Potential Cycling Experiments Research Articles

Revealing the critical role of low voltage excursions in enhancing PEM fuel cell catalyst degradation by automotive hydrogen/air potential cycling experiments

The durability of Polymer Electrolyte Membrane Fuel Cells under dynamic operation still needs to be improved. To understand the automotive voltage cycling-induced catalyst degradation, the loss of the electrochemically active surface area (ECSA) is investigated through an experimental campaign on Membrane Electrode Assemblies. Ad-hoc hydrogen/air accelerated protocols were designed to evaluate the voltage profile impact in a range relevant for both automotive and heavy duty transport application (<0.90 V). Besides the well-known aging dependence on the upper potential limit, this work evidences the critical role of the short-stops, characterized by low voltage transients. Effort was spent in studying this procedure parameters (voltage level, duration, scan rate, humidification). The accelerated ECSA loss is due to Pt nanoparticles coarsening as proved by transmission electron microscopy and is suspected dominated by Pt cathodic dissolution, incentivized during excursions towards very low potentials (<0.4 V). These findings help the development of system mitigation strategies.

Accelerated fuel cell tests of anodic Pt/Ru catalyst via identical location TEM: New aspects of degradation behavior

In the present work the stability, chemical composition and structure of a Pt/Ru catalyst alloy with a nominal ratio of 1/1 is investigated. The same catalyst particles are analyzed before and after potential cycling experiments using identical location transmission electron microscopy. The experiments were performed at room temperature at [0–1.0] VRHE and [0–1.2] VRHE to simulate conditions occurring during ramping up of fuel cells. With decreasing maximum potential value a higher stability is found. Dissolution and dealloying are identified to be the main degradation mechanisms during potential cycling with Ru being dissolved preferably. Also agglomeration and Ostwald ripening are taking place, the frequency decreasing the longer the experiment is performed. Advanced in-depth analysis of potential-dependent reshaping mechanisms are performed by calculating the three-dimensional volume of single particles both in the as-prepared state and after potential cycling experiments using electron tomography data. Evaluation of the volume-specific change of the accessible surface area of the catalyst helps to understand fuel cell performance deterioration.

Highly active nanostructured palladium-ceria electrocatalysts for the hydrogen oxidation reaction in alkaline medium

We report an interesting new class of bifunctional electrocatalysts, Pd/C-CeO2, with excellent activity and stability for the hydrogen oxidation reaction (HOR) under alkaline conditions. The unique structure of palladium deposited onto a mixed support of Vulcan XC-72 carbon and CeO2 consists of Pd metal preferable deposited on the ceria regions of the catalyst. The CeO2-Pd interaction leads to enhanced HOR kinetics and increased stability. Here we compare catalysts with three different Pd loadings and show that the 10wt% Pd sample has optimized activity. Hydrogen pumping and fuel cell experiments based on this catalyst show higher activities as compared to a Pd/C sample without ceria. Metal dissolution tests and identical location transmission microscopy experiments show that the catalyst stability under harsh potential cycling experiments in alkaline medium is significantly improved as compared to Pd/C, making this material one of the best options for use as highly active and highly stable electrocatalysts for the HOR in anion exchange membrane fuel cells.

