Increased Oxygen Reduction Reaction Activity Research Articles

(Digital Presentation) ORR Activities on Hydrophobic Phosphonium Ionic Liquid Modified Pt/C Catalysts

PEFCs have been attracted as one of the clean and efficient energy conversion systems. Platinum nanoparticles are commonly used as cathode catalysts in Polymer electrolyte fuel cells (PEFCs) due to their high activity for oxygen reduction reaction (ORR). However, platinum is rare and expensive, therefore it needs to improve the mass activity and durability for widespread fuel cells. Recently, ionic liquids (ILs) have been studied to enhance the ORR activity of platinum catalysts1). We try to improve the catalytic activity of Pt/C catalysts by using Ionic Liquids (ILs). RTILs are salts that melt at temperatures below 100 oC and generally consist of bulky cations and anions. It has been studied in a wide range of fields, including the energy sector because of its highly conductive and nonvolatile. Recently, it has been reported that ILs layer enhance the ORR catalytic activity on Pt surface, due to the suppress the coverage of OHad from water molecules1). However, the correlation between increased ORR activity and the structure of the ionic liquid has not been clarified. In this study, we focused on ILs quaternary phosphonium cations and a bis(trifluoromethylsulfonyl)amide (TFSA) anion. The phosphonium cations based ILs have been reported to have higher thermal stability than imidazolium or ammonium cations2). In addition, the phosphonium ILs show higher electrochemical stability, conductivity, and hydrophobicity than the ammonium ILs. The ORR activity was analyzed using the rotating disk electrode (RDE) method3). Commercially available 20 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The Pt/C electrodes were modified with phosphonium ILs (PXXXY+TFSA- [alkyl chain X= 4 Y= 1, 12, 16]). The thickness of the ILs layer was calculated by using the BET surface area of carbon supports. The electrochemical measurements were conducted under Ar or O2 saturated conditions, respectively. Electrochemical surface area (ECSA) was estimated from hydrogen adsorption wave in cyclic voltammograms (CVs). Mass activity (MA) and surface area specific activity (SA) of ORR at 0.9 V on Pt nanoparticles were calculated from the Koutecky-Levich equation. The ILs layer enhanced ORR activity. However, ORR activity decreased when the ILs layer was excessively thick. These results suggest that the ORR activity decreases in a thicker ILs layer due to increased resistance to oxygen mass transport or proton conductivity. Moreover, the ORR activity of the ILs modified Pt/C catalyst increased as the asymmetric part alkyl chains of the ILs became longer. Our previous reports show that as the size -is larger, both the molar volume and oxygen solubility increases. Our results show that these physicochemical properties originating from the structure of the cation affect ORR activities.[Reference]1) Gui-Rong Zhang, et al., ACS Catal., 8, 8244-8254 (2018).2) K. Tsunashima and M. Sugiya, Electrochem. Commun., 9, 2353, (2007).3) H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Appl. Catal. B Environ. 56, 9. (2005).

First-Principles Study on Pt-Based Sub-Nanocluster Carbon-Metal Hybrid Catalysts for the Oxygen Reduction Reaction

Recently, the polymer electrolyte membrane fuel cell (PEMFC) has been drawing huge attention as an eco-friendly energy conversion device which can replace an internal combustion engine without environmental contaminant such as CO2 and NOx. However, the use of expensive noble metal catalysts such as Pt is essential for the cathode because the oxygen reduction reaction (ORR) at cathode are kinetically much slower than the hydrogen oxidation at anode. For this reason, many of researchers have vigorously worked on the development of Pt-based alloy catalysts such as Pt-Co and Pt-Ni alloys to reduce noble metal usage, but there are still limitations in catalytic activity and durability for the commercialization. Herein, to overcome the limitations, we developed Pt-based carbon-metal hybrid catalyst showing high performance for ORR using density functional theory (DFT) calculation. The slab model of Pt-based carbon-metal hybrid catalyst was constructed by Pt sub-nanocluster supported on graphite with graphene-shaped carbon shell on top of the Pt sub-nanocluster. The most stable geometry of the hybrid catalyst was determined by the formation energy, and the ORR mechanism on surface of hybrid catalyst as well as electrochemical activity was investigated. As a result of investigation, we confirmed that the oxygen reduction occurs on active graphene-shaped carbon shell sites. On pure Pt sub-nanocluster hybrid catalyst, once the hydrogen superoxide (OOH) is adsorbed on carbon shell surface after the oxygen adsorption and protonation, the hydrogen peroxide (H2O2) is preferentially formed at higher onset potential than that of four-electron O2 reduction, which the final product is water (H2O). However, the formation of hydrogen peroxide in PEMFC should be avoided owing to creation of radical, resultant degradation of membrane and low energy conversion efficiency. Surprisingly, we identified that the hydrogen peroxide formed on pure Pt sub-nanocluster hybrid catalyst can be decomposed into OH and H2O with low energy barrier, whose value is much lower than desorption energy of produced hydrogen peroxide, so that subsequent reduction reaction can occur. When this four-electron ORR via hydrogen peroxide decomposition takes place, the ORR activity is improved compared to Pt(111) catalyst, where the onset potential of pure Pt sub-nanocluster hybrid catalyst presents is higher than that of Pt(111). Moreover, we introduced Pt-based alloy sub-nanocluster to reduce Pt usage further. The Pt-based alloy sub-nanocluster hybrid catalyst also exhibited increased ORR activity. For Pt-based alloy sub-nanocluster hybrid catalyst, we carried out analysis of density of state and figured out that the more orbital overlap between carbon shell and sub-nanocluster is generated, the higher catalytic activity is shown due to enrichment of electrons on carbon shell surface. We expect this work can help understanding the mechanism of activity improvement of carbon metal hybrid catalyst, suggesting promising way to design high-performance ORR catalyst with low Pt loadings.

