Oxygen Reduction Reaction Onset Potential Research Articles

Bifunctional electrocatalysts based on hierarchical graphene/iron hybrid architectures branched by N-doped CNT

A noble metal-free, highly active and stable bifunctional electrocatalyst with high activity and stability is needed to replace existing precious ones for electrochemical energy conversion and storage systems, such as fuel cells and metal-air batteries. Herein, we report hierarchical steam-activated reduced graphene oxide (srGO)/Fe hybrid architectures branched by nitrogen-doped carbon nanotube (N-CNT) for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts. Through the adequate choice of ferrocene precursor and microwave method, the CNT branches, the active iron phase, and N-doping are accomplished simultaneously to construct a bifunctional hybrid electrocatalyst. The Fe2O3/N-CNT@srGO hybrid architecture achieves an ORR onset potential of 0.72 V, a mass activity of 605.30 mA·mgactive⁻1, an OER onset potential of 1.63 V, and a Tafel slope of 61 mV·dec⁻1, which are much higher compared to those of the Fe2O3/CNT@srGO, Fe2O3@N-srGO, and Fe2O3@N-CNT catalysts. These results indicate the enhancement of bifunctional catalytic activity due to hierarchical porous structure, CNT branches, and N-doping.

The Activity of Oxygen Reduction Reaction on the Noble Metal Doped Titanium Oxide in Acidic Electrolyte

Titanium oxide with the oxygen vacancies is expected as one of the candidates for the non-platinum catalyst of polymer electrolyte fuel cells. The reasons why the activity of titanium oxide improves by the presence of oxygen vacancies are assumed the formation of the active site and the micro electron-conducting path [1, 2]. From the viewpoint of enhancement of the electron conductivity, it is well known that doping a transition metal to titanium oxide is also effectiveness method. Thus, many papers had reported about the niobium doped titanium oxide to increase the electron conductivity. However, not only the doping transition metals but also the oxygen vacancies are necessary to obtain high activity of the titanium oxide. We put forward a possibility that the crystal distortion acts as the active site [3]. Now, we think that the majority role of transition metal doping is the formation of the micro electro-conducting path to the active site. Hence, we were attending to the electron conductivity of transition metal-doped titanium oxide. From the results of a theoretical calculation, the electron conductivity of niobium doped titanium oxide, which is researched well as the non-platinum catalyst, is relatively low [4]. And, ruthenium and iridium as doping species give high electron conductivity to titanium oxide. Ruthenium and iridium are famous for interacting with titanium oxide in the electrolysis field. In this study, the activity of oxygen reduction reaction (ORR) of ruthenium or iridium doped titanium oxide was evaluated. Ruthenium doped titanium oxide (Ru-TiO2) and iridium doped titanium oxide (Ir-TiO2) were synthesized by the sol-gel method, and both doping titanium oxides were supported on the Ketjen Black 300 J. The loading of both doping titanium oxides was less than 20 wt%. In addition, the molar ratio of titanium and ruthenium or iridium was 95:5. Electrochemical studies were conducted in Ar- or O2-saturated 0.1 M HClO4 at 60ºC with carbon plate counter-electrode and Ag/AgCl reference. The ORR activity was evaluated by the difference current between the current in Ar-saturated condition from the current in O2-saturated condition. The crystal structure of Ru-TiO2 was almost the same as the anatase structure of TiO2. The intensity of diffraction peaks of Ir-TiO2 was weaker than that of Ru-TiO2, although its crystal structure was the anatase. Both samples showed no diffraction peaks attributed to the noble metal oxide and metallic phase. The atomic valence of ruthenium in Ru-TiO2 was mainly tetravalent from the results of XPS analysis. But a full width at half maximum of Ru 2p was wide. Thus, the ruthenium atom may have many oxidation states. Titanium atom at the surface showed tetravalent, and it inside of a particle indicated the reduction state. Now, we thought that the ruthenium atom was atomically mixing with titanium oxide structure because no evidence observed ruthenium oxide and ruthenium metal by our analysis method.ORR activity of titanium oxide improved by the addition of 5 mol% ruthenium. ORR activity of titanium oxide is well known to increase by introducing the oxygen vacancies. ORR activity of Ru-TiO2 was higher than that of titanium oxide with the oxygen vacancies. Additionally, Ru-TiO2 showed also high ORR activity after the reduction process using hydrogen gas as well as titanium oxide. Ir-TiO2 showed the highest ORR current in this study, and its onset potential of ORR was also a high value at about 0.87 V.From the above results, the addition of a small quantity of noble metal was very effective for the enhancement of ORR activity of titanium oxide.[1] A. Ishihara, et al., J. Phys. Chem. C, 117, 18837 (2013).[2] A. Ishihara, et al., J. Phys. Chem. C, 123, 18150 (2019).[3] T. Saida, et al., MRS Advances, 4, 1851 (2019).[4] M.-C. Tsai, et al., Mol. Syst. Des. Eng., 2, 449 (2017).

