Tantalum (oxy)nitrides prepared using reactive sputtering for new nonplatinum cathodes of polymer electrolyte fuel cell

Tantalum (oxy)nitrides (TaO_xN_y) have been investigated as new cathodes for polymer electrolyte fuel cells without platinum. TaO_xN_y films were prepared using a radio frequency magnetron sputtering under Ar + O₂ + N₂ atmosphere at substrate temperatures from 50 to 800 °C. The effect of the substrate temperature on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the TaO_xN_y films were examined. The catalytic activity of the TaO_xN_y for the ORR increased with the increasing substrate temperature. The ORR current density at 0.4 V vs. RHE on TaO_xN_y prepared at 800 °C was approximately 20 times larger than that on TaO_xN_y prepared at 50 °C. The onset potential of the TaO_xN_y for the ORR was obtained at the ORR current density of −0.2 μA cm⁻². The onset potential of the TaO_xN_y prepared at 800 °C was ca. 0.75 V vs. RHE. The X-ray diffraction patterns revealed that Ta₃N₅ structure grew as the substrate temperature increased. While, the ionization potentials of all specimens were lower than that of Ta₃N₅, and decreased with the increasing substrate temperature. The TaO_xN_y which had Ta₃N₅ structure and lower ionization potential might have a definite catalytic activity for the ORR.     

Fe–Nx/C electrocatalysts synthesized by pyrolysis of Fe(II)–2,3,5,6-tetra(2-pyridyl)pyrazine complex for PEM fuel cell oxygen reduction reaction

2,3,5,6-Tetra(2-pyridyl)pyrazine (TPPZ) was employed as a ligand to prepare an iron(II) complex (Fe–TPPZ) that served as a precursor to synthesize carbon-supported catalysts (Fe–N_x/C) through heat-treatment at 600, 700, 800 and 900 °C under N₂ atmosphere. Both the structure and composition of the synthesized Fe–N_x/C were analyzed by X-ray diffraction and energy-dispersive X-ray microanalysis, respectively. The rotating disk and ring-disk electrode measurements showed that these catalysts have strong ORR activity with an overall 4-electron transfer process through a (2 + 2)-electron transfer mechanism, which was assigned to the catalytic function of the Fe–N_x center. A study on the heat-treatment temperature on the ORR activity showed that 800 °C is the optimal temperature for the synthesized catalysts. Furthermore, the effect of both catalyst and Nafion^® ionomer loadings in the catalyst layer on the corresponding ORR activity was also investigated. The kinetic parameters such as the chemical reaction rate between O₂ and Fe–N_x/C (adduct formation reaction), the rate constant for the rate-determining step (RDS), and the electron numbers in the ORR, were obtained. The methanol tolerance of the catalyst was also tested. To validate the ORR activity, a membrane electrode assembly in which the cathode catalyst layer contained Fe–N_x/C was constructed and tested in a real fuel cell. The results obtained are encouraging when compared with similar non-noble catalysts.     

Ammonia-treated carbon-supported cobalt tungsten as fuel cell cathode catalyst

The ammonia-treated carbon-supported cobalt tungsten (Co-W/C) was prepared by a reaction employing a temperature program in a stream of NH₃ between 773 and 1073 K. The effects of the NH₃ heat-treatment temperature, Co-W ratio and the preparation method were investigated. The activity of Co-W/C for the oxygen reduction reaction (ORR) was evaluated using a rotating disc electrode and single fuel cell measurements. The Co-W/C prepared by the impregnation method with the same atomic ratio of Co and W, and NH₃ heat treated at 823 K, exhibited the highest ORR activity with an onset potential of 0.74 V (vs. RHE at 5 μA cm⁻²). The XRD and temperature-programmed measurements revealed that the catalyst active species were ascribed to the presence of CoW oxynitride and Co nitride. The catalyst surface was characterized as nitrided metal (accommodation of the N-atom in the host Co-W lattice resulting from a sufficient distance between the Co-W atoms and N) and N-containing carbon (Co surrounded by N-atoms and attached to the C support) by X-ray photoelectron spectroscopy. The Co-W oxynitrides, Co nitride and pyrrolic-type nitrogen of the N-containing carbons are likely responsible for the good ORR activity.     

