Oxygen Reduction Reaction Activity In Alkaline Media Research Articles

ZIF-8 derived iron-, sulphur-, and nitrogen-doped catalysts for anion-exchange membrane fuel cell application

This study presents the preparation of tridoped Fe, N, S carbon-based catalysts using ZIF-8 as the main carbon source and iron(II)acetate, 1,10-phenanthroline, ammonia gas and thiourea as precursors for Fe, N and S elements. Thiourea was mixed via planetary ball-milling with a Fe–N–C catalyst prepared by inert gas pyrolysis, and different pyrolysis conditions were applied thereafter. The main goal of the work is to synthesise active non-precious metal electrocatalysts for oxygen reduction reaction (ORR). High electrocatalytic ORR activity in alkaline media was observed for the two materials obtained after thiourea addition and subsequent pyrolysis. Physico-chemical characterisation revealed successful S- and N-doping for all three catalysts with enhanced porosity for the two materials that were subjected to additional pyrolysis after the thiourea addition. The sulphur doping was highest for the FeNSC3 material that was subjected to a second pyrolysis in N2 and a third pyrolysis in NH3, compared to the FeNSC2 material that was subjected to a single pyrolysis in NH3 flow after thiourea addition. The ORR activity of FeNSC2 with half-wave potential of 0.88 V was slightly higher than that of FeNSC3, and the former was selected for evaluation as a cathode catalyst in an anion-exchange membrane fuel cell.

Hybrid high-performance oxygen reduction reaction Fe–N–C electrocatalyst for anion exchange membrane fuel cells

Atomically dispersed active sites and hierarchical porosity in Fe–N–C catalyst materials are essential for efficient oxygen reduction reaction (ORR) electrocatalysis and effective mass transport, respectively. However, the majority of these catalysts fail to deliver outstanding electrocatalytic activity and stumble when used as cathode materials in fuel cells. Herein, we developed a catalyst with partially transformed metallic Fe into Fe-Nx sites embedded in the carbonaceous matrix, and its excellent electrocatalytic performance is attributed to the hybrid nanoparticles and built-in Fe-Nx site structures. The optimized Fe@Fe–N–C catalyst was tested for ORR performance both ex-situ using the rotating (ring)-disk electrode technique and in an anion exchange membrane fuel cell (AEMFC). The Fe@Fe–N–C electrocatalyst exhibited remarkably enhanced ORR activity in alkaline media with a half-wave potential of 0.822 V vs. reversible hydrogen electrode (RHE) and showed the maximum power density (Pmax) of 242 mW cm−2 in an AEMFC test, surpassing the peak power density obtained from the Pt/C cathode catalyst (Pmax = 220 mW cm−2) under H2–O2 conditions. Therefore, the study provides a promising prospect for designing improved, cost-effective, and noble metal-free electrocatalysts for achieving high performance in AEMFCs.

Fe–N–C nanostick derived from 1D Fe-ZIFs for electrocatalytic oxygen reduction

The slow oxygen reduction process at the cathode and the scarcity of platinum-based metals lead to limited applications in fuel cells and metal-air cells. Recently, transition metal and nitrogen co-doped carbon-based catalysts (M–N–C) are regarded as the most prospective non-precious metal catalysts for future fuel cell applications. It is verified theoretically and experimentally that the metal and nitrogen coordination structure is the main catalytic activity center of oxygen reduction reaction (ORR), so constructing M–N–C materials with high available surface area and structural stability is an effective way to accelerate ORR. Herein, we deliberately synthesize a one-dimensional ZIF structure to fabricate one-dimensional porous Fe–N–C nanostick via two-step pyrolysis. Excitingly, the as-synthesized exhibited an outstanding ORR activity in alkaline medium (E1/2 of 0.928 V), as well as superior stability (only changed 7 mV after 10,000 cycles in alkaline medium). Our results show that the reduction of electrocatalyst dimensionality can promote mass transport and increase the accessibility of active sites, thus optimizing their performance in ORR. This work is a good demonstration of the importance of a rational design of catalyst structure for efficient ORR.