Promoting H2O2 production via 2-electron oxygen reduction by coordinating partially oxidized Pd with defect carbon

Electrochemical synthesis of H2O2 through a selective two-electron (2e−) oxygen reduction reaction (ORR) is an attractive alternative to the industrial anthraquinone oxidation method, as it allows decentralized H2O2 production. Herein, we report that the synergistic interaction between partially oxidized palladium (Pdδ+) and oxygen-functionalized carbon can promote 2e− ORR in acidic electrolytes. An electrocatalyst synthesized by solution deposition of amorphous Pdδ+ clusters (Pd3δ+ and Pd4δ+) onto mildly oxidized carbon nanotubes (Pdδ+-OCNT) shows nearly 100% selectivity toward H2O2 and a positive shift of ORR onset potential by ~320 mV compared with the OCNT substrate. A high mass activity (1.946 A mg−1 at 0.45 V) of Pdδ+-OCNT is achieved. Extended X-ray absorption fine structure characterization and density functional theory calculations suggest that the interaction between Pd clusters and the nearby oxygen-containing functional groups is key for the high selectivity and activity for 2e− ORR.

B, N‐codoped Cu–N/B–C Composite as an Efficient Electrocatalyst for Oxygen‐Reduction Reaction in Alkaline Media

AbstractAs an alternative for platinum‐based electrocatalysts, the development of non‐precious metal catalysts for oxygen reduction reaction (ORR) is highly desirable for fuel cell applications. In this paper, we propose a facile preparation method for a B, N‐codoped Cu–N/B–C nanomaterial as an efficient electrocatalyst for ORR in alkaline electrolytes. One‐step heat treatment of cyanamide/melamine, boric acid, and cupric chloride loaded on carbon black produces a Cu–N/B–C composite with a high specific surface area. The Cu–N/B–C‐800 composite pyrolyzed at 800 °C has the best ORR performance among all tested composites. Cu–N/B–C‐800 saw an ORR onset potential at 0.95 V and a half‐wave potential (E1/2) at 0.84 V vs. reversible hydrogen electrode (RHE) in 0.1 M KOH solution, which is comparable to the commercial 20 wt% Pt/C catalyst. Moreover, Cu–N/B–C‐800 has a small negatively shifted E1/2 value (−8.0 mV) under the accelerated‐durability test condition, demonstrating superior stability and higher tolerance to the methanol‐crossover effect compared with the Pt/C catalyst. Furthermore, the anion exchange membrane fuel cell (AEMFC) loaded with Cu–N/B–C‐800 as the cathode catalyst has an open cell voltage of 0.85 V and a peak power density of 80 mW/cm2 at 60 °C without backpressure, which is in the list of optimal non‐precious metal catalysts for ORR in AEMFC.

FeNx/FeSx-Anchored Carbon Sheet–Carbon Nanotube Composite Electrocatalysts for Oxygen Reduction

