On Oxygen Reduction Reaction Activity Research Articles

First-principles studies of enhanced oxygen reduction reactions on graphene- and nitrogen-doped graphene-coated platinum surfaces.

Developing innovative platinum-based electrocatalysts and enhancing their efficiency are crucial for advancing high-performance fuel cell technology. In this study, we employed DFT calculations to provide a theoretical basis for interpreting the impact of graphene coatings on various Pt surfaces on oxygen reduction reaction (ORR) catalytic activity, which are currently applied as protective layers in experiments. We comprehensively assess the geometric and electronic properties of Pt(100), Pt(110), and Pt(111) surfaces in comparison to their graphene-coated counterparts, revealing different adsorption behaviors of O2 across these surfaces. The ORR mechanisms on different Pt surfaces show distinct rate-determining steps, with Pt(111) showing the highest ORR activity, followed by Pt(110) and Pt(100). Graphene coatings play a key role in enhancing charge transfer from the surface, resulting in modifications of O2 adsorption. Despite influencing ORR kinetics, these graphene-coated surfaces demonstrate competitive catalytic activity compared to their bare counterparts. Notably, Pt(111) with a graphene coating exhibits the lowest activation energy among graphene-coated surfaces. Our calculations also suggest that the ORR can occur directly on non-defective Pt@graphene surfaces rather than being restricted to exposed Pt centers due to point defects on graphene. Furthermore, our work highlights the potential of nitrogen doping onto the Pt(111)@C surface to further enhance ORR activity. This finding positions nitrogen-doped Pt@C as a promising electrocatalyst for advancing electrochemical technologies.

Synthesis of MnOOH and its application in a supporting hexagonal Pd/C catalyst for the oxygen reduction reaction.

With the expansion of global energy problems and the deepening of research on oxygen reduction reaction (ORR) in alkaline media, the development of low cost and high electrocatalytic performance catalysts has become a research hotspot. In this study, a hexagonal Pd-C-MnOOH composite catalyst was prepared by using the triblock copolymer P123 as the reducing agent and protective agent, sucrose as the carbon source and self-made MnOOH as the carrier under hydrothermal conditions. When the Pd load is 20% and the C/MnOOH ratio is 1 : 1, the 20% Pd-C-MnOOH-1 : 1 catalyst obtained by the one-step method has the highest ORR activity and stability in the alkaline system. At 1600 rpm, the limiting diffusion current density and half-wave potential of the 20% Pd-C-MnOOH-1 : 1 electrocatalyst are -4.78 mA cm-2 and 0.84 V, respectively, which are better than those of the commercial 20%Pd/C catalyst. According to the Koutecky-Levich (K-L) equation and the linear fitting results, the electron transfer number of the 20%Pd-C-MnOOH-1 : 1 electrocatalyst for the oxygen reduction reaction is 3.8, which is similar to that of a 4-electron process. After 1000 cycles, the limiting diffusion current density of the 20%Pd-C-MnOOH-1 : 1 catalyst is -4.61 mA cm-2, which only decreases by 3.7%, indicating that the 20%Pd-C-MnOOH-1 : 1 catalyst has good stability. The reason for the improvement of the ORR performance of the Pd-C-MnOOH composite catalyst is the improvement of the conductivity of the carbon layer formed by original carbonization, the regular hexagonal highly active Pd particles and the synergistic catalytic effect between Pd and MnOOH. The method of introducing triblock copolymers in the synthesis of oxides and metal-oxide composite catalysts is expected to be extended to other electrocatalysis fields.

