Oxygen Reduction Reaction Current Density Research Articles

Investigation of Co3O4 modified carbon black as an efficient non-noble metal oxygen reduction catalyst in self-driven phosphorus recovery systems

Phosphorus and nitrogen recovery from urine in the form of struvite via a Mg-air fuel cell (MAFC) platform has previously been reported as a promising approach to achieving sustainable development goals. However, the applicability of this technology has been hindered by the expensive cathodic catalyst, i.e., Pt/C. In this study, an efficient and cost-effective non-precious metal catalyst, the Co3O4 doped carbon black (CB-ZIF-900), was developed to maximize nitrogen and phosphate recovery from urine while reducing investment costs. CB-ZIF-900 exhibited an oxygen reduction reaction current density of 2.95 mA cm−2 and an electron transfer number between 3.7 and 3.9 in urine solution, which approached those of commercial Pt/C at the same testing conditions. Using the MAFC, over 99 % of phosphorus was removed from urine, and >95 % of phosphorus was recovered in the precipitates. Stable phosphorus removal efficiency and precipitates composition were observed over five consecutive batch experiments. Furthermore, the MAFC with CB-ZIF-900 exhibited a maximum power density of 594.89 W m−3, surpassing that with commercial Pt/C by 9.3 %. These findings underscore the utility of the highly efficient and cost-effective CB-ZIF-900 for advancing the scalability and widespread applications of self-driven phosphorus recovery systems.

Metal-Support Interaction between Titanium Oxynitride and Pt Nanoparticles Enables Efficient Low-Pt-Loaded High-Performance Electrodes at Relevant Oxygen Reduction Reaction Current Densities.

In the present work, we report on a synergistic relationship between platinum nanoparticles and a titanium oxynitride support (TiOxNy/C) in the context of oxygen reduction reaction (ORR) catalysis. As demonstrated herein, this composite configuration results in significantly improved electrocatalytic activity toward the ORR relative to platinum dispersed on carbon support (Pt/C) at high overpotentials. Specifically, the ORR performance was assessed under an elevated mass transport regime using the modified floating electrode configuration, which enabled us to pursue the reaction closer to PEMFC-relevant current densities. A comprehensive investigation attributes the ORR performance increase to a strong interaction between platinum and the TiOxNy/C support. In particular, according to the generated strain maps obtained via scanning transmission electron microscopy (STEM), the Pt-TiOxNy/C analogue exhibits a more localized strain in Pt nanoparticles in comparison to that in the Pt/C sample. The altered Pt structure could explain the measured ORR activity trend via the d-band theory, which lowers the platinum surface coverage with ORR intermediates. In terms of the Pt particle size effect, our observation presents an anomaly as the Pt-TiOxNy/C analogue, despite having almost two times smaller nanoparticles (2.9 nm) compared to the Pt/C benchmark (4.8 nm), manifests higher specific activity. This provides a promising strategy to further lower the Pt loading and increase the ECSA without sacrificing the catalytic activity under fuel cell-relevant potentials. Apart from the ORR, the platinum-TiOxNy/C interaction is of a sufficient magnitude not to follow the typical particle size effect also in the context of other reactions such as CO stripping, hydrogen oxidation reaction, and water discharge. The trend for the latter is ascribed to the lower oxophilicity of Pt-based on electrochemical surface coverage analysis. Namely, a lower surface coverage with oxygenated species is found for the Pt-TiOxNy/C analogue. Further insights were provided by performing a detailed STEM characterization via the identical location mode (IL-STEM) in particular, via 4DSTEM acquisition. This disclosed that Pt particles are partially encapsulated within a thin layer of TiOxNy origin.

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3 /C Composites

Electrochemical energy storage and conversion systems are the appealing replacement for nonrenewable resources to provide clean and sustainable energy. Of particular interest are fuel cells and metal-air batteries, both of which rely on the oxygen reduction reaction (ORR) for their operation. Fe-N-C catalyst has been widely used as a prominent catalyst towards ORR, but there is still some ambiguity regarding its active sites. This study describes the role of Fe2O3, an inevitable species formed in Fe-N-C catalyst towards the ORR. The Fe2O3 is carefully synthesised on various carbon supports and characterised using XPS spectra, HRTEM images, and surface area analysis. The particle size of the Fe2O3 nanoparticles synthesised on the pores of the carbon support was determined to be ~10 nm from the HRTEM analysis. The current density of ORR and the number of electrons transferred on Fe2O3/C catalysts is increased compared with bare carbon supports. Based on the kinetic analysis, the ORR on Fe2O3/C follows pseudo-4-electron (or) 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. The role of Fe2O3 is further studied using the Theoretical method, which reveals the decreased formation of free energy of O2 adsorption. The Fe2O3 reduces the energy barrier for the reduction of *OOH to OH-. The Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20,000 cycles. Figure 1

(Invited) Enhanced Material Durability of Transition Metal-Antimony X-Ide Nanoparticles in Oxygen Reduction Electrocatalysis

Hydrogen fuel cells (FCs) are key energy devices that can accelerate our transition away from non-renewable, carbon-intensive fuels especially in the grid-scale storage and transportation sectors. FC cathodes perform the oxygen reduction reaction (ORR), and despite decades of progress towards higher performance electrocatalysts, the ORR remains kinetically sluggish and limits the efficiency of assembled FC devices. Modern, commercially available FC cathodes typically contain platinum (Pt)-based catalysts that can cause the overall FC to become prohibitively expensive for some applications so identifying alternate non-platinum group ORR catalyst formulations could increase deployment of FCs. Physics-based calculations with density functional theory (DFT) have predicted many first-row transition metal oxides (MOx, M = Mn, Fe, Cr, Ni, Co) as having favorable ORR performance trends. Additionally, there is experimental validation of nanoscale ternary and quaternary transition metal oxide systems with a mixture of transition metals used for catalyzing the ORR where relatively modest activity enhancements are reported. Despite this progress, an important limitation is present in the low material stability of non-precious transition metal oxides especially in acidic environments such as those encountered in proton exchange membrane FCs. Using synthetic material strategies to overcome such stability limitations in a variety of FC operating conditions will provide a deeper understanding of the relationship between structure, activity, and degradation.In this work, we employ two simultaneous strategies to work towards enhancing the activity and stability of transition metal oxides: (1) addition of antimony (Sb) as a ligand and (2) functionalization with nitrogen or sulfur. We utilized a previously reported colloidal synthesis for uniform ~ 50 nm oxide nanoparticles for manganese, nickel, cobalt, and iron to produce four sets of nanoparticles based on each of these metals. By using a colloidal synthesis that concurrently introduces Sb with the secondary metal and drying the particles afterwards, we incorporate Sb evenly throughout the polycrystalline structure of the of these nanoparticle catalysts. A high temperature surface modification reaction with reactive gas such as air, ammonia, or hydrogen sulfide was used to convert any remaining transition metal antimonate into an oxide, nitride, or sulfide, respectively. In total, twelve materials are used in this study: the oxide, nitride, and sulfide (collectively “X-ides”) of each of the four transition metal containing, antimony incorporated nanoparticles. We optimize this material synthesis and evaluate the nanoparticle catalyst’s ability to enhance electrochemical ORR activity in both acidic (pH 1) and alkaline (pH 13) conditions which are simulating operating conditions of proton-exchange membrane and anion-exchange membrane FCs, respectively. In addition to activity modulation, we measure the ORR selectivity change towards hydrogen peroxide formation for each material and note the role of nitrogen and sulfur modification in suppressing hydrogen peroxide formation in alkaline environments. In addition to electrochemical characterization, we employed a vast array of bulk and surface characterization techniques to understand catalyst dynamics and contextualize observed activity trends. Transmission electron microscopy (TEM) enabled determination of particle sizes and morphologies alongside electron diffraction patterns to validate the catalyst synthesis. X-ray diffraction (XRD) then provided further information about crystallinity that connected bulk-level insights to the local information from TEM. X-ray photoelectron spectroscopy (XPS) enabled determination of oxidation state changes in the transition metal and in Sb that serve as important descriptors for estimating the activity of these catalysts. We also employed in situ/operando techniques to probe catalyst dynamics and stability under various conditions. Using manganese K-edge synchrotron x-ray absorption spectroscopy (XAS) on the manganese antimony X-ides, we find significant changes to oxidation state and coordination of manganese in the nitride and sulfide as a function of potential indicating processes that displace nitrogen and sulfur in the lattice under certain conditions. Lastly, we use an in situ electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) to measure corrosion as a function of ORR current density, potential sweeping, and electrolyte gas saturation to determine degradation patterns. Insights from all these techniques provide a fascinating perspective on what factors in a material and its environment are responsible for enhanced activity, selectivity, and stability. We perform DFT modeling of these systems to further elucidate material properties that are principally responsible for achieving key metrics in activity and stability especially as non-precious catalysts are evaluated for their performance in assembled device conditions. Combining electrochemical experiments, physics-based modeling, and material characterization to better understand structure-performance relationships is a promising path towards accelerating the process of materials discovery for eventual deployment in critical energy applications for the future.

Boosting oxygen reduction via MnP nanoparticles encapsulated by N, P-doped carbon to Mn single atoms sites for Zn-air batteries

An electrocatalyst of single-atomic Mn sites with MnP nanoparticles (NPs) on N, P co-doped carbon substrate was constructed to enhance the catalytic activity of oxygen reduction reaction (ORR) through one-pot in situ doping-phosphatization strategy. The optimized MnSA-MnP-980℃ catalyst exhibits an excellent ORR activity in KOH electrolyte with a half-wave potential (E1/2) of 0.88 V (vs. RHE), and the ORR current density of MnSA-MnP-980℃ maintained 97.9 % for over 25000 s chronoamperometric i-t measurement. When using as the cathode, the MnSA-MnP-980℃ displays a peak power density of 51 mW cm−2 in Zinc-Air batteries, which observably outperformed commercial Pt/C (20 wt%). The X-ray photoelectron spectroscopy reveal that the doped P atoms with a strong electron-donating effectively enhances electron cloud density of Mn SAs sites, facilitating the adsorption of O2 molecules. Meanwhile, the introduction of MnP NPs can regulate the electronic structure of Mn SAs sites, making Mn SAs active sites exist in a low oxidation state and are less positively charged, which can supply electrons for ORR process to narrow the adsorption energy barrier of ORR intermediates. This work constructs novel active sites with excellent ORR properties and provides valuable reference for the development of practical application.

Metal-organic framework derived porous N, P co-doping carbon microcubes toward aqueous and flexible quasi-solid-state rechargeable Zn-air batteries

The sluggish kinetics of oxygen reduction and/or evolution reactions in air cathodes of Zn-air batteries (ZABs) limits their practical applications. Herein, a facile strategy of phosphating metal-organic framework is developed to fabricate porous defect-enriched nitrogen and phosphorus co-doing carbon microcubes (NPC) both toward efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as a cost-efficiency bi-functional electrocatalyst. With detailed regulation in annealing temperatures, the optimized NPC sample (NPC-10) with nitrogen (N, 1.0 at.%) and phosphorus (P, 1.6 at.%) co-doping possesses bi-model micro-/meso-pores, and large specific surface area of 1627.1 m2 g−1. Thanks to unique structural merits and N/P synergetic contribution, the NPC-10 is endowed with abundant active sur-/interfaces and rapid kinetics for efficient ORR (limited ORR current density of 5.4 mA cm−2, half-wave potential of 0.89 V at 10 mA cm−2, Tafel slope of 141.6 mV dec−1 and a four-electron selectivity of >94.7 %) and OER (overpotential of 220 mV at 10 mA cm−2 and Tafel slope of 57.3 mV dec−1). More encouragingly, the NPC-10 assembled aqueous and flexible quasi-solid-state (QSS) rechargeable ZABs both deliver high power density and long-cycle stability. The work provides highly efficient, durable, and low-cost ORR/OER electrocatalysts toward rechargeable aqueous and/or QSS ZABs.

Superoxide intermediate in the oxygen reduction on a zinc hydroxide model corrosion product.

The inhibition of the electrochemical oxygen reduction reaction (ORR) by zinc corrosion products plays an important role in the corrosion protection of galvanized steel. Hence, the electrocatalytic mechanism of the ORR on electrodeposited zinc hydroxide-based model corrosion products was investigated by in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy, supplemented by density functional theory (DFT) calculations. Model corrosion products containing flake-like crystalline Zn5(NO3)2(OH)8 were cathodically electrodeposited on germanium(100) electrodes from a zinc nitrate precursor electrolyte. Substantial amounts of the films are non-crystalline, and their surfaces predominantly consist of zinc oxide and hydroxide species, as evidenced by x-ray photoelectron spectroscopy. ATR-IR spectra show a peak at 1180 cm-1 during cathodic currents in O2-saturated NaClO4 solution. This peak is assigned to a surface-bound superoxide, the only ORR intermediate detected. Absorbance from the intermediate increases with increasing cathodic current, indicating an increase in surface concentration of superoxide intermediates at larger ORR current densities. The zinc hydroxide ages in the experiments, most likely by a transformation into zinc oxide, consistent with the observed decrease in absorbance over time of the OH bending mode of zinc hydroxide at 1380 cm-1. This aging is a time-dependent chemical process, implying that pure chemical aging is important in actual corrosion products as well. DFT calculations of adsorbed superoxide yield a Zn-O bond length similar to the bond length in Zn-O, thus enhancing superoxide interaction with undercoordinated tetrahedral Zn2+ sites on the surface. Thus, such active sites catalyze the first reduction step in the ORR.

Pd-Supported Co3O4/C Catalysts as Promising Electrocatalytic Materials for Oxygen Reduction Reaction

This paper describes the activity of PdCo3O4/C obtained by wet impregnation towards the oxygen reduction reaction (ORR). For this purpose, the Co3O4/C substrate was synthesized using the microwave irradiation heating method with further annealing of the substrate at 400 °C for 3 h (Co3O4/C-T). Then, the initial Co3O4/C substrate was impregnated with palladium chloride (Pd-Cl2-Co3O4/C), and then part of the obtained Pd-Cl2-Co3O4/C catalyst was annealed at 400 °C for 3 h (PdOCo3O4/C). The electrocatalytic activity of the prepared catalysts was investigated for the oxygen reduction reaction in alkaline media and compared with the commercial Pt/C (Tanaka wt. 46.6% Pt) catalyst. It was found that the annealed PdOCo3O4/C catalyst showed the largest ORR current density value of −11.27 mA cm−2 compared with Pd-Cl2-Co3O4/C (−7.39 mA cm−2) and commercial Pt/C (−5.25 mA cm−2).

An In-Depth Exploration of the Electrochemical Oxygen Reduction Reaction (ORR) Phenomenon on Carbon-Based Catalysts in Alkaline and Acidic Mediums

Detailed studies of the electrochemical oxygen reduction reaction (ORR) on catalyst materials are crucial to improving the performance of different electrochemical energy conversion and storage systems (e.g., fuel cells and batteries), as well as numerous chemical synthesis processes. In the effort to reduce the loading of expensive platinum group metal (PGM)-based catalysts for ORR in the electrochemical systems, many carbon-based catalysts have already shown promising results and numerous investigations on those catalysts are in progress. Most of these studies show the catalyst materials’ ORR performance as current density data obtained through the rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) experiments taking cyclic voltammograms (CV) or linear sweep voltammograms (LSV) approaches. However, the provided descriptions or interpretations of those data curves are often ambiguous and recondite which can lead to an erroneous understanding of the ORR phenomenon in those specific systems and inaccurate characterization of the catalyst materials. In this paper, we presented a study of ORR on a newly developed carbon-based catalyst, the nitrogen-doped graphene/metal-organic framework (N-G/MOF), through RDE and RRDE experiments in both alkaline and acidic mediums, taking the LSV approach. The functions and crucial considerations for the different parts of the RDE/RRDE experiment such as the working electrode, reference electrode, counter electrode, electrolyte, and overall RDE/RRDE process are delineated which can serve as guidelines for the new researchers in this field. Experimentally obtained LSV curves’ shapes and their correlations with the possible ORR reaction pathways within the applied potential range are discussed in depth. We also demonstrated how the presence of hydrogen peroxide (H2O2), a possible intermediate of ORR, in the alkaline electrolyte and the concentration of acid in the acidic electrolyte can maneuver the ORR current density output in compliance with the possible ORR pathways.

Defected NiFe layered double hydroxides on N-doped carbon nanotubes as efficient bifunctional electrocatalyst for rechargeable zinc–air batteries

Layered double hydroxides (LDHs) are highly active materials for the oxygen evolution reaction (OER); however, they have low activity for the oxygen reduction reaction (ORR). The surface modification by the defects engineering is a great strategy to boost their bifunctionality for rechargeable Zn-air batteries (ZABs). In this work, NiFe LDHs were synthesized varying the composition (Ni:Fe compositions of 1:1, 2:1 and 3:1), and BET surface areas of 262.3, 165.9 and 79.5 m2 g−1 were found. Despite the differences in BET surface areas, the NiFe LDH 3:1 presented higher activity for the OER, and thus, oxygen vacancies (Ov)–type surface defects were introduced to this LDH. This surface engineering decreased the oxygen atomic percentage from 75.25 to 57.4 %, and as result, the resistance to charge transfer drastically decreased due to electronic modifications, which can be rationalized in terms of the theoretical density of states obtained by density functional theory for a model system including oxygen vacancies. The combination between defected NiFe LDH with N-doped carbon nanotubes allowed to obtain the same current density for ORR than benchmarked Pt/C with a potential difference of 95 mV, and better activity for OER (1.46 V @ 10 mA cm−2). In the ZAB, a cell voltage of 1.44 V was achieved with a power density close to 36 mW cm−2, being this power similar to that of Pt/C.

Pt/C-TiO2 as Oxygen Reduction Electrocatalysts against Sulfur Poisoning

Proton exchange membrane (PEM) fuel cells using Pt-based materials as electrocatalysts have achieved a decent performance, represented by the launched Toyota Mirai vehicle. The ideal PEM fuel cells consume stored pure hydrogen and air. However, SO2, as a primary air contaminant, may be fed along with air at the cathode, leading to Pt site deactivation. Therefore, it is important to improve the SO2 tolerance of catalysts for the stability of the oxygen reduction reaction (ORR). In this work, we develop the Pt/C-TiO2 catalyst against SO2 poisoning during ORR. Impressively, the hybrid Pt/C-TiO2 catalyst with 20 mass % TiO2 shows the best ORR and anti-toxic performance: the kinetic current density of ORR is 20.5% higher and the degradation rate after poisoning is 50% lower than Pt/C. The interaction between Pt and TiO2 as well as the abundant hydroxyl groups on the surface of TiO2 are both revealed to account for the accelerated removal of poisonous SO2 on Pt surfaces.

Proliferation of pH-universal oxygen reduction performance by morphology modulation in NiS-N,S doped carbon microflowers

Transition metal/doped carbon hybrid nanostructures with precisely controlled morphological features are emerging as the prospective electroactive materials to meet the futuristic sustainable energy demands. Here, gas template-assisted hydrothermal preparation of NiS impregnated on N, S-rich nanocarbon composite is reported for pH-universal oxygen reduction reaction (ORR) electrocatalysis for hydrogen fuel cell and metal-air battery applications. The unique microflower-like superstructures formed by assembly of interconnected nanosheets as visualized through electron microscopic studies pave the way towards achieving the proliferation of ORR performance by providing pathways for gas transport and charge transfer during the reaction. The onset potential of 0.95 V vs. reversible hydrogen electrode (RHE), half-wave potential of 0.74 V vs. RHE, and ORR current density of 5.96 mA/cm2 (at 0.05 V vs. RHE) in an alkaline medium are achieved in this study. The most active electrocatalyst is tested for ORR activity in an acidic medium, as well. The prepared electrocatalyst exhibits excellent current durability and stability in a methanol-saturated environment compared to the state-of-the-art electrocatalyst in both media. In a nutshell, this study presents a precursor-morphology-performance correlation in pH-universal ORR electrocatalysis in addition to serving as a reference study for gas template-assisted hydrothermal synthesis.

In-Situ derived Co1-xS@nitrogen-doped carbon nanoneedle array as a bifunctional electrocatalyst for flexible Zinc-air battery

• Co 1-x S@NC is prepared via a facile “CBD deposition-PDA coating-sulfurization” method. • The excellent OER and ORR electrocatalytic activities both mainly derive from Co 1-x S. • The 3D array structure of Co 1-x S@NC facilitates OER/ORR-related charge-and-mass transport. • There is wrapping-protection of NC on Co 1-x S and strong electron coupling between them. • Promising quasi-solid-state rechargeable Zn-air battery based on Co 1-x S@NC is achieved. It is of great significance to develop high-performance bifunctional electrocatalysts for catalyzing two vital reactions in rechargeable zinc-air battery (ZAB). Transitional metal sulfides and N-doped carbon materials are widely reported as effective electrocatalysts of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively. Herein, an 3D-array-structured catalyst composed of Co 1-x S nanoneedles enwrapped by nitrogen-doped carbon (NC) is in-situ grown on carbon fibre paper substrate. It is revealed that the metastable hexagonal Co 1-x S itself possesses attractive electrocatalytic activities toward both OER and ORR. Additionally, owing to the wrapping-protection effect of polydopamine-derived NC on Co 1-x S, the strong electron coupling between Co 1-x S and NC, and the 3D array structure, Co 1-x S@NC presents excellent OER/ORR catalytic activities and long-term stabilities, superior to Co@NC, Co 1-x S, and NC, and comparable to noble metals and other reported state-of-the-art bifunctional oxygen electrocatalysts. A small energy gap (Δ E ) of 0.66 V between the ORR half-wave potential (0.86 V vs RHE) and OER potential (1.52 V vs RHE) at a current density of 10 mA cm −2 , and a diffusion-limit ORR current density of 11.37 mA cm −2 are achieved. Notably, the flexible ZAB using the catalyst exhibits a markedly higher peak power density of 242 mW cm 2 , and a small charge/discharge voltage gap of ∼ 0.68 V with a high round-trip efficiency of ∼ 64% when cycling at 5 mA/cm 2 .

Electrocatalytic generation of hydrogen peroxide on cobalt nanoparticles embedded in nitrogen-doped carbon

Electrocatalytic reduction of oxygen is a growing synthetic technique for the sustainable production of hydrogen peroxide (H2O2). The current challenges concern seeking low-cost, highly active, and selective electrocatalysts. Cobalt-nitrogen-doped carbon containing catalytically active cobalt-nitrogen (Co-Nx) sites is an emerging class of materials that can promote the electrochemical generation of H2O2. Here, we report a straightforward method for the preparation of cobalt-nitrogen-doped carbon composed of a number of Co-Nx moieties using low-energy dry-state ball milling, followed by controlled pyrolysis. This scalable method uses inexpensive materials containing cobalt acetate, 2-methylimidazole, and Ketjenblack EC-600JD as the metal, nitrogen, and carbon precursors, respectively. Electrochemical measurements in an acidic medium show the present material exhibits a significant increase in the oxygen reduction reaction current density, accompanied by shifting the onset potential into the positive direction. The current catalyst has also demonstrated an approximate 90 % selectivity towards H2O2 across a wide range of potential. The H2O2 production rate, as measured by H2O2 bulk electrolysis, has reached 100 mmol gcat.−1 h−1 with high H2O2 faradaic efficiency close to 85% (for 2 h at 0.3 V vs. RHE). Lastly, the catalyst durability has been tested (for 6 h at 0.3 V vs. RHE). The catalyst has shown relatively consistent performance, while the overall faradic efficiency reaches approximate 85% throughout the test cycle indicating the promising catalyst durability for practical applications. The formed Co-Nx moieties, along with other parameters, including the acidic environment and the applied potential, likely are the primary reasons for such high activity and selectivity to H2O2 production.

CoNi Nanoparticles Supported on N-Doped Bifunctional Hollow Carbon Composites as High-Performance ORR/OER Catalysts for Rechargeable Zn-Air Batteries.

Searching for high-quality air electrode catalysts is the long-term goal for the practical application of Zn-air batteries. Here, a series of coexistent composite materials (CoNi/NHCS-TUC-x) of cobalt-nickel supported on nitrogen-doped hollow spherical carbon and tubular carbon are obtained using a simple pyrolysis strategy. Co and Ni in the composites are mainly present in the form of alloy nanoparticles, M-Nx and M-Cx (M = Co or Ni) species, with high oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electroactivity. The materials containing different proportions of spherical carbon and tubular carbon obtained by simply adjusting the raw materials for generating tubular carbon exhibit interesting bifunctional performance: samples with an abundant tubular content have the highest ORR onset potential (0.91 V vs reversible hydrogen electrode), while those with a rich spherical content have the highest ORR current density (5.13 mA·cm-2). Furthermore, CoNi/NHCS-TUC-3 provides the lowest potential difference (ΔE = Ej=10 - E1/2) of 0.806 V. We then test the potential possibility of CoNi/NHCS-TUC-3 as an air electrode for primary and rechargeable Zn-air batteries. The primary battery delivers an open-circuit potential of 1.59 V, a peak power density of 361.8 mA·cm-2, and a specific capacity of 756.5 mA h·gZn-1. The rechargeable battery could be cycled stably for more than 55 h at 10 mA·cm-2. These characteristics make CoNi/NHCS-TUC-3 a superior electrocatalyst for both the ORR and OER, as well as a suitable bifunctional electrode applied to a rechargeable Zn-air battery.

Acid-directed preparation of micro/mesoporous heteroatom doped defective graphitic carbon as bifunctional electroactive material: Evaluation of trace metal impurity

Extensive research to explore cost-effective carbon materials as electrocatalysts and electrode materials for energy conversion and storage has been conducted in the recent literature. This raised a crucial question regarding the origin of this electrocatalytic activity from the heteroatom doping/ hierarchical porous defect-rich architecture and/ or the trace metal impurities introduced during synthesis/ inherent to the precursor. In this work, an insight into this issue is provided by considering a lignocellulosic biowaste, Euryale Ferox (foxnut) shells as a precursor to derive micro/ mesoporous defective graphitic carbon sheets by the phosphoric acid (H3PO4) dictated in-situ carbonization for oxygen reduction reaction (ORR) and supercapacitor applications. The sample synthesized at 900 °C (FP900) shows an onset potential of 0.98 V vs. reversible hydrogen electrode (RHE), ORR current density of 5.5 mA cm−2, and current stability of 93% (in 10 h measurement) in 0.1 M KOH. In addition, a symmetric supercapacitor device is assembled using the prepared material and the specific capacitance of 207.5 F g−1 at 1 A g−1 is obtained. An attempt to explain the origin of the electrochemical performance is made by establishing parallels with the physicochemical characterizations. The inherently doped heteroatoms give rise to electroactive functionalities and the wide enough pore size distribution improves the active sites utilization efficiency by enhancing the accessibility to electrolytic ions resulting in better electrochemical performance. Furthermore, the contribution from the intrinsic trace metal impurities is evaluated using X-ray fluorescence (XRF) spectroscopy. The presented research clarifies the non-contributing nature of trace metal species owing to the inaccessibility of active sites and lower abundance in F900 and FP900, respectively.

Observing, tracking and analysing electrochemically induced atomic-scale structural changes of an individual Pt-Co nanoparticle as a fuel cell electrocatalyst by combining modified floating electrode and identical location electron microscopy

Upon exposure to an electrochemical environment, structural properties of nanoparticulate electrocatalysts at the atomic scale are not stagnant but rather dynamic. These have a direct effect on catalysts' performance via structure-property relationships. The active surface structure is constantly changing via complex phenomena dependant on their nature and reaction conditions. State-of-the-art transmission electron microscopy (TEM) can already provide us with atomically precise structures of individual nanoparticles, which are a key to exploring structure-property relations. However, with the analysis of random nanoparticles with unknown structural history, it is impossible to realise the exact structural alternation mechanisms. In order to study these phenomena operando, in-situ or quasi-in-situ methods need to be developed and used. In the present study, we highlight a recently introduced methodological approach named modified floating electrode (MFE), which enables the assessment of (i) proton exchange membrane fuel cell (PEMFC) cathode oxygen reduction reaction (ORR) at the industry-relevant current densities and (ii) atomic-level structural changes of the same nanoparticle, via identical location scanning electron microscopy (SEM) and TEM approach (IL-SEM and IL-TEM), in one measurement. Careful analysis and comparison of atomically resolved high-resolution scanning TEM (HR-STEM) images of the same nanoparticle before and after MFE measurements were conducted via homemade microscopy image analysis algorithms. We reveal structural changes on the atomic-scale of the industrial benchmark Pt-Co nanoalloy ORR electrocatalyst upon exposure to electrochemical activation and high ORR current densities. Observing and comparing the detailed structure and morphology of the same nanoparticle reveals atomic-scale processes such as particle anisotropic etching and redeposition, besides other processes such as particle necking, anti-necking, pore formation, particle movement, coalescence, etc. The understanding of the dynamics behind these changes is crucial for the interpretation of ORR electrocatalyst's activity and stability. Our bottom-up approach enables direct investigation of nanoparticles’ structure-stability relationships.

Carbon nanowalls functionalization for efficient O2 reduction catalyzed by laccase using design of experiment

An enzymatic biofuel cell cathode was elaborated by nano-structuration and functionalization of graphite using two consecutive plasma treatments before immobilization of laccase as a biocatalyst to perform oxygen reduction reaction (ORR) by direct electron transfer ((DET). Nano-structuration of graphite surfaces by synthesis of vertical graphen-like carbon nanowalls (CNWs) was performed by microwave excited plasma enhanced chemical vapor deposition (PECVD). Atmospheric pressure plasma jet (APPJ) system was then used to functionalize chemically the CNWs surface via a rapid and dry route. A partial fractional design evidenced two significant parameters of the APPJ treatment in terms of ORR current density: the treatment time and the inter-distance between the plasma jet and the surface of the carbon material. With the optimized APPJ treatment parameters, a four-fold enhancement of the ORR current density (-440 ± 70 μA/cm2) could be obtained on CNWs, compared to graphite electrodes. The best current density obtained was about −1 mA/cm2 with oxidized laccase covalently immobilized on the electrode without any agitation of the solution, confirming the main importance of enzyme orientation on the electrode surface for efficient DET transfer, the relevance of electrode nanostructuration and the advantage of covalent linking of the protein on the electrode surface.

Morphology and Structure Controls of Single-Atom Fe–N–C Catalysts Synthesized Using FePc Powders as the Precursor

Understanding the origin of the high electrocatalytic activity of Fe–N–C electrocatalysts for oxygen reduction reaction is critical but still challenging for developing efficient sustainable nonprecious metal catalysts used in fuel cells. Although there are plenty of papers concerning the morphology on the surface Fe–N–C catalysts, there is very little work discussing how temperature and pressure control the growth of nanoparticles. In our lab, a unique organic vapor deposition technology was developed to investigate the effect of the temperature and pressure on catalysts. The results indicated that synthesized catalysts exhibited three kinds of morphology—nanorods, nanofibers, and nanogranules—corresponding to different synthesis processes. The growth of the crystal is the root cause of the difference in the surface morphology of the catalyst, which can reasonably explain the effect of the temperature and pressure. The oxygen reduction reaction current densities of the different catalysts at potential 0.88 V increased in the following order: FePc (1.04 mA/cm2) < Pt/C catalyst (1.54 mA/cm2) ≈ Fe–N–C-f catalyst (1.64 mA/cm2) < Fe–N–C-g catalyst (2.12 mA/cm2) < Fe–N–C-r catalyst (2.35 mA/cm2). By changing the morphology of the catalyst surface, this study proved that the higher performance of the catalysts can be obtained.

Carbon Supported Platinum-Molybdenum Alloy Nanoparticles for Oxygen Reduction Reaction

Fuel cells are gaining importance in the emerging area of power generation. However, sluggishness of the cathodic oxygen reduction reaction (ORR) and usage of expensive electrocatalysts are hindering its widespread application. Hence, an effort has been made in the present study to synthesize efficient electrocatalysts based on Pt-Mo alloys with varying atomic ratios (0-100 at. %) by thermal decomposition method. The synthesized samples were characterized using XRD, SEM, TEM and XPS techniques. The electrocatalytic activity for ORR was measured using cyclic voltammetry and rotating disk electrode for all the samples and Pt-Mo (1:1) electrocatalyst performed better among the synthesized electrocatalysts with ORR current density of 63 mA/cm2 at an applied potential of 0.6 V vs. Hg/HgSO4. The present study suggests that Pt-Mo studied are proven to be a superior catalyst than a costly Pt catalyst with high ORR activity.