Even though various Pt-free electrocatalysts for oxygen reduction reaction (ORR) have been introduced, many of them are found to be active only in alkaline conditions. Considering Nafion, phosphoric acid-doped polybenzimidazole (PBI), and so on as the prominent ionomer membranes, used in the commercially available polymer electrolyte membrane fuel cells (PEMFCs), it becomes important that any development on the Pt-free catalysts should ensure the better ORR performance under acidic conditions. The present work effectively tackles this issue, where an ORR-based catalyst could be prepared with simultaneous incorporation of both Fe–N and Fe–S active sites on in situ generated carbon sheets which are spatially separated by the carbon nanotube (CNT) network. This catalyst shows ability to perform under both acidic and basic conditions. This has been achieved by growing a polyethylenedioxythiophene polymer network in the presence of CNT and melamine followed by its pyrolysis under an inert atmosphere. The catalyst formed at 900 °C (PMCNT-900) displays 0.94 V onset potential for ORR under acidic electrolyte conditions, which corresponds to 60 mV overpotential compared to its 40 wt % Pt/C counterpart. Interestingly, in single cell demonstration of Nafion-based PEMFC with PMCNT-900 as the cathode catalyst, the system delivered a maximum power density (PD) of 500 and 275 mW/cm2 at 60 °C under H2–O2 and H2–air feed conditions, respectively. On the other hand, in a single cell test in the anion exchange membrane fuel cell (AEMFC) mode, a maximum power density of 65 mW/cm2 at 50 °C could be achieved with the same cathode catalyst, which is a comparable value obtained while employing Pt/C as the cathode. These results, thus, infer to the efficiency of the catalyst to facilitate ORR under the extreme pH conditions, and particularly its performance under acidic condition reveals its prospect as a potential Pt-free electrocatalyst to serve in the Nafion-based systems.

Au/Ta(110) and Au/Nb(110) as Highly Active, Stable, and Inexpensive Catalysts for Oxygen Reduction Reaction on Hydrogen Fuel Cell Cathodes: Prediction from First Principles

AbstractWe propose here structures – a gold monolayer on early transition metal surfaces – as a promising alternative to the prohibitively expensive platinum‐based electro‐catalysts for the oxygen reduction reaction (ORR), which occurs on hydrogen fuel cell cathodes. Based on existing knowledge and educated guesses, we preselect Au/Nb, Au/Ta, Au/Mo, and Au/W as potential catalysts. Our calculations show that these materials are stable and have very high dissolution potentials. A deviation from the linear scaling for the binding energy of ORR intermediates, revealed for Au/Nb and Au/Ta, has a desired impact on the ORR thermodynamics. We find it to be a result of hybridization of p‐electronic states of oxygen belonging the OOH‐radical and Au‐pz states. The calculated ORR free energy diagrams show that the ORR onset potentials for Au/Nb and Au/Ta are significantly higher than that for Pt. We thus predict two inexpensive, thermodynamically and electrochemically stable, and highly active ORR catalysts.

Role of strontium as doping agent in LaMn0.5Ni0.5O3 for oxygen electro-catalysis

Doping is an effective way to trigger various behaviors in ceramics. LaMn0.5Ni0.5O3 perovskites with or without strontium (Sr) doping were prepared using a sol-gel process and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) gas adsorption techniques. A structural transition in the LaMn0.5Ni0.5O3 perovskite was observed upon doping, suggesting its pinning effect on the crystal structure. The SEM and BET results showed that the incorporation of Sr2+ into this oxide material markedly decreased the particle size into the nanoscale range and increased the active surface area. The oxide electrocatalytic properties toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an alkaline medium were measured using a rotating disk electrode. The results revealed that La0.8Sr0.2Ni0.5Mn0.5O3 is a more active catalyst than LaNi0.5Mn0.5O3 for the ORR and OER. The improved electrocatalytic performance of the Sr-doped oxide is related to the increase in ORR/OER current density and the positive shift in the ORR onset potential compared with that of the LaNi0.5Mn0.5O3 catalyst. The overall electron transfer number was measured as approximately 4. The basis of this enhanced performance is discussed in terms of the material electronic structure and morphology.

Communication—Platinum and Tin Oxide Dispersed in a Fluffy TiO2 Nanolayer for Electrocatalytic Reduction of Oxygen

Pt catalysts perform well in the oxygen reduction reaction (ORR), but they suffer weak bonding with carbon supports, leading to catalyst degradation. We introduce a fluffy titanium dioxide (TiO2) nanolayer with a very high specific surface area as support, made possible with a new condensed layer deposition technique. When Pt and SnO2 were incorporated in it as co-catalyst, a remarkable improvement in ORR onset potential was achieved at 930 mV vs RHE in sulfuric acids. The mass activity was 2.66 times that of Pt/C at 900 mV. The fluffy layer stabilized SnO2 and together, they enabled a pronounced ligand effect.

Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts.