(Digital Presentation) Rotating Ring-Disk Electrodes Studies on Oxygen Reduction Reaction at Hydrophobic Ionic Liquids Layer Modified Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are expected as one of the clean, highly effective energy conversion systems, and have been actively studied. Pt nanoparticles on carbon supports (Pt/C) are commonly used as a cathode catalyst for PEFCs, because of the high activity on oxygen reduction reaction (ORR). However, its durability in long term driving and high mass activity for ORR have not sufficiently fulfilled at the present. More improvements of Pt/C catalyst for ORR activity and the durability are required for the spread of PEFCs To improve the activity and durability of Pt/C, ionic liquids (ILs) modified catalysts, called Pt/C-Solid catalysis with an IL layer (SCILL) catalysts, have recently attracted much attention [1]. ILs are liquid salts around room temperature, which have high ionic conductivity and electrochemical stability. We focused on hydrophobic ionic liquids with quaternary phosphonium cations [2]. Quaternary phosphonium cations ionic liquids have higher ionic conductivity and hydrophobicity than quaternary ammonium ionic liquids with the same structure and the same counter ions. Because of interference in the oxygen reduction reaction on platinum by water, increasing the hydrophobicity of the surface is expected to improve the activity on Pt/C catalysis. Hence, by using the quaternary phosphonium cations ILs for Pt/C-SCILL catalysts, it promises to improve the ORR activities for the catalyst. On the other hand, proton conduction and oxygen permeation rate are also important factors for ORR activity enhancement. In this study, we report the ORR activity for Pt/C-SCILL catalysts with ILs composed of highly hydrophobic cations and hydrophobic anions. Quaternary phosphonium based ILs (PXXXY+TFSA-[alkyl chain X= 4 Y= 1, 12, 16]) were used for Pt/C-SCILL modified electrodes. Commercially available 20 wt% or 30 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The ultrasonicated Pt/C catalyst suspension was dropped on the mirror polished glassy carbon disk electrodes. Pt/C-SCILL modified electrodes were prepared by recasting the ILs-2-propanol solution on the bare Pt/C modified electrodes or adding ILs to the catalyst suspension. Moreover, the mixture of the ILs and bis(trifluoromethanesulfonyl)imide (HTFSA) were also examined. The thickness of the ILs layer was calculated relative to the BET surface area of the carbon support. A platinum wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. CV and LSV were carried out in 0.1 M HClO4 solution under Ar (CV) or O2 (LSV) atmosphere, respectively. The Pt/C-SCILL catalyst was prepared by recasting (thin layers, ~0.4 nm) or mixing (thick layers, 1nm~) methods. In the case of the thin layer (~0.4 nm) of ILs, the ORR activity increased as the size of the cations increased. However, for the thicker (1 nm ~) ILs layers, the ORR activity was lower than for the bare Pt/C catalysts. It seems that the aprotic ILs interfered with the proton supply, making the four-electron reduction reaction of oxygen less likely to occur. Therefore, we attempted to supply protons to the thick ILs layer by mixing an ionic liquid with a proton source. HTFSA was dissolved in P444(12)TFSA, which showed relatively good ORR activity in thin film studies. The Pt/C-SCILL catalyst was prepared by adding the mixture with a catalyst dispersion. This proton mixing Pt/C-SCILL catalyst showed a significant improvement in ORR activity compared to bare Pt/C catalyst.

(Digital Presentation) Electrocatalytic Evaluation of Atomically Dispersed Cobalt on to Boron-Nitrogen Co-Doped Carbon as Chloride Tolerant Cathodes for Desalination Fuel Cells

In recent years, the demand on drinking water escalates the research on desalination technology. Existing desalination technologies utilize fossil fuel driven electricity which gains the researchers attention to develop the use of renewable resources. To address this, we have developed a novel device named desalination fuel cell (DFC), [1] that simultaneously desalinate the feed water and generate electricity by the anodic hydrogen oxidation and cathodic oxygen reduction. The bottleneck of DFC performance largely depends on oxygen reduction reaction (ORR) [2] and requires highly active catalysts. Herein, platinum nanoparticles deposited on activated carbon (Pt/C) is recognized as the state-of-the-art catalysts, however, the scarcity, cost and stability limited the usage in the real application. In addition, the catalyst surface poisoning due to chloride ions (Cl−) from the feed water deteriorates Pt/C catalytic activity in DFC. Therefore, the seek for chloride tolerant and highly active ORR catalysts for DFC are of interest. Several electro-catalysts have been investigated for chloride tolerance ORR activity [3-5]. Very recently, we have reported Fe/N/C as Cl- tolerant ORR catalyst with an on-set potential of 0.84 V vs. RHE in the presence of Cl- ions was on par with that of the state-of-the-art Pt/C in acidic medium [6]. The optimized Fe/N/C catalyst was utilized in DFC cathodes and delivered the highest open circuit voltage of 1.6 V on comparison with Pt/C cathode (1.46 V).The present work describes the synthesis and ORR evaluation of atomically dispersed Co nanoparticles on to boron-nitrogen co-doped carbon catalyst. The catalyst was derived using high temperature pyrolysis of Co and B incorporated zeolitic imidazole framework (ZIF) and systematically evaluated with respect to oxygen reduction activities. The effect of B and Co concentration, and pyrolysis temperature were analyzed using XRD, BET sorption studies, XPS and Raman spectroscopic techniques. Fig. 1a shows transmission electron microscopic image of Co/B/N/C catalyst. It is noteworthy that the black spots indicate the presence of atomically dispersed Co nanoparticles on to carbon support material. Flake-like surface morphology is observed for the catalyst synthesized in the present study (Fig. 1a inset). Fig. 1b shows the comparative ORR linear sweep voltammograms (LSVs) of Co/B/N/C and Pt/C catalysts with and without chloride ions. Notably, almost 140 mV difference in onset potential is observed for Pt/C in the presence of Cl– ions. However, Co/B/N/C catalyst exhibits nearly 10 mV difference in onset potential after the addition of Cl– ions in both acidic and alkaline media. The results show that Co/B/N/C catalyst is tolerance towards Cl– ions in acidic as well as alkaline media.The optimized Co/B/N/C catalyst was utilized as cathode in an indigenous DFC device and the performance was evaluated at room temperature. The DFC device was constructed as three compartments viz., anode, desalination and cathode as shown in the Fig. 2a [6]. In an identical operating condition, the highest OCV of 1.58 V was observed for Co/B/N/C cathode compared with that of Pt/C cathode (1.49 V). Notably, the Pt/C cathode delivered a peak power density of 15.7 mW cm-2 at a load current density of 19.0 mA cm-2. However, Co/B/N/C cathode exhibit a peak power density of 12.1 mW cm-2 at a load current density of 16.5 mA cm-2 (Fig. 2b) which is on-par with the state-of-the-art Pt/C catalyst. Simultaneously, the desalted water concentration was measured using conductivity meter and the results are presented in Fig. 2c. At OCV, the desalted water concentrations were measured to be 0.39 and 0.37 M for Co/B/N/C and Pt/C catalysts, respectively. At 16.5 mA cm-2 load current density, concentration of the desalted water was 0.2 M for Co/B/N/C catalyst and for Pt/C, the concentration at 16.5 mA cm-2 current density was 0.15 M. From the above polarization and concentration profiles, Pt/C shows enhanced performance over in-house synthesized, however, considering non-precious catalyst, the observed results for Co/B/N/C are much appreciable. We are hopeful that the results discussed in this study will provide insights in designing anion tolerant catalyst for the improved DFC performance. Reference Atlas, S. Abu Khalla, M. E. Suss, J. Electrochem. Soc. 2020, 167, 134517.Abdalla, S. Abu Khalla, M. E. Suss, Electrochem. commun. 2021, 132, 107136.Malko, T. Lopes, E. Symianakis, A. R. Kucernak, J. Mater. Chem. A 2016, 4, 142.Mamtani, D. Jain, A. C. Co, U. S. Ozkan, Catal. Lett. 2017, 147, 2903.Tylus, Q. Jia, H. Hafiz, R. J. Allen, B. Barbiellini, A. Bansil, S. Mukerjee, Appl. Catal. B Environ. 2016, 198, 318.A. Asokan, S. Abu-Khalla, S. Abdalla, M. E. Suss, ACS Appl. Energy Mater. 2022, 5, 1743. Figure 1