Defects engineered hierarchical porous iron-nitrogen-enriched carbon derived from pyridyl conjugated microporous polytriphenylamine networks for efficient oxygen reduction reaction

Iron-nitrogen-carbon catalysts are promising non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) used in fuel cells. However, the rational design and fabrication of iron–nitrogen-carbon catalysts with tunable ORR activity remain challenging. Herein, iron–nitrogen-enriched carbon materials (FeNCs) were produced using the precursor of pyridyl conjugated microporous polytriphenylamine with the impregnation of FeCl3via two cycles of high-temperature pyrolysis with acid leaching in-between. The Fe doping content and pyrolysis temperature were varied to manipulate the state of active sites, the degree of graphitization and electrocatalytic performance. The optimized FeNCs exhibit enhanced stability, methanol tolerance and superior ORR activity in alkaline media with a higher onset potential (0.990 V vs RHE), half-wave potential (0.878 V vs RHE) and diffusion current density JL (5.95 mA cm−2) compared to commercial Pt/C catalyst (0.974 V, 0.860 V, 5.10 mA cm−2). The excellent electrocatalytic performance is ascribed to the co-existing highly active sites of Fe4N and Fe3C, the hierarchical porous structures for full action of active sites and a high surface area (1397 m2 g−1) with defect engineered graphitic and amorphous carbon for efficient electron conduction of ORR. Accordingly, the high-performance FeNCs can become an alternative low-cost catalyst to precious Pt/C catalyst for fuel cells.

Deactivation Mechanism of Pyridinic Nitrogen-Doped Carbon for ORR in Acidic Media

Nitrogen doped (N-doped) carbons are attracting attention as Pt-free catalysts for oxygen reduction reaction (ORR) in fuel cells. N-doped carbon catalysts exhibit high activity comparable to that of platinum in basic media whereas the activity is significantly reduced in acidic media. To solve the deactivation mechanism based on kinetics, we have investigated the reaction pathway of N-doped carbon catalysts. The active site of pyridinic nitrogen (pyri-N) is protonated to be pyridinium (pyri-NH+) in acidic media due to acid-base equilibrium ofpyri-N + H+ ⇄ pyriNH+ (1)Following to Eq. (1), we recently exhibited thatpyriNH+ + e- + O2⇄ O2a + pyri-NH(2)proceeds using model catalysts with homogenous active sites. In the step, the electrochemical reduction from pyri-NH+ to pyri-NH and thermal O2 adsorption simultaneously occurs wherein the adsorption of O2 promotes the electron transfer into π* orbital of pyri-NH. We proposed that the hydration of pyri-NH+ lowers the redox potential of Eq. (2) decreasing the ORR activity in acidic media. In the present study, we further considered the acid-base equilibrium of pyri-N in Eq. (1) to comprehensively answer why the ORR activity of N-doped carbon catalyst is high in alkaline media and is decreased in acidic media.To investigate the relationship between acid-base equilibrium and the activity, the electrochemical measurements were performed in electrolyte solutions of various pH. Figure 1 shows the onset potential (voltage at -100 μA/cm2) as a function of pH measure with a model catalyst of 1,10-phenanthroline/(carbon black). In the range from pH 5 to 13, the onset potential decreased with a slope of ~59 mV/pH, while at below pH 5, the onset potentials were almost constant. This indicates that the reaction mechanism changes due to the shift of the acid-base equilibrium. In low pH, the reactions determining the onset potential is assumed to be Eq. (2). On the other hand, at high pH, the reaction should start from pyri-N but not from pyri-NH+.We have figured out that pyri-N + H2O +e- + O2⇄ O2a + pyri-NH + OH- (3) takes place in alkaline madia. Figure 2 shows the N1s peak of the model catalyst applied voltage at pH 13 in N2-(left) and O2-(right)saturated electrolyte. A peak at 400.5 eV appeared at higher applied voltage in O2-saturated electrolyte. This means that pyri-N (399.2 eV) is reduced at a higher potential in the presence of O2 to form pyri-NH. In other words, oxygen adsorption and reduction of pyri-N simultaneously occurred in the presence of O2.The redox potentials of Eq. (2) and (3) explain the pH dependences of the onset potentials. The Nernst equations of Eq. (2) and Eq. (3) show pH-independent redox potential vs. RHE and pH-dependent redox potential vs. RHE with the slope of 59 mV/pH, respectively. This also means that the activity depends on the acid-base equilibrium of pyri-N.We also calculated the redox potentials based on DFT as shown in Figure 3. The additional important point is that the redox potentials of Eq. (2) is strongly influenced by the hydration whereas the redox potentials of Eq. (3) is not influenced by the hydration because the stability of pyri-N is not changed by the hydration. Therefore, the ORR activity in alkaline media is high because the redox potential of Eq. (2) vs. RHE is higher for the higher pH and is not influenced by the hydration. On the other hand, the activity in acidic media is lowered by the hydration. This is that the suppression of the hydration by the introduction of the hydrophobicity at the vicinity of the active site can significantly promote the activity in acidic media. Figure 1

