Rate-determining Step Of Oxygen Reduction Reaction Research Articles

A New Catalyst System for the Oxygen Reduction Reaction (ORR) at Fuel Cell Cathodes: P-Block Precious Group Metal-Free Tin and Nitrogen-Doped Carbon (SnNC) Catalyst

The sluggish kinetics of the Oxygen Reduction Reaction (ORR) and the consequently large amount of platinum catalyst required for cathode material in Proton Exchange Membrane Fuel Cell (PEMFC) is today’s primary hurdle for commercializing fuel cell technology in automotive application.1 A class of carbon-doped with metal and nitrogen electrocatalysts (labelled MNC) is promising substitute for platinum-group metals (PGMs) in oxygen reduction reaction (ORR) in PEMFC.2 Particular attention was placed on a specific family of MNC catalysts derived from 3d transition metal precursors with FeNC and CoNC being the most promising for PEMFC applications requiring high power density.3-5 In the present contribution, we report on a new MNC catalyst system, different by its chemical nature from all others catalysts previously reported as ORR-active in acidic medium. The highlights of this work are the successful design and synthesis of SnNC as a non-noble metal catalyst for the first time, and the finding that SnNC hosts Sn-based active sites that can achieve the same turnover frequency as Fe-based active sites in FeNC, and much higher turnover frequency than Co-based active sites in CoNC. FeNC and CoNC materials have, until now, been the state-of-art PGM-free catalysts for ORR in acidic medium. The present SnNC material shows also high and similar selectivity for the four-electron ORR pathway as FeNC, much higher than CoNC materials. The prepared SnNC catalysts meet and exceed the FeNC catalysts in terms of hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a Fenton-inactive character of Sn. Among other techniques, density functional theory and 119Sn Mössbauer spectroscopy were used in combination to investigate SnNC, providing for the first time insights on the structure of its active sites, their rate-determining step for ORR and selectivity for the four-electron reduction pathway. References Holton, O. T.; Stevenson, J. W., The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Metals Review 2013, 57 (4), 259-271.Luo, F.; Wagner, S.; Ju, W.; Primbs, M.; Li, S.; Wang, H.; Kramm, U. I.; Strasser, P., Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors. Journal of the American Chemical Society 2022, 144 (30), 13487-13498.Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8 (1), 957.Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature materials 2015, 14 (9), 937-942.Luo, F.; Roy, A.; Silvioli, L.; Cullen, D. A.; Zitolo, A.; Sougrati, M. T.; Oguz, I. C.; Mineva, T.; Teschner, D.; Wagner, S.; Wen, J.; Dionigi, F.; Kramm, U. I.; Rossmeisl, J.; Jaouen, F.; Strasser, P., P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature materials 2020, 19 (11), 1215-1223.

Importing Antibonding-Orbital Occupancy through Pd-O-Gd Bridge Promotes Electrocatalytic Oxygen Reduction.

The active-site density, intrinsic activity, and durability of Pd-based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd2 O3 , Sm2 O3 , Eu2 O3 , and CeO2 ) by using RE metal-organic frameworks to tune the ORR performance of the Pd sites through the Pd-REx Oy interface interaction. Taking Pd-Gd2 O3 /C as a representative, it is identified that the strong coupling between Pd and Gd2 O3 induces the formation of the Pd-O-Gd bridge, which triggers charge redistribution of Pd and Gd2 O3 . The screened Pd-Gd2 O3 /C exhibits impressive ORR performance with high onset potential (0.986 VRHE ), half-wave potential (0.877 VRHE ), and excellent stability. Similar ORR results are also found for Pd-Sm2 O3 /C, Pd-Eu2 O3 /C, and Pd-CeO2 /C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd2 O3 promotes electron transfer through the Pd-O-Gd bridge, which induces the antibonding-orbital occupancy of Pd-*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd-Gd2 O3 is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd-Gd2 O3 /C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.