Air Electrode Property of Doped NiCo2O4 Based Oxide for Rechargeable Zn-Air Battery

There is a strong demand for rechargeable battery with large energy density and capacity because of increase in performance of mobile equipment such as mobile phone, laptop personal computer (PC) and also electric vehicle. At present, Li ion battery (LIB) have been widely used for such purpose. However, there are several issues such as the insufficient capacity and safety required to solve. Zinc-air batteries have been used commercially as a non rechargeable battery at present, however, recently, there is strong interest for development of rechargeable Zn-air battery because of its theoretical large capacity, low cost, and safety. However zinc-air rechargeable battery still has limits in cycle file and energy efficiency. In particular, increase in air electrode performance is strongly required. In this study, nickel cobalt spinel oxide, NiCo2O4, was studied as reversible air electrode for Zn-air battery. In addition, effects of dopant on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were studied. NiCo2O4 based spinel oxide is active to ORR and OER, however, surface area is still insufficient by the preparation of conventional hydrothermal method. Therefore, application of splay pyrolysis method for preparation of NiCo2O4 oxide is also studied for increasing ORR/OER activity and stability.NiCo2O4 based oxide was prepared by hydrothermal synthesis method and partial substitution was performed by using Mn, Fe, and Cu. On spray pyrolysis method, precursor solvent turned into nano-sized droplet by ultrasonic wave vaporizer, and the droplets are transferred to the tubular electric furnaces by air as carrier gas. The solvent evaporates from the droplets resulting in the formation of mesopore and after calcination, the fine powder is gathered in filters. Air electrode performance was measured by using gas diffusion layer, and PTFE and graphitic carbon were mixed with NiCo2O4 spinel for electrode. 8MKOH aqueous solution at 313K was used for electrolyte and a constant current of 20 mA/cm2 was applied by battery charge discharge equipment.Various spinel oxides were successfully prepared by hydrothermal synthesis method from XRD measurement. It was found that ORR and OER activity of MnCo2O4 and NiCo2O4 was reasonably high among the spinel oxides prepared. ORR activity is slightly higher on MnCo2O4, however, NiCo2O4 shows much longer cycle stability of ORR/OER. In addition, elution of Mn was observed in case of MnCo2O4, in contrast, almost no change in composition was observed for NiCo2O4 after cycle measurement. Therefore, NiCo2O4 shows reasonable activity to ORR and OER and high chemical stability. Effects of dopant on NiCo2O4 on ORR and OER activity and doping Cu and Fe is effective for increasing ORR potential and decreasing OER potential. Although the most superior performance of air electrode was observed on Cu doped one, elution of Cu is obviously observed. Therefore, from chemical stability, Fe doped NiCo2O4 is the most active to air electrode catalyst. The optimization of Fe amount in Ni site was also studied and it was found that Ni0.65Fe0.35Co2O4 is the most active and stable air electrode catalyst. On this electrode, overpotential for ORR reaction is around 0.25V at 20mA/cm2, 313 K and almost 500 cycles of ORR/OER which is 1 h ORR and OER each. In addition, introduction of mesoporous structure was achieved by splay pyrolysis method and increase in surface area is also effective for increasing stability to ORR/OER activity.This work is based on results obtained from a project, "Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2)", JPNP16001, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Development of Bifunctional Electrocatalyst By Palladium Deposition on Modified Perovskite La0.6Sr0.4Co0.2Fe0.8O3-Δ for Li−O2 Battery

