Electrocatalytic Activity For Oxygen Reduction Research Articles

Highly improved ionic conduction in reverse spinel structured CoAl2O4 and ZnO heterostructure through interfacial disordering

For ceramic fuel cells with low temperatures operation (LT-CFCs; 350°-550 °C), increasing the ionic conductivity and improving the oxygen reduction electrocatalytic activity response at low operating temperatures would be highly desired. This Study presents a novel semiconductor heterostructure composite with a competent electrolyte membrane for LT-CFCs. The composite comprises CoAl2O4 (CA) in a spine-like structure and wurtzite-structured ZnO. We have demonstrated a button-sized CFC under H2 and ambient air environment delivering a high power density of 835 mW-cm−2 at 550 °C, with potential operating up-to 400 °C. Compared to ZnO and CA alone, the CA-ZnO heterostructure composite has reduced oxygen vacancy production and activation energy, facilitating ion transport more quickly through the CA-Zn interface. Density functional theory (DFT) computations, photoelectron spectroscopy, U-visible, X-ray diffraction, and HR-TEM are the characterization techniques used to study the enhanced ionic conduction of the CA-ZnO heterostructure composite. These results imply that the heterostructure approach works better than the simple bulk structure for the electrolyte application of LT-SOFCs.

Ni-Substituted Sr2FeMoO6-δ as an Electrode Material for Symmetrical and Reversible Solid-Oxide Cells.

This work develops a novel perovskite Sr2FeNi0.35Mo0.65O6-δ (SFN0.35M) simultaneously using as a fuel electrode and oxygen electrode in a reversible solid oxide cell (RSOC). SFN0.35M shows outstanding electrocatalytic activity for hydrogen oxidation, hydrogen evolution, oxygen reduction, and oxygen evolution. In situ exsolution and dissolution of Fe-Ni alloy nanoparticles in SFN0.35M is revealed. In a reducing atmosphere, SFN0.35M shows in situ exsolution of Fe-Ni alloy nanoparticles, and then the Fe-Ni alloy is reoxidized into SFN0.35M while converting into an oxidizing atmosphere. The polarization resistances of SFN0.35M electrode are 0.043 Ω cm2 in 20% O2-N2 and 0.064 Ω cm2 in H2 at 850 °C. Moreover, symmetric fuel cells using the SFN0.35M electrode achieves a maximum power density of 0.501 W cm-2 at 850 °C in H2 fuel, while the symmetric electrolysis cell has an electrolysis current density of 0.794 A cm-2 at 1.29 V in 90% H2O-10% H2 at 850 °C. It is the first time we demonstrate that the cell voltage of symmetrical cell at 0.5 A cm-2 in the fuel cell mode and -0.5 A cm-2 in the electrolysis cell mode can be fully recovered in 10 electrode alternating cycles and therefore demonstrate the possibility that SFN0.35M can be used in a fully symmetric RSOC stack with electrode alternating functions.

Renewable lignin-derived heteroatom-doped porous carbon nanosheets as an efficient oxygen reduction catalyst for rechargeable zinc-air batteries

Lignin upgrading to various functional products is promising to realize high-value utilization of low-cost and renewable biomass waste, but is still in its infancy. Herein, using industry waste lignosulfonate as the biomass-based carbon source and urea as the dopant, we constructed a heteroatom-doped porous carbon nanosheet structure by a simple NaCl template-assisted pyrolytic strategy. Through the synergistic effect of the NaCl template and urea, the optimized lignin-derived porous carbon catalyst with high content of active nitrogen species and large specific surface area can be obtained. As a result, the fabricated catalysts exhibited excellent electrocatalytic oxygen reduction activity, as well as good methanol tolerance and stability, comparable to that of commercial Pt/C. Moreover, rechargeable Zn-air batteries assembled with this electrocatalyst have a peak power density of up to 150 mW cm-2 and prominent long-term cycling stability. This study offers an inexpensive and efficient way for the massive production of highly active metal-free catalysts from the plentiful, inexpensive and environmentally friendly lignin, offering a good direction for biomass waste recycling and utilization.

Nanobioelectrocatalysis Using Human Liver Microsomes and Cytochrome P450 Bactosomes: Pyrenyl-Nanocarbon Electrodes.

Human liver microsomes containing various drug-metabolizing cytochrome P450 (P450) enzymes, along with their NADPH-reductase bound to phospholipid membranes, were absorbed onto 1-pyrene butylamine pi-pi stacked with amine-functionalized multiwalled carbon nanotube-modified graphite electrodes. The interfaced microsomal biofilm demonstrated direct electrochemical communication with the underlying electrode surface and enhanced oxygen reduction electrocatalytic activity typical of heme enzymes such as P450s over the unmodified electrodes and nonenzymatic currents. Similar enhancements in currents were observed when the bioelectrodes were constructed with recombinant P450 2C9 (single isoform) expressed bactosomes. The designed liver microsomal and 2C9 bactosomal bioelectrodes successfully facilitated the electrocatalytic conversion of diclofenac, a drug candidate, into 4'-hydroxydiclofenac. The enzymatic electrocatalytic metabolite yield was several-fold greater on the modified electrodes than on the unmodified bulk graphite electrodes adsorbed with a microsomal or bactosomal film. The nonenzymatic metabolite production was less than the enzymatically catalyzed metabolite yield in the designed microsomal and bactosomal biofilm electrodes. To test the throughput potential of the designed biofilms, eight-electrode array configurations were tested with the microsomal and bactosomal biofilms toward electrochemical 4'-hydroxydiclofenac metabolite production from diclofenac. The stability of the designed microsomal bioelectrode was assessed using nonfaradaic impedance spectroscopy over 40 h, which indicated good stability.