Improved ORR activity of non-noble metal electrocatalysts by increasing ligand and metal ratio in synthetic complex precursors

In an effort to improve oxygen reduction reaction (ORR) activity by increasing the catalytic active site density in carbon-supported non-noble metal catalysts, several nitrogen-containing catalysts were synthesized through a heat treatment process at 900 °C using precursor complexes of Fe(II) and tripyridyl triazine (TPTZ). Fe to TPTZ mole ratios of 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 were used to prepare the precursor complexes. X-ray diffraction and surface electrochemical techniques were used to characterize these catalysts (Fe–N_x/C), and revealed that when the amount of TPTZ in the precursor complex was increased, the decomposition of Fe–N_x sites, which are considered active sites for the ORR, was effectively reduced, resulting in higher Fe–N_x site density and thus improving the catalysts’ ORR activity. This beneficial effect was validated through rotating disk electrode tests and analysis of the ORR kinetics catalyzed by these catalysts. The obtained results showed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–N_x/C increased monotonically in the mole ratio range of 1:2–1:6. Therefore, increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to reduce the decomposition of ORR active site density and thereby enhance the ORR activity of non-noble metal catalysts.     

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

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

Oxygen reduction reaction on electrodeposited Pt100−x−yNixPdy thin films

The kinetics of the oxygen reduction reaction (ORR) were examined on a series of Pt_100−x−yNi_xPd_y ternary alloys. Films were produced by electrodeposition that involved a combination of underpotential and overpotential reactions. For Pt-rich Pt_100−x−yNi_xPd_y alloy films (x < 0.65) Ni co-deposition occurred at underpotentials while for Ni-rich films (x > 0.65) deposition proceeded at overpotentials. Rotating disk electrode (RDE) measurements of the ORR kinetics on Ni-rich Pt_100−x−yNi_xPd_y thin films revealed up to ∼6.5-fold enhancement of the catalytic activity relative to Pt films with the same Pt mass loading. More than half of the electrocatalytic gain may be attributed to surface area expansion due to Ni dealloying. Surface area normalization based on the H_upd charge reduced the enhancement factor to a value less than 2. The most active ternary alloy film for ORR was Pt₂₅Ni₇₃Pd₂. Comparison of the ORR on Pt, Pt₂₀Ni₈₀, Pt₂₅Ni₇₃Pd₂ thin films indicate that the binary alloy is the most active with a H_upd normalized ORR enhancement factor of up to 3.0 compared to 1.6 for the ternary alloy.     

The oxygen reduction reaction on Pt/TiO x N y -based electrocatalyst for PEM fuel cell applications

Titanium oxy-nitride was developed for the first time as Pt electrocatalyst support for the ORR in PEM fuel applications. The conditions of the support preparation and the Pt/TiO _x N _y -based electrodes’ elaboration by chemical reduction method were determined. Comparison of the polarization curves of the carbon and the TiO _x N _y supported how clearly TiO _x N _y was more stable than the Vulcan XC-72R. It was found that the 40 wt% Pt/TiO _x N _y -based electrocatalyst is active for the ORR in acid medium, but the activity was less than that of Pt/C. The normalized electrochemical surface area degradation of Pt/TiO _x N _y was significantly less than that of Pt/C. The kinetics of the ORR on Pt/TiO _x N _y proceeded through a four-electron transfer process. The single-cell hydrogen/oxygen PEM fuel cell performances based on Pt/TiO _x N _y cathode electrocatalyst exhibited the same range of characteristics as those based on Pt/C.     

Carbon corrosion characteristics of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanostructures in acidic media and implications for ORR performance

Accelerated electrochemical corrosion of nitrogen-containing carbon (CN _x ) oxygen reduction catalysts was performed by a chronoamperometric hold at 1.2 V versus NHE in acidic electrolyte using a rotating disk electrode system. Cyclic voltammograms were used to measure the electrochemically active quinone/hydroquinone redox reaction couple indicating the degree of carbon corrosion. Half-cell testing of CN _x oxygen reduction catalyst materials showed superior carbon corrosion resistance compared to Vulcan carbon, the most ubiquitous cathode catalyst support. When oxygen reduction activity was measured before and after carbon corrosion, carbon corrosion resilience trended with the oxygen reduction activity. CN _x catalysts subjected to carbon corrosion testing did not show a change in the onset of oxygen reduction reaction (ORR) activity potentials with only a slight reduction in current density, but showed improved ORR selectivity to the complete reduction of dioxygen to water.     