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskites have previously emerged as efficient cathode catalysts for various electrochemical devices such as lithium-air cathode, since its superior chemical stability and mixed electron-ion conductivity. LSCF exhibits good activity for the oxygen evolution reaction (OER) as cathode catalysts. However, the activity on Oxygen Reduction Reaction (ORR) is still a barrier to be utilized as a catalyst for low-temperature electrochemical devices such as lithium-air battery. Bifunctional electrocatalyst is important to reduce the large overpotential involved the sluggish kinetics of the ORR and OER at the air electrode in lithium-air battery. In this work, palladium (Pd) contents were deposited on the nano-sized LSCF surface to improve its ORR activity in lithium-air battery. When the 20 wt.% Pd/Nano-LSCF were applied to cathode catalyst in Li-O2 battery, higher initial discharge capacity (16912 mAh g-1) compared with Nano-LSCF (6707 mAh g-1) was obtained. These results demonstrate the deposition of Pd can be an effective catalyst to enhance the ORR activity of perovskite LSCF.

Interrelations between sulfur, iron, nitrogen, pore and graphite matrix for oxygen reduction reaction

Nitrogen doped lotus-stem carbon and that being promoted by sulfur or iron (N-LC, NS-LC, NFe-LG and NSFe-LG) are synthesized. The interrelations between pore size, graphitization degree, N content, Fe–N content, S doping, and their effects on oxygen reduction reaction (ORR) activity and stability in alkaline media are studied. The NSFe-LG shows the most excellent performances for ORR. The Fe-free NS-LC shows close ORR activity and comparable stability to the NSFe-LG, which may make metal-free catalyst attractive.

Exploring Different Synthesis Parameters for the Preparation of Metal-Nitrogen-Carbon Type Oxygen Reduction Catalysts

The influence of various synthesis conditions of a metal-nitrogen-carbon (M-N/C) catalyst material on oxygen reduction reaction (ORR) kinetics is discussed. Seven M-N/C catalysts based on cobalt are obtained by changing various synthesis conditions, such as the mixing environment, pyrolysis gas, and post-treatment. The ORR activity and stability measurements are performed using the classical three-electrode configuration in a 0.1 M HClO4 solution. The most active and stable ORR catalyst proves to be the material obtained by mixing a cobalt salt, 2,2’-bipyridine, and a high surface area silicon carbide derived carbon together in water and pyrolyzing the mixture in argon. In a fuel cell test, however, a maximum power density value of 135 mW cm−2 is achieved with the catalyst mixed together in a planetary ball-mill at a low catalyst loading of 1.0 ± 0.1 mg cm−2 and at a test cell temperature of 60 oC despite of the fact that preparing the catalyst via dry ball-milling reduces the surface area of the material roughly 40% more than in the case of using a solution-based method. Consequently, mixing the catalyst precursors together without any additional chemicals in a planetary ball-mill instead of in a solution appears to be the most promising choice.

Redox-state modulated ORR activity of Cd-based Prussian blue analog frameworks transformed via anion exchange with controlled redox-state from CdCO3 cuboids

This work reports a facile solution based chemical transformation of thin film of CdCO3 cuboids into Cd-based Prussian blue analogue structured thin film with controlled redox-state by manipulating the anion exchange reaction between the solid film and the aqueous solution of K3[Fe(CN)6] at 60°C. The progress of the transformation reaction is monitored via electron microscopy, X-ray difractometry and X-ray photoelectron spectroscopy. Based on the scenario of the reaction progress revealed by these analysis, the CdCO3 cuboids are first found to be transformed into the smaller cuboids of cadmium ferricyanide, which, on further reaction, are transformed into the cadmium ferrocyanide cuboids. Thus, by controlling the redox-state of the Fe-metal center on the frameworks, the electrocatalytic activity of the frameworks on oxygen reduction reaction (ORR) is studied. The electrocatalytically inert CdCO3 cuboids after transformation to the frameworks with Fe3+-centers demonstrate an enhanced catalytic activity on ORR, while the Fe2+-centers of the frameworks are found to relegate the catalytic activity, thereby manifesting the redox-sate modulated ORR activity of the frameworks. To the best of the authors' knowledge, this is the first report of this kind on redox-state specific ORR activity.

