Step For Oxygen Reduction Reaction Research Articles

DFT Study of Oxygen Reduction Reaction on Pt-Zr Catalysts

The sluggish oxygen reduction reaction (ORR) kinetics at cathode and expensive Pt electrocatalysts are two main issues that retard fuel cell technology commercialization.1-3 To overcome these problems, alloying Pt with transition metals, core-shell structures, and non-Pt catalysts are applied to improve the catalytic activity and reduce the catalyst cost.4-8 It was found that Pt3Ni, Pt3Co, and Pt3Fe have better activity than pure Pt.9-11 However, stability of these alloys needs to be improved. Pt3Zr, Pt3Y, Pt3Sc show greater stability and high catalytic activity,9,10,12but these materials need further experimental and theoretical investigations.We used DFT calculations to study the origins of the better ORR performance and greater stability of Pt-Zr catalysts. To accurately evaluate the activity of Pt-Zr catalysts, we examined binding energies of ORR intermediates and reaction energy barriers for ORR steps both in gas phase and solution. Based on the results obtained, ORR mechanisms on Pt-Zr catalysts will be considered and compared to pure Pt and Pt alloys. Some related experimental results will be discussed as well. Acknowledgement. This work is financially supported by the National Science Foundation (grant CBET-1067848).

ORR on the Different Surfaces with Different Adsorbents: First Principles Molecular Dynamics Simulations

Oxygen Reduction Reaction (ORR) is the hard task in the fuel cell operations and has been studied extensively not only from practical point of view but also from fundamental one. Although various calculations by both MO and DFT calculations were reported, many of them are under the static conditions (i.e. at 0 K) or they are on simple surfaces. We are currently working for it with more realistic models at a certain temperature under the electric bias. First Principles Molecular Dynamics (FPMD) combined with Effective Screening Medium (ESM) [1,2] to apply the potential on the electrode in the similar manner as the real double layer region. We first used it for the simple electrode reaction of the first step of the hydrogen evolution reaction (Volmer reaction) on Pt(111) [3] and then we are extending it to ORR [4,5,6] on various surfaces with various adsorbents.Electronic structure calculations in the FPMD are based on DFT with ultrasoft-pseudopotentials and PBE functional for the exchange correlation. Program code "STATE" is used. MD is performed at constant temperature 353 K by a velocity scaling thermostat. Electrode potential is controlled by the charges added by the ESM method shown in the figure.The last step of ORR (=the first step of oxygen evolution) isH3O+ + OH(ad) + e– --> 2 H2OThe reversibility of this reaction was confirmed on Pt(111) surface with surface charge density σ = 7 µC/cm2(corresponding to ca. 0.6–0.7 V vs. SHE) [4,5] with a pre-adsorbed OH. With higher or lower σ, reaction was remained in only one side. If we introduce a defect (a pit) on the surface, the reaction did not take place [6].In order to see an effect of the different surface, we simulated a full ORR from O2adsorption to H2O formation on (111), (110) and (100) surfaces. The main reaction scheme observed in the several time simulations for each surface is (dissociation scheme):O2 --> 2O(ad)O(ad) + H3O+ + e– --> OH(ad) + H2OH3O+ + OH(ad) + e– --> 2 H2OIn some cases, following H2O2 formation takes place as an intermediate species (association scheme).2O(ad) + e– + H3O+ --> H2O2 We are also testing another kind of coadsorption species as a reaction promoter.In summary, we could confirm that water liquid layer (not only single or bilayer) on the Pt surface plays an important role to proceed to a complete ORR.[1] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006). [2] O. Sugino, I. Hamada, M. Otani, Y. Morikawa, T. Ikeshoji Y. Okamoto, Surf. Sci., 601 (2007) 5237. [3] M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, T. Ikeshoji, J. Phys. Soc. Jpn.. 77 (2008) 024802, Phys. Chem. Chem. Phys., 10 (2008) 3609. [4] T. Ikeshoji, M. Otani, I. Hamada and Y. Okamoto, Phys. Chem. Chem. Phys., 13 (2011) 20223. [5] T. Ikeshoji, M. Otani, I. Hamada, O. Sugino, Y. Morikawa, Y. Okamoto, Y. Qian, and I. Yagi, AIP Advances, 2, 032182 (2012). [6] Y. Qian, I. Hamada, M. Otani, and T. Ikeshoji, Catal. Today, 202, 163-167 (2013).