Gelation Behavior of Nafion Dispersion

Perfluorinated Nafion possesses unique properties that are valuable for electrochemical devices.1 Nafion is industrial standard material for the proton exchange membrane and electrode binder of proton exchange membrane fuel cells (PEMFCs). The solution processing techniques for Nafion have been developed since the mid-1980s and currently several versions of solution-cast Nafion membranes and liquid dispersions are commercially available. For solution processing techniques, understanding Nafion particle morphology in liquid media2,3 and the gelation process are of particular interest. Recently small angle neutron scattering (SANS) and 19F NMR studies found that the Nafion in dilute solvent systems can have three distinctive morphology, depending on the dispersing solvent4: i) a well-defined cylindrical dispersion in polyhydric alcohols ii) a random-coil conformation in aprotic solvents and iii) a less-defined highly solvated large particle in water-monohydric alcohol mixtures. In this presentation, the gelation behavior of Nafion from dilute dispersions and their impact on fuel cell membrane and electrode properties are investigated. Three types of gelation for Nafion dispersion were observed, depending on the dispersion solvent. For Nafion dispersion in polyhydric alcohols such as ethylene glycol or glycerol, inhomogeneous thermal irreversible gelation was observed. In this process, thermal irreversible gel with remaining concentrated liquid is formed during the solvent evaporation. The formed gel was not transformed back to the sol phase upon temperature increase or dilution. For Nafion dispersion in aprotic solvents, thermal reversible homogeneous gelation was observed. In this gelation process, an instant gelation occurred at a certain polymer concentration. At this concentration, the sol-gel transition occurred reversibly with temperature change. For Nafion dispersion in water/monohydric alcohols, Nafion was precipitated during the solvent evaporation process. The white fine string/flake precipitates were aggregated in the bottom of the vial and solidified with further solvent evaporation. The gelation process of Nafion strongly impacts the properties of solution-cast membrane and electrodes. The tensile toughness of Nafion changed from 0.001 to 20 MPa (over 4 orders of magnitude difference), depending on gelation type. Nafion membranes cast from homogeneous gelation exhibited the highest mechanical properties while Nafion membranes cast from precipitation process exhibited an extreme brittle nature. The electrode durability during potential cycling also strongly depends on gelation type. The electrode durability prepared from inhomogeneous thermal irreversible gelation exhibited the best durability. Microelectrode experiments indicated that the Nafion ionomers cast from irreversible gelation have more resistance to the polymer deformation in the presence of water, that allowed maintaining the electrode structure during the potential cycling experiments. This presentation explains further aspects of the importance of gelation behavior. Acknowledgment U.S. Department of Energy Fuel Cell Technologies Program (Technology Development Manager: Dr. Nancy Garland) funded this research. References Grot, W. G. Fluorinated Ionomers, Elsevier, 2007.Aldebert, P.; Dreyfus, B.; Pineri, M. Macromolecules, 1986, 19, (10), 2651-2653.Loppinet, B.; Gebel, G.; Williams, C. E. J. Phys. Chem B. 1997, 101 (10), 1884-1892.Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S. ACS Macro Letters, 2012, 1, 1403-1407.Kim, Y. S.; Welch, C. F.; Mack, N. H.; Hjelm, R. E.; Orler, B.; Hawley, M.; Lee, K. S.; Yim S.-D.; Johnston, C. M. Phy. Chem. Chem. Phy. 16, (13) 5927-5932.

A Study of Platinum Alloy Fuel Cell Catalyst Degradation in Aqueous and Membrane Electrode Assembly Environments Using in Situ Anomalous Small-Angle X-Ray Scattering

Numerous ex-situ and in-situ spectroscopic and microscopic techniques have aided in advancing research efforts aimed at improving the oxygen reduction reaction activity and durability of the platinum-based cathode electrocatalysts for polymer electrolyte fuel cells. One such technique is small angle X-ray scattering (SAXS), which can be used to determine, in situ, mean particle size and particle size distributions (PSDs) on the nanometer scale.1 When scattering patterns are acquired at multiple energies near the absorption edge of the element of interest, a technique termed anomalous SAXS (ASAXS), the PSD of just this element can be resolved.2 In this study, we are using ASAXS to determine PSDs of nanoparticle platinum alloy electrocatalysts while under potential control in aqueous and membrane-electrode assembly (MEA) environments. The information from this technique is correlated with electrochemically-active surface area (ECA) losses to investigate the mechanisms of catalyst degradation. Our studies have focused on the effects of environment (i.e., MEA or aqueous) and potential profile on the extent and mechanisms of particle growth and corresponding ECA loss. The catalyst studied was 40 wt% Pt3Co nanoparticles supported on an Akzo Nobel Ketjen EC300J carbon black support. The as-prepared Pt3Co catalyst had a mean particle size of ~5.6 nm as found through transmission electron microscopy (TEM) analysis. As an example of the results that will be discussed, we show a comparison of the ASAXS results for the Pt3Co catalyst for two potential profiles in the aqueous environment. ASAXS patterns were taken as a function of triangle wave cycling between 0.6 and 1.0 V at 50 mV/s and square wave cycling between 0.4 and 1.05 V (20 s/cycle). Fitting of the ASAXS patterns in the ~0.02 to ~0.3 Å-1 Q range resulted in evolution of the PSDs, shown in Figure 1, for both cycling protocols. Figure 2 shows the change in the mean particle diameter as a function of the number of potential cycles for both the triangle and square wave protocols. After 1,500 potential cycles the mean diameter of the Pt3Co catalyst increased 0.35 nm for the triangle wave case and 0.45 nm for the square wave case. The triangle wave potential cycling experiment also showed lower losses in the geometric surface area (GSA), which has been shown to be directly related to ECA,3 and mass as calculated from the PSDs. The triangle wave experiment resulted in a GSA loss of 6% and mass loss of 7% over the first 1,500 potential cycles. Meanwhile, the square wave experiment resulted in a GSA loss of 9% and a mass loss of 8%. This increased degradation for the square wave cycling is similar to previously reported results showing increased loss of ECA with increasing anodic sweep rate.4 These results along with the results from potential cycling experiments performed in the MEA environment will be compared and analyzed in order to investigate Pt3Co catalyst degradation mechanisms in both environments.Acknowledgements: The authors acknowledge funding from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program (Nancy Garland, Technology Development Manager) and the U.S. Department of Energy, Office of Basic Energy Sciences for support of the Advanced Photon Source.ReferencesT. Narayanan, Soft Matter: Scattering, Imaging, and Manipulation. 1st ed.; Springer: Berlin, 2007.C. Yu, S. Koh, J. Leisch, M.T. Toney, and P. Strasser, 2008, Faraday Discuss., 140, 283.E. F. Holby, W. C. Sheng, Y. Shao-Horn, D. Morgan, 2009, Energy Environ. Sci. 2, 865.M. Uchimura, S. Kocha, 2007, ECS Trans, 11, 1215.

Electrocatalytic activity and stability of Co and Mn-based oxides for the oxygen reduction reaction in alkaline electrolyte

The oxygen reduction reaction (ORR) was investigated on electrocatalysts composed of Co and Ni-modified Mn oxides, on Mn-modified Co oxides (spinel structure) and on these pure metal oxides, all supported on carbon powder. The transition metal oxides were synthesized via thermal decomposition of the respective nitrates under oxidizing atmosphere, and characterized by X-ray diffraction, before and after electrochemical potential cycling experiments. The ORR electrocatalytic activity was measured using steady state polarization curves using a rotating disk electrode. The comparison of the ORR polarization curves obtained for the pure metal oxides showed higher electrocatalytic activity for the Mn oxide electrocatalyst. However, durability tests showed an activity loss of this material due to the formation of the less active Mn3O4 phase. Co and Ni oxide showed similar ORR onset potential, but Co oxide presented much higher limiting current, evidencing higher number of electrons involved in the ORR. This was ascribed to its higher ability for the disproportionation reaction and for the O–O bond breaking, conducting the ORR via the 4-electron pathway. The Ni-modified Mn oxide material resulted in an increased stability during the ORR electrocatalysis. This was attributed to the introduction of Ni atoms into the Mn oxide structure, which leads to an electronic interaction between the Ni and Mn atoms, avoiding the formation of the less active Mn3O4. In the case of the Mn-modified Co oxide electrocatalyst, the addition of Mn into the Co oxide structure facilitated the electroreduction of the Co3+ to Co2+ during the ORR polarization curves. The faster electroreduction of Co(III) to Co(II) increased the mediated step, increasing the overall ORR rate.

Tungsten materials as durable catalyst supports for fuel cell electrodes

Durable platinum catalyst support materials, e.g. tungsten carbide (WC), tungsten oxide (WOx) and self-synthesized tungsten oxide (WOxs) were evaluated for the use in High-Temperature Proton Exchange Fuel Cells (HT-PEM) based on phosphoric acid doped polybenzimidazole as electrolyte. The support materials and the catalyst loaded support materials were characterized ex-situ by cyclic voltammetry in HClO4, potential cycling, CO-stripping, electron microscopy and X-ray diffraction measurements. The tungsten oxide and tungsten carbide based supported catalysts were compared to High Surface Area Carbon (HSAC), each coated with platinum via the same in-house manufacturing procedures. The in-house manufacturing procedures resulted in catalyst particle sizes on HSAC of 3–4 nm with a uniform distribution.The in-situ Potential Cycling experiments of WOx or WOxs supported catalysts showed much lower degradation rates compared to High Surface Area Carbons. The formation of WOx species on WC was proven by ex- and in-situ cyclic voltammetric studies and thermogravimetric analyses. X-ray diffraction, ex-situ cyclic voltammetry and in-situ cyclic voltammetry showed that WOx is formed from WC as starting material under oxidizing conditions. Finally a 1000 h durability test with WOx as catalyst support material on the anode was done in a HT-PEM fuel cell with reformed methanol on the anode.

Half-Cell, Steady-State Flow-Battery Experiments

An experimental approach designed to separately test the positive and negative sides of a redox flow battery is discussed. Circulating a single electrolyte through a cell so that oxidation occurs on one electrode and reduction occurs on the other electrode eliminates the problem of reactant crossover that occurs when different electrolytes are placed on either side of the cell. This approach allows phenomena associated with the positive and negative electrode-electrolyte combinations to be studied in isolation. Polarization curves, steady-state potential holds, and potential-cycling experiments are presented as examples of results that can be obtained with this approach. The impact of side reactions on the results is also discussed.

In Situ Anomalous Small-Angle X-ray Scattering Study of Fuel Cell Catalyst Degradation in Aqueous and Membrane Electrode Assembly Environments

Anomalous small-angle X-ray scattering (ASAXS) is a powerful technique for probing catalyst particle sizes on the nanometer scale making it ideal for monitoring particle size distributions (PSDs) of Pt-based cathode electrocatalysts for polymer electrolyte fuel cells. Potential cycling experiments were performed on Pt nano-particles in both aqueous and membrane-electrode assembly (MEA) environments. PSDs were resolved through in-situ ASAXS and used to calculate geometric surface areas (GSA) and mass losses. A comparison of the losses in the GSA and mass of Pt along with changes in the evolutions of the PSDs over 900 potential cycles elucidated differences between degradation mechanisms in the two different environments. The MEA environment induced greater GSA and mass losses while maintaining similar growth in the mean particle size as compared to the aqueous environment. Results show changes in PSDs consistent with dominant mechanisms of Pt dissolution/coarsening in the aqueous environment and dissolution in the MEA environment.

In situ electrochemical STM study of platinum nanodot arrays on highly oriented pyrolythic graphite prepared by electron beam lithography

Model electrodes consisting of platinum dots with a mean diameter of (30±5) nm and heights of 3–5nm upon highly oriented pyrolytic graphite (HOPG) were prepared by electron beam lithography and subsequent sputtering. The Pt nanodot arrays were stable during scanning tunnelling microscopy (STM) measurements in air and in sulphuric acid electrolyte, indicating the presence of “anchors”, immobilising the dots on the HOPG surface.Electrochemical STM was used to visualise potential induced Pt, carbon and Pt-influenced carbon corrosion in situ in 0.5M sulphuric acid under ambient conditions. Potentiostatic hold experiments show that the Pt dots start to disappear at electrode potentials of E>1.4V vs. SHE. With increasing time and potential a hole pattern congruent to the original dot pattern appears on the HOPG basal planes. Corrosion and peeling of the HOPG substrate could also be followed in situ.Dissolution of Pt dots appears to be accelerated for potential cycling experiments compared to the potential hold statistics.

Quantitative analysis of carbon corrosion during fuel cell start-up and shut-down by anode purging

The corrosion of the carbon support during start-up and shut-down of polymer electrolyte fuel cells was systematically quantified to elucidate effects related to the propagation of the gas front through the anode compartment and to determine the influence of relevant operating parameters. Measurements with different reactant gas concentrations and gas flow rates suggest that the gas exchange process is dominated by a homogeneous displacement of the anode gas in case of start/stop operation at OCV. The influence of additional effects such as direct recombination of hydrogen and oxygen, diffusion on the channel and land scale and pseudocapacitive effects seems to be negligible under the investigated conditions. With an external load applied during the gas exchange process (discharge mode), shut-down induced carbon corrosion strongly differs from the behavior at OCV. The virtually constant corrosion rates obtained below a critical purge gas flow rate in the discharge mode are attributed to a significant contribution of electrochemical hydrogen consumption. Temperature and humidity variation measurements demonstrate that the carbon corrosion behavior during real start/stop processes can differ considerably from the correlations obtained under controlled potential conditions in H2/N2 mode. Thus, potential cycling experiments only provide limited information about start/stop durability under application relevant conditions.

Effect of Scan Range on Pt Surface Area Loss in Potential Cycling Experiments

Effect of upper and lower potential limits on Pt stability in potential cycling experiments is studied for 20 wt. % Pt/C E-TEK catalyst in acidic medium. The Pt electrochemical surface area (ECSA) loss is observed as a parameter to study different potential ranges. It is observed that at upper potential limits the ECSA loss is influenced by electrochemical Pt dissolution while at lower potential limits reduction of β Pt oxide phase plays important role in ECSA loss.

Effect of Scan Range on Pt Surface Area Loss in Potential Cycling Experiments

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Fuel Cell Catalyst Particle Size Growth Characterized by X-Ray Scattering Methods

X-ray diffraction experiments coupled with whole profile pattern refinement and Warren Averbach data analysis methods were used to quantify Pt electrode crystallite growth in polymer membrane fuel cell cathode samples. Both X-ray data analysis methods produced similar results for average particle sizes. The cycling of cathodes to cell potentials above 1 volt greatly increases the loss of Pt cathode surface area by Pt grain growth. The fuel cell anode catalysts exhibit little to no growth after potential cycling experiments. Changes in Pt to carbon peak integrated intensity ratios indicate carbon corrosion is occurring upon cycling to high potentials.

PEM fuel cell electrocatalyst durability measurements

Electrode material durability is an important factor in limiting the commercialization of polymer electrolyte membrane fuel cells (PEMFCs). PEMFCs typically use carbon supported nanometer sized Pt and/or Pt alloy catalysts for both anode and cathode. Electrocatalyst surface area loss is due to the growth of platinum particles. Particle size growth is accelerated by potential cycling whether due to artificial potential cycling or by cycling during fuel cell operation. Catalysts were analyzed by X-ray diffraction (XRD) to determine the degree of electrocatalyst sintering. Cathode Pt particle size growth is a function of temperature, test length and potential. The largest increase in cathode Pt particle size was observed during potential cycling experiments and increased with increasing potential. During single cell durability testing, the cathode catalyst particle size grew from about 1.9 to 3.5 nm during the drive cycle experiments over 1200 h of testing. This extent of growth was greater than that observed during steady-state testing, where the particles grew to 2.6 nm at 900 h and 3.1 nm over 3500 h. During cycling measurements, catalyst coarsening rates exhibited a linear increase with temperature. Low relative humidity decreased platinum particle growth, but substantially increased carbon loss. Carbon corrosion of the electrode catalyst layer was found to increase with increasing potential and decreasing humidity.

Voltammetric Reduction ofα- andγ*-[S2W18O62]4-and α-, β-, and γ-[SiW12O40]4-: Isomeric Dependence of Reversible Potentials of Polyoxometalate Anions Using Data Obtained by Novel Dissolution and Conventional Solution-Phase Processes

Comparative studies on the voltammetric reduction of the alpha and gamma isomers of Dawson [S(2)W(18)O(62)](4)(-) and alpha, beta, and gamma forms of Keggin [SiW(12)O(40)](4)(-) polyoxometalate anions have been undertaken. For the six reversible one-electron [S(2)W(18)O(62)](4)(-)(/5)(-)(/6)(-)(/7)(-)(/8)(-)(/9)(-)(/10)(-) processes in acetonitrile, reversible potentials (E(0)(')) were found to be independent of isomeric form within experimental error (+/-5 mV). However, because both the alpha and gamma* isomers of [Bu(4)N](4)[S(2)W(18)O(62)] are insoluble in water, solid-state voltammetric studies with microcrystals adhered to electrode surfaces in contact with aqueous Et(4)NCl and Bu(4)NCl electrolyte media were also possible. Although no isomeric distinction was again detected in the solid-state studies, it was found that reduction of adhered solid by four or more electron equivalents led to rapid dissolution. When Et(4)NCl was the electrolyte, this dissolution process coupled with potential cycling experiments enabled conventional solution-phase data to be obtained in water for the analogous six one-electron reduction steps previously detected in acetonitrile. A strong medium effect attributed to Lewis acidity effects was apparent upon comparison with E(0)(') data obtained in water and acetonitrile. In contrast, with the [SiW(12)O(40)](4)(-) system, E(0)(') values for the [SiW(12)O(40)](4)(-)(/5)(-)(/6)(-)(/7)(-) processes in acetonitrile exhibited a larger (about 70 mV) dependence on isomeric form, and the isomerization step, [gamma-SiW(12)O(40)](6)(-)--> [alpha-SiW(12)O(40)](6)(-), was detected on the voltammetric time scale. The influence of isomeric form on reversible potential data is considered in terms of structural and charge density differences exhibited in the [S(2)W(18)O(62)](4)(-) and [SiW(12)O(40)](4)(-) systems studied in this paper and published data available on the alpha, beta, gamma, and gamma isomers of [As(2)W(18)O(62)](6)(-) and [P(2)W(18)O(62)](6)(-) Dawson anions and Keggin systems.