Oxygen reduction reaction activity and the microbial community in response to magnetite coordinating nitrogen-doped carbon catalysts in bioelectrochemical systems

A Fe-N-C catalyst is supposed to drive commercialization of microbial fuel cells (MFCs) because of its remarkable catalytic performance on oxygen reduction reaction (ORR). However, the catalyst suffers from unclear active site structure and unknown responses of the cathodic microbial community. Here, we prepared a mesoporous core-shell structure catalyst with a nitrogen doped matrix carbon shell, and a Fe3O4 core (Fe3O4@N-mC) as efficient cathode catalysts for the oxygen reduction reaction (ORR). The resulting hybrid electrocatalyst (Fe3O4@N-mC) showed higher limiting current density (2.94 mA cm−2), lower H2O2 yield (7.2–12.7%), and a higher electron transfer number (3.74–3.85) for ORR activity than its intermediates, including Fe3O4, polyaniline (PANI), nitrogen doped carbon (N-C), and magnetic polyaniline composite (Fe3O4@PANI). In MFC tests, Fe3O4@N-mC produced a power density of 0.73 W m−2, which was 2.14 times of N-C (0.34 W m−2), 3.84 times of Fe3O4@PANI (0.19 W m−2), and more than four times of Fe3O4 (4.29, 0.17 W m−2) and PANI (4.87, 0.15 W m−2). Illumina sequencing of 16S rRNA gene amplicons and non-metric multi-dimensional scaling (NMDS) indicated distinct separation of the cathode biofilm bacterial communities between MFCs with different cathodic catalysts. MFCs with the Fe3O4@N-mC cathode facilitated the enrichment of putative exoelectrogenic Dietzia. Our findings suggest that enhancing ORR performance of Fe3O4@N-mC in MFCs can be attributed to the co-effects of Fe and N, the core-mesoporous shell structure, and various nitrogen functionalities.

(Invited) Impacts of Anions on Oxygen Reduction Reaction Kinetics on Platinum and Palladium Surfaces

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of F-, Cl-, ClO4 -, CO3 2-, SO4 2-, Nafion ionomer, and citrate anions on Pt/C, Pd/C and transition metal, N co-doped carbon-based (Me-N-C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO4 - had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The non-covalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This non-covalent interaction, however was not observed on Pd/C, which showed the same ORR activity in all three electrolytes (LiOH, NaOH and KOH). The ORR activity of Me-N-C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C. Self-dissociation of citrate anion was found to occur on both Pt/C and Pd/C in the H adsorption/desorption and double layer regions. For the first time, surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique was used to investigate the self-dissociation of citrate on Pt and Pd thin film electrodes. The break of carboxylic groups and carbon backbone were proposed as possible dissociation pathways for citrate. The adsorbed species have a negligible effect on ORR activity on Pt/C as they are removed by oxidation before 0.75 V. In contrast, their oxidation on Pd/C surfaces is not completed until 0.91 V, which causes a lower ORR activity observed in rotating disk electrode measurements.

Ligand-Regulated ORR Activity of Au Nanoparticles in Alkaline Medium: The Importance of Surface Coverage of Ligands

The functionalization of organic ligands on nanoparticles (NPs) to create metal-organic interfaces can tune the activity and selectivity of catalytic reactions. Comparing to well-documented electronic and steric contribution of organic ligands, the coverage of ligands is also a significant factor but has been rarely investigated. Herein, we employ two methods, potential cycling (Figure 1c) and thermal annealing, to judiciously remove the surface ligands from gold nanoparticles (AuNPs) and systematically investigate the influence of surface coverage of ligands on the catalytic performance. Potential cycling enables a mild removal of ligands without changing the size of AuNPs (Figure 1a and Figure 1b) and thus provides ideal models to identify the intrinsic influence of surface coverage of ligands on oxygen reduction reaction (ORR). The ORR activity and selectivity is quantitatively evaluated by plotting half potential and transferred electron number versus the surface coverage of ligand. Thermal annealing at different temperatures produces different sized AuNPs with various surface ligand coverage. However, it is suggested that the surface coverage of ligand plays a more significant role on ORR than the particle size of AuNPs. Capping AuNPs with oleylamine and sodium citrate and the reverse poisoning clean AuNPs with thiol, butylamine and CTAB further reveal the difference of ORR activity/selectivity of the capped AuNPs is directly related to the surface coverage of the ligands regardless the diverse chemical nature of these ligands (Figure 1d). This work highlights that the surface coverage of ligands should be considered to be an important factor accounting for metal-organic interfaces in addition to chemical nature of the ligand itself. Figure 1

(Invited) Impacts of Ions on Oxygen Reduction Reaction Kinetics on Platinum and Palladium Surfaces

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this paper, the specific adsorption of various anions and cations on platinum and palladium catalysts and their effects on the ORR kinetics were summarized. It was found that ClO4 - had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers can significantly poison the bulk Pt surfaces, but its impact on Pt/C is negligible. Self-dissociation of citrate anion was found to occur on both Pt/C and Pd/C.