Catalytic Reactions on the Open-Edge Sites of Nitrogen-Doped Carbon Nanotubes as Cathode Catalyst for Hydrogen Fuel Cells

Recent experimental reports proposed that pyridinic-type sites on the open edges of carbon nanotubes (CNTs) may contribute to the high catalytic activity for oxygen reduction reaction (ORR) on nitrogen-doped CNTs (N-CNTs). Herein, we performed first-principles spin-polarized density functional theory calculations to examine the catalytic steps for ORR and water formation reaction (WFR) on the open edges of N-CNTs. For half-N doping on the open edge of CNTs (HN-CNTs), O2 and OOH can be chemisorbed and partially reduced on the C–N bridge site without an energy barrier. The subsequent WFR for reduced O2/OOH with ambient H+ and additional electrons can be finished without energy barrier for the formation of two H2O molecules. The second H2O molecule needs an energy of ∼0.49 eV to be desorbed from the catalytic site, which completes an electrocatalytic reaction cycle on the cathode catalyst for hydrogen fuel cells (HFCs). For H-saturated open-edge sites of HN-CNT, ORR and WFR can also be completed energeticall...

Characterization and electrochemical performances of Pr2CuO4 as a cathode material for intermediate temperature solid oxide fuel cells

K2NiF4-type structure oxide Pr2CuO4 is synthesized and evaluated as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The electrode forms good contact with the electrolyte after sintering at 950°C for 4h in air. The lowest polarization resistance (Rp) (0.56Ωcm2 at 700°C in air) and cathodic overpotential values are achieved when the material is sintered at 950°C on CGO electrolyte. Two main oxygen reduction reaction (ORR) processes are identified. The rate limiting step for ORR is the charge transfer process. The excellent electrochemical performance indicates that Pr2CuO4 can be a promising cathode material for IT-SOFCs.

Evaluation of layered perovskites YBa1−xSrxCo2O5+δ as cathodes for intermediate-temperature solid oxide fuel cells

This study is focused on the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of YBa1−xSrxCo2O5+δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The high oxygen nonstoichiometry, δ = 0.18–0.43 at 700 °C, indicates the large oxygen vacancy concentrations in oxides. The electrical conductivity is improved due to the greater amount of electronic holes originated from the increased interstitial oxygen, and the conductivities of all samples are above 100 S cm−1 at 400–700 °C in air. The results demonstrate the promising performance of YBa1−xSrxCo2O5+δ cathodes at intermediate temperatures, as evidenced by low area-specific resistances (ASRs) e.g. 0.21–0.59 Ω cm2 at 700 °C. The lowest ASR, 0.44 Ω cm2, and the cathodic overpotential, −40 mV at a current density of −136 mA cm−2, are obtained in YBaCo2O5+δ cathode at 650 °C. The dependence of polarization resistance on oxygen partial pressure suggests that the charge transfer process is the rate-limiting step for oxygen reduction reaction in YBaCo2O5+δ cathode.

Comparative Study on the Catalytic Activity of the TM–N2 Active Sites (TM = Mn, Fe, Co, Ni) in the Oxygen Reduction Reaction: Density Functional Theory Study

We investigate the interaction of transition metal–nitrogen (TM–N2; TM = Mn, Fe, Co, and Ni) active sites with oxygen reduction reaction (ORR) related molecules using the density functional theory (DFT) calculations. Generally, the trend of molecular adsorption energy in TM–N2 systems is Mn–N2 > Fe–N2 > Co–N2 > NiN2. In the case of O2 adsorption, the O2 molecule is adsorbed with a symmetric side-on configuration on the TM–N2 systems, regardless the type of TM atom. We also find that when the reaction of the adsorbed HO2 molecule and an H atom takes place, instead of forming H2O2 molecule the reaction produces two OH radicals. From the evaluation of the potential energy surface profiles of the oxygen reduction reaction (ORR), we find that a direct four-electron reduction pathway could be facilitated on the TM–N2 active sites. However, all of the TM–N2 systems share the same main rate-limiting reaction, which is the OH reduction step. Generally, the edge-like TM–N2 active site has stronger molecular adsorption energy as compared to the TM–pyrrole-like active site. However, the strong molecular adsorption energy in the edge-like TM–N2 active site results in uphill energy profiles for some of ORR steps, even at a low electrode potential. This suggests that edge-like TM–N2 active site will have a relatively slower ORR rate as compared to the TM–pyrrole-like active site.