Synthesis and Characterisation of High Performing Heteroatom Doped Carbon Catalyst for Oxygen Reduction Reaction (ORR) in Alkaline Fuel Cells

Alkaline fuel cell (AFC) routing platinum-free electrocatalysts are one of the potential frontiers to enable its use in widespread applications. PGM catalysts have alwaysbeen preferred andwidely utilised due to their enhanced intrinsic activity and superior performance. Its high cost and less abundance brought attention to heteroatom doped carbon-based electrocatalysts that offer comparable performance, tunable selectivity and simple synthesis route. The lack of efficient catalysts hampers the performance, peak power density and longevity of the cell due to sluggish ORR activity.In this article, Heteroatom-doped carbon (F, N/F and N/S/F) is prepared using PVDF-HFP polymer as a precursor. The polymer is mixed with urea or ThioUrea concerning the required dopant and pyrolyzed at 900 ºC for 3 hours at a heating rate of 5 deg per min in an inert atmosphere. The enhanced performance is obtained from the N/F- doped carbon which shows an onset potential (E = 1.04V vs. RHE), a diffusion-limited current density(j) of 6.12 mA/cm2 in 0.1M KOH, which is comparatively better than the F-doped and N/S/F-doped carbon. The number of electrons (n) involved in the ORR activity was calculated to be 3.99, 3.98, 3.85 for N/F-C, N/S/F-C and F-C respectively. This suggests a 4e- pathway indicating efficient operation for the Alkaline electrochemical cell. The systematic studies exhibit an increased ORR activity in the case of the N/F-doped carbon electro-catalyst due to a combined synergistic effect of hetero-atoms and high surface area. Heteroatom doping and the activity in the carbon material were confirmed by all the necessary physical and electrochemical characterisations. Keywords: Hydrogen, Alkaline Fuel cell, AFC, Oxygen reduction reaction, ORR catalyst

Insights into MXenes-based electrocatalysts for oxygen reduction

Due to the urgent need for a sustainable energy economy, fuel cells, which use clean and renewable fuels (hydrogen and methanol) for efficient power generation, have attracted more and more attention in recent years. The oxygen reduction reaction(ORR) on the cell's cathode is the core of its electrochemical conversion process, and the performance of the ORR electrocatalyst determines the overall performance and efficiency of related devices. Developing efficient, durable, and low-cost electrocatalysts for ORR has always been the goal of people's exploration. In recent years, MXenes, a new family of two-dimensional (2D) metal carbides, nitrides, and carbonitrides, have emerged and attracted great interest due to their excellent electrical conductivity and high surface area. The MXene-based materials inspire the design and preparation of electrocatalysts with increased ORR activity, high selectivity, and long load life. This review has provided an overview of MXenes, then described their synthesis methods, and finally explained the progress of MXene (mainly Ti3C2Tx) in the field of oxygen reduction.

Exploiting the Synergistic Electronic Interaction between Pt-Skin Wrapped Intermetallic PtCo Nanoparticles and Co-N-C Support for Efficient ORR/EOR Electrocatalysis in a Direct Ethanol Fuel Cell.

The development of low-Pt catalysts with high activity and durability is critical for fuel cells. Here, Pt-skin wrapped sub-5nm PtCo intermetallic nanoparticles are successfully mounted on single atom Co-N-C support by exploiting the barrier effect of Co-anchor. According to a collaborative experimental and computational investigation, the increased oxygen reduction reaction activity of PtCo/Co-N-C arises from the direct electron transfer from PtCo to Co-N-C, and the resulting optimal d-band center of Pt. Owing to such unique electronic structure interaction and synergistic effect, the specific and mass activities of PtCo/Co-N-C are up to 4.20mA cm-2 and 2.71 A mgPt-1 , respectively, with barely degraded stability after 40 000 CV cycles. The PtCo/Co-N-C also exhibits outstanding activity as an ethanol electrocatalyst. This work shows a new and effective route to boost the overall efficiency of direct ethanol fuel cells in acidic media by integrating intermetallic low-Pt alloys and single atom carbon support.

Ionomer-free electrocatalyst using acid-grafted carbon black as a proton-conductive support

In the developments of the polymer membrane electrolyte fuel cells (PEMFCs), strategic design of their catalysts layer is a key to improve the performance and durability. Especially, interfacial structure of the electrocatalysts constructed by carbon support, platinum (Pt)-based nanoparticles and polymer electrolyte so called ionomer dominates the performance. It is known that the surface coverage of the ionomers onto the carbon-supported Pt nanoparticles generates overpotentials in the catalyst layer. To avoid this, an ionomer-free electrocatalyst is developed by functionalized surface of carbon supports, in which the support is covalently grafted to benzenesulfonic acid to facilitate the proton conduction on the carbon surface and Pt nanoparticles are attached to the acid-grafted carbon support. In single-cell measurements, although the ionomer-free electrocatalyst exhibits larger proton resistance than a conventional ionomer-based electrocatalyst, higher activity at low (<0.01 A cm−2) and high (>1.7 A cm−2) current densities were achieved owing to increased oxygen reduction reaction activity and decreased oxygen diffusion resistance, respectively. Elimination of the ionomer reduces both the interfacial overpotential as well as diffusion overpotential of the oxygen.