DFT Study of the Oxygen Reduction Reaction Activity on Fe-N₄-Patched Carbon Nanotubes: The Influence of the Diameter and Length.

The influences of diameter and length of the Fe−N4-patched carbon nanotubes (Fe−N4/CNTs) on oxygen reduction reaction (ORR) activity were investigated by density functional theory method using the BLYP/DZP basis set. The results indicate that the stability of the Fe−N4 catalytic site in Fe−N4/CNTs will be enhanced with a larger tube diameter, but reduced with shorter tube length. A tube with too small a diameter makes a Fe−N4 site unstable in acid medium since Fe−N and C−N bonds must be significantly bent at smaller diameters due to hoop strain. The adsorption energy of the ORR intermediates, especially of the OH group, becomes weaker with the increase of the tube diameter. The OH adsorption energy of Fe−N4/CNT with the largest tube diameter is close to that on Pt(111) surface, indicating that its catalytic property is similar to Pt. Electronic structure analysis shows that the OH adsorption energy is mainly controlled by the energy levels of Fe 3d orbital. The calculation results uncover that Fe−N4/CNTs with larger tube diameters and shorter lengths will exhibit better ORR activity and stability.

Impacts of anions on the oxygen reduction reaction kinetics on platinum and palladium surfaces in alkaline solutions.

The fundamental understanding of the impacts induced by anions on oxygen reduction reaction (ORR) in alkaline media is of great importance in the design of more advanced catalysts for alkaline fuel cells (AFC). In this study, the specific adsorption of F-, Cl-, ClO4-, CO32-, SO42-, and citrate anions on Pt/C and Pd/C catalysts, and their impacts on the ORR kinetics in alkaline solutions were systematically studied. It was found that F-, Cl- and ClO4- did not specifically adsorb on Pt or Pd surfaces and had no poisoning effect on ORR. CO32- and SO42- had significant effects on Pt/C and lowered the activity even at a very low concentration. On the other hand, their impacts on Pd/C were negligible. Self-dissociation of citrate anions was found to occur on both Pt/C and Pd/C in the H adsorption/desorption and double layer regions. For the first time, surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique was used to investigate the self-dissociation of citrate on Pt and Pd thin film electrodes. The breaking of carboxylic groups and the carbon backbone was proposed as a possible dissociation pathway for citrate. The adsorbed species have a negligible effect on ORR activity on Pt/C as they are removed by oxidation before 0.75 V. In contrast, their oxidation on Pd/C surfaces is not completed until 0.91 V, which causes a lower ORR activity observed in rotating disk electrode measurements. The findings in this paper emphasize the importance of specific adsorption of anions and double-layer interfacial effects on the ORR activity measurement in alkaline solutions.

Impacts of Perchloric Acid, Nafion, and Alkali Metal Ions on Oxygen Reduction Reaction Kinetics in Acidic and Alkaline Solutions

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of ClO4–, Nafion ionomer, and cations on Pt/C, Pd/C, and transition metal, N codoped carbon-based (Me–N–C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO4– had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand, showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however, is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The noncovalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This nonc...

Effect of Sulfate Contaminant on ORR Activity and PEMFC Performanc

Lower cost materials for stack hardware and system components help reduce the overall cost of the automotive and stationary fuel cell systems and make these competitive in the market. However, low-cost system component materials need to provide similar function, performance and durability. Intelligently selecting low cost materials for application in polymer electrolyte membrane fuel cell (PEMFC) systems requires understanding the potential adverse effects that system contaminants may have on the fuel cell performance and durability. There are many prospective balance of plant (BOP) materials that can be used in fuel cell systems. Families of material, based on input from OEMs, fuel cell system manufacturers and other attributes like cost, physical properties, have been studied. These include structural materials, elastomers for seals and (sub)gaskets, and assembly aids (adhesives, lubricants). The contaminants from the BOP materials were leached out via an accelerated aging procedure. The leachates obtained from these plastics were a mixture of organics, inorganics, and ions. The sulfate anion was one of the anions identified in the leachates and was chosen as the model compound for further studies. Sulfate anions can also come from membrane degradation. Ex-situ electrochemical measurements were carried out to understand the impact of sulfate anion concentration on oxygen reduction reaction (ORR). The ORR scan rate and direction (low to high potential and vice versa) were studied to determine the effect of the initial state of the Pt surface on sulfate contamination. In-situ fuel cell experiments were also carried out. Sulfate anion was introduced to a working fuel cell to determine their effect on fuel cells performance. Our previous work showed that sulfate anion did not result in performance loss when it was infused into the cathode at 0.2 A/cm2. The cathode potential was greater than 0.8 V at this current density. It is postulated that sulfate contamination is potential dependent and that sulfate adsorption only occurs when Pt is in its metallic state. In the current study, the effect of potential on sulfate contamination will be studied by infusing sulfate into the cathode at several constant current densities. Several in-situ diagnostics such as infusion, cyclic voltammetry, electrochemical impedance spectroscopy, and I-V curves were carried out to better characterize the contaminant effects of sulfate anion. The goal is to better understand the contamination mechanisms of specific species so that mitigation strategies can be determined. The authors would like to acknowledge funding from the U.S. Department of Energy EERE Fuel Cell Technologies Office, under Contract No. AC36-08GO28308 with the National Renewable Energy Laboratory and collaborations with colleagues at GM. Structural plastic materials and leachates were provided by GM for this study.