Multi-Walled Carbon Nanotube Supported Manganese Selenide As Highly Active Bifunctional OER and ORR Electrocatalyst

Transition metal selenides have attracted intensive interest as cost-effective electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) because of the continuous thrust in sustainable energy conversion. In this article a Mn-based bifunctional electrocatalyst, MnSe has been identified which shows efficient OER and ORR activity in alkaline medium. The catalytic activity could be further enhanced by using multiwalled carbon nanotubes (MWCNT) asadditives which increases the charge transfer and electronic conductivity of the catalyst composite. This MnSe@MWCNT catalyst composite exhibits a very low overpotential of 210 mV at 50 mA cm-2 when deposited on Ni foam, which outperforms state-of-the-art RuO2 as well as other oxide and Mn-based electrocatalysts. Furthermore, the composite’s facile OER kinetics was evidenced by its small Tafel slope of 54.76 mV dec–1 and low charge transfer resistance, indicating quick transport of the reactant species. The MnSe@MWCNT also exhibited efficient electrocatalytic activity for ORR with a Eonset of 0.94 V, which is among the best reported till date for chalcogenide based ORR electrocatalysts. More importantly, this MnSe-based ORR electrocatalyst exhibits high degree of methanol tolerance, showing no degradation of catalyst performance in presence of copious quantities of methanol, thereby out-performing state-of-the-art Pt electrocatalyst. The catalyst compositie also exhibited exceptional functional and compositional stability for OER and ORR after prolonged period of continuous operation in alkaline medium. The surface Raman analysis after OER revealed the retention of manganese selenide surface with evidence of Mn-oxo coordination confirming the formation of mixed anionic (oxy)selenide as the active surface for OER. Such efficient bifunctional OER and ORR activity makes this MnSe based catalyst attractive for overall electrolysis in regenerative as well as direct methanol fuel cells.

Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide.

The electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H2O2), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H2O2. The optimized Pb(NiWMnNbZrTi)1/6O3 ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H2O2 including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide

AbstractThe electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H2O2), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H2O2. The optimized Pb(NiWMnNbZrTi)1/6O3 ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H2O2 including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

Fe/Co/N-C/graphene derived from Fe/ZIF-67/graphene oxide three dimensional frameworks as a remarkably efficient and stable catalyst for the oxygen reduction reaction.

