Effect of annealing temperature on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrode

The effect of annealing temperature T_ann on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrodes used in polymer electrolyte membrane fuel cells (PEMFCs) was investigated in 1 M H₂SO₄ solution using AC-impedance spectroscopy and potentiostatic current transient technique. For this purpose, the gas diffusion electrodes were annealed at different temperatures ranging from 140° to 180 °C in order to control the proton transport resistance distribution across the active catalyst layer (ACL). For the annealed gas diffusion electrodes with different proton transport resistance distributions, the measured impedance spectra exhibited a straight line inclined at a constant angle higher in absolute value than 45° to the real axis at high frequencies, followed by a depressed arc at low frequencies.From the quantitative analysis of the measured impedance spectra based upon the transmission line model (TLM) modified with the proton transport resistance distribution, it was found that as T_ann increased, the average proton transport resistance R_ave and the standard deviation σ of the proton transport resistance distribution increased as well. Furthermore, as T_ann rose, the charge transfer resistance R_ct increased and simultaneously the double layer capacitance C_dl decreased due to the smaller electrochemical active area A_ea. From the analysis of the cathodic current transients measured during nitrogen blowing, it was noted that as T_ann increased, the current decayed more rapidly with time, suggesting that the larger values of R_ave and σ kinetically impede proton transport through the Nafion membrane within the ACL due to the wider RC time constant distribution.     

Rotating disk electrode analysis of oxygen reduction at platinum particles under limiting diffusion conditions

An analysis is carried out of oxygen reduction under limiting diffusion conditions on a rotating disk electrode partially covered with platinum particles (‘particulate electrode’). First a model is developed for the current response at a rotating particulate electrode, because Levich equation, used for the classical continuous disk electrodes, is not applicable in this case. The model allows for the calculation of the limiting diffusion current by an iterative algorithm, as a function of the density and size of the particles, and the constants for the adsorption/desorption and diffusion of reactants over non-covered areas of the substrate. The model is valid when there is no overlapping of surface diffusion areas around the particles. In a second part, the oxygen reduction on platinum particles electrodeposited on a glassy carbon disk is studied. Platinum particulate electrodes with a variable density and size of particles are prepared by single pulse electrodeposition technique. Limiting diffusion currents for oxygen reduction are analysed on the light of the proposed model. Values for the oxygen surface diffusivity and the equilibrium adsorption/desorption constant on the glassy carbon substrate are obtained from the analysis.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Kinetics of oxygen reduction at composite electrodes with controlled three-phase boundaries by patterning YSZ column

The oxygen reduction mechanism was investigated at the porous LSM-patterned YSZ composite electrode by employing the ac-impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, the dense YSZ pellet was patterned by a laser beam, and was then coated with the LSM slurry. The length of three-phase boundaries (TPBs) per unit area l_TPB was effectively controlled by varying the width of the YSZ column. From analyses of the ac-impedance spectra and the cathodic PCTs obtained from the electrodes based upon the modified transmission line model (TLM), it was first experimentally confirmed that the effective migration length l_m decreased with increasing l_TPB under the mixed migration and charge-transfer control. Secondly, as the value of l_TPB increases, the charge-transfer resistance R_ct is decreased to a more extent but the ion migration resistance R_i is reduced to a less extent. Finally, from a comparison of the cathodic PCTs measured on the porous LSM-YSZ composite electrode to those measured on the porous LSM-patterned YSZ composite electrode, the oxygen reduction kinetics at that porous composite electrode was discussed in terms of the steady-state current density i_st and the time to reach the steady-state current density t_st.     

Application of conventional activated carbon loaded with dispersed Pt to PEFC catalyst layer

Improvement of the polymer electrolyte fuel cell (PEFC) requires development of highly active electrodes of low cost to facilitate its widespread use. In the present study, the possibility of applying conventional activated carbon particles loaded with Pt to the electrode catalyst layer was tested because the particles were promising in dispersion of Pt and preparation cost. The catalyst layer was formed from the particles and Nafion® and was supported as a thin film on a rotating glassy carbon disk electrode (GC RDE). Activity for oxygen reduction was evaluated by the hydrodynamic voltammetry in perchloric acid to give a current free of the influence of mass transfer in the solution. Compared with a conventional catalyst layer formed from carbon black loaded with Pt, the new catalyst layer exhibited a significant, approximately 6-fold increase in current in the high potential region corresponding to a 100 mV increase in electrode potential. Activity, however, was retarded in the low potential region. This disadvantage was overcome by mixing a conductive agent into the layer and covering it with another layer containing carbon black loaded with Pt. This double catalyst layer exhibited increased activity across all potential regions, indicating the availability of the activated carbon in the electrodes.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