Ionomer-Free Electrode Catalyst Using Acid-Grafted Carbon Black As a Proton-Conductive Support

In the developments of the polymer membrane electrolyte fuel cells (PEMFCs), strategic design of their catalysts layer is a key to improve the efficiency and durability. Especially, interfacial structure of the electrode catalysts constructed by carbon support, platinum (Pt)-based nanoparticles and polymer electrolyte so called ionomer dominates the efficiency. It is known that the surface coverage of the ionomers onto the carbon-supported Pt nanoparticles generates overpotentials in the catalyst layer. To avoid this, an ionomer-free electrode catalyst is developed by functionalized surface of carbon supports, in which the support is covalently grafted to benzenesulfonic acid to facilitate the proton conduction on the carbon surface and Pt nanoparticles are attached to the acid-grafted carbon support. In single-cell measurements, although the ionomer-free electrode catalyst exhibits larger proton resistance than a conventional ionomer-based electrode catalyst, higher activity at low (< 0.01 mA cm−2) and high (>1.7 mA cm−2) current densities were achieved owing to increased oxygen reduction reaction activity and decreased oxygen diffusion resistance, respectively. Elimination of the ionomer reduces both the interfacial overpotential as well as diffusion overpotential of the oxygen.

Non-precious metal cathodes for anion exchange membrane fuel cells from ball-milled iron and nitrogen doped carbide-derived carbons

Iron and nitrogen doping of carbon materials is one of the promising pathways towards replacing Pt/C in polymer electrolyte fuel cell cathodes. Here, we show a synthesis method to produce highly active non-precious metal catalysts and study the effect of synthesis parameters on the oxygen reduction reaction (ORR) activity in high-pH conditions. The electrocatalysts are prepared by functionalizing silicon carbide-derived carbon (SiCDC) with 1,10-phenanthroline, iron(II)acetate and, optionally polyvinylpyrrolidone, by ball-milling with ZrO2 in dry or wet conditions, followed by pyrolysis at 800 °C. The catalysts are characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, N2 physisorption and inductively coupled plasma mass spectrometry. By optimizing the ball-milling conditions, we achieved a reduction in the size of SiCDC grains from >1 μm to 200 nm without negatively affecting the high BET area of catalysts derived from SiCDC. This resulted in increased ORR activity in both rotating disk electrode and anion exchange membrane fuel cell (AEMFC) environments, and improved mass-transport properties of the cathode layer in fuel cell. The ORR activity at 0.9 V in AEMFC of the optimized iron and nitrogen-doped SiCDC reaches 52 mA cm−2, exceeding that of a Pt/C cathode at 36.5 mA cm−2.

Improvement of ORR Activity and Durability of Pt Electrocatalyst Nanoparticles Anchored on TiO2/Cup-Stacked Carbon Nanotube in Acidic Aqueous Media

Platinum nanoparticles (Pt NPs) have been selectively anchored by photodeposition on titanium oxide (TiO2) matrix which is formed by hydrolysis of titanium isopropoxide on cup–stacked carbon nanotubes (CSCNT) in isopropanol, producing the composite Pt catalyst useful for oxygen reduction reaction (ORR) in acidic media, i.e., Pt NPs/TiO2/CSCNT (Fig. 1). Using scanning transmission electron microscopy (STEM) with a high–angle annular–dark–field (HAADF) detector it has been clarified that Pt NPs are sunk into the TiO2 moieties and have the unique polyhedral shape surrounded mainly by the Pt (1 1 1) and Pt (1 0 0) facets. X–ray photoelectron spectroscopy (XPS) allowed us to confirm changes in electronic properties of both Pt NPs and TiO2 support, induced by the so-called strong metal–support interactions (SMSI) and the significantly increased ORR activity was attained in 0.1 M HClO4, compared with the Pt NPs deposited on CB (Vulcan carbon) and CSCNT. The surface structure of the Pt NPs was characterized by transmission electron microscopy (TEM), indicating the improved durability of the Pt NPs deposited on the TiO2/CSCNT, i.e., the only slight increase in the particle size after the durability test (typically 2000 times’ potential cycling at 10 mV s-1 in the potential ranges of 0.05 to 1.1 V and 1.0 to 1.5 V vs. RHE in 0.1 M HClO4). The results obtained demonstrate that the anchoring of Pt NPs on the TiO2 support material deposited on CSCNT is an effective way to enhance the ORR activity of Pt NPs by the SMSI as well as to prohibit Pt NPs from aggregating, i.e., the degradation of the ORR activity of Pt NPs.Figure １ HAADF STEM images of Pt NPs/TiO2/CSCNT sample with (A) low- and (B) high-magnification. The high-magnification image in (B), which shows the assignment of the surface geometry of the Pt NPs deposited on TiO2/CSCNT, was collected from the selected area denoted by the dotted square in (A). Figure 1

The effect of water vapor on surface oxygen exchange kinetics of thin film (La,Sr)(Co,Fe)O3-δ

Oxygen reduction and transport in electrochemical applications are strongly affected by the reactant gas composition, including impurities such as water. Understanding the relationship between oxygen reactivity and humidity is key for attaining stability and high efficiency of solid oxide fuel cells, which are especially attractive for transportation applications as low emission power generation devices. Both short- and long-term effects of moisture have previously been associated with use of ambient air. To better understand these effects, we study oxygen exchange kinetics at the cathode surface and the cathode/electrolyte interface using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) epitaxial thin films as a model-surface system. In situ synchrotron X-ray techniques evaluate the oxygen reduction reaction (ORR) kinetics in response to environmental variables. The results suggest a clear enhancement of the ORR rate upon short term exposure of the perovskite-structured films to water. On the basis of these measurements, along with in situ X-ray characterization of Sr segregation and computational first-principles studies, we suggest a model for this increased ORR activity that may lead to further improvements by stabilizing active cathode surface configurations.