Evaluation of a brownmillerite oxide as cathode for solid oxide fuel cells

Brownmillerite oxide Ca2Fe1.3Mn0.7O5+δ is synthesized and evaluated as a new cathode material for solid oxide fuel cells (SOFCs). The average thermal expansion coefficient (TEC) of Ca2Fe1.3Mn0.7O5+δ is 13.3±0.2×10−6K−1, close to that of the typical electrolyte materials. The lowest polarization resistance (Rp) (0.22±0.02Ωcm2 at 750°C in air) and cathodic overpotential values are achieved when the material is sintered at 1000°C on Ce0.9Gd0.1O1.95 (CGO) electrolyte. Two main oxygen reduction reaction (ORR) processes are identified. The rate limiting step for ORR is found to depend on the oxygen partial pressure. The excellent electrochemical performance indicates that Ca2Fe1.3Mn0.7O5+δ can be a promising cathode material for SOFCs.

The Effect of Adding Ce1–xSmxO2–x/2 with Different Sm Contents on the Electrochemical Performance of GdBaCo2O5+δ Based Composite Cathode

AbstractIn this paper, the Ce1–xSmxO2–x/2 (x = 0.025, 0.05, 0.1, 0.2) samples were synthesized and then mixed with GdBaCo2O5+δ (GBCO) to form GBCO–Ce1–xSmxO2–x/2 composite cathodes. The electrochemical performance of the composite cathodes was investigated by the electrochemical impedance spectroscopy (EIS) as a function of temperature and oxygen partial pressure. The impedance spectra results demonstrated that the introduction of proper Ce1–xSmxO2–x/2 phase remarkably enhanced the electrochemical performance of GBCO cathode and caused a reduction in the total polarization resistance (Rp). Furthermore, as the amount of Ce1–xSmxO2–x/2 in composite cathode was fixed, the variation of Sm content in Ce1–xSmxO2–x/2 also had a significant influence on the electrochemical performance of the GBCO–Ce1–xSmxO2–x/2 cathodes. For example, the Rp of GBCO cathodes containing 10 wt.% Ce1–xSmxO2–x/2 considerably reduced from 0.37 to 0.17 Ω cm2 at 600 °C with the decreasing Sm content x from 0.2 to 0.025. The improvement in performance of the GBCO–Ce1–xSmxO2–x/2 cathodes compared to pure GBCO cathode could be mainly attributed to the catalytic activity of Ce1–xSmxO2–x/2 towards the surface diffusion related processes, which was an elementary step in oxygen reduction reaction at cathode.

Electrochemical performance of La 2Cu 1− xCo xO 4 cathode materials for intermediate-temperature SOFCs

K 2NiF 4-type structure oxides La 2Cu 1− x Co x O 4 ( x = 0.1, 0.2, 0.3) are synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. The results show that no reaction occurs between La 2Cu 1− x Co x O 4 electrode and Ce 0.9Gd 0.1O 1.95 (CGO) electrolyte at 1000 °C. The electrode forms good contact with the electrolyte after sintering at 800 °C for 4 h in air. The electrode properties of La 2Cu 1− x Co x O 4 are studied under various temperatures and oxygen partial pressures. The optimum composition of La 2Cu 0.8Co 0.2O 4 results in 0.51 Ω cm 2 polarization resistance ( R p) at 700 °C in air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer process. La 2Cu 0.8Co 0.2O 4 cathode exhibits the lowest overpotential of about 50 mV at a current density of 48 mA cm −2 at 700 °C in air.

Improved electrochemical performance of Ca2Fe1.4Co0.6O5–Ce0.9Gd0.1O1.95 composite cathodes for intermediate-temperature solid oxide fuel cells

The performance of Ca2Fe1.4Co0.6O5–Ce0.9Gd0.1O1.95 (CFC–CGO) composite cathode has been investigated for potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The composite cathodes are prepared and characterized by XRD and SEM, respectively. The electrochemical properties of the composite cathodes are investigated using AC impedance and DC polarization methods from 500 to 700 °C under different oxygen partial pressures. The polarization resistance (Rp) decreases with the increase of CGO content in the composite electrode. The addition of 40 wt.% CGO in CFC results in the lowest Rp of 0.48 Ω cm2 at 700 °C in air. Oxygen partial pressure dependence study indicates that the charge-transfer process is the rate limiting step for oxygen reduction reaction. CFC-40CGO composite cathode exhibits the lowest overpotential of about 67 mV at a current density of 85 mA cm−2 at 700 °C in air.