The development of low-cost bifunctional electrocatalysts with both a high activity and long durability is critical for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The reasonable design and construction of bifunctional electrocatalysts is the key to energy storage and energy conversion technologies. In this study, transition metal carbon nitrides were used as a substitute for the precious metal catalyst, the Ni-Co-BTC (metal organic framework (MOF)) mixed with polyacrylonitrile (PAN) using electrostatic spinning technology to prepare the bamboo-like nanofibers precursor (Ni-Co-BTC@PAN). A series of electrocatalytic materials (NiCo-X@N-CNFs-Ts, T = 700, 800, 900 °C) were synthesized with nitrogen-doped carbon nanofibers coated with NiCo alloy nanoparticles using high temperature carbonization at different temperatures. We studied the effects of different calcination temperatures and different Ni/Co molar ratios of NiCo-X@N-CNFs-Ts (T = 700, 800, 900 °C) on the bifunctional catalytic performance of the ORR/OER. The composite, NiCo-0.8@N-CNFs-800, exhibited a highly doped-N level, uniform NiCo alloy nanoparticle dispersion and decentralized NiCo-Nx active sites, therefore affording an excellent bifunctional electrocatalytic performance. The ORR onset potential on NiCo-0.8@N-CNFs-800 was 0.91 V and the half-wave potential (E1/2) was 0.82 V, the NiCo-0.8@N-CNFs-800 corresponded to the minimum potential of 1.61 V at the current density of 10 mA cm-2 among all of the NiCo-X@N-CNFs-Ts hybrids under the OER condition. The NiCo-0.8@N-CNFs-800 catalyst exhibited a low reversible overpotential of 0.79 V between the ORR (E1/2) and OER (Ej = 10 mA cm-2) with excellent stability, durability and methanol tolerance, even surprisingly superior to the commercial Pt/C and RuO2 catalysts. This work provides a general strategy and useful guidance for the design and development of a variety of multifunctional non-noble metal catalysts for energy applications.

Design and synthesis of MnN4 macrocyclic complex for efficient oxygen reduction reaction electrocatalysis

The Pt-based materials were considered to be the most promising catalysts for the oxygen reduction reaction (ORR) in fuel cell technology but poor abundance and prohibitive cost limit their further use. Herein, we designed and synthesized a novel MnN4 macrocyclic complex and its nanocomposite, MnN4@rGO, with reduced graphene oxide (rGO). Both the MnN4 complex and MnN4@rGO composite were characterized by using multiple spectroscopies. On the basis of spectral analysis, octahedral geometry was assigned to MnN4 complex. Relevant to their performance for ORR electrocatalysis, the composite MnN4@rGO showed significant good ORR performance (+0.91 V and +0.86 V, onset and formal potential, respectively). Further, the Mn3+/Mn2+ redox potential and its role as activity indicator for ORR were correlated to the low energy gap between HOMO of Mn metal ion and LUMO of dioxygen, leading to effective overlapping between Mn3+/Mn2+ and ORR onset potentials for 4e− ORR. The SCN− poison effect on the ORR active sites showed the establishment of the Mn3+ state that could anticipate the redox (Mn3+/Mn2+) process, and decrease in available active site for oxygen adsorption, ORR declined.

Co3O4/C and Au supported Co3O4/C nanocomposites – Peculiarities of fabrication and application towards oxygen reduction reaction

The purpose of this paper was to investigate the influence of fabrication conditions of the carbon based cobalt(II, III) oxide (Co3O4/C) and Au supported Co3O4/C (AuCo3O4/C) catalysts on their electrocatalytic activities towards the oxygen reduction reaction (ORR). The catalysts were prepared using impregnation and microwave irradiation methods. It has been found, that the combination of both of these methods was a more efficient way for fabrication of high catalytically active Co3O4/C and AuCo3O4/C catalysts. The synthesized Co3O4/C and AuCo3O4/C catalysts showed high electrocatalytic activity towards the oxygen reduction reaction in an alkaline medium. The ORR onset potential for the investigated catalysts was comparable to theoretical ORR potential values between 0.7 and 0.9 V vs. reversible hydrogen electrode (RHE).