Tuning oxygen vacancy of the Ba0·95La0·05FeO3−δ perovskite toward enhanced cathode activity for protonic ceramic fuel cells

Innovation of highly active cathode is of great significance to the development of protonic ceramic fuel cells (PCFCs). Herein, tailoring oxygen vacancies in Zn-doped Ba0·95La0·05FeO3−δ (BLFZ) perovskite is proved to be beneficial for promoting the formation of proton defects. Hydration ability of the triple conducting BLFZ perovskites is confirmed by electrical conductivity relaxation (ECR). The results demonstrate that BLFZ exhibits a proton surface exchange coefficient of 1.34 × 10−3 cm s−1 at 600 °C, which greatly extends active sites from the electrolyte/cathode interface to the entire electrode. Mechanism and process elementary steps of the oxygen reduction reaction (ORR) of BLFZ-BaCe0.7Zr0·1Y0.1Yb0.1O3−δ (BCZYYb) are detailedly studied. It is found that the rate-determining step of ORR is surface dissociative adsorption of oxygen on BLFZ-BCZYYb cathode. A maximum power density of 673 mW cm−2 at 700 °C is achieved and BLFZ-BCZYYb based single-cell shows no obvious degradation at 600 °C for 200 h. The good performance is ascribed to the rapid proton diffusion of BLFZ-BCZYYb composite electrode by regulating the oxygen vacancies.

(Invited) Kinetic Isotope Effect As a Tool to Investigate the Oxygen Reduction Reaction on Pt-Based Electrocatalysts

Oxygen reduction reaction (ORR) mechanisms on Pt electrocatalyst was investigated with kinetic isotope effect (KIE). Specifically, the rate-determining step for ORR on dispersed Pt/C electrocatalyst and polycrystalline Pt (Pt-poly) was studied. KIE is defined as the ratio of the kinetic current measured in protonated electrolyte versus deuterated electrolyte, where KIE values larger than one indicate the proton participation in the rate-determining step. We have investigated the effect of poisoning anions on the Pt on the rate determining step by assessing the KIE in perchloric (non-poisoning) and sulfuric acid-based electrolytes. The kinetics currents were calculated using both Koutechy-Levich and Tafel analysis. A KIE of 1 was observed for high loading, Pt/C (with a 40 wt.% Pt loading) and Pt-poly, thus indicating that, on 40 wt. % Pt/C and Pt-poly, the rate determining step is proton independent. We have extended the study to various carbon black supports with specific surface area ranging from 80 to 900 m2/g. When the density of Pt nanoparticles on the surface of carbon support was high, we observed KIE to be 1. However, at lower Pt nanoparticles densities the KIE was higher than one, indicating that carbon support has some impact on the rate determining step. Figure 1

Fe atoms anchored on defective nitrogen doped hollow carbon spheres as efficient electrocatalysts for oxygen reduction reaction

Defective electrocatalysts, especially for intrinsic defective carbon, have aroused a wide concern owing to high spin and charge densities. However, the designated nitrogen species favorable for creating defects by the removal of nitrogen, and the influence of defects for the coordination structure of active site and oxygen reduction reaction (ORR) activity have not been elucidated. Herein, we designed and synthesized a pair of electrocatalysts, denoted as Fe-N/C and Fe-ND/C for coordination sites of atomic iron-nitrogen and iron-nitrogen/defect configuration embedded in hollow carbon spheres, respectively, through direct pyrolysis of their corresponding hollow carbon spheres adsorbed with Fe(acac)3. The nitrogen defects were fabricated via the evaporation of pyrrolic-N on nitrogen doped hollow carbon spheres. Results of comparative experiments between Fe-N/C and Fe-ND/C reveal that Fe-ND/C shows superior ORR activity with an onset potential of 30 mV higher than that of Fe-N/C. Fe-ND sites are more favorable for the enhancement of ORR activity. Density functional theory (DFT) calculation demonstrates that Fe-ND/C with proposed coordination structure of FeN4−x (0<x<4) anchored by OH as axial ligand during ORR, weakens the strong binding of OH* intermediate and promotes the desorption of OH* as rate-determining step for ORR in alkaline electrolyte. Thus, Fe-ND/C electrocatalysts present much better ORR activity compared with that of Fe-N/C with proposed coordination structure of FeN4.

Confinement of Fe2O3 nanoparticles in the shell of N-doped carbon hollow microsphere for efficient oxygen reduction reaction

Fe2O3 nanoparticles is confined in the mesoporous shell of N-doped carbon microsphere for oxygen reduction reaction (ORR). A series of hollow catalysts together with corresponding control samples have been constructed, characterized and evaluated. The structure, composition and state of the as-prepared catalysts were characterized by Scanning electron microscope, Transmission electron microscope, X-Ray diffraction, X-Ray photoelectron spectroscopy, Raman and nitrogen adsorption-desorption. The electrochemical activity and selectivity of Fe2O3-based catalyst toward the 4e− pathway has been greatly improved (Eonset of −50 mV, E1/2 of −150 mV and average electron transfer number of 3.87). The current density is 18.5% higher than that of commercial 20 wt% Pt/C. Density functional theory calculations reveal that the confinement of Fe2O3 nanoparticles in the interlayer of N-doped graphite can significantly promote the activity of N-doped graphitic surface in alkaline media for ORR, due to facilitated O2 adsorption and reduced energy of OOH formation which is the rate-determining step of ORR. Such a confinement strategy provides a general and effective route to push the limit of transition metal oxides to a higher level for ORR and realize the controllable construction of highly active non-noble-metal catalysts with well-defined mechanism understanding.