Roles of radical characters of pristine and nitrogen-substituted hydrographene in dioxygen bindings

We investigate by means of density functional theory (DFT) calculations how hydrogen-terminated graphenes (hydrographenes) with and without nitrogen impurities interact with dioxygen. The current study aims at searching whether hydrographenes can be utilized as cathode catalysts in fuel cell with a focus on dioxygen binding, the first step in oxygen reduction reaction (ORR). If hydrographenes have a nanometer-size rhombic structure with zigzag edges, unpaired electrons are localized at their edges with or without the nitrogen impurities. Spin localization comes from frontier orbitals of the nanometer-size hydrographenes whose amplitudes appear only at their edges. Due to their radical characters, dioxygen can bind to an edge carbon atom of the hydrographenes under the condition where fuel cell is usually operated. There are two types of dioxygen binding into a hydrographene: one is a Pauling fashion where one C-O bond is formed and the other is a bridging fashion with two formed C-O bonds. In the bridging fashion, the formation of the two C-O bonds activates dioxygen, and then radical characters of the oxygen atoms completely disappear. In contrast, the Pauling fashions retain an unpaired electron on the oxygen atom that does not participate to the C-O bond formation. The existence of radical oxygen atoms would facilitate the next step in ORR (the initial proton transfer to an adsorbed dioxygen), whereas such facilitative effects cannot be seen in its absence. According to DFT calculations, the Pauling-type bindings are always energetically preferred over the bridging-type bindings. In particular, the C→N substitution enhances the preferences of the Pauling-type binding over the bridging-type binding compared with the pristine case. Accordingly DFT calculations demonstrate that radical characters of edge carbons of a nanometer-sized rhombic hydrographene play a crucial role in dioxygen bindings in a Pauling fashion that would be responsible for enhancing the catalytic activity in fuel cell.

Electrochemical properties of nanostructured lanthanum strontium manganite cathode fabricated by electrostatic spray deposition

Electrostatic spray deposition was applied to prepare nanoporous lanthanum strontium manganite (LSM) films with high specific surface area (37.34 m 2/g) for the cathode application in solid oxide fuel cell (SOFC). The electrochemical characteristics were investigated at a temperature range from 546 to 777 °C and oxygen partial pressure from 0.01 to 1.0 atm. The diffusion of atomic oxygen and oxygen ion transfer from three-phase boundary to the YSZ electrolyte were found to be the rate-determining steps for oxygen reduction reaction on LSM cathode. The polarization resistance of the LSM prepared using electrostatic spray deposition decreased from 15 to 1.2 Ωcm 2 with increasing temperature from 546 to 777 °C and the activation energy was 0.81 eV. It was demonstrated that the ESD method offers a promising approach for the preparation of electrochemically active nanoporous layers, particularly applicable for solid oxide fuel cells.

Electrode properties of Co-doped Ca 2Fe 2O 5 as new cathode materials for intermediate-temperature SOFCs

Brownmillerite oxide Ca 2Fe 2− x Co x O 5 ( x = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca 2Fe 2− x Co x O 5 has no reaction with Sm 0.2Ce 0.8O 1.9 (SDC) electrolyte at 1100 °C for 10 h in air. The thermal expansion coefficient (TEC) of Ca 2Fe 2− x Co x O 5 increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca 2Fe 2− x Co x O 5 were studied under various temperatures and oxygen partial pressures. The polarization resistance ( R p) of Ca 2Fe 2− x Co x O 5 with x = 0.2, 0.4 and 0.6 are 0.23, 0.48 and 1.05 Ω cm 2 at 700 °C in air, respectively. The rate-limiting step for oxygen reduction reaction was the charge transfer process. Ca 2Fe 1.8Co 0.2O 5 cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm −2 at 700 °C in air.

Electrochemical reduction of oxygen on gold and boron-doped diamond electrodes in ambient temperature, molten acetamide–urea–ammonium nitrate eutectic melt

The electrochemical reduction of oxygen has been studied on gold, boron-doped diamond (BDD) and glassy carbon (GC) electrodes in a ternary eutectic mixture of acetamide (CH 3CONH 2), urea (NH 2CONH 2) and ammonium nitrate (NH 4NO 3). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and rotating disk electrode (RDE) voltammetry techniques have been employed to follow oxygen reduction reaction (ORR). The mechanism for the electrochemical reduction of oxygen on polycrystalline gold involves 2-step, 2-electron pathways of O 2 to H 2O 2 and further reduction of H 2O 2 to H 2O. The first 2-electron reduction of O 2 to H 2O 2 passes through superoxide intermediate by 1-electron reduction of oxygen. Kinetic results suggest that the initial 1-electron reduction of oxygen to HO 2 is the rate-determining step of ORR on gold surfaces. The chronoamperometric and RDE studies show a potential dependent change in the number of electrons on gold electrode. The oxygen reduction reaction on boron-doped diamond (BDD) seems to proceed via a direct 4-electron process. The reduction of oxygen on the glassy carbon (GC) electrode is a single step, irreversible, diffusion limited 2-electron reduction process to peroxide.