Kinetics and PEMFC performance of RuxMoySez nanoparticles as a cathode catalyst

Kinetics of Ru_xMo_ySe_z nanoparticles dispersed on carbon powder was studied in 0.5 M H₂SO₄ electrolyte towards the oxygen reduction reaction (ORR) and as cathode catalysts for a proton exchange membrane fuel cell (PEMFC). Ru_xMo_ySe_z catalyst was synthesized by decarbonylation of transition-metal carbonyl compounds for 3 h in organic solvent. The powder was characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Catalyst is composed of uniform agglomerates of nanocrystalline particles with an estimated composition of Ru₆Mo₁Se₃, embedded in an amorphous phase. The electrochemical activity was studied by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Tafel slopes for the ORR remain invariant with temperature at −0.116 V dec⁻¹ with an increase of the charge transfer coefficient in dα/dT = 1.6 × 10⁻³, attributed to an entropy turnover contribution to the electrocatalytic reaction. The effect of temperature on the ORR kinetics was analyzed resulting in an apparent activation energy of 45.6 ± 0.5 kJ mol⁻¹. The catalyst generates less than 2.5% hydrogen peroxide during oxygen reduction. The Ru_xMo_ySe_z nanoparticles dispersed on a carbon powder were tested as cathode electrocatalyst in a single fuel cell. The membrane-electrode assembly (MEA), included Nafion^® 112 as polymer electrolyte membrane and commercial carbon supported Pt (10 wt%Pt/C-Etek) as anode catalyst. It was found that the maximum performance achieved for the electro-reduction of oxygen was with a loading of 1.0 mg cm⁻² Ru_xMo_ySe_z 20 wt%/C, arriving to a power density of 240 mW cm⁻² at 0.3 V and 80 °C.     

Chalcogenide electrocatalysts for oxygen-depolarized aqueous hydrochloric acid electrolysis

Several Rh- and Ru-based carbon-supported chalcogenide electrocatalysts were evaluated as oxygen-depolarized cathodes for HCl electrolysis applications. The roles of both crystallinity and morphology of the electrocatalysts were explored by investigating several synthetic processes for materials, specifically patented E-TEK methods and the non-aqueous method. The activity of the electrocatalysts for ORR was evaluated via RDE studies in 0.5 M HCl, and compared to state of the art Pt/C and Rh/C systems. Rh_xS_y/C, Co_xRu_yS_z/C, and Ru_xS_y/C materials synthesized from the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The amorphous non-aqueous moieties, while exhibiting little depolarization due to the presence of high concentrations of Cl⁻ in the RDE studies, were unsuitable for operation in a true ODC HCl electrolyzer cell because of irreversible dissolution resulting from the high concentration (∼5 M) of HCl. In contrast, the Ru-based materials from the E-TEK methods were unaffected by the depolarizing conditions of an uncontrolled shutdown. These Ru-based electrocatalysts, being on the order of seven times less expensive than the state of the art Rh_xS_y material, may prove to be of economic benefit to the HCl electrolysis industry.     

Influence of sputtering power on oxygen reduction reaction activity of zirconium oxides prepared by radio frequency reactive sputtering

Zirconium oxides (ZrO_2−x) have been investigated as new cathodes for direct methanol fuel cells without platinum. ZrO_2−x films were prepared using a radio frequency (RF) magnetron sputtering at RF powers from 75 to 175 W. The influence of the RF power on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the ZrO_2−x films were examined. The ORR activity of the ZrO_2−x catalyst increased with the RF power in the range we studied. The onset potential for ORR over ZrO_2−x deposited at 175 W was 0.88 V vs RHE. In addition, the relationship between the ORR activity and the composition, crystallinity, electric conductivity, as well as the ionization potential has been investigated. The zirconium oxide with an oxygen defected state and the higher electric conductivity showed the higher ORR activity, and the electrocatalytic activity for ORR increased with the decreasing in the ionization potential of the ZrO_2−x catalyst.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Influence of chloride ions on the stability of PtNi alloys for PEMFC cathode