The development of non-noble metal catalysts with high-performance, long stability and low-cost is of great importance for fuel cells, to promote the oxygen reduction reaction (ORR). Herein, Fe/Co/N–C/graphene composites were easily prepared by using Fe/ZIF-67 loaded on graphene oxide (GO). The Fe/Co/porous carbon nanoparticles were uniformly dispersed on graphene with high specific surface area and large porosity, which endow high nitrogen doping and many more active sites with better ORR performance than the commercial 20 wt% Pt/C. Therefore, Fe/Co/N–C/graphene composites exhibited excellent ORR activity in alkaline media, with higher initial potential (0.91 V) and four electron process. They also showed remarkable long-term catalytic stability with 96.5% current retention after 12 000 s, and outstanding methanol resistance, compared with that of 20 wt% Pt/C catalysts. This work provides an effective strategy for the preparation of non-noble metal-based catalysts, which could have significant potential applications, such as in lithium–air batteries and water-splitting devices.

Synergistic Binary Fe-Co Nanocluster Supported on Defective Tungsten Oxide as Efficient Oxygen Reduction Electrocatalyst in Zinc-Air Battery.

Rational design of metal oxide supported non‐precious metals is essential for the development of stable and high‐efficiency oxygen reduction reaction (ORR) electrocatalysts. Here, an efficient ORR catalyst consisting of binary Fe/Co nanoclusters supported by defective tungsten oxide and embedded N‐doped carbon layer (NC) with a 3D ordered macroporous architecture (3DOM Fe/Co@NC‐WO2− x ) is developed. The oxygen deficient 3DOM WO2− x not only serves as a porous and stable support, but also enhances the conductivity and ensures good dispersion of the binary Fe/Co nanocluster, benefiting its ORR catalytic activity. Theoretical calculation shows that there exists a synergistic effect of electron transfer from Fe to Co in the supported binary Fe/Co cluster, promoting the ORR reaction energetics. Accordingly, the 3DOM Fe/Co@NC‐WO2− x catalyst exhibits excellent ORR activity in alkaline medium with a half wave potential (E 1/2) of 0.87 V higher than that of Pt/C (0.85 V). The zinc–air batteries assembled by 3DOM Fe/Co@NC‐WO2− x cathode deliver a higher power density and specific capacity than that of Pt/C. A new strategy of combining synergistic binary‐metal nanoclusters and conductive metal oxide support design is provided here to develop efficient and durable ORR electrocatalyst.

Improvement of Oxygen Reduction Performance in Alkaline Media by Tuning Phase Structure of Pd-Bi Nanocatalysts.

Tuning the crystal phase of bimetallic nanocrystals offers an alternative avenue to improving their electrocatalytic performance. Herein, we present a facile and one-pot synthesis approach that is used to enhance the catalytic activity and stability toward oxygen reduction reaction (ORR) in alkaline media via control of the crystal structure of Pd-Bi nanocrystals. By merely altering the types of Pd precursors under the same conditions, the monoclinic structured Pd5Bi2 and conventional face-centered cubic (fcc) structured Pd3Bi nanocrystals with comparable size and morphology can be precisely synthesized, respectively. Interestingly, the carbon-supported monoclinic Pd5Bi2 nanocrystals exhibit superior ORR activity in alkaline media, delivering a mass activity (MA) as high as 2.05 A/mgPd. After 10,000 cycles of ORR durability test, the monoclinic structured Pd5Bi2/C nanocatalysts still remain a MA of 1.52 A/mgPd, which is 3.6 times, 16.9 times, and 21.7 times as high as those of the fcc Pd3Bi/C counterpart, commercial Pd/C, and Pt/C electrocatalysts, respectively. Moreover, structural characterizations of the monoclinic Pd5Bi2/C nanocrystals after the durability test demonstrate the excellent retention of the original size, morphology, composition, and crystal phase, greatly alleviating the leaching of the Bi component. This work provides new insight for the synthesis of multimetallic catalysts with a metastable phase and demonstrates phase-dependent catalytic performance.