Oxygen Reduction Activity of Nb-Doped Titanate Nanosheets in an Acidic Electrolyte

Introduction Polymer electrolyte fuel cells (PEFCs) are of great importance as consumer use energy devices, such as fuel cell vehicles and residential micro-cogeneration systems. To reduce usage of platinum in PEFCs, non-noble metal oxide catalysts for oxygen reduction reaction (ORR) have extensively been investigated to date. From the viewpoint of stability, titanium oxides are promising candidates, though conductive path between support and active site should properly be designed because of their low conductivity. In this study, we focus on the use of titanate nanosheets as a cathode catalyst. Because titanate nanosheets are very thin, they can reduce conductive length between support and active sites. In addition, they are advantageous to form uniform composite with support materials because titanate nanosheets can be handled as a colloidal solution. From our previous study on titanium oxide nanoparticles,1) hetero-atom doping is expected to enhance the ORR activity. Here, we report the ORR activity of TNS and Nb-doped TNS (Nb-TNS) supported on carbon nanotube in an acidic electrolyte. Experimental TNS was synthesized according to the literature.2) Titanium isopropoxide (TIP) was refluxed in an aqueous solution of tetramethylammonium hydroxide for 30 min to provide titanate nanosheets with small lateral size. To synthesize Nb-TNS, niobium ethoxide was used instead of 10 mol% of TIP. Electrode materials were prepared as follows. A carbon nanotube (Flotube® 9110, denoted as CNT) was dispersed in the nanosheet dispersion, followed by stirring, sonication, and filtration with a membrane (pore size: 0.20 μm). The obtained composite was again dispersed in 1-hexanol containing Nafion® to prepare a catalyst ink. The catalyst ink was dropped onto a glassy carbon (GC) rod and dried at 60 °C for 1 h. The GC rod was used as a working electrode without rotation. The samples without titanate nanosheets, with TNS, and with Nb-TNS were named as CNT, TNS/CNT, and Nb-TNS/CNT, respectively. Electrochemical measurement was performed in a 3-electrode cell at 30 °C. 0.1 mol L–1 H2SO4 was used as an electrolyte. A reversible hydrogen electrode (RHE) and a GC plate were used as reference and counter electrodes, respectively. As a pre-treatment, 300 cyclic voltammetry (CV) cycles (0.05–1.2 V vs. RHE, 200 mV s–1) were performed under N2 atmosphere. CV (0.2–1.2 V vs. RHE, 50 mV s–1) was performed under N2 atmosphere to evaluate electric double layer capacitance (C dl) from the charges in 0.8–1.0 V vs. RHE. CV (0.2–1.2 V, 5 mV s–1) under N2 and O2 atmospheres were performed to evaluate ORR current (i ORR) which is calculated by subtracting the currents under N2 atmosphere from those under O2 atmosphere. Results and discussion The XRD pattern of the powdery samples showed diffraction peaks assignable to layered titanate (ICSD79800) without any peaks related to impurity. The d 020 value of Nb-TNS was larger than that of TNS by 0.5%. Because the d 020 reflects only in-plane crystal structure of a titanate sheet, these results imply the lattice expansion due to substitution of Ti atom with Nb. The Nb/Ti ratio of Nb-TNS was 13 mol% by XRF, which is consistent with the incorporation of Nb in Nb-TNS. Consequently, Nb-doped TNS was successfully synthesized. Fig. 1 shows the polarization curves of CNT, TNS/CNT, and Nb-TNS/CNT, normalized by the weight of CNT. The onset potential of TNS/CNT and Nb-TNS/CNT were 0.45 and 0.51 V, respectively. These were higher than that of CNT, suggesting the ORR activity due to titanate nanosheets. To compare ORR activities of TNS and Nb-TNS, the polarization curves normalized by the weight of titanate nanosheets are also shown in Fig. 2. Nb-TNS showed higher onset potential than that of TNS by 0.09 V. The Tafel plots whose current density is normalized by C dl is shown in Fig. 3. The activity of TNS/CNT was similar to that of CNT only; thus, the increased ORR activity of TNS/CNT was possibly due to increased C dl. On the other hand, the ORR activity of Nb-TNS was obviously higher than the others. Therefore, the hetero-atom doping is shown to be a useful way to improve the activity of titanate nanosheets as cathode catalyst for PEFCs. Conclusion In conclusion, TNS are advantageous to form powdery electrode composites because they are obtained as dispersions. TNS was shown to have ORR activity and the activity is enhanced by the hetero-atom doping. References 1) A. Ishihara et al, Electrochim. Acta, 283, 1779–1788 (2018). 2) E. L. Tae et al, J. Am. Chem. Soc., 130, 6534–6543 (2008). Figure 1

Effect of Underlying Cobalt Content on Oxygen Reduction Reaction Activity at Pt-Skin/Pt100-XCox (111) Single Crystal Electrodes

The R&D of highly active and durable cathode catalysts for the oxygen reduction reaction (ORR) is very important for polymer electrolyte fuel cells (PEFCs). Bimetallic alloys of Pt such as Pt-Co exhibit higher ORR activities than that of pure Pt.1 For polycrystalline Pt alloy electrodes, the kinetically-controlled area-specific current densities (jk ) for the ORR strongly depend on the bulk Co composition.2 It is essential to analyze the mechanism of the increased ORR activity at well-defined Pt-alloys by multilateral techniques. Recently, we have demonstrated the composition dependence of jk at a series of (111), (110), and (100) faces of Pt100−xCox single crystal electrodes, on which well-defined Pt-skin layers were formed by annealing in H2.3 The Pt-skin/Pt73Co27(111) electrode exhibited the highest jk value, which was about 27 times higher than that on pure Pt(111). In the present research, we have examined the effect of layer-by-layer composition, measured under electrochemical conditions, of three different Pt100−xCox(111) electrodes with varying Co bulk content (x = 17, 25, 30 at.%) on the ORR activity. The Pt100−xCox(111) single crystal electrodes (ca. 3.5 mm in diameter and 1.5 mm in thickness) were prepared in the same manner as described in our previous work.3 The Co composition in bulk was evaluated by use of X-ray photoelectron spectroscopy (XPS). Prior to the all measurements, the Pt100−xCox(111) electrodes were heat-treated at 1170 K in 100% H2 to prepare the well-defined surface. In-situ surface X-ray scattering (SXS) measurements were carried out at synchrotron radiation facilities (SPring-8 and the Photon Factory) in the same manner as described previously.4 After filling the electrochemical cell with N2-purged 0.1 M HClO4, the working electrode was polarized at 0.4 V vs. RHE. Structure parameters from the topmost to fourth layers were determined from the least-squares fitting to X-ray reflectivity data [(00) crystal truncation rod (CTR)] with a kinematical calculation based on a specific interfacial model. Figure 1 shows the dependence of jk at 0.9 V vs. RHE on x, together with layer-by-layer composition of Pt and Co analyzed by in situ SXS. The composition of the topmost layer was found to be Pt 98 at.% for all three alloys, regardless of Co content, i.e., a Pt-skin layer was formed on the surface of the Pt-Co(111) single crystal electrodes. In contrast, the Co content in the second layer changed significantly with the Co content in bulk. It is striking that the second layer of Pt75Co25(111) (Pt3Co(111)) was nearly pure Co, 98 at.%, where the jk value was close to the maximum value, ca. 25 times higher than that of pure Pt(111). At Co 30 at.% in bulk, jk decreased, and the Co content in the second layer (32 at.%) was very similar to that in bulk. The Co content in the third layer ranged from ca. 20 at.% to ca. 30 at.% in the three kind electrodes. Thus, the ORR activities of Pt-skin/Pt100−xCox(111) electrodes were strongly correlated with the enrichment of Co in the second layer. It was found by ex-situ angle-resolved, grazing-incidence XPS (ARGI-XPS) for Pt-skin/ Pt73Co27(111) that Co in the subsurface layers was positively charged, which was completely different from the electronic state of Co(II)O.4 Such an electronically positive state of Co is reasonably explained by an electron transfer from Co into the Pt-skin layer, which has been linked to a d-band center downshift. This work was supported by funds for the SPer-FC project from NEDO and a Grant-in-Aid (No. 25410007) for Scientific Research MEXT of Japan. We wish to thank Dr. K. Tamura of the Japan Atomic Energy Agency and Drs. I. Hirosawa and T. Watanabe of SPring-8 for their help with the CTR fitting program and SXS measurements, respectively. The synchrotron radiation experiments were performed as projects approved by the PAC (No. 2016G059) and by JASRI (2016A1184 and 2016B1551). References V. R. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross, N. M. Markovic, J. Rossmeisl, J. Greeley, J. K. Nørskov, Angew. Chem. Int. Ed., 45, 2897 (2006).T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146, 3750 (1999).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, H. Uchida, J. Phys. Chem. C, 121,11234 (2017).S. Kobayashi, M. Aoki, M. Wakisaka, T. Kawamoto, R. Shirasaka, K. Suda, D. A. Tryk, J. Inukai, T. Kondo, H. Uchida, ACS Omega, 3, 154 (2018). Figure 1

Increased activity of nitrogen-doped graphene-like carbon sheets modified by iron doping for oxygen reduction

Rational design and synthesis of Fe-N-codoped carbon materials are promising for replacing commercial Pt/C for oxygen reduction reaction (ORR). Herein, we develop a simple two-step pyrolysis approach to synthesize highly active Fe-N-codoped graphene-like carbon sheets (FeNGC) with active Fe-N-based species for ORR. In this strategy, two-dimensional nitrogen-doped graphene-like carbon sheets (NGC) with a high N-doping level (8.1 at%) and abundant mesoporosity (3.8 nm) are firstly synthesized by co-pyrolysis of biomass carbon source and dicyandiamide, in which dicyandiamide simultaneously serves as a trifunctional role of in situ reaction template, nitrogen source and porogen. Secondly, FeNGCs are prepared by additional iron doping of NGC at high temperatures, in which sheet-like structure is in favor of increased accessibility of N-functional groups to more Fe atoms, thus giving rise to formation of high-density Fe-N-based active sites. The optimized catalyst synthesized at 950 °C (FeNGC-950) demonstrates significantly increased ORR activity with a dominant 4e− reduction process compared to pure NGC in alkaline and acidic solutions, which evidently shows the comparable activity to Pt/C due to the synergy of simultaneously optimized structures and multi-active sites. Moreover, FeNGC-950 has better long-term stability and methanol tolerance than Pt/C both in alkaline and acidic electrolytes. The present strategy paves a new venue to design and prepare various metal-doped carbon materials with great potentials in energy applications.