Mesoporous Iron Doped Carbons Based on Protic Ionic Salt with Tunable Oxygen Reduction Activity

Electrochemical energy systems such as fuel cells and metal–air batteries [1, 4] are based on oxygen reduction reaction (ORR) which is intrinsically sluggish in kinetics. So, the importance of efficient, low-cost and stable electrocatalysts for such systems is paramount. In this study, we prepared metal (Fe) and nitrogen (N) doped mesoporous carbons (Fe/N/C) as an ORR electrocatalyst by using low cost precursors, which are protic ionic salt, 1,10-phenanthrolinium dibisulfate [Phen][2HSO4], and iron (III) chloride (FeCl3) via facile and scalable strategy termed as nanocasting. Colloidal silica was used as a hard template for the introduction of mesopores [2] in the catalyst structure. Structural characterizations and electrochemical features i.e., performance and stability were investigated and compared with the state of the art Pt/C. Mesoporous Fe/N/C catalysts were synthesized by direct carbonization (Scheme 1: PR-I & PR-II) of the mixture of [Phen][2HSO4], FeCl3 and silica for 2h under an Ar flow of 100 mL/min from 600 oC to1000 oC. Composition of iron precursor was also varied. Also, after the removal of template by HF the second carbonization (selected samples) was done for 1h. To test the catalyst in acidic media, it was washed with 6M HCl; dried and carbonized. Electrochemical measurements were carried out in 0.1 M KOH and 0.1M HClO4(in progress) using a temperature controlled water-jacketed three-electrode cell. Ag/AgCl and Pt coil were used as the reference and counter electrodes, respectively. The mesoporous catalyst was dispersed in a mixture of deionized water, ethanol, and Nafion (5 wt%) and sonicated for 1h. Definite amount of the ink was then dropped for the desired loading onto the surface of a glassy carbon electrode (4 mm) and dried naturally. Successful fabrication of mesopores was observed by HR-TEM and BET. XRD pattern of Fe/N/C showed (002) and (100) diffraction peaks at 2θ = 24.7o and 43.7o, respectively for the graphitic structure. In addition, broad Raman bands were observed at ν =1589 and 1349 cm-1 which could be attributed to the G mode and D mode peak, respectively, and the low value of I G/I D ratio indicates the presence of defects and disorders in the carbons. EDX elemental analysis revealed the presence of C, N, O, S, and Fe in the prepared mesoporous carbon also supported by XPS survey scan. HR-XPS exhibited that the N1s spectrum of Fe/N/C is comprised of pyridinic N, pyrrolic/pyriodonic N and graphitic-type quaternary N structures. The slight impurity that appeared at a higher binding energy was assigned to oxidized N species [2]. The S2p spectrum was also resolved into well-defined peaks related to S2p3/2 and S2p1/2 in the lower binding energy and those at the higher binding energies were ascribed to oxidized species stemming from the anion. But, it was not possible to obtain well-resolved spectra for iron owing to the low amount of iron confirmed from ICP-AES measurements. Electrochemical measurements revealed that the Fe/N/C has an electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline solution. Figure 1 shows rotating disk electrode (RDE) voltammograms of Fe/N/C prepared under carbonization profile, i.e., PR-I and PR-II by keeping other parameters constant in 0.1M KOH solution under O2 atmosphere at 30 oC. In comparison with Pt/C (Sigma-Aldrich) catalyst, mesoporous Fe/N/C synthesized by following PR-II showed improved activity in terms of onset potential and half-wave potential under the same catalyst loading. Chronoamperometry measurements (20000s) and cyclic performance (1500 cycles) study vindicated that the Fe/N/C mesoporous carbon was stable in alkaline media. Electrochemical measurement in acidic media is currently under study. We found that in alkaline electrolyte the feed composition of iron, carbonization procedure (temperature, profile, and second carbonization) and acid treatment played pivotal role to fine-tunethe ORR activity of the metal doped carbons. The performance of Fe/N/C electrocatalyst was affected by the balance between surface area, graphitic degree, and Fe/N doping [1-3] which could be controlled by the synthetic parameters. Moreover, protic ionic salt platform could be considered as a viable candidate for utilization of non-precious metal based carbon materials for fuel cells. References (1) H.-W. Liang, X. Zhuang, S. Bruller, X. Feng, and K. Mullen, Nat. Commun. 5, 4973 (2014). (2) S. Zhang, A. Ikoma, K. Ueno, Z. Chen, K. Dokko, and M. Watanabe, ChemSusChem 8, 1608 (2015). (3) K. Strickland, E. Miner, Q. Jia, U. Tylus, N. Ramaswamy, W. Liang, M-T Sougrati, F. Jaouen and S. Mukerjee, Nat. Commun. 6, 7343 (2015). (4) Y. Gogotsi, A. Nikitin, H. H. Ye, W. Zhou, J. E. Fischer, B. Yi, H. C. Foley, M. W. Barsoum, Nat. Mater. 2, 591 (2003). Figure 1