Probing the active sites of site-specific nitrogen doping in metal-free graphdiyne for electrochemical oxygen reduction reactions

The development of highly active and low-cost catalysts for electrochemical reactions is one of the most attractive topics in the renewable energy technology. Herein, the site-specific nitrogen doping of graphdiyne (GDY) including grap-N, sp-N(I) and sp-N(II) GDY is systematically investigated as metal-free oxygen reduction electrocatalysts via density functional theory (DFT). Our results indicate that the doped nitrogen atom can significantly improve the oxygen (O2) adsorption activity of GDY through activating its neighboring carbon atoms. The free-energy landscape is employed to describe the electrochemical oxygen reduction reaction (ORR) in both O2 dissociation and association mechanisms. It is revealed that the association mechanism can provide higher ORR onset potential than dissociation mechanism on most of the substrates. Especially, sp-N(II) GDY exhibits the highest ORR electrocatalytic activity through increasing the theoretical onset potential to 0.76 V. This work provides an atomic-level insight for the electrochemical ORR mechanism on metal-free N-doped GDY.

Climbing the Apex of the ORR Volcano Plot via Binuclear Site Construction: Electronic and Geometric Engineering.

Great enthusiasm in single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) has been aroused by the discovery of M-NX as a promising ORR catalysis center. However, the performance of SACs lags far behind that of state-of-the-art Pt due to the unsatisfactory adsorption-desorption behaviors of the reported catalytic centers. To address this issue, rational manipulation of the active site configuration toward a well-managed energy level and geometric structure is urgently desired, yet still remains a challenge. Herein, we report a novel strategy to accomplish this task through the construction of an Fe-Co dual-atom centered site. A spontaneously absorbed electron-withdrawing OH ligand was proposed to act proactively as an energy level modifier to empower easy intermediate desorption, while the triangular Fe-Co-OH coordination facilitates O-O bond scission. Benefiting from these attributes, the as-constructed FeCoN5-OH site enables an ORR onset potential and half-wave potential of up to 1.02 and 0.86 V (vs RHE), respectively, with an intrinsic activity over 20 times higher than the single-atom FeN4 site. Our finding not only opens up a novel strategy to tailor the electronic structure of an atomic site toward boosted activity but also provides new insights into the fundamental understanding of diatomic sites for ORR electrocatalysis.

Lignin derived multi-doped (N, S, Cl) carbon materials as excellent electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Abstract Nowadays, hierarchically macro-/meso-/microporous 3D carbon materials have been paid more attention due to their imaginative application potential in specific electrochemistry. Here, we report a dual-template strategy using eutectic NaCl/ZnCl2 melt as airtight and swelling agent to obtain 3D mesoporous skeleton structured carbon from renewable lignin. The prepared lignin-derived biocarbon material (LN-3-1) has a high specific surface area (1289 m2 g−1), a large pore volume (2.80 cm3 g−1), and a well-connected and stable structure. LN-3-1 exhibits extremely high activity and stability in acidic medium for oxygen reduction reaction (ORR), superior to Pt/C catalyst and most non noble-metal catalysts reported in recent literatures. The prepared carbon material was used as a cathode catalyst to assemble a H2-O2 single fuel cell, and its excellent catalytic performance has been confirmed with the maximum power density of 779 mW cm−2, which is one of the highest power densities among non-metallic catalysts so far. Density functional theory (DFT) calculations indicate that the synergy of chlorine and nitrogen reconciles the intermediate adsorption energies, leading to an appropriate theoretical ORR onset potential. We develop a cost-effective and highly efficient method to prepare biocarbon catalyst for ORR in proton-exchange membrane fuel cells.

Potentiostatic regeneration of oxygen reduction activity in MnOx @ graphene hybrid nanostructures

The oxygen reduction reaction (ORR) activity for Manganese oxide (MnOx) based nanostructures primarily depend upon their morphology, phase and surface Mn valency. While used in fuel cells or metal air batteries, MnOx undergoes irreversible change with degradation in ORR activity. In the present study, we developed a methodology for electrochemical regeneration of their ORR activity by potentiostatic conditioning. MnOx nanostructures with three different morphologies were grown on N doped graphene (NG) and their ORR activity was measured in alkaline medium by rotating ring disc electrode (RRDE). The catalysts were subjected to two step electrochemical treatment and the ORR activity after each step was monitered by rotating disc electrode (RDE). In the first step, the catalysts were put to 300 electrochemical cycles in the potential window 1.12 to 0.12 V (vs. RHE), and after this step, the ORR onset potential showed negative shift, with decrease in limiting current density. On subsequent potentiaostatic treatment at −0.58 V in O2 atmosphere, partial regeneration of onset potential, electron transfer number (n) and current density were observed. The characteristic redox peaks for MnOx disappeared in the first step and reappeared after the second step. After each step, the catalysts were characterised ex-situ by X-ray diffraction (XRD), Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). It showed that after the first step the decay in ORR activity was due to phase transformation to Mn3O4 with decrease in nominal Mn valency and drastic change in microstructure. Then the activity regeneration after the second step was due to the regeneration of MnOOH phase and Mn valency.