Effect of Surface-Specific Treatment by Infiltration into LaNi6Fe4O3- δ Cathodic Backbone for Solid Oxide Fuel Cells

In this study, the impact of various infiltrates on the kinetics and rate-limiting step of oxygen reduction reaction (ORR) is examined with LaNi0.6Fe0.4O3- δ (LNF) as the cathode backbone of a solid oxide fuel cells (SOFC). Five materials were infiltrated: LaNi1-xFexO3- δ, LaxSr1-xCoyFe1-yO3-δ (LSCF), La1-xSrxMnO3-δ (LSM), Gd1-xCexO2-δ (GDC) and Pr6O11. The dependence of electrode polarization resistance on the precursor concentration, temperature, oxygen partial pressure is presented with related discussions on ORR kinetics and rate-determining step for ORR. A one-step infiltration of all five materials resulted in a dramatic decrease in polarization resistance (Rp) and related activation energy (Ea), which is ascribed to a significant facilitation of surface processes such as dissociative adsorption with electron transfer and diffusion of oxygen intermediates. Finally, a short-term durability test in a Cr-containing environment indicated that the LSCF-infiltrated cell is susceptible to Cr poisoning unlike the GDC- or LNF-infiltrated cells as expected but advantageous in terms of morphological stability during operation.

Effect of Spin Ordering on Oxygen Reduction Reaction on Af/Pm-NiO Surface

The reduction reaction of diatomic oxygen molecules (O2(g)) to split into monatomic oxide ions (O2-) is attracting attention for the oxide cathode reactivity to reduce cathodic over-potential in solid oxide fuel cells and other electrochemical applications. Various experimental and theoretical approaches have been reported to understand the rate determining step (RDS) of the oxygen reduction reaction (ORR) on the surface of oxide cathodes, mainly composed of 3d transition metal like (La, Sr)CoO3. Although it is generally accepted that RDS of ORR on oxide cathodes is the process for a surface adsorbate of to meet with surface oxygen vacancy (Vö(s)) [i], an atomistic analysis on the adsorption of oxygen molecule and the splitting reaction is still under debated; how the electrons are transferred and how the splitting of oxygen molecule proceeds. On the other hand, the most stable form of O2(g) is so-called “triplet” oxygen, which has a pair of up -spin electrons in two orbitals and the highest molecular orbital of is completely unoccupied. This electron configuration indicates that, the oxygen molecule, during ionization process, initially accept only two down -spin electrons to form bonded to surface Ni ions, and subsequently a pair of up - and down -spin electrons to dissociated into two O2-on surface. One may, then, come up with a simple question whether or not oxygen molecules have such spin preference during ORR. If it is true, ORR on oxide is, to a certain extent, influenced by the spin polarization of donated electrons, and therefore, the spin configuration of the oxide surface. We employed ab initio molecular dynamics (ai-MD) simulation and a nudged elastic band (NEB) density function theory simulation to make precise analysis on the electron transfer process. The material we chose was nickelous oxide (NiO), which is well known to exhibit an anti-ferromagnetic (af) order below Néel temperature around 523 K and becomes the disordered paramagnetic (pm) state above the temperature. The ai-MD simulation was carried out using Ni24O24 and Ni23O23 supercells with and without Vö(s), respectively, connected to a vacuum cavity placed with 8 O2(g) molecules above NiO slab. In the latter case, Vö(s) was introduced as a Schottky pair to maintain charge neutrality. The ai-MD simulation was made with and without spin polarization corresponding to af- and pm-NiO. The simulation results show that, when a O2(g) molecule approaches to the NiO surface, O2(g) is immediately charged with electrons, primarily owing to the increase of the coordination number of Ni2+/3+ ions, from monodentate to bidentate regardless of the presence of Vö(s) and spin polarization [ii]. No splitting reaction was observed on surface without Vö. The adsorbates further dissociated on the surface of pm-NiO with Vö, but no splitting reaction proceeds on the surface of af-NiO even with the presence of Vö. The NEB simulation suggests that almost monotonous decrease of total energy with the increase of coordination number for pm-NiO, but a thermally activated process with its activation energy of 0.7 eV is necessary for the af-NiO case, because of additional spin dipole moment for the latter. Also observed was spin reversal during the electron transfer. The results indicate significant influence of spin polarization on ORR. A strong effect of af-order on the oxygen chemisorption on undoped and Li-doped NiO was discovered in late 60’s, but no reasonable explanation has been given [iii]. A similar strong influence of spin order of oxide cathode is anticipated in other 3d transition metal oxide systems with ferromagnetic and antiferromagnetic configuration, and the extent of “spin buffering capacity” seems a quite important factor that govern the reactivity of ORR on oxide surface. [i] M. Kukja et al, PCCP, 15(2013), 5443–5471: S. Miyoshi et al., Solid State Ionics, 285(2016), 202–208. [ii] S. Sugiura et al., Solid State Ionics, 285(2016), 209–214: S. Sugiura et al., To be submitted. [iii] E.R.S. Winter, J. of Catalysis 6(2016), 35–49: B. De Rosa et al., Reactivity of Solids 4(1987), 53-72.

