeric precursor method, which involves the formation of a gel containing the metal precursors and subsequent heat-treatment in air, was studied for the IrxRu1−xO2 system. An in-depth analysis of X-ray diffraction (XRD) and X-ray absorption (XAS) data, including EXAFS and linear combination of XANES, shows that the polymeric precursor synthesis method is capable of providing an intimate mixing of Ir and Ru in the catalyst. In addition to the oxide phase, metal phases, i.e. with Ru or Ir or both in oxidation state zero (Ir(fcc) and Ru(hcp)), were also found in the product materials. Facing complex structures such as some of those synthesized here, we have shown that a representation of shells with more than one atom type are efficiently represented using mixed sites, i.e. including scattering contributions from several elements in a site corresponding to a partial occupancy of the site by these elements, this method forming a very efficient basis for analyzing EXAFS data.

Electrocatalysis of the oxygen reduction reaction (ORR), oxidation of carbon monoxide, and the methanol oxidation reaction (MOR) are all critical to the performance of direct-methanol fuel cells (DMFC). In this work we analysed the activity and mechanism for these reactions at carbon-supported Ru@Pt core-shell catalysts based on measurements of the potential of zero total charge (PZTC) and cyclic voltammetry. The PZTC measured at Ru@Pt is approximately 140 mV lower than at Pt. An analysis of charging curves and complex-capacitance data shows that the lower PZTC cannot be explained exclusively by a lower potential of zero free charge (PZFC). The lower PZTC at the core-shell catalyst therefore indicates that OH adsorbs more strongly on the Ru@Pt than on Pt. From a scaling relation between the free energies of adsorbed oxygen and OH we infer that the lower PZTC value for Ru@Pt implies a larger potential-determining step for the ORR at this catalyst than at Pt. For oxidation of CO and the MOR the stronger binding of OH to the surface at Ru@Pt than at Pt is expected to increase the activity of the former. These predictions are in agreement with the results of measurements of the catalytic activity of the catalysts, and serve to rationalise the catalytic activity of Ru@Pt with respect to those of Pt for these reactions.

Iridium oxide powders with a surface area of more than 1 m

The microporous layer (MPL) and the gas diffusion layer (GDL) in a polymer electrolyte membrane (PEM) fuel cell assembly are often treated as separate layers in the literature. However, there exists a considerable third region where the two different materials merge in the coating process. This region has properties that differ from either of the materials that it consists of. Through-plane thermal conductivity and thickness variation under different compaction pressures were measured for such a composite region of two different commercial GDLs, Freudenberg H1410 and Toray Paper TGP-H-030, each treated with a custom-made MPL ink. Thermal conductivity at 15 bar compaction pressure for untreated Freudenberg H1410 GDL is 0.124 ± 0.009 W K-1 m−1 and for the custom-MPL-coated Freudenberg H1410 materials it was increased by the treatment to 0.141 ± 0.004 W K−1 m−1 and 0.145 ± 0.004 W K-1 m−1 for 9.9 wt% and 11.9 wt% ink, respectively. For Toray paper TGP-H-030 the thermal conductivity at 15 bar compaction pressure for GDL only is 0.449 ± 0.009 W K−1 m−1 and for the custom-MPL-coated Toray TGP-H-030 materials it was decreased by the treatment to 0.39 ± 0.05 W K−1 m−1 and 0.39 ± 0.00 W K−1 m−1 for 9.9 wt% and 11.9 wt% ink, respectively. © 2018. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: http://jes.ecsdl.org/content/165/7/F514

A method for measuring downstream concentration effects through electrochemical impedance spectroscopy at double channel electrodes is demonstrated. An ac current perturbation is applied at an upstream working electrode and the resulting ac potential response at a downstream sensing electrode is measured. This generator-detector scheme is implemented with a single potentiostat. Experimental data for a reversible redox couple are presented and good agreement is found with numerical simulations. A qualitative explanation of the features is given which lays the foundation for a more rigorous theoretical treatment. Relationships with flow rate and frequency are found that can scale the data to lie on a universal curve. This is the authors' manuscript to the article (preprint).

An on-chip thin-film palladium hydride reference electrode is described for use with microfluidic electrochemical cells. A Pd electrode fabricated by photolithographic methods is charged with hydrogen in-situ with a simple current step technique. The placement of the reference electrode in a side channel means that it is unaffected by species in the main channel. Placement upstream of the working and sense electrodes is shown to be a significant improvement over cells with the reference electrode placed externally. The adverse effects of solution resistance are investigated quantitatively, and it is shown that the described configuration allows for high current experiments while maintaining accurate potential measurement. A simple formula for calculating the maximum current density for a certain cell geometry and electrolyte resistivity is presented. © 2016 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Author's post-print must be released with a Creative Commons Attribution Non-Commercial No Derivatives License.