MIL-101-Derived Mesoporous Carbon Supporting Highly Exposed Fe Single-Atom Sites as Efficient Oxygen Reduction Reaction Catalysts.

Fe single-atom catalysts (Fe SACs) with atomic FeNx active sites are very promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR). The pyrolysis of metal-organic frameworks (MOFs) is a common approach for preparing Fe SACs, though most MOF-derived catalysts reported to date are microporous and thus suffer from poor mass transfer and a high proportion of catalytically inaccessible FeNx active sites. Herein, NH2 -MIL-101(Al), a MOF possessing a mesoporous cage architecture, is used as the precursor to prepare a series of N-doped carbon supports (denoted herein as NC-MIL101-T) with a well-defined mesoporous structure at different pyrolysis temperatures. The NC-MIL101-T supports are then impregnated with a Fe(II)-phenanthroline complex, and heated again to yield Fe SAC-MIL101-T catalysts rich in accessible FeNx single atom sites. The best performing Fe SAC-MIL101-1000 catalyst offers outstanding ORR activity in alkaline media, evidenced by an ORR half-wave potential of 0.94V (vs RHE) in 0.1 m KOH, as well as excellent performance in both aqueous primary zinc-air batteries (a near maximum theoretical energy density of 984.2Wh kgZn -1 ) and solid-state zinc-air batteries (a peak power density of 50.6mW cm-2 and a specific capacity of 724.0 mAh kgZn -1 ).

Shape inducer-free polygonal angle platinum nanoparticles in graphene oxide as oxygen reduction catalyst derived from gamma irradiation

In this report, polygonal angle platinum nanoparticles (PtNPs) anchored on nitrogen doping reduced graphene oxide (NrGO) as oxygen reduction reaction (ORR) catalyst was synthesized by gamma irradiation assisted with in situ hydrolysis of urea without using any shape inducer, seed, or template. Urea was not only employed as the nitrogen source, but also offered more reductive radicals in the gamma system. The uniform dispersion and homogeneous size distribution of PtNPs are obtained on reduced graphene oxide (rGO), which is attributed to the synergy of restriction effects of GO and crush capacity of high energy gamma rays. In addition, the method simultaneously offers PtNPs with polygonal angle structure and doping nitrogen in rGO, thus provides more surface and corner defects on PtNPs and heteroatomic defects on rGO, which synergistically improve the ORR performance of the samples. The obtained polygonal angle PtNPs modified NrGO exhibit fantabulous ORR activity in alkaline media with enhanced onset potential (906 mV), half-wave potential (783 mV) and superior limit current density (6.74 mA·cm−2) compared to the commercial Pt/C and those PtNPs supported on rGO composites. The results indicate that gamma irradiation assisted with in situ hydrolysis of urea can be a promising candidate method for preparation of high performance Pt-based catalysts in practical application.

Atomic Dispersion and Surface Enrichment of Palladium in Nitrogen-Doped Porous Carbon Cages Lead to High-Performance Electrocatalytic Reduction of Oxygen.

Metal-nitrogen-carbon (MNC) nanocomposites have been hailed as promising and efficient electrocatalysts toward oxygen reduction reaction (ORR), due to the formation of MNx coordination moieties. However, MNC hybrids are mostly prepared by pyrolysis of organic precursors along with select metal salts, where part of the MNx sites are inevitably buried in the carbon matrix. This limited accessibility compromises the electrocatalytic performance. Herein, we describe a wet-impregnation procedure by facile thermal refluxing, whereby palladium is atomically dispersed and enriched onto the surface of hollow, nitrogen-doped carbon cages (HNC) forming Pd-N coordination bonds. The obtained Pd-HNC nanocomposites exhibit an ORR activity in alkaline media markedly higher than that of metallic Pd nanoparticles, and the best sample even outperforms commercial Pt/C and relevant Pd-based catalysts reported in the literature. The results suggest that atomic dispersion and surface enrichment of palladium in a carbon matrix may serve as an effective strategy in the fabrication of high-performance ORR electrocatalysts.