Studies on the heterogeneous electron transport and oxygen reduction reaction at metal (Co, Fe) octabutylsulphonylphthalocyanines supported on multi-walled carbon nanotube modified graphite electrode

Heterogeneous electron transfer dynamics and oxygen reduction reaction (ORR) activities using octabutylsulphonylphthalocyanine complexes of iron (FeOBSPc) and cobalt (CoOBSPc) supported on multi-walled carbon nanotube (MWCNT) platforms have been described. The MWCNT-based electrodes (MWCNT-CoOBSPc and MWCNT-FeOBSPc) showed larger Faradaic current responses than the electrodes without the MWCNTs, interpreted as a consequence of the trapped electrolyte species within the porous layers of MWCNTs undergoing a redox process. The EPPGE-MWCNT-FeOBSPc showed onset potential (−0.01 V vs Ag|AgCl) which is comparable and even much lower than recent reports. The MWCNT-FeOBSPc showed the best ORR activity involving a direct 4-electron mechanism, with a Tafel slope of about 124 mV, indicating a 1-electron process in the rate-determining step.     

Electrosynthesis of hydrogen peroxide by partial reduction of oxygen in alkaline media. Part II: Wall-jet ring disc electrode for electroreduction of dissolved oxygen on graphite and glassy carbon

A wall-jet ring disc electrode was constructed by adapting a wall-jet flow through electrochemical cell. Commercially available spectral graphite and glassy carbon were used as working disc electrodes and the ring electrode was made of stainless steel. The efficiency and rate constants, measured in a planar parallel flow hydrodynamic regime, indicated the partial electroreduction of dissolved oxygen as a quasi-reversible two-electron process for both electrode materials tested.     

An investigation of lithium transport through vanadium pentoxide xerogel film electrode by analyses of current transient and ac-impedance spectra

Lithium transport through vanadium pentoxide xerogel film electrode has been investigated in a 1 M solution of LiClO₄ in propylene carbonate by employing potentiostatic current transient technique and ac-impedance spectroscopy. From the comparison of the initial current experimentally measured with those initial currents theoretically calculated from the Ohm’s law and the Cottrell equation, it was confirmed that the cell-impedance-controlled constraint at the electrode surface is changed to the real potentiostatic boundary condition (diffusion-controlled constraint) when the potential step exceeds a critical value over the whole range of the lithium content. It was also found that the slope of the logarithmic current transient obtained at the lithium contents above 0.4 positively deviates in absolute value from 0.5 even under the real potentiostatic boundary condition, but the phase angle of the diffusion impedance under the semi-infinite diffusion condition negatively deviates in absolute value from 45° with increasing lithium content. With the aid of the X-ray diffractometry, the anomalous behaviours of the current transient and the diffusion impedance were discussed in terms of lithium transport through the interlayers with widely distributed spacings across the quasi-ordered xerogel film electrode. Furthermore, the current transient theoretically determined by employing the concept of interlayer spacing distribution coincided fairly well in form with that current transient experimentally measured.     

Effect of support type and synthesis conditions on the oxygen reduction activity of RuxSey catalyst prepared by the microwave polyol method

Ru_xSe_y nanoparticles supported on different carbon substrates were synthesized by microwave heating of ethylene glycol solutions of Ru(III) chloride and sodium selenite at different pH and Ru/Se mole ratios. The resulting catalysts were used for the electrochemical oxygen reduction reaction (ORR) in acidic solution. The electrochemical activity was highest for the supported catalyst synthesized at pH 8. Increasing the Se concentration of the catalyst up to 15 mol% increased the catalytic activity for the ORR; at this Se concentration, the activity of the catalyst was considerably higher than that observed for pure Ru catalyst synthesized at exactly the same conditions. The influence of the type of carbon support on the activity of the electrocatalyst was also investigated. Among the different supports, including carbon black (Vulcan XC-72R) (C1), and nanoporous carbons synthesized from resorcinol- (C2) and phloroglucinol-formaldehyde (C3) resins, the Ru_xSe_y catalyst supported on C3 exhibited highest activity for ORR.     

Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for efficient power generation in the 21st century. Currently, high temperature proton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. Owing to the aforementioned features, high temperature proton exchange membrane fuel cells have been utilized more widely compared to low temperature proton exchange membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technological constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid-base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temperature operation of proton exchange membrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membrane fabrication and physicochemical characterizations is provided. The proton conductivity and cell performance of the polymeric membranes can be improved by high temperature treatment. The mechanical and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chemical stability of the polymer, acid management, and the integral interface between the electrode and membrane.