Facile one-pot aqueous fabrication of interconnected ultrathin PtPbPd nanowires as advanced electrocatalysts for ethanol oxidation and oxygen reduction reactions

The catalytic features of Pt-based advanced materials closely correlate with the compositions, morphology and structure. Hence, interconnected trimetallic PtPbPd ultrathin nanowires (PtPbPd NWs) were synthesized by octylphenoxypolyethoxyethanol (NP-40)-mediated one-pot aqueous method, using in-situ generated hydrogen bubbles as the dynamic template. It is found that the types of the precursors and the amount of NP-40 are critical in this synthesis. The as-obtained architectures showed remarkable improvement in the electrocatalytic properties for ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR), surpassing those of commercial Pt/C (20 wt%), homemade PtPd NWs, PtPb NWs and PdPb NWs. Specifically, the mass activity (MA)/specific activity (SA) of PtPbPd NWs (1.20 A mg−1/2.78 mA cm−2) is higher than those of Pt/C (0.86 A mg−1/1.79 mA cm−2) in 0.5 M KOH solution. Furthermore, the as-synthesized catalyst displayed a positive-shift of the onset potential (Eonset, 0.993 V) for ORR over Pt/C (0.895 V) in 0.1 M KOH electrolyte. These scenarios manifest that this approach provides some new valuable guidelines for preparing novel trimetallic nanocatalysts in energy storage and conversion applications.

Developing an advanced electrocatalyst derived from triangular silver nanoplates@polyvinylpyrrolidone-polyaniline nanocomposites

There is a need for developing new and perspective materials for oxygen reduction reaction (ORR) as one of the most essential reactions in the life processes, energy storage, and conversion, in order to exchange the most often used materials based on platinum supported/unsupported substrates, especially carbon supports. Herein, we present low-cost alternative electrode materials consisting of two silver@polyvinylpyrrolidone-polyaniline (Ag@PVP-PANI) nanocomposites that showed great potential as Pt-free ORR electrocatalysts. Simple and effective polymerization processes of aniline in the methanol, using PVP as shape-mediated stabilizator and stimulator of its oxidation by silver ions, led to the formation of nanocomposites with truncated triangular silver nanoparticles dispersed throughout granular and wrinkle-like surfaces of PANI matrix. The behavior of PVP chains in acidic conditions showed great influence on nanocomposites’ conductivities, regardless of the high silver content. Electrocatalytic survey of nanocomposites examined in alkaline media toward ORR pointed out their appreciable activities with high ORR onset potentials. Moreover, for the nanocomposite with lower silver content (18.9 wt. %), the four-electron ORR pathway was evidenced.

Hydrogen Evolution Reaction and Oxygen Reduction Reaction of Titanium Oxiynitride Catalyzed Activated Carbon in Acid Aqueous Solution