N-Doped Porous Carbons Derived from Cross-Linked Polybenzoxazine as Efficient Catalysts for ORR in Alkaline Electrolyte

As a promising class of metal-free oxygen reduction reaction (ORR) catalysts, N-doped carbon (NDC) catalysts have attracted widespread attention owing to their high stability, high conductivity, good poison tolerance, and environmental friendliness. We herein report the fabrication of NDC catalysts by using triphenylimidazole-containing polybenzoxazine (TPB) as the precursor. The TPB is firstly Friedel–Crafts cross-linked in CCl4 by using anhydrous AlCl3 as the catalyst. The cross-linked TPB is then pyrolyzed at 800 °C–1000 °C, giving O-containing NDC catalysts. The as-fabricated NDC-900 catalyst has a particulate morphology together with a high BET surface area (947 m2 g−1) and a highly ORR active N content (3.39 at%). Compared to the commercial Pt/C catalyst, the NDC-900 catalyst exhibits a higher ORR activity in alkaline media as evidenced by the higher onset potential (E o, 0.986 vs 0.964 V), higher half-wave potential (E 1/2, 0.845 vs 0.815 V), and higher current density at 0.92 V (J @0.92, 0.251 vs 0.141 mA cm−2). Moreover, the NDC-900-based Zn-air battery achieves a maximum power density of 131.4 mW cm−2 at 204.8 mA cm−2 and a high specific capacity of 818.4 mAh g−1 Zn at 5.0 mA cm−2.

In‐Situ Electrodeposition of Rhodium nanoparticles Anchored on Reduced Graphene Oxide nanosheets as an Efficient Oxygen Reduction Electrocatalyst

A simple, versatile, and cost‐effective one‐pot electrochemical deposition is used to fabricate rhodium (Rh) nanoparticles decorated surface of reduced graphene oxide (rGO) functionalized glassy carbon electrode (GCE) for oxygen reduction reaction (ORR) in alkaline media. The chemical and physical structure of the sample is probed via transmission electron microscopy, rotating disk electrode (RDE), X‐ray photoelectron spectroscopy, linear sweep voltammetry, and Raman spectroscopy. The synergistic effects between the unique properties of Rh nanoparticles and rGO creates such innovative hybrid that exhibits a catalytic activity comparable to that of the commercial platinum electrocatalyst (Pt/C). As a result, the as‐electrodeposited Rh@rGO hybrid exhibits outstanding ORR activity in alkaline media, as evidenced by a larger diffusion‐limited current, greater positive onset potential, much better stability and methanol tolerance than Pt/C under the same conditions.

Porous carbon framework derived from N-rich hypercrosslinked polymer as the efficient metal-free electrocatalyst for oxygen reduction reaction

The development of high-performance non-precious metal electrocatalysts is of great importance for construction of next generation fuel cells. Herein, we report for the first time the creation of new metal-free porous carbon framework catalysts derived from N-rich hypercrosslinked polymers (HCPs) using pyrrole as building blocks for oxygen reduction reaction (ORR) (denoted as PCF-HCPs). The PCF-HCPs obtained at the pyrolysis temperature of 900 °C (PCF-HCP-900), displayed outstanding catalytic activity towards the ORR compared with other PCF-HCPs pyrolysis temperature. Due to PCF-HCP-900 with large specific surface, excellent porosity and, importantly, exposed active sites caused by N doping with high density, the catalysts exhibited outstanding ORR activity in alkaline medium, i.e., the PCF-HCP-900 yielded a half-wave potential of 0.84 V, which was 10 mV higher than that of the state of-the-art Pt/C catalyst (0.83 V), and onset potential of 0.95 V (vs. RHE), a high diffusion limiting current density of 4.8 mA cm−2, better methanol tolerance and long-term stability (2 mV negative shift after 3000 potential cycles) than commercial Pt/C. Furthermore, PCF-HCP-900 shows a favorable 4-electron transfer process (n ≈ 3.96) and lower H2O2 yield. Based on these merits, the PCF-HCP-900 may open a new possibility for design of ORR metal-free catalyst of fuel-cell technologies.