(Invited) Rate-Determining Step for Oxygen Reduction Reaction on Oxide Cathode in SOFC and Its Interpretation Based on Band Energy Diagram

It is well known that the rate-determining step (RDS) of highly reactive oxide cathodes like La1-xSrxCoO3 (LSC), La1-xSrxCo1-yFeyO3 (LSCF), and Ba1-xSrxCo1-yFeyO3 (BSCF), for solid oxide fuel cells (SOFCs) is the surface reaction of oxygen reduction reaction (ORR), shown as 1/2O2(g) + Vö -> Oo + 2h, while its elementary step is still under discussion to date. The authors focus on the surface reactivity, defined as interfacial conductivity, σe, which is inverse of area-specific resistance, using dense thin film cathode. In the present study, σe was estimated using conventional triple-electrode cells composed of thin dense cathodes with additional thin surface modified layer, either polycrystalline pellet of Gd-doped CeO2 (GDC) or single crystalline yttria-stabilized zirconia (YSZ) with GDC barrier layer, and porous platinum anode. Various type of cathodes of LaCoO3 (LC) and LSC with variable (La+Sr)/Co ratios as well as pure and BaPrO3 coated PrOx, which shows stoichiometric composition of Pr6O11 at low temperatures below ≈480˚C in high oxygen pressure regime. The compact cathode films of LC/LSC with typically 500 nm thick coated with surface modification layer of 0, 10, 20 and 40nm are sputtered at RT, followed by a post heat-treatment at 600 ˚C. PrOx is selected to examine the influence of electronic band structure by comparison with LC and LSC systems, since PrOx is known as a mixed Vö and electron conductor in contrast to Vö and electron hole dominance in nonstoichiometric LC and LSCs. The measurements were made from 400 to 600˚C in pure O2 gas flow and the oxygen partial pressure (Po2) dependence of σe was examined at 600˚C from 10-2 to 1 atm. All the impedance data show three distinct semicircles in the impedance spectra, and specific types of resistance, corresponding to the electrolyte impedance, REl, middle frequency impedance, RMF, and low frequency impedance, RLF were extracted for further analyses. Although the origin of RMF is not well understood, it is reasonable to assume as the impedance that occurs out of cathode region; presumably an interfacial impedance between cathode and electrolyte. Although RMF of all the samples does not show the Po2 dependence, RLF clearly exhibits 1/2 power dependence for all the cathode materials from LC/LSC to PrOx system. The σe values of PrOx were 3-5 and 1-2 orders of magnitude lower than LSC and LC, respectively, which were greatly improved by adding merely 1nm thick surface layer of BaPrO3, presumably due to increased surface basicity. The rate equation with square root dependence of Po2 can be interpreted that the RDS is the reaction between the peroxide ion (O2 2-) adsorbed on the oxide cathode surface and Vö diffusing from cathode/electrolyte interface. Therefore, σe is proportional to the probability for bi-dentate immobile O2 2- to meet with Vö, expressed by the concentration product of [O2 2-][Vö]. Poor performance of PrOx can be attributed to a low [O2 2-] due to low basicity. The ab initio molecular dynamics were employed to observe the surface reaction of oxygen molecules on NiO with and without Vö at the surface. NiO was selected as a model system and GGA functional without spin correlation were employed. O2 molecules are adsorbed on NiO surface in step-by-step manner from mono-dentate O2 - to bi-dentate O2 2-. The increase of coordination number of O2 immediately increase the number of donated electron, maintaining one electron per coordination as average, and no further electron donation occurs without a help of Vö. Upon meeting of O2 2- and Vö, O2 2- splits and the third electron is donated to decompose O2 2- into OoX and O- ad. The observation above justify our conclusion from the experiments that the reaction between O2 2- and Vö is the RDS for ORR. A combination of O1s XAS and XPS measurements were made to estimate the electronic structure of oxide cathode materials to understand the energy level alignment between adsorbed O2 2- and cathode oxide surface. A higher EF of cathode material contributes to easy transfer of electrons, which is assumed to be represented, as a zeroth approximation, by the energy separation between EF and the maximum occupied partial DOS of O2p orbital in the valence band, ΔE, which corresponds to the basicity of the oxides. The σe for ORR observed shows excellent proportionality with ΔE, suggesting an important role of O2 2- in the cathodic ORR.