A post-modification strategy to precisely construct dual-atom sites for oxygen reduction electrocatalysis

Dual-atom catalysts (DACs) afford promising potential for oxygen reduction electrocatalysis due to their high atomic efficiency and high intrinsic activity. However, precise construction of dual-atom sites remains a challenge. In this work, a post-modification strategy is proposed to precisely fabricate DACs for oxygen reduction electrocatalysis. Concretely, a secondary metal precursor is introduced to the primary single-atom sites to introduce direct metal–metal interaction, which ensures the formation of desired atom pair structure during the subsequent pyrolysis process and allows for successful construction of DACs. The as-prepared FeCo-NC DAC exhibits superior oxygen reduction electrocatalytic activity with a half-wave potential of 0.91 V vs. reversible hydrogen electrode. Zn–air batteries equipped with the FeCo-NC DAC demonstrate higher peak power density than those with the Pt/C benchmark. More importantly, this post-modification strategy is demonstrated universal to achieve a variety of dual-atom sites. This work presents an effective synthesis methodology for precise construction of catalytic materials and propels their applications in energy-related devices.

Molecular Recognition Regulates Coordination Structure of Single‐Atom Sites

AbstractCoordination engineering for single‐atom sites has drawn increasing attention, yet its chemical synthesis remains a tough issue, especially for tailorable coordination structures. Herein, a molecular recognition strategy is proposed to fabricate single‐atom sites with regulable local coordination structures. Specifically, a heteroatom‐containing ligand serves as the guest molecule to induce coordination interaction with the metal‐containing host, precisely settling the heteroatoms into the local structure of single‐atom sites. As a proof of concept, thiophene is selected as the guest molecule, and sulfur atoms are successfully introduced into the local coordination structure of iron single‐atom sites. Ultrahigh oxygen reduction electrocatalytic activity is achieved with a half‐wave potential of 0.93 V versus reversible hydrogen electrode. Furthermore, the strategy possesses excellent universality towards diversified types of single‐atom sites. This work makes breakthroughs in the fabrication of single‐atom sites and affords new opportunities in structural regulation at the atomic level.

Molecular Recognition Regulates Coordination Structure of Single-Atom Sites.

Coordination engineering for single-atom sites has drawn increasing attention, yet its chemical synthesis remains a tough issue, especially for tailorable coordination structures. Herein, a molecular recognition strategy is proposed to fabricate single-atom sites with regulable local coordination structures. Specifically, a heteroatom-containing ligand serves as the guest molecule to induce coordination interaction with the metal-containing host, precisely settling the heteroatoms into the local structure of single-atom sites. As a proof of concept, thiophene is selected as the guest molecule, and sulfur atoms are successfully introduced into the local coordination structure of iron single-atom sites. Ultrahigh oxygen reduction electrocatalytic activity is achieved with a half-wave potential of 0.93 V versus reversible hydrogen electrode. Furthermore, the strategy possesses excellent universality towards diversified types of single-atom sites. This work makes breakthroughs in the fabrication of single-atom sites and affords new opportunities in structural regulation at the atomic level.

Noncovalent composites of meso-substituted A2B2-porphyrins and carbon nanotubes for enhanced electrocatalytic oxygen reduction

Exploring highly active oxygen reduction electrocatalysts with low precious metals content is imperative but remains a considerable challenge. Herein, a series of heterobimetallic multi-walled carbon nanotubes (MWCNTs) electrocatalysts based on metal complexes are presented. These electrocatalysts feature diverse transition metals (M=Mn, Fe, Co, Ni) 5,15-bromophenyl-10, 20-methoxyphenyl porphyrin (MBMP) and tetrakis(triphenylphosphine)palladium (0) (Pd[P(Ph3)4]) anchored non-covalently on its surface. The resulting NiBMP-based MWCNTs with Pd[P(Ph3)4] (PdNiN4/MWCNTs) display outstanding electrocatalytic oxygen reduction activity (onset potential, 0.941 V; half wave potential, 0.830 V) and robust long-term durability in alkaline electrolyte. While in neutral condition, the MnBMP-based MWCNTs with Pd[P(Ph3)4] (PdMnN4/MWCNTs) are the most active heterobimetallic ORR catalyst and produce ultra-low concentration hydrogen peroxide (H2O2 yield, 1.2%–1.3%). Synergistically tuning the ORR electrocatalytic activity and electron