Cobalt-Free Perovskite-Type Oxygen Reduction Reaction Catalysts for Energy Conversion

Alkaline fuel cells and metal–air batteries are emerging as an alternative energy conversion devices which can use non-precious and earth-abundant materials as electrocatalysts for oxygen reduction reaction (ORR) [1-3]. Perovskite-type oxides have been widely investigated due to their high activity, long durability and low cost [4]. Mn and Co-based perovskites have shown stable and high ORR performance and their electrochemical properties was explained using the molecular orbital theory. It was shown that the transition-metal-oxide electrocatalysts exhibit maximum catalytic activity when the occupancy of the eg orbital is ~1 which is the primary activity descriptor, whereas spin-state and electronegativity are secondary activity descriptors [5]. However, Co-based oxides have several disadvantages such as high volatility, thermal expansion coefficient, and cost [6-8]. Thus, there is need for alternative Co-free materials for next generation fuel cells. Here, we report Co-free perovskite-type Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as ORR electrode for fuel cells. Effect of Cu-doping in Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) towards the ORR activity in alkaline media has been studied. Partial substitution of Cu in the Fe-site in (Sr,Ba)FeO3 could influence the transition-metal oxygen covalency which replaces and regenerates OH— ions in alkaline ORR process. Thin film rotating-disk electrode (RDE) method with well-defined oxygen transport was previously developed to study the ORR activity of perovskite catalysts [4, 9]. Evaluating the effects of such molecular level engineering on the desired outcome is challenging, since the ORR process consists of multiple adsorption/desorption step involving oxygen containing species. There are fewer literature on understanding the ORR process on transition metal oxides involving the transfer of four electrons and four OH— ions to O2 with subsequent cleavage of double bond O2. Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) powders were prepared through sol gel synthesis method. Powder X-ray diffraction was employed for structural characterization and the activity towards ORR was studied using RDE method. RDE experiments were performed in oxygen saturated electrolyte at 100-1600 rpm. The voltage scan was performed from 0.4-0.9 V vs. RHE. The relation between current and rotational speed was evaluated through a Koutechy-Levich (K-L) plot to deduce the number of electrons (n) involved in the ORR process, i.e. n = (0.62∙F∙D0 2/3∙ v-1/6∙Co∙slope)-1 Where, F is Faraday’s constant, F = 96485 C.mol-1, (Do) diffusion coefficient of oxygen in 0.1M KOH = 1.93x10-5 cm2.s-1, (ν) Kinematic viscosity of 0.1 M KOH = 1.09x10-2 cm2.s-1, (Co) saturation concentration of O2 in 0.1 M KOH at 1 atm O2 pressure = 1.26x10-6 mol.cm-3, and slope – slope from K-L plot. Preliminary results showed that Cu doping in the lattice is affecting the ORR properties of perovskite in alkaline media. The slope of K-L plot gradually increases with increase in Cu content and decreases beyond 0.05 Cu until 0.2 Cu. The number of electron involved in the ORR process is mostly restricted to two. Acknowledgements This research was supported by the Canada First Research Excellence Fund (CFREF) at the University of Calgary. References Lu, J. Pan, A. Huang, L. Zhuang, J. Lu, Proc. Natl. Acad. Sci. U. S. A. 105, 20611 (2008).Pan, C. Chen, L. Zhuang, J. Lu, Acc. Chem. Res. 45, 473 (2012).Oz, K. Singh, D. Gelman, V. Thangadurai, Y. Tsur, J. Phys. Chem. C. 122 (27) 15097 (2018).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W.T. Goh, T.S.A. Hor, Y. Zong, Z. Liu, ACS Catal. 5 , 4643 (2015).Suntivich, H. A. Gasteiger, N.Yabuuchi, H. Nakanishi, J.B. Goodenough, Y. Shao-Horn, Nat. Chem. 3, 546 (2011).McIntosh, J.F.Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Chem. Mater. 18, 2187 (2006).Teraoka, H. Shimokawa, C. Y. Kang, H. Kusaba, K. Sasaki, Solid State Ionics 177 , 2245 (2006).Wang, C. Tablet, A. Feldhoff, J. Caro, Adv. Mater. 17, 1785 (2005).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, et al., ACS Catal. 5, 4643 (2015). Figure 1