Oxygen Reduction Reaction at Multi-Copper Complexes Investigated by Modified Electrode and In Situ XAFS

The oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells (PEFCs), requires a large overpotential (more than 200 mV) even at the highly active platinum group metal (PGM) catalysts, since this reaction contains 4-electrons and 4-protons transfer to yield two water molecules from a dioxygen molecule. However, the biocathodes which are constructed by covalently attaching multi-copper enzymes, such as laccase (Lac), at electrode surfaces show nonexsistent overpotential (ca. 20 mV) toward ORR[1-2]. Unfortunately, the pH region, where the Lacs works well as an electrocatalyst for ORR, is limited and the spatial density of reactive sites at such the biocathode seems to be insufficient as compared with the state-of-the-art PGM electrocatalysts. Thus, the extraction of reactive sites in Lac and the effective arrangement on catalyst supports have been desired[3]. Recently, Gewirth and co-workers have achieved a preparation of binuclear copper complex, Cu2(Hdatrz)2, as an electrocatalyst for ORR[4-6]. Although the catalyst shows relatively large overpotential for ORR in acidic solution, the activity in alkaline solution is quite high. The active site for ORR in the catalyst has been proved to be bi-copper centers [6-7] and the investigation of ORR mechanism at the catalyst seems to be valuable.In the present study, we have adopted in situ X-ray absorption fine structure (XAFS) spectroscopy to monitor the electronic structure of Cu centers during ORR and to determine the molecular structure of Cu2(Hdatrz)2 complexes on electrodes. In situ XANES measurement has shown that the ORR at the Cu centers requires the formation of CuI species and the rate-determining step for ORR is dependent on pH.For further development of metal complex catalysts by replacing Cu centers to other metal ions, a easy preparation of model complex electrode should be required. Chemically modified model electrodes are also constructed on gold surface to investigate ORR at multi-metal complex using electrochemistry and spectroscopic methods. This work was supported by New Energy, Industrial Technology Development Organization (NEDO). XAFS experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G200 and 2013G173)[1] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B, 110, 11180 (2006)[2] M.S. Thorum, C.A. Anderson, J.J. Hatch, A.S. Campbell, N.M. Marshall, S.C. Zimmerman, Y. Liu, A.A. Gewirth, J. Phys. Chem. Lett, 1, 2251 (2010)[3] C.H. Kjargaard, J. Rossmeisi, J.K. Norskov, Inorg. Chem., 49, 3567 (2010)[4] M.S. Thorum, J. Yadav, A.A. Gewirth, Ang. Chem. Int. Ed., 48, 165 (2008)[5] F.R. Brushett, M.S. Thorum, N.S. Lioutas, M.S. Naughton, C. Tornow, H-R. M. Jhong, A.A. Gewirth, P.A. Kenis, J. Am. Chem. Soc., 132, 12185 (2010)[6] M.S. Thorum, J.M. Hankett, A.A. Gewirth, J. Phys. Chem. Lett., 2, 295 (2011)[7] C.C.L. McCrory, A. Devadoss, X. Otterwaelder, R.D. Lowe, T.D.P. Stack, C.E.D. Chidsey, J. Am. Chem. Soc., 133, 3696 (2011)

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.