A Study of Depletion Layer Effects on Equivalent Circuit Parameters Using an Electrochemical Quartz Crystal Impedance System

An electrochemical quartz crystal impedance system (EQCIS) was used to investigate depletion layer effects on equivalent circuit parameters of piezoelectric quartz crystal resonance in electrochemical processes of CoSO4 aqueous solutions containing 0.4 mol/L ethylenediamine + 0.5 mol/L Na2SO4, 0.2 mol/L 1,10-phenanthroline + 0.5 mol/L Na2SO4 and K3Fe(CN)6, or K4Fe(CN)6 aqueous solutions containing 1.0 mol/L KCl in potential cycling experiments, respectively. The impedance data were analyzed according to Martin's model. For the former two systems, motional resistance R1, series resonant frequency fs, and motional inductance L1 changed reversibly with potential, and the observed values of ΔR1, ΔL1, and Δfs roughly satisfied equations reflecting liquid loading effects, suggesting that changes in these parameters might be mainly governed by local variations in solution density and viscosity near the electrode surface. For the latter two systems, reversible changes in R1 with potential coincided well with the simulated results from variations in solution density and viscosity near the electrode surface; however, besides reversible changes of ΔL1 and Δfs with potential, rising drifts of fs and decreasing drifts of L1 were found at oxidation potentials. The drifts of fs and L1 observed are believed to result from mass changes of the gold electrode, and cyanide and chloride corrosion of the gold electrode at oxidation potentials may play an important role for the drifting phenomena. It is concluded that quantitative analyses of ΔR1, ΔL1, and Δfs obtained from the EQCIS provide the possibility for differentiating the net depletion layer effect with the change in electrode mass, and the ΔR1 response may be well used for evaluating local changes in liquid loading inside the depletion layer compared with responses of Δfs and ΔL1.

Electrochemically- and Photo-Induced IR Absorption of Low Band-Gap Polydithienothiophenes: A Comparative Study

ABSTRACTPoly(dithieno[3,4-b:3',4'-d]thiophene) (pDTT1), poly(dithieno[3,4-b:3',2'-d]thiophene (pDTT2) and poly(dithieno[3,4-b:2',3'-d]thiophene) (pDTT3) are low band-gap polymers that undergo both p- (oxidation) and n- (reduction) type electrochemical reversible doping in organic electrolytes. In this work we report on the in-situ IR spectroscopic characterization of both doping types using attenuated total reflection (ATR) FTIR spectroelectrochemistry. Thin polymer films were prepared on Ge reflection elements by electrosynthesis. During electrochemical potential cycling experiments in a spectroelectrochemical cell, the evolution of infrared active vibrational (IRAV) modes, correlated with the generation of charge within the polymer chain, was studied. In addition to electrochemical doping, the polymers exhibit also charged excitations by photodoping. FTIR spectra recorded by illumination/dark cycles show photoinduced IRAV bands which are similar to those found for the electrochemically p-doped materials. The infrared spectroscopic results are compared with Raman spectra of the neutral polymers. The relationship between vibrational properties and structure of the polymers is discussed in terms of different contribution of the φ-electrons within the repeating units to the electronic structure of the conjugated backbone.

Stability of Prussian Blue films on ITO electrodes: effect of different anions

Transformation of insoluble newly deposited Prussian Blue (PB) into the soluble structure stabilizes the film and allows its total oxidation to Prussian Yellow. The stability of PB films in electrochemical processes in successive potential cycling experiments and under long-term chemical attack by an electrolyte depends not only on the cation that takes part but also on the nature of the anion present. In aqueous media, the sulphate ion clearly destabilizes the crystalline structure whereas the nitrate ion favours stabilization. Initial changes in the voltammograms recorded immediately after the spectroscopic tests have been detected in all media. During the immersion period water molecules and ions enter the film and affect the redox processes. With time the ions arrange themselves in the heart of the lattice and the water molecules are lost, and the blocking of the redox processes disappears.