Electrochemical hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are important reactions for realization a renewable energy society. At the present, platinum group metals are known to be the bifunctional electrode catalysts in the acid medium. Unfortunately, these metals are rare, which leads to increase in the cost of the electrochemical reactors such as a water electrolysis and a fuel cell. Recently, we found that titanium oxiynitride catalyzed activated carbons (TiON/ACs) shows a catalytic activity for both HER and ORR in acid aqueous solution [1, 2]. In this presentation, we report the preparation of TiON/ACs (TiON/AC and TiON/ACF), and their activity for these reactions. TiON/AC is successfully obtained through loading of titanium dioxide (TiO2) on AC powder or ACF (activated carbon fiber) and subsequent NH3 nitridation. At first, TiO2 was deposited on the activated carbons by a modified liquid-phase deposition (LPD) method, in where H3BO3 and (NH4)2TiF6 aqueous solutions were used. A desired amount of AC or ACF was added into H3BO3 aqueous solution and then, (NH4)2TiF6 aqueous solution was added with stirring [1, 2]. NH3 nitridation was carried out at over 700oC with flowing NH3 gas. The characterization was performed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS). In order to evaluate the HER activity, the water electrolysis experiment was demonstrated in 0.5 mol dm-3 of H2SO4 solution. The working electrode (cathode) was prepared by using TiON/AC and carbon paper as the substance. A carbon paper and Ag|AgCl were used as the counter and the reference electrode, respectively. Hydrogen was quantified by a gas chromatography during chronoamperometry. We carried out a rotating ring electrode technique to evaluate the ORR activity. In this case, the electrolyte solution was 0.1 mol dm-3 of HClO4 solution saturated with O2. TiO2 particles were confirmed on AC or ACF before NH3 nitridation. After NH3 nitridation, the diffraction peaks assigned to TiN and TiO and the broad peak of carbon were found in XRD patterns. The formation of Ti-O-N and O-Ti-N bonding were confirmed in XPS spectra. Therefore, we considered that TiO2 was transformed to TiON during NH3 nitridation. FE-SEM images show that TiON particles were found on AC or ACF substrates. These particles were considered the products composed of titanium, nitrogen and oxygen. The hydrogen production rate was estimated from the results of gas chromatography. The dependency on the loading ratio of TiON was shown in Figure 1. Hydrogen production was confirmed at overpotential = 0.1 V. The TiON/AC with 19 wt% of TiON shows the highest hydrogen production rate. The onset potential for ORR was estimated ca. 0.78 V vs. RHE and it is higher than that of AC. This result shows that TiON has also a catalytic activity for ORR. Thus, we consider that TiON/ACs have a potential to be the bifunctional catalyst for HER and ORR. This work was partially supported by the Matching Planner Program (MP28116808401) from Japan Science and Technology Agency , JST and JSPS KAKENHI Grant Number 16K05940. References T. Kinumoto, et al., Journal of Power Sources, 273(1), 136 -141 (2015).T. Furushiro, et al., The meeting abstract of AiMES2018, #Z01-2091 (2018). Figure 1

LaSr2Mn2O7 Ruddlesden-Popper manganites for oxygen reduction and electrochemical capacitors

LaSr2Mn2O7, a Ruddlesden-Popper type bilayered perovskite, was investigated as oxygen catalyst for oxygen reduction reaction (ORR) and active electrode material for electrochemical capacitors in KOH solutions. XPS results show that Mn takes a mixed Mn2+, Mn3+ and Mn4+ valence state with an average valence of 3.4. As catalyst towards ORR, LaSr2Mn2O7 and LaSr2Mn2O7/acetylene black composite favor four electron pathways for ORR. LaSr2Mn2O7/C composite shows a comparable onset potential for ORR to that of Pt/C electrode. The improved ORR activity of LaSr2Mn2O7/acetylene black composite is attributed to the synergy between catalytic LaSr2Mn2O7 and conductive acetylene black. As electrode material for electrochemical capacitors, LaSr2Mn2O7 exhibits a maximum specific capacitance of 167.2 F/g with 62% faradic contribution at a scan rate of 1 mV/s in cyclic voltammetry test.

Engineering the multiscale structure of bifunctional oxygen electrocatalyst for highly efficient and ultrastable zinc-air battery

Rechargeable zinc-air batteries (ZABs) are attracting enormous attention owing to their low cost and high theoretical energy density. However, their practical applications are impeded by the low lifetimes and inferior energy conversion efficiency as a result of poor catalyst stability and sluggish oxygen reaction kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at their air cathode. Herein, a multiscale structural engineering of the novel catalyst of Fe/Co single atoms co-doped graphite nanoarrays mounted on carbon microspheres (FeCo(a)-ACM) is reported towards ORR and OER for ZABs. In micro-scale, 3D hierarchically porous carbon microspheres facilitate the mass and electron transport; in nano-scale, ordered graphite nanoarrays increase the accessible chances of the active sites; in atomic-scale, chemical doping of Fe/Co single atoms boost the intrinsic ORR and OER activity and finally enhance the performance of ZABs. The FeCo(a)-ACM exhibits excellent ORR (onset potential: 1.03 V, half-wave potential: 0.90 V) and OER (1.60 V at 10 mA cm−2) activity. In a practical demonstration, a low charge-discharge gap and a record rechargeable lifetime lasting 900 h at 10 mA cm−2 are achieved by zinc-air battery with FeCo(a)-ACM within the air electrode.