Thermal conductivity of the polymer electrolyte membrane fuel cell (PEMFC) components has achieved increased attention over the past decade. Despite the fact that the PEMFC itself (between the gas flow plates) is less than a millimetre in thickness, several °C temperature differences can arise inside it during operation. These temperature differences mainly arise across the porous transport layers (PTL) often also referred to as gas diffusion layers (GDL). Several research efforts have led to a good understanding of the thermal conductivity of the PTL; in particular to how this property changes with compression, temperature, PTFE content, different fabrics, and water content. Far less attention has been given to the thermal conductivity of the much thinner layered micro porous layer (MPL) and in particular to the thermal conductivity of the transitional region between the PTL and the MPL. In this study we have used X-ray computer tomography (XCT), scanning electron microscopy (SEM), and thermal conductivity measurements to show that a MPL coated PTL is actually a three layered structure where the PTL is on one side, the MPL on the other, and a composite region consisting of the MPL as a matrix with the PTL fibres in the middle. We have shown that the thermal conductivity of the MPL-PTL-composite region is much larger than for the two others and that temperature differences inside this layer can be neglected compared to the regions where it is MPL-only and PTL-only. We have also shown that the MPL has a significantly lower thermal conductivity than the other two layers. In light of this research, the MPL of the commercial SGL should be integrated into the GDL in order to have lower temperature deviations in the PEMFC. A relevant literature review is included.

For reasons of electrode integrity and durability it is desirable to add zirconium hydrogen phosphate to electrocatalytic layers in PEM water electrolysis. Common synthesis methods are frequently associated with an accompanying addition of phosphoric acid. The presence of free phosphoric acid is known to be detrimental for catalytic activity of the common oxygen evolution catalysts. In this work composite electrocatalytic layers of iridium oxide, Nafion® and zirconium hydrogen phosphate were fabricated on glassy carbon disk electrodes. Zirconium hydrogen phosphate was prepared by mixing zirconium oxychloride and phosphoric acid. The effect of the electrocatalyst constituents were electrochemically investigated with respect to the oxygen evolution reaction in 0.5 M sulfuric acid electrolyte at room temperature. Slow linear sweep voltammograms showed that the electrocatalyst performance depends only to a small extent on the way the composite electrodes were manufactured. However, the coexistence of zirconium hydrogen phosphate particles in the electrode structure offsets the destructive effect of any phosphoric acid remaining in the electrode more or less completely. Inherent catalyst activity is unaffected by the addition of zirconium hydrogen phosphate, but apparently worsen the current per mass emphasizing the necessity of optimizing the catalytic layers with respect to performance and mechanical properties. © 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Electrocatalyst support affects not only catalytic activity of a catalyst, but also mass transportation and electron transfer in the catalyst layers of an electrode for proton exchange membrane fuel cells. Multi-dimensional and combined carbon materials such as Vulcan XC-72, carbon nanotubes (CNTs), and home-made coiled carbon nanotubes (CCNTs) are applied to enhance the catalyst activity and utilization. Three-dimensional CCNTs with large specific surface area and graphitic characteristic are synthesized by solid-state catalytic method. This obtained CCNTs and commercial CNTs are used as support to prepare platinum catalysts via modified ethylene glycol method, respectively. And the electrochemical surface areas (ECSAs) of the as-prepared Pt/CNTs, Pt/CCNTs and commercial Pt/C (JM) catalyst are evaluated by cyclic voltammetry. Then each two and three kinds of above catalysts mixed with different mass ratios are investigated. The ECSAs of Pt/C–Pt/CCNTs (95:5) and Pt/C–Pt/CNTs–Pt/CCNTs (80:10:10) are calculated to be 106 m2 gPt−1 and 111 m2 gPt−1, with respect to 70 m2 gPt−1 of Pt/C (JM) catalyst. And these mixed catalysts also demonstrate improved oxygen reduction reaction activities. This is mainly attributed to the unique structure of CCNTs, which can construct a multi-dimensional network to facilitate the mass transportation and electrons/protons transfer. © 2014. This is the authors’ accepted and refereed manuscript to the article. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

A carbon-polymer composite coating on stainless steel 316L substrates was investigated for the use as bipolar plate material for polymer electrolyte membrane fuel cells. The coating consisted of 45 vol% graphite, 5 vol% carbon black and 50 vol% epoxy binder. The coating was applied by a spraying technique followed by hot-pressing while the binder cured. An interfacial contact resistance of 9.8 mΩ cm2 at a compaction pressure of 125 N cm−2 was measured. Ex-situ electrochemical tests showed that the carbon-polymer composite coated plates had smaller increases in the interfacial contact resistance after polarization than bare stainless steel plates at potentials of 0.0191 and 0.6191 VSHE. At 1.0 VSHE, the resistance increased similarly for both the coated plate and the bare stainless steel plate, and reached unacceptable values. The porosity of the coating was estimated with scanning electron microscope imaging of the cross-section of the coating to be about 50%.