New Avenues Towards Designing Transition Metal Phosphides for Efficient Electrocatalysis

Transition metal phosphides (TMPs) have emerged as an intriguing class of efficient electrocatalysts finding widespread application in hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) etc. Unfortunately, however, currently adopted wet chemistry and solid-gas synthesis techniques rely on the use of surfactants/ toxic organic solvents or toxic gas which restricts facile fabrication of TMPs. Moreover, hybridizing TMPs with suitable carbon support (graphene, carbon nanotubes etc.) which favorably optimizes the electrochemical activity of TMPs is often achieved via post-synthesis modification further complicating the synthesis procedure. In an attempt to overcome these limitations, we have recently developed a solid state, phosphine free method to fabricate Co2P nanoparticles encapsulated N,P co-doped CNTs. These as prepared catalysts were applied as a bi-functional electrocatalyst for HER and OER. In order to increase the surface area and loading of Co2P nanoparticles, we next designed Co2P encapsulated CNTs grafted N,P co-doped hollow spheres via MOF-templated strategy with superior catalytic activity compared to Co2P@CNTs. We then introduced sub-2nm IrP2 nanoparticles encapsulated bilayered N, P co-doped graphene as a novel electrocatalyst which exhibits better HER activity than benchmark Pt/C in acid, neutral as well as the alkaline medium. Interestingly, careful tuning of the reaction parameters also enables the synthesis of phase pure Ir2P. Finally, a MOF-templated strategy was adopted to fabricate N, P co-doped hollow carbon nanocages grafted with ~ 2nm Ru2P nanoparticles which exhibited superior bifunctional HER and ORR activity in alkaline medium. In conclusion, the versatile strategy developed here can be extended to fabricate a variety of TMPs with widely varied applications.

Nitrogen-Doped Carbon Nano-Onions as a Metal-Free Electrocatalyst

Growing environmental and economical concern necessitates novel materials with inherent electrochemical activity in as-prepared. Herein, we report, the electrochemical reactivity of the nitrogen-doped carbon nano-onions (N-CNO) prepared by a simple flame pyrolysis method. The as-obtained N-CNO has high N-content (4 at.%) and a large Brunauer-Emmett-Teller surface area (842.3 m2 g−1). The as-prepared N-CNO were probed as a cathode material for the oxygen reduction reaction (ORR) and as an electrocatalyst for sensitive nitrite ( $$ {\mathrm{NO}}_2^{-} $$ ) determination. The results demonstrate that, compared to N-CNT, as-prepared N-CNO exhibit a significantly enhanced ORR activity in alkaline medium and long-term stability with negligible change in activity. The N-CNO-based electrochemical nitrite sensor exhibits nitrite oxidation with two linear ranges 0.5 to 10 μM (R = 0.996) and 10 to 1000 μM (R = 0.978) with a sensitivity of 811.22 and 400.51 μA mM−1 cm−2, respectively. The detection limit (LOD) was 0.3 μM. The relative standard deviation (RSD) for eight replicate determinations was found to be 2.53%. The values of observed analytical parameters (linear range, sensitivity, and LOD) are comparable or superior to previously reported carbon-based $$ {\mathrm{NO}}_2^{-} $$ sensors.