Comparing Novel PGM-Free, Platinum, and Alloyed Platinum Catalysts for HT-PEM Fuel Cells

The Nyquist plot of impedance spectra of high-temperature polymer electrolyte fuel cells (HT-PEMFCs) is recorded to characterize the polarization losses. However, without further analysis, it only gives qualitative information about the overpotential of the inspected fuel cell, as the overlapping arcs in the Nyquist plot cannot be distinguished by mere eye. The Distribution of Relaxation Times (DRT) analysis is a powerful tool used to resolve the individual contributions to the impedance spectrum and to allocate them to their respective frequencies [1]. The peaks in the DRT spectrum can be sectioned into frequency ranges related to mass transport, the oxygen reduction reaction (ORR) and high frequency impedance related to anode processes and proton transport [2], as seen in Figure 1 a). In this work, we evaluate the performance of three catalysts by analyzing their impedance at different frequencies using DRT and explaining the catalytic behaviour. The three catalysts are a standard platinum catalyst (Pt on Vulcan X-72, Heraeus), a platinum alloy catalyst (Pt0.7Co0.3, Tanaka), and a novel Fe-C-N catalyst, a highly active and durable catalyst of the platinum-free, transition metal-nitrogen-carbon family, derived from Nicarbazin and possessing an open frame structure which is controlled by a silica sacrificial support method [3]. For each material, we assembled a HT-PEM fuel cell MEA and recorded polarization curves and EIS spectra at 100 mAcm-2. This data is shown in Figure 1 b) and c). The EIS spectra were processed using the DRT method (Figure 1 a) and yielded the frequency resolved impedance, giving insight into the processes causing the performance differences. Quantitative results were obtained by DRT analysis for the operation of the cells at 100 mAcm-2. We found the platinum/cobalt alloy catalyst to be outperforming platinum with an increased ORR activity, explained by the facilitated adsorption and reduction of oxygen resulting from the lower interatomic distance in the alloyed catalytic system [4]. The Fe-N-C system exhibits a lower performance compared to the noble metal catalysts, showing a significantly increased mass transport region and a higher ORR resistance. Contrary to our expectation, the mass transport resistance is dominating the ORR resistance. This can be explained by the narrow spaces in between the nitrogen-doped graphenic layers in which the reactive sites are present [3]. The reactivity of the non-PGM active sites is not the primarily inhibiting factor of the performance. DRT proves to be a powerful tool to inspect the impedance of a fuel cell in great detail and to learn about the inner mechanisms, highlighting positive and negative aspects of different catalysts. This tool allows us to systematically investigate the performance of catalysts and to elucidate the influence of single parameters, making the optimization straight forward and focused instead of depending on trial and error experiments. 1. F. Ciucci, C. Chen, Electrochim. Acta, 167, 439 (2015). 2. A. Weiß, S. Schindler, S. Galbiati, M.A. Danzer, R. Zeis, Electrochim. Acta, 230, 391 (2017). 3. A. Serov, K. Artyushkova, P. Atanassov, Adv. Energy Mat., 4, 7 (2014). 4. C.V. Rao, J. Parrondo, S.L. Ghatty, B. Rambabu, J. Pow. Sources, 195, 3425 (2010). Figure 1

Nanometric Fe-substituted ZrO2 on Carbon Black: a Novel PGM-Free ORR Catalyst for PEMFCs

The independence from Platinum-Group-Metal (PGM) is one of the major goals towards the commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs), due to the limited availability of PGM and their considerable loading on the Oxygen-Reduction-Reaction (ORR), increasing their cost contribution to the overall system. On the other hand, the challenges for PGM-free ORR catalysts are an activity approaching that of Pt and a long-term operational stability in the strong acidic environment of a PEMFC. Low-cost FeNC catalysts fulfill the high activity requirement, but they still face fast degradation in acidic environment [1, 2]. On the contrary, pure valve-metal oxides as non-PGM ORR catalysts need a significant improvement in ORR activity and H2O yield but they are very promising in terms of stability during PEMFC operation [3, 4]. The aim of this contribution is to combine the intrinsic high ORR activity of Fe with the stability of valve-metal oxides, via partial substitution of Zr4+ with Fe3+ in the ZrO2 structure [5]. The formation of a solid solution of oxides of the two aliovalent-metals should also provide oxygen vacancies or uncoordinated metal sites at the oxide surface, hypothesized to be correlated to an increased ORR activity of valve-metal oxides [6, 7]. In this study, to obtain Fe-substituted ZrO2 we used a homogeneous mixture of the two metals at the molecular level, by employing two soluble organometallic precursors, i. e., zirconium (IV) tetra-tert-butyl dichloro phthalocyanine (ZrCl2Pc(tBu)4) and iron(II) tetra-tert-butyl phthalocyanine (FePc(tBu)4). Graphitized Ketjen-Black carbon is first impregnated with the precursors and then heat-treated as described in [8, 9]. XRD and TEM characterization show that our catalysts consist of ZrO2 particles of about 3 nm (Fe-substituted) and 7 nm (pure ZrO2). Mössbauer spectroscopy reveals that the Fe moieties are isolated and in Fe3+ electronic configuration at high spin, typical of an oxidic environment. This is confirmed by soft XAS data at the Fe L-edge and XPS data, pointing to the desired FexZr1-xO2-δ phase. We already published an evaluation of the ORR mass activity of FexZr1-xO2-δ in comparison to Fe-only and ZrO2-only catalysts, using a thin-film rotating (ring) disk electrode (RRDE) setup [9]. Using the preferred catalyst Fe0.07Zr0.93O2-δ, the variation of its mass activity and selectivity as a function of the loading is further discussed, with a H2O2 yield even lower in comparison to an FeNC catalyst [10]. The catalyst mass activity and its Arrhenius analysis in a single PEMFC at ≈0.4 mgcat/cm² is compared to the RDE results (Figure 1) [11]. The Arrhenius analysis resulted in an ORR activation energy of 16 kJ/mol (RDE) and 18 kJ/mol (PEMFC) at 0.4 VRHE, significantly lower than our best pure-ZrO2 catalyst reported (21 kJ/mol from RDE and 29 kJ/mol from PEMFC data) [4]. Stability tests and loading optimization of nanometric FexZr1-xO2-δ will be the future outlook. Acknowledgements: This work was supported by the Bayerische Forschungsstiftung (Project ForOxiE², AZ 1143-14).

Tailoring the porous structure of N-doped carbon for increased oxygen reduction reaction activity

A facile one-pot method using molten salt medium is developed to fabricate an open structured N-doped carbon (ONC) derived from biomass. SEM and TEM show the ONC is made up of interconnected thin amorphous carbon layers. N2 isotherm shows that ONC has hierarchical porous structure with a high specific surface area of 1893.5m2g−1. Compared to commercial Pt/C (20wt%), the ONC has comparable ORR activity and better durability. Due to its high ORR activity and durability, producing an opened N-doped carbon in this molten salt medium can provide a promising approach for synthesizing heteroatom doped carbons with electrochemical performance.

Nano-engineered intrapores in nanoparticles of PtNi networks for increased oxygen reduction reaction activity

Network-like metallic alloys of solid nanoparticles have been frequently reported as promising electrocatalysts for fuel cells. The three-dimensional structure of such networks is rich in pores in the form of voids between nanoparticles, which collectively expose a large surface area for catalytic activity. Herein, we present a novel solution to this problem using a precursor comprising a flocculent core-shell PtNi@Ni to produce PtNi network catalysts with nanoparticle intraporosity after carefully controlled electrochemical dealloying. Physical characterization shows a hierarchical level of nanoporosity (intrapores within nanoparticles and pores between them) evolves during the controlled electrochemical dealloying, and that a Pt-rich surface also forms after 22 cycles of Ni leaching. In ORR cycling, the PtNi networks gain 4-fold activity in both jECSA and jmass over a state of the art Pt/C electrocatalyst, and also significantly exceed previously reported PtNi networks. In ORR degradation tests, the PtNi networks also proved stable, dropping by 30.4% and 62.6% in jECSA and jmass respectively. The enhanced performance of the catalyst is evident, and we also propose that the presented synthesis procedure can be generally applied to developing other metallic networks.

Nitrogen-Coordinated Iron-Carbon and Nitrogen-Doped Carbon-Encapsulated Iron Nanoparticle Electrocatalysts for the Oxygen Reduction Reaction