The photocurrent of hydrogen generating solid-state photoelectrochemical cell utilising a polybenzimidazole proton-conducting membrane and gaseous anode reactants has been enhanced by operation at higher temperatures. With a bias of 0 V for example, photocurrent increased from 15 to 30 μA/cm 2 on moving from 25 °C to 45 °C. The increase in photocurrent, which was limited by the dehydration of the cell, was shown to have contribution from improved electrode kinetics. Modification of TiO 2 surface with triazole, a conjugated heterocyclic compound, led to significant increase in photocurrent up to 4 fold increase at 0 V and 25 °C. This was attributed to improved separation of photogenerated charge carriers, as confirmed by correspondingly increased carrier lifetimes from 50 ns to 90 ns for triazole-modified TiO 2 . Assembly of the photoelectrochemical cell by hot-pressing induced a 0.3 eV red shift in optical absorption edge of TiO 2 , in agreement with a shift of its valence band maximum to higher binding energy.

A PEM fuel cell membrane–electrode assembly (MEA) was characterized by dynamic electrochemical impedance spectroscopy (dEIS) before and after cycling in a single cell configuration. The cell was subjected to 100 cycles between 0.6 V and 1.5 V vs. RHE in N2/5% H2 and 80 °C and 100% RH. Initially, the impedance-plane plots contained first- and fourth-quadrant behavior, which is resulting from a reaction mechanism at the cathode involving adsorbed intermediates. After the cycling, the impedance spectra changed to display first-quadrant behavior only. This is suggested to be due to particle growth and possibly the formation of edges between agglomerated particles. The results show that dEIS is a sensitive technique to detect even very moderate changes in electrocatalyst structure.

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H2SO4 solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F− and 10 ppm Cl−) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl− to 100 ppm, pitting was initiated on the stainless steel surface.

Bulk CO oxidation on smooth polycrystalline platinum was studied with a rotating disk electrode and cyclic voltammetry in CO-saturated 0.5 M sulfuric acid. The reaction rate was modelled with differential equations. The modelling goal was to build a model that reproduces the experimental current–potential characteristics, with special attention to the platinum oxide region. Two different models were tested: one containing conventional chemical and electrochemical reaction steps, and one built around an empirical equation describing the rate of platinum oxide formation and reduction. The latter model produced good fits to the experimental results. Both models showed that CO oxidation in the platinum oxide range proceeds via direct reaction between an adsorbed species such as PtOH and dissolved CO.

article i nfo Reverse Electro-Dialysis, RED, is an electrochemical technique to extract work by mixing aqueous solutions of different salinities. Electro-Dialysis, ED, is an electrochemical technique to extract potable water from seawa- ter. Both RED and ED typically employs noble metal oxide catalysts in the conversion between electronic current and ionic current. By testing several different electrode materials and red-ox salts, this paper demon- strates that these electrochemical reactions are controlled by mass transfer in the electrolyte rather than by the electrocatalytic properties of the electrode material. By comparing two different carbon materials and standard noble metal oxides, we show, for two red-ox salts, that the electrochemical performances of the in- expensive and expensive materials are similar. Relatively inexpensive graphite along with environmentally benign FeCl2 and FeCl3 in moderate concentrations was demonstrated to lower the electrode concentration overpotential.

The electrooxidation of methanol was studied at elevated temperature and pressure by cyclic voltammetry and constant potential experiments at real fuel cell electrocatalysts. Ruthenium core and platinum shell nanoparticles were synthesized by a sequential polyol route, and characterized electrochemically by CO stripping at room temperature to quickly confirm the structure of the synthesized core–shell structure as compared to pure commercial Pt/C and Pt–Ru/C alloy catalysts. A significant promotional effect of Pt decorated Ru cores in the methanol oxidation was found at elevated temperatures and rather high-electrode potentials. A negative potential shift of the methanol oxidation peak is observed for the Ru@Pt/C core–shell catalyst at moderate temperatures, while a significant shift to positive potentials of the methanol oxidation peak occurs for Pt/C catalysts. The onset potential for methanol oxidation is lowered some 200 mV from room temperature and up to 120 °C for all electrocatalysts, indicating that it is the thermal activity of water adsorption that dictates the onset potential. Direct methanol fuel cell experiments showed only small performance differences between Ru@Pt/C and Pt/C anode electrocatalysts, suggesting the necessity of render possible the formation of surface oxygen species at lower electrode potentials.