The dependence of the rate of Ni dissolution from PtNi alloys on the chloride concentration was studied electrochemically in 0.5 M HClO₄ at room temperature. Electrodeposited PtNi catalysts were subjected to extensive potential cycling between 20 mV and 1.3 V at various Cl⁻ concentrations and the cyclic voltammograms (CV) response and the oxygen reduction reaction (ORR) activity of the catalysts were determined at different intervals. Energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma spectroscopy (ICP) analyses were carried out to determine the elemental composition of the alloys and the amount of dissolved Ni at different stages of the potential cycling. It was found that the presence of Cl⁻ increases the rate of Ni dissolution and by this accelerates the dealloying process relative to potential cycling in chlorine-free solutions. Dealloying is most pronounced during the initial stages of potential cycling. Already a small amount of Cl⁻ is sufficient to dissolve the majority of the non-noble metal from the alloys. Even then, under oxygen reduction conditions, the blockage of Pt surface by Cl⁻ is less pronounced for the alloys than for pure Pt catalysts, leading to marginally improved ORR activity for the PtNi alloys at low Cl⁻ concentrations. From a practical point of view, the effect of chloride ion leakage from a commercially available saturated KCl reference electrode on the electrocatalytic activity was also investigated.     

Oxygen reduction reaction on carbon-supported CoSe2 nanoparticles in an acidic medium

We investigated the effect of CoSe₂/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H₂O₂ production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe₂ nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H₂O₂ production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe₂ loading on the disk electrode. H₂O₂ production also decreased with increasing catalytic site density, a phenomenon related to the CoSe₂ loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe₂/C nanoparticles at 22 μg cm⁻², we determined that the transfer process involves about 3.5 electrons.     

Effect of Cr dopant on the cathodic behavior of LiCoO2

Compounds of the formula LiCo_1−yCr_yO₂ (0.0≤y≤0.20 and y=1.0) have been synthesized by high temperature solid-state reaction and were characterized by XRD and FT-IR. Hexagonal a and c lattice parameters increase with increasing y as expected from ionic size effects. Cyclic voltammograms reveal that the phase transformation occurring at x=0.5 in Li_1−x(Co_1−yCr_y)O₂ is suppressed for y=0.05 and 0.10. Low-current (0.01 C; 1 C=140 mA g⁻¹) galvanostatic charging curves show that the deintercalation voltage for y=0.05 and 0.10 decrease for a given x as compared to LiCoO₂. Galvanostatic charge-discharge cycling of the Li(Co_1−yCr_y)O₂ cathodes at 0.14 C and 2.7-4.3 V (vs. Li) show that increasing amount of chromium content in the LiCoO₂ lattice drastically reduces the amount of Li that can be reversibly cycled. Ex-situ XRD of the cycled cathodes show that slight cation-mixing occurs in the layered structure for y=0.05 and 0.10 and could be the reason for their poor electrochemical performance. Reversible Li intercalation/deintercalation is not possible in LiCrO₂ in the voltage range 2.7-4.3 V.     

Direct electrochemical detection of pyruvic acid by cobalt oxyhydroxide modified indium tin oxide electrodes

An enzyme-free electrode was fabricated by anodic electrodeposition of cobalt oxyhydroxide film on an ITO electrode (CoO_x(OH)_y/ITO) for direct electrochemical detection of pyruvic acid (PA) in solution. Scanning electron microscopy (SEM) and atom force microscopy (AFM) were employed to characterize the morphology of CoO_x(OH)_y film. Cyclic voltammetry (CV) was used to investigate the electrochemical properties of PA on CoO_x(OH)_y/ITO in order to select the optimal potential for the chronoamperometric detection of PA. It was found that the CoO_x(OH)_y/ITO electrode served as an excellent PA sensor with a linear detection range of 1.00 μM to 1.91 mM, a detection limit of 0.55 μM, and a high sensitivity of 417.1 μA mM⁻¹ cm⁻². Moreover, the response time of CoO_x(OH)_y/ITO to PA is less than 10 s, which is the shortest for PA detection reported in literature using electrochemical method. These properties and the high stability of CoO_x(OH)_y/ITO made it a good candidate for developing electrochemical enzyme-free PA sensing device.     

Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties

In this study, a new method of carbonizing hafnium nitride was proposed to synthesize ultrahigh-temperature hafnium carbonitride (HfC_xN_y) powders. The new method helps to maintain both the purity of phases and control the content of nitrogen in the HfC_xN_y. The results show that the as-prepared HfC_xN_y powders have a single phase, with an average particle size of approximately 2 $\mu m$, and Hf, C and N are evenly distributed. Moreover, the microstructures, phase compositions, ablation properties and mechanism of the HfC_0.62N_0.38 composites under a plasma ablation environment were studied in detail. The results show that the HfC_0.62N_0.38 composites exhibited excellent ablation resistance at 3073 K for 60 s and the ablation mechanism of HfC_0.62N_0.38 can be identified as HfC_0.62N_0.38→HfC_xO_y→HfO₂. The mass ablation rate of the HfC_0.62N_0.38 composite is evaluated to be 1.36 mg/cm²∙s, which is lower than that of HfC ceramics. Our work is intended to provide new insight regarding the development of ultrahigh-temperature ceramics and widen their applications.     

Highly active TiO2−x−yNxFy visible photocatalyst prepared under supercritical conditions in NH4F/EtOH fluid

A novel N and F co-doped TiO₂ (TiO_2−x−yN_xF_y) photocatalyst is prepared by treating the TiO₂ precursor in NH₄F/ethanol fluid under supercritical conditions. During photocatalytic degradation of methylene blue under visible light irradiation, the as-prepared TiO_2−x−yN_xF_y exhibits higher activity than the undoped TiO₂, N-doped TiO₂ (TiO_2−xN_x), and F-doped TiO₂ (TiO_2−yF_y). Based on the characterizations including XRD, Raman, FTIR, TEM, PLS, UV–vis DRS, N₂ adsorption–desorption isotherms, XPS and NH₃-TPD, the synergetic promotions of N- and F-dopants incorporated into the TiO₂ lattice are discussed based on the enhanced spectral response in visible region, oxygen vacancies, and surface acidic sites. Meanwhile, the supercritical treatment also promotes the activity owing to the increase in both the surface area and the crystallization degree of anatase, and the enhanced incorporation of N- and F-dopants into the TiO₂ lattice.     

Kinetics and electrocatalytic activity of nanostructured Ir-V/C for oxygen reduction reaction

Active, carbon-supported Ir-V nanoparticle catalysts have been synthesized by an ethylene glycol reduction method under controlled conditions at pH 10-13 and 120 °C, then further reduced at elevated temperature from 150 to 500 °C using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts have been characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). Ir nanoparticles, after modification with V, show a narrow particle size distribution in the range 0.5-4.5 nm, centered at 1.8 nm, and are uniformly dispersed on Vulcan XC-72. No particle agglomeration was observed, not even at high V loadings (V:Ir = 4:1 in atomic ratio). Investigation of the catalytic activity of the Ir-V/C by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) employing a rotating disk electrode (RDE) has revealed that the presence of V may suppress the electrochemical oxidation of Ir and stabilize the Ir active centers. About six times higher kinetic current density was obtained for Ir-V/C compared to that of the pure Ir/C catalyst at 0.8 V versus RHE for the oxygen reduction reaction (ORR). The ORR in acid solution proceeds by an approximately four-electron pathway, through which molecular oxygen is directly reduced to water. The performance of a membrane electrode assembly (MEA) prepared with the most active 40% Ir-10% V/C as the cathode catalyst in a single proton-exchange membrane fuel cell (PEMFC) generated a maximum power density of 517 mW cm⁻² at 0.431 V and 70 °C, and 100 h of stable cell operation due to no loss of catalyst sites on the cathode.     

Polypyrrole and La1−xSrxMnO3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La_1−xSr_xMnO₃)/PPy electrodes made of the perovskite La_1−xSr_xMnO₃ embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La_1−xSr_xMnO₃) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm⁻³ pyrrole (Py) plus 0.05 mol dm⁻³ K₂SO₄ with and without containing a suspension of 8.33 g L⁻¹ oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm⁻³ K₂SO₄ plus 5 mmol dm⁻³ KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.