The development of fuel cells is hampered by the sluggish oxygen reduction reaction (ORR), which is responsible for high overpotentials, even on Pt-based electrocatalysts. Therefore, it is mandatory the search for more active materials and, at the same time, composed by earth-abundant elements for attaining the widespread commercialization of the fuel cell systems. Non-noble metal electrocatalysts based on iron, nitrogen and carbon composites have gained considerable attention mainly due to recent discoveries of two different active structures: (i) nitrogen-coordinated iron-carbon materials (represented here by Fe-N-C), in which the active center contains the nitrogen-iron coordination and; (ii) nitrogen-doped carbon-encapsulated iron nanoparticles (Fe@N-C), in which the electrocatalyst is formed by iron nanoparticles protected by a graphitic shell of carbon layers, doped with nitrogen atoms [1,2]. In the present study, non-noble metal electrocatalysts composed by iron, nitrogen and carbon were synthesized by the pyrolysis of a mixture of Vulcan carbon, iron chloride and a nitrogen precursor in N2 atmosphere. The results showed that when the pyrolysis was conducted at 700 oC, the use of imidazole as the nitrogen precursor resulted in the formation of the encapsulated nanoparticle structure, as revealed by the presence of a peak at ca. 2.1 Å (metallic Fe nanoparticles) in the Fourier Transform (FT) of the EXAFS oscillations. On the other hand, when phenanthroline was used as the nitrogen precursor, a FT peak centered by ca. 1.5 Å was obtained, evidencing the formation of the nitrogen-iron coordinated structure. When the pyrolysis was conducted at 1050 oC, the resulting structure was the same for the case of imidazole, but a mixture of Fe@N-C and Fe-N-C was obtained for phenanthroline. The ORR polarization curves for the materials prepared at 700 oC showed much higher activity for the Fe-N-C structure, but with similar and low stability for both materials. The pyrolysis at 1050 oC also showed higher activity for the material prepared with phenanthroline, but the stability was considerably improved, with similar behavior for both electrocatalysts. An additional pyrolysis step at 950 oC in NH3 atmosphere resulted in an increased ORR activity and stability for both electrocatalysts, but still with much higher activity for the Fe-N-C-containing material. Interestingly, the EXAFS results showed that the prolonged NH3 treatment of the Fe@N-C structure conducted to the appearance of the FT peak at 1.5 Å (nitrogen-iron coordination), and the ORR polarization curves indicated further increase on its original activity and stability. These results evidenced that the increase in the overall nitrogen content of the electrocatalyst by using a efficient iron chelating agent (phenanthroline), or via NH3 treatment, improved the ORR activity due to the increase in the iron-nitrogen interaction for both Fe-N-C and Fe@N-C initial structures. (Further enhancement in the ORR activity may come from the increase in the nitrogen doping level of the graphitic matrix). The increase in the nitrogen content for both Fe-N-C and Fe@N-C structures may promote a more embraced carbon matrix and, so, it may avoid the demetallation phenomenon, increasing the stability during the ORR. [1] U.Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C 2014, 118, 8999−9008. [2] K. Strickland, E. Miner, Q. Jia, U. Tylus, N. Ramaswamy, W. Liang, M-T. Sougrati, F. Jaouen, S. Mukerjee, Nat. Commun. 6:7343 (2015) 1-7. Acknowledgements The authors gratefully acknowledge financial support from FAPESP (2013/16930-7, and 2016/13323-0) and CNPq.

Relationship Between ORR Catalytic Activity and D-Band Center of Pt Nanoparticle Deposited on Metal Oxide Support Materials

Platinum nanoparticles (Pt NPs) have been selectively anchored by photodeposition on titanium oxide (TiO2) matrix which is formed by hydrolysis of titanium isopropoxide on cup–stacked carbon nanotubes (CSCNT) in isopropanol, producing the composite Pt catalyst useful for oxygen reduction reaction (ORR) in acidic media, i.e., Pt NPs/TiO2/CSCNT. Using scanning transmission electron microscopy (STEM) with a high–angle annular–dark–field (HAADF) detector it has been clarified that Pt NPs are sunk into the TiO2 moieties and have the unique polyhedral shape surrounded mainly by the Pt (1 1 1) and Pt (1 0 0) facets. X–ray photoelectron spectroscopy (XPS) allowed us to confirm changes in electronic properties of both Pt NPs and TiO2 support, induced by the so-called strong metal–support interactions (SMSI) and the significantly increased ORR activity was attained in 0.1 M HClO4, compared with the Pt NPs deposited on CB (Vulcan carbon) and CSCNT. The surface structure of the Pt NPs was characterized by transmission electron microscopy (TEM), indicating the improved durability of the Pt NPs deposited on the TiO2/CSCNT, i.e., the only slight increase in the particle size after the durability test (typically 2000 times’ potential cycling at 10 mV s-1 in the potential ranges of 0.05 to 1.1 V and 1.0 to 1.5 V vs. RHE in 0.1 M HClO4). The results obtained demonstrate that the anchoring of Pt NPs on the TiO2 support material deposited on CSCNT is an effective way to enhance the ORR activity of Pt NPs by the SMSI as well as to prohibit Pt NPs from aggregating, i.e., the degradation of the ORR activity of Pt NPs.

Improvement of ORR Activity and Durability of Pt Electrocatalyst Nanoparticles Anchored on TiO2/Cup-Stacked Carbon Nanotube in Acidic Aqueous Media

Platinum nanoparticles (Pt NPs) have been selectively anchored by photodeposition on titanium oxide (TiO2) matrix which is formed by hydrolysis of titanium isopropoxide on cup–stacked carbon nanotubes (CSCNT) in isopropanol, producing the composite Pt catalyst useful for oxygen reduction reaction (ORR) in acidic media, i.e., Pt NPs/TiO2/CSCNT. Using scanning transmission electron microscopy (STEM) with a high–angle annular–dark–field (HAADF) detector it has been clarified that Pt NPs are sunk into the TiO2 moieties and have the unique polyhedral shape surrounded mainly by the Pt (1 1 1) and Pt (1 0 0) facets. X–ray photoelectron spectroscopy (XPS) allowed us to confirm changes in electronic properties of both Pt NPs and TiO2 support, induced by the so-called strong metal–support interactions (SMSI) and the significantly increased ORR activity was attained in 0.1M HClO4, compared with the Pt NPs deposited on CB (Vulcan carbon) and CSCNT. The surface structure of the Pt NPs was characterized by transmission electron microscopy (TEM), indicating the improved durability of the Pt NPs deposited on the TiO2/CSCNT, i.e., the only slight increase in the particle size after the durability test (typically 2000 times’ potential cycling at 10mVs−1 in the potential ranges of 0.05 to 1.1V and 1.0 to 1.5V vs. RHE in 0.1M HClO4). The results obtained demonstrate that the anchoring of Pt NPs on the TiO2 support material deposited on CSCNT is an effective way to enhance the ORR activity of Pt NPs by the SMSI as well as to prohibit Pt NPs from aggregating, i.e., the degradation of the ORR activity of Pt NPs.