The paper reports an application of CO stripping-voltammetry as a diagnostic tool for the determination of surface composition of various PtRu nanoparticles (NPs). A series of electrochemical experiments on different systems, including monometallic Pt and Ru, a commercially available PtRu alloy, Ru@Pt (Ru NPs covered by a Pt shell), and their physical mixtures verified that the surface of the Ru@Pt particles does not contain significant amounts of ruthenium. A large cathodic shift (200 mV) of the main CO stripping peak of the Ru@Pt with respect to that of Pt was observed, consistent with what has been found at other catalysts with a Pt skin but of a different architecture. An additional small separate stripping peak at a potential corresponding to the oxidation of adsorbed CO at Pt/C is interpreted as being due to either separate Pt particles in the catalyst or thick Pt shells at a minor fraction of the Ru@Pt particles. Since the method probes the whole sample it will be useful for assessment of catalyst homogeneity.

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.

In order to make fuel cells with high power density the structure and morphology for the three-dimensional gas diffusion electrodes (GDEs) are very important. A preparation technique for GDEs for phosphoric acid doped polybenzimidazole (PBI) is presented. Teflon treatment of the backing material was found to be beneficial for the performance of the electrodes, and explained by higher total porosity. In general the open circuit voltage (OCV) with PBI-based cells is 0.9 V. The observed low OCV was explained by slow kinetic for the oxygen reduction and cross over of the reactants. The performance of the fuel cells is found to increase with increasing temperature; this was explained by faster reaction kinetic and higher membrane conductivity. A typical power output was 0.3–0.4 W cm −2 at 0.6 V and 175 °C.

Fuel-cell dynamics have been investigated with a variable-resistance board applied to a high-temperature polymer fuel cell, operating on pure oxygen and hydrogen at atmospheric pressure and about 150 °C. A particular pattern has been identified in the voltage–current density diagram. A model has been developed, using a characteristic equation to represent the external circuit as a resistive load, possibly nonlinear and time-varying, rather than assuming the current density to be an independent variable. The model successfully explains the shape of the transient current–voltage paths, including the different time constants observed when reversing a transient.

Fast cyclic voltammetry and a combined chronoamperometry/cyclic voltammetry pulse-sweep technique are used to study methanol oxidation on platinum in sulphuric acid. Measurement of suppression of hydrogen desorption charges simultaneously with charges for stripping of the CO poison allows for accurate baseline correction of the CO stripping peaks. Saturation coverages of CO formed from methanol, formic acid and dissolved CO are not all the same and differ in their potential dependence. Only dissociative adsorption of methanol to give adsorbed CO and subsequent oxidation of CO to CO 2 occurs during continuous cycling, but in the pulse-sweep method evidence was found for the existence of a parallel pathway occurring independently of adsorbed CO production, provided there is some surface not covered by CO or oxide.

This paper describes the testing of the gas-diffusion electrodes for polymer electrolyte membrane fuel cells utilizing phosphoric acid doped polybenzimidazole (PBI) electrolyte, which allows for an operating temperature as high as 200 °C. In order to determine the optimum structure of our anodes and cathodes, the platinum content in the Pt/C catalyst and catalyst loading were varied, as well as the loading of the PBI electrolyte dispersed in the catalyst layer. The different MEAs were tested in terms of their performance by recording polarization curves using pure oxygen and hydrogen. It was found that a high platinum content and a thin catalyst layer on both anode and cathode, gave the overall best performance. This was attributed to the different catalyst surface areas, the location of the catalyst in relation to the electrolyte membrane and particularly the amount of PBI dispersed in the catalyst layer. Scanning electron microscopy (SEM) was used in order to examine the cross-section of the MEAs and measure the thickness of the catalyst layers. With this information, it was possible to give an estimate of the porosity of the catalyst layer.

Methanol oxidation on Pt electrodes is studied by ac voltammetry. Data from voltammograms at frequencies from 0.5 Hz to 20 kHz are assembled into electrochemical impedance spectra and analysed using equivalent circuits. Inductive behavior and negative relaxation times are attributed to nucleation and growth behavior. The rate-determining step is proposed to be the reaction of adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites.