Electrochemical Oxygen Reduction Activity Research Articles

Ultrasmall Mo2C Embedded in N‐Doped Holey Carbon for High‐Efficiency Electrochemical Oxygen Reduction Reaction

AbstractThe Oxygen reduction reaction (ORR) plays an important role in fuel cells, but the synthesis of efficient and low‐cost catalysts remains challenging. We report the synthesis of ultra‐small Mo2C nanoparticles with an average size of 1.5 nm embedded in a N‐doped holey carbon (NHC) matrix by employing self‐assembled macrocycle cucurbit[6]uril (CB[6]) and melamine as carbon precursors. The interaction between Mo precursors and CB[6] effectively prevented the aggregation of Mo2C NPs during the pyrolysis procedure. Besides, the holey structural carbon has a larger electrocatalytic activity area and improves mass transfer efficiency and charge transfer rate. The NHC features abundant defective sites and thus effective anchoring and electronic regulation for Mo2C nanoparticles. The electrochemical oxygen reduction activity of Mo2C/NHC is comparable to that of noble metal Pt/C benchmark catalyst with an initial potential, half‐wave potential and limiting current density of 0.95 V, 0.84 V and 5.0 mA cm−2, respectively. Mo2C/NHC shows a good long‐term durability during 25 h chronoamperometry.

Characterisation of Novel Nitrogen Doped Reduced Graphene Oxide

The physical and electrochemical properties of a novel nitrogen-doped reduced graphene oxide (N-rGO) material were studied. The novel material was synthesized using biological microorganisms and non-toxic chemicals. In addition, the effect of the high shear treatment on material particle size and suspension stability was investigated. The material was characterized by particle size distribution analysis, X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric analysis, and gas adsorption methods. The N-rGO material has low porosity but its specific surface area and total pore volume increased fourfold and tenfold, respectively, due to the high shear treatment. Electrochemical oxygen reduction activity measurements were conducted in 0.1 M HClO₄ and 0.1 M KOH solutions using rotating disc electrode and cyclic voltammetry methods.

Surface enhanced performance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes by infiltration Pr-Ni-Mn-O progress

The present study reports the enhancement of electrochemical oxygen reduction activity of porous La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes by coating a thin film of Pr-Ni-Mn oxide (PNM5) using a multi-step infiltration process. XRD examination reveals that PNM5 mainly contains a multiphase mixture of Pr6O11, PrNiO3, MnO and NiO. SEM morphology shows a thin PNM5 film and small particles are formed on the surface of LSCF backbone particles. Impedance spectrum analysis indicates that PNM5 infiltrated LSCF exhibits dramatically reduced polarization resistance (Rp), reaching Rp of 0.244 Ω cm2 at 973 K, which is one-half of the baseline LSCF cathode. The activation energy of LSCF cathodes infiltrated with PNM5 is 1.45 eV, slightly lower than the baseline LSCF cathode (1.77 eV). Distribution of relaxation time (DRT) function analysis shows PNM5 infiltration layer significantly promotes the oxygen reduction reaction (ORR) of cathode surface. With the increase in firing temperature, the total resistance increases and Rp changes from ion transport to oxygen reduction reaction. Degradation rate of the PNM5-infiltrated LSCF is also lower, 0.02168% vs 0.07093% for the baseline LSCF over a 200-h period. A single cell testing indicated that the peak power density of the PNM5-infiltrated cell was increased by 140.66%. Overall, PNM5 could be a potential catalyst for boosting the performance of a commercial LSCF cathode for solid oxide fuel cells (SOFC).

Effects of Catalyst Layer and Porous Transport Layer Properties on Performance of Unitized Regenerative Fuel Cells

A unitized regenerative fuel cell (URFC) that uses a single cell that functions in electrolysis or fuel cell mode provides potential advantages including lower cost, mass, and volume compared to separate discrete systems. However, significant challenges for URFCs include lower performance, lower round-trip efficiency, and higher degradation compared to separate systems. The lower performance and enhanced degradation of URFCs are generally considered to consist of catalyst degradation and water management issues that arise during continuous operation and switching between electrolyzer and fuel cell modes. Underlying URFC performance and degradation are complex interconnected processes that involve the catalysts, catalyst layers, gas diffusion layer (GDL), porous transport layer (PTL), and operating conditions within both electrolyzer and fuel cell modes. To improve performance and stability, we modified the structure of the bifunctional oxygen catalyst layer (CL) and porous-transport layer (PTL) within proton exchange membrane (PEM) URFC membrane electrode assemblies (MEAs). We investigated the effects of different experimental conditions (e.g. catalyst (Pt-IrO2) ratio catalyst loading, spraying/drying conditions, porosity, and content of polytetrafluoroethylene within the CL and/or PTL) and their influence on catalyst layer thickness, porosity, wettability, mass transport, and cell resistance which were correlated to round-trip efficiency and stability over repeated switching between electrolyzer and fuel cell modes. The understanding of the effects of CL and PTL structure on PEM URFC MEAs performance and stability underlies the ability to fabricate improved URFCs and lays the foundations of experimental conditions necessary for the application of advanced oxygen electrocatalysts, as recently reported by our group.1,2,3 References Godínez-Salomón, F.; Albiter, L.; Mendoza-Cruz, R.; Rhodes, C.P. Bimetallic Two-dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts. ACS Applied Energy Materials 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051 Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171 Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043

Defected molybdenum disulfide catalyst engineered by nitrogen doping for advanced lithium–oxygen battery

Rechargeable lithium–oxygen (Li–O2) batteries have great potential for electric vehicles and grid energy storage system. The cathode catalysts play a crucial role in accelerating the oxygen reaction kinetics for improving battery performance. Herein, we employ hetero-atom doping method by combining hydrothermal reaction and ammonia-atmosphere calcination to introduce defects in MoS2 catalyst. The experimental results reveal the fabricated N-doped MoS2 (denoted as N-MoS2) catalysts introduced with abundant lattice defects while maintains the crystal structure of MoS2. The lithium–oxygen batteries with N-MoS2 catalyst as cathode show high initial specific capacity of 5500 mAh g–1 at 100 mA g–1 with good reversibility and efficiency. And the N-MoS2 cathode exhibits good rate capability and cycle stability up to 100 cycles. We reveal that the porous structure and lattice defects promote the exposition of active sites, resulting in higher electrochemical activity for oxygen reduction and the decomposition of discharge products. This work provides a promising avenue for constructing a bi-functional defected molybdenum disulfide catalyst by nitrogen doping in Li–O2 batteries.

Structural and electronic engineering of biomass-derived carbon nanosheet composite for electrochemical oxygen reduction

Carbon nanosheets from sustainable biomass via chemical pretreatment and heterogeneous doping exhibit exceptional electrochemical oxygen reduction activity in alkaline solution.

O-Doping Boosts the Electrochemical Oxygen Reduction Activity of a Single Fe Site in Hydrophilic Carbon with Deep Mesopores.

Carbon-based electrocatalysts with single metal sites hold great potential for mechanism exploration via mimicking molecular catalysts, due to their distinct catalytic sites. In addition to metal atoms, the neighboring nonmetal heteroatoms such as N, S, and O atoms, which are widely detected in carbon-based single-atom catalysts, may also contribute to enhancing the electrochemical activity of single-metal centers. In this work, the boosting effect of O-doping toward the electrochemical oxygen reduction reaction (ORR) was evaluated by both experimental studies and DFT calculations. O-doped carbon-supported single-Fe-site catalysts possessing deep mesopores and desirable hydrophilic surface were achieved by confined carbonization in an inert or reductive atmosphere (SAFe-NDC and SAFe-NDC-H). As compared to the state-of-the-art Pt/C, these catalysts showed superior catalytic activity toward the ORR in terms of half-wave potential, Tafel slope, and long-term stability. In particular, SAFe-NDC-H outperformed its SAFe-NDC counterpart. Considering that these two catalysts possess a comparable porous structure, surface properties, and local electronic structure of a single Fe site, the dopant nonmetal O atoms, specifically, carbonyl group (C═O), are revealed to affect the ORR activity of the single Fe site exclusively. The introduced C═O facilitates the formation of *OOH as well as the reduction of *OH, thereby reducing the catalysts' overpotential.

Electrochemical Oxygen-Reduction Activity and Carbon Monoxide Tolerance of Iron Phthalocyanine Functionalized with Graphene Quantum Dots: A Density Functional Theory Approach

We examined the catalytic activities of iron phthalocyanine integrated with graphene quantum dots (FePc/GQDs) and the pure iron phthalocyanine (FePc) system toward oxygen reduction from both thermo...

Effects of Catalyst and Electrode Composition on Performance of Unitized Regenerative Fuel Cells

A unitized regenerative fuel cell (URFC) that uses a single cell that functions in electrolysis or fuel cell mode provides potential advantages including lower cost, mass, and volume compared with discrete electrolyzer and fuel cell systems. However, significant challenges for URFCs include lower performance, lower round-trip efficiency, and higher stability/degradation issues compared with separate systems. The lower performance and enhanced degradation of URFCs results from a combination of factors including catalyst degradation and water management issues that result from switching between electrolyzer and fuel cell modes. To address these challenges and improve performance of proton-exchange membrane (PEM) URFCs in both fuel cell and electrolyzer modes, we evaluated the effects of catalyst and electrode composition of the bifunctional oxygen electrode. To improve catalyst activity and efficiency of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), we explored using bifunctional oxygen catalysts composed of bimetallic nanoframes derived from our prior studies.1,2 Electrochemical testing using a rotating disk electrode configuration showed that the nanoframe bifunctional oxygen electrocatalysts have higher ORR mass activity, OER mass activity, and round-trip efficiency and reasonable stability compared with baseline Pt-IrO2 catalysts. URFC membrane electrode assemblies (MEAs) were fabricated and tested to determine the effect of the composition of the oxygen electrode on fuel cell and electrolyzer performance and evaluate the effect of switching between operating in fuel cell and electrolyzer modes. Performance comparisons were made with MEAs composed of baseline commercial Pt black-IrO2 catalysts. References Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171

Two-Dimensional Nanoframes for High Activity Bifunctional Acidic Oxygen Reduction and Evolution Electrocatalysts

Bifunctional oxygen electrocatalysts that facilitate the acidic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the same electrode are critical components of unitized regenerative fuel cells (URFCs) and metal-air batteries. Obtaining bifunctional oxygen catalysts that simultaneously provide high ORR and OER activity, high stability, and lower precious metal content remains a significant challenge. Our recent work has explored bimetallic two-dimensional (2D) nanoframes as an approach to obtain carbon-free, unsupported nanostructures and demonstrated nickel-platinum and nickel-iridium 2D nanoframes function as high activity, separate ORR and OER catalysts respectively.1,2 To obtain high activity bifunctional oxygen catalysts, we investigated the combination of NiPt and CoIr 2D nanoframes. Bimetallic 2D nanoframes were prepared by microwave-assisted synthesis of noble metal-decorated transition metal hydroxide nanosheets followed by thermal reduction and chemical leaching steps. The 2D nanoframes utilize noble metal (Pt,Ir)-transition metal (Ni, Co) interactions to alter surface electronic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional accessibility to the catalytically active sites. Electrochemical testing using a rotating disk electrode configuration showed the two-component 2D nanoframe bifunctional oxygen electrocatalysts have higher ORR mass activity, OER mass activity, and round-trip efficiency compared with baseline Pt-IrO2 catalysts. Accelerated durability testing consisting of repeated voltage cycles over ORR/OER potential ranges showed that the 2D nanoframe catalysts exhibit similar ORR stability and improved OER stability compared with Pt-IrO2. The ability to combine highly catalytically active surfaces within a carbon-free 3D nanoarchitecture provides the opportunity to design bifunctional catalysts with improved activity and stability. References Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171

A One‐Pot Synthesis of Platinum Nanoparticles on Electrochemically Exfoliated Graphite

AbstractGraphite rods were electrochemically exfoliated in an aqueous solution of H2PtCl6 at potentials ranging from 2.5 V to 10 V. The solution was then directly reduced in the same pot with hydrazine hydrate. Scanning Electron Microscopy (SEM) confirmed the platinum deposition onto the exfoliated graphene while Raman spectroscopy and X‐Ray Diffraction (XRD) displayed varying degrees of disorder and oxidation in the graphitic lattices exfoliated at the various potentials. Half‐cell electrochemical tests showed the reduced graphene oxide supported platinum catalysts exfoliated at 5 V displayed the highest electrochemical activity for oxygen reduction in the acidic media while the catalyst exfoliated at 8 V displayed the lowest activity.

Synthesis of Bimetallic Conductive 2D Metal-Organic Framework (Cox Niy -CAT) and Its Mass Production: Enhanced Electrochemical Oxygen Reduction Activity.

The development of new electrocatalysts for electrochemical oxygen reduction to replace expensive and rare platinum-based catalysts is an important issue in energy storage and conversion research. In this context, conductive and porous metal-organic frameworks (MOFs) are considered promising materials for the oxygen reduction reaction (ORR) due to not only their high surface area and well-developed pores but also versatile structural features and chemical compositions. Herein, the preparation of bimetallic conductive 2D MOFs (Cox Niy -CATs) are reported for use as catalysts in the ORR. The ratio of the two metal ions (Co2+ and Ni2+ ) in the bimetallic Cox Niy -CATs is rationally controlled to determine the optimal composition of Cox Niy -CAT for efficient performance in the ORR. Indeed, bimetallic MOFs display enhanced ORR activity compared to their monometallic counterparts (Co-CAT or Ni-CAT). During the ORR, bimetallic Cox Niy -CATs retain an advantageous characteristic of Co-CAT in relation to its high diffusion-limiting current density, as well as a key advantage of Ni-CAT in relation to its high onset potential. Moreover, the ORR-active bimetallic Cox Niy -CAT with excellent ORR activity is prepared at a large scale via a convenient method using a ball-mill reactor.

Boosting the electrochemical oxygen reduction activity of hemoglobin on fructose@graphene-oxide nanoplatforms.

A metal-free oxygen reduction reaction (ORR) electrocatalyst with outstanding performance was obtained through an easy and one-pot synthesis of hemoglobin functionalized fructose@graphene-oxide (GO) nanocomposites. The active pyridinic nitrogen sites of the highly unfolded proteins together with the excellent electronic properties of GO appears to be the main factors causing the improved electrocatalytic activity.

Pt[sbnd]CoN supported on TiN-modified carbon nanotubes (Pt[sbnd]CoN/TiN–CNT) as efficient oxygen reduction reaction catalysts in acidic medium

Herein, new catalysts for the oxygen reduction reaction (ORR) (PtCoN nanoparticles supported on TiN–carbon nanotubes (CNTs)) were successfully prepared via the simple polymerized complex method and heat treatment in NH3. Through this facile two-step synthesis, metal chlorides were first transformed to the corresponding oxides, then reduced to metal particles or nitrides. The synthesized PtCoN/TiN–CNT electrocatalysts showed excellent electrochemical activity for oxygen reduction and high tolerance against methanol in acidic medium. The ORR onset potential of Pt0.3CoN/TiN–CNT (0.812 V vs. RHE) is higher than that of 40 wt% commercial Pt/C (0.789 V vs. RHE) due to the synergistic effect between nano-platinum particles and metal nitrides. The TiN-coated CNT support provided high electron transfer rate and catalytic tolerance in methanol solution. The number of transferred electrons was examined by rotating ring-disk electrode measurements, which concurred with the result of the Koutecky–Levich equation for a four-electron mechanism.

(Invited) Facets of Nanocrystal: A Knob to Tune Electrocatalytic Activity

The electrocatalytic reactions strongly depend on the surface atomic structures of the electrocatalysts. Thus, tuning the surface facets of the nanoparticles can be a powerful tool to control the activity for a specific electrocatalytic reaction. In previous studies, we employed high-resolution transmission electron microscopy to reveal the surface atomic structure of Pt nanoparticles, where the amount of high-index surface facets on Pt nanoparticles was quantified.1,2 Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly showed the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation.1 On the other hand, the oxygen reduction reaction (ORR) activity of Pt nanoparticles is not influenced by surface steps in contrast to methanol oxidation reaction (MOR) activity.2 More recently, we demonstrate the synthesis of Pd@Ir core−shell concave nanocubes as an advanced electrocatalyst for acidic oxygen evolution reaction (OER). The Pd@Ir core−shell concave nanocubes were successfully prepared by atomic layer-by-layer deposition of Ir atoms on the surface of free-standing Pd nanocubes in a liquid phase. Using X-ray photoelectron spectroscopy (XPS), the right composition and oxidation states of these nanocubes are confirmed. In the 0.1 M HClO4 electrolyte, the synthesized Pd@Ir concave nanocubes showed significantly increased activity compared to the IrO2 nanoparticles. The improved activity is mainly due to well-controlled surface structures based on high-index facets. We also observed that the specific activity of Pd@Ir concave nanocubes increased after the chronoamperometry test for 10 hours. S. W. Lee, S. Chen, W. Sheng, N. Yabuuchi, Y.-T. Kim, T. Mitani, E. Vescovo, and Y. Shao-Horn, Roles of surface steps on Pt nanoparticles in electro-oxidation of carbon monoxide and methanol, Journal of the American Chemical Society, 2009; 131(43): p. 15669-15677.S. W. Lee, S. Chen, J. Suntivich, K. Sasaki, R. R. Adzic, and Y. Shao-Horn, Role of surface steps of Pt nanoparticles on the electrochemical activity for oxygen reduction, The Journal of Physical Chemistry Letters, 2010; 1(9): p. 1316-1320.

A Polymer Encapsulation Strategy to Synthesize Porous Nitrogen-Doped Carbon-Nanosphere-Supported Metal Isolated-Single-Atomic-Site Catalysts.

A novel polymer encapsulation strategy to synthesize metal isolated-single-atomic-site (ISAS) catalysts supported by porous nitrogen-doped carbon nanospheres is reported. First, metal precursors are encapsulated in situ by polymers through polymerization; then, metal ISASs are created within the polymer-derived p-CN nanospheres by controlled pyrolysis at high temperature (200-900 °C). Transmission electron microscopy and N2 sorption results reveal this material to exhibit a nanospheric morphology, a high surface area (≈380 m2 g-1 ), and a porous structure (with micropores and mesopores). Characterization by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure confirms the metal to be present as metal ISASs. This methodology is applicable to both noble and nonprecious metals (M-ISAS/p-CN, M = Co, Ni, Cu, Mn, Pd, etc.). In particular, the Co-ISAS/p-CN nanospheres obtained using this method show comparable (E1/2 = 0.838 V) electrochemical oxygen reduction activity to commercial Pt/C with 20 wt% Pt loading (E1/2 = 0.834 V) in alkaline media, superior methanol tolerance, and outstanding stability, even after 5000 cycles.

Improving the Electrochemical Oxygen Reduction Activity of Manganese Oxide Nanosheets with Sulfurization‐Induced Nanopores

AbstractLow‐cost and high‐activity electrocatalysts for the oxygen reduction reaction (ORR) are necessary for the development of fuel cells and metal‐air batteries. Manganese oxide would be a good candidate because of its low cost, abundant supply, and environmental benignity if not for its relatively low activity compared with noble metals. To improve the ORR activity of manganese oxide, we developed a sulfurization process to create pores in 2 D manganese oxide nanosheets. The nanoporous MnO2 nanosheets (np‐MnO2‐ns) prepared as such contain 7 nm pores in the nanosheets and their half‐wave potential (0.73 V) is 40 mV more positive than that of pore‐free MnO2 nanosheets (0.69 V). The higher ORR activity of np‐MnO2‐ns may be attributed to the combination of a large surface area and the presence of high‐ORR‐activity Mn3+/4+ sites. The np‐MnO2‐ns also showed an enhanced oxygen evolution reaction activity and delivered a good performance in rechargeable Zn‐air batteries.

Nitrogen and sulfur co-doped mesoporous hollow carbon microspheres for highly efficient oxygen reduction electrocatalysts

Heteroatom-doped porous carbons are considered as one of the most promising metal-free electrocatalysts for highly efficient electrocatalytic reactions. Herein, nitrogen and sulfur co-doped mesoporous hollow carbon microspheres are fabricated using a feasible process involving the pyrolysis of polydopamine in the presence of sodium 1,5-naphthalenedisulfonate (1,5-NDSNa) and the use of mesoporous silica SBA-15 as hard template. The resultant carbon microspheres have a well-developed hierarchical porosity with large surface area, showing high-efficiency electrochemical oxygen reduction activity in alkaline media, along with much enhanced stability and methanol tolerance, as compared with those of commercial platinum catalysts. The superior electrocatalytic performance and stability of the catalyst might be due to the synergistic effect between N and S elements and the well-defined meso- and macroporosity, presenting great potential in the fields of fuel cells and metal-air batteries.

Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability.

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH)2 nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cmPt-2) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 VRHE showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.

Electrochemical Oxygen Reduction Activity of Metal Embedded Nitrogen Doped Carbon Nanostructures Derived from Pyrolysis of Nitrogen-Rich Guanidinium Salt

Nitrogen and transition metal containing carbon composites are considered as promising non-platinum oxygen reduction catalysts with potential to replace Pt in fuel cell cathodes and metal-air batteries. Nitrogen-rich precursor guanidinium chloride, transition metal salts (Fe/Co) and high surface area carbon spheres are pyrolyzed to yield non-platinum composite electrocatalysts with new and interesting morphological features. Based on rotating ring-disk electrode (RRDE) experiments, both cobalt, and iron containing composite catalysts exhibited significant oxygen reduction activity in alkaline solutions with E1/2 values (∼60 mV lower) of best catalyst comparable to Pt/C. The kinetic parameter analysis, using Koutecky-Levich relation, for the best catalyst made of cobalt, nitrogen and carbon (Co-N-C) show specific exchange current densities of 0.1 mA mg−1, Tafel slopes of 75–80 mV dec−1 and %H2O2 of 10–13% of the total current in alkaline solution. XRD, BET, XPS and Raman spectral study serve as a complementary tool to evaluate the structure of the composite electrocatalysts.