Role of Surface Steps of Pt Nanoparticles on the Electrochemical Activity for Oxygen Reduction

The design of highly active nanoscale catalysts for the oxygen reduction reaction (ORR) and oxidation of small molecules such as the methanol oxidation reaction (MOR) constitutes the grand scientific and technological challenge in fuel cells. Although increasing surface steps on Pt nanoparticles was shown recently to enhance the activity of electrochemical oxidation of carbon monoxide and methanol electro-oxidation, little is known about the role of surface steps of nanoparticles on ORR activity. Here, we report that the ORR activity of Pt nanoparticles of ∼2 nm is not influenced by surface steps, in contrast to MOR activity. Further, CO annealing experiments and in situ X-ray adsorption measurements clearly compare the role of surface step for ORR and MOR on Pt nanoparticles. The findings provide new insights to design highly active catalysts for proton exchange membrane fuel cells and direct methanol fuel cells.

Characterization and optimization of La 0.8Sr 0.2Sc 0.1Mn 0.9O 3− δ-based composite electrodes for intermediate-temperature solid-oxide fuel cells

Composite electrodes composed of a perovskite-type La 0.8Sr 0.2Sc 0.1Mn 0.9O 3− δ (LSSM) and a fluorite-type scandium-stabilized zirconia (ScSZ) were prepared and evaluated as potential cathodes for intermediate-temperature solid-oxide fuel cells. Characterization was made by phase reaction, electrochemical impedance spectroscopy, step current polarization and I– V tests. The phase reaction between LSSM and ScSZ occurred at 1150 °C or higher; however, it had a minor effect on the electrode performance. The formation of a composite electrode led to an obvious improvement in both charge transfer and surface-related processes. With the increase of ScSZ content, the rate-limiting step of oxygen reduction reaction steadily changed from mainly a surface diffusion process to an electron transfer process. The optimal ScSZ content and sintering temperature of the electrode layer were found to be 20 wt.% and 1100–1150 °C, respectively. Under optimal conditions, an anode-supported single cell with LSSM + ScSZ composite cathode showed high power densities of ∼1211 and 386 mW cm −2 at 800 and 650 °C, respectively.

Preparation and electrochemical properties of Sr-doped Nd 2NiO 4 cathode materials for intermediate-temperature solid oxide fuel cells

Cathode materials Nd 2 − x Sr x NiO 4 were prepared by the glycine-nitrate process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impendence spectroscopy and DC polarization method, respectively. The results show that no reaction occurred between the electrode and the CGO electrolyte at 1100 °C and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The rate-limiting step for oxygen reduction reaction on Nd 1.6Sr 0.4NiO 4 electrode changed with oxygen partial pressure and measurement temperature. The Nd 1.6Sr 0.4NiO 4 electrode gave a polarization resistance of 0.93 Ω cm 2 at 700 °C in air, which indicates that Nd 2 − x Sr x NiO 4 electrode is a promising cathode material for intermediate-temperature solid oxide fuel cell (IT-SOFC).

La substituted Sr 2MnO 4 as a possible cathode material in SOFC

Sr 2− x La x MnO 4+ δ ( x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr 2− x La x MnO 4+ δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm −1 was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr 1.4La 0.6MnO 4+ δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm 2 at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr 1.4La 0.6MnO 4+ δ electrode.

Promotion of Oxygen Reduction Reaction on Vitreous Carbon Electrode by DTAB

The effect of cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), on oxygen reduction reaction was studied by rotating vitreous carbon disk electrode. The results showed that the catalytic activity of vitreous carbon electrode on oxygen reduction reaction could be improved significantly by DTAB. From the analyses on the relation between the current of oxygen reduction and rotating speed of the electrode and the Tafel relation between kinetic current and potential, it was found that DTAB promoted the transfer coefficient of charge transfer step for oxygen reduction reaction on vitreous carbon electrode, thus improved the kinetics of oxygen reduction reaction.

Electrode properties of Sr doped La 2CuO 4 as new cathode material for intermediate-temperature SOFCs

High performance La 2− x Sr x CuO 4− δ ( x = 0.1, 0.3, 0.5) cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFCs) were prepared and characterized. The investigation of electrical properties indicated that La 1.7Sr 0.3CuO 4 cathode has low area specific resistance (ASR) of 0.16 Ω cm 2 at 700 °C and 1.2 Ω cm 2 at 500 °C in air. The rate-limiting step for oxygen reduction reaction on La 1.7Sr 0.3CuO 4 electrode changed with oxygen partial pressure and measurement temperature. La 1.7Sr 0.3CuO 4 cathode exhibits the lowest overpotential of about 100 mV at a current density of 150 mA cm −2 at 700 °C in air.