Atomically Resolved Dealloying of Structurally Ordered Pt Nanoalloy as an Oxygen Reduction Reaction Electrocatalyst

The positive effect of intermetallic ordering of platinum alloy nanoparticles on oxygen reduction reaction (ORR) activity has been well established. What is still missing is an understanding of selective leaching of the less noble metal from the ordered structure and its correlation to long-term ORR performance. Using a combination of kinetic Monte Carlo simulations and advanced characterization techniques, we provide unprecedented insight into dealloying of intermetallic PtCu3 nanoparticles—a well-known binary alloy. Comparison of ordered and disordered samples with identical initial compositions and particle size distributions reveals an unexpected correlation: whereas the copper dealloying rates in the ordered and disordered counterparts are almost the same, in the ordered structure Pt atoms are surrounded by 15–30% more Cu atoms throughout all the stages of acid leaching. This more convenient Pt–Cu coordination explains the statistically significant increase of 23–37% in ORR activity of the ordered st...

Suppression of Oxygen Reduction Reaction on Pt-Based Electrocatalysts from Ionomer Interaction

Nafion ionomer is incorporated into cathode catalyst layers of PEMFCs to provide protonic pathways for the ORR reaction. Although it possible to probe the effect of I/C ration in cathode catalyst layers of PEMFCs, the ionomer cannot be completely eliminated. In contrast, when catalysts are evaluated as thin films fabricated on RDE tips and studied in a non-adsorbing electrolyte such as perchloric acid, ionomer is not essential for protonic conduction and can be eliminated. Recently, the effect of Nafion on oxygen reduction reaction (ORR) activity of catalysts has been studied using rotating disk electrode (RDE) methods on single crystalline, polycrystalline Pt surfaces and nanostructured thin film (3M NSTF) in 0.1 M HClO4 [1-5]. In these studies, thin films (nm to 10’s of nm) were applied to the electrode surfaces by capping with ionomer. A positive shift on the onset potential for OH and other oxide species formation and a suppression of the ORR activity is typically observed after capping. Subbaraman et al. [1] attributed the features observed on cyclic voltammograms under inert atmosphere to sulfonate anion adsorption on Pt surfaces using a CO displacement method. Moreover, the activity trend: Pt (111) < (100) < (110) is identical for Nafion-free surfaces in sulfuric acid and Nafion-capped surfaces in perchloric acid solution that led them to conclude that sulfonate anion adsorption was the main cause for lowered ORR activity. In contrast to these detailed kinetics studies on well-defined Pt surfaces, little work has been reported on carbon supported nanoparticle platinum (Pt/C) catalysts [6]. The lack of studies is a consequence of following barriers: (i) ionomer inclusion in a catalyst ink often helps to increase ink dispersion (ii) elimination of ionomer can result in poor catalyst film quality (iii) ionomer is often used for adhesion of catalysts films on substrate surfaces [7]. However, we have recently discovered that the use of extremely thin and uniform films is indispensable to the kinetics study of ORR catalysts without hindrance from O2 diffusion limitations within the catalyst layer and ionomer is not essential to obtain well-dispersed stable thin catalyst films [8]. In this work, we present results on the effect of Nafion ionomer on the ORR activity as well as CVs under inert atmosphere for several commercial Pt/C and Pt Alloy/C catalysts by employing thin-uniform catalyst films obtained by advanced film fabrication techniques [8]. Nafion capped catalyst films were obtained by impregnating diluted Nafion solution onto pre-formed Nafion-free ultra-thin films. The sensitivity of the specific activity of various catalysts including Pt/HSC, Pt/Vulcan, PtxCo1-x/C at 0.9 V vs. RHE in 0.1 M HClO4 to the ionomer to carbon (I/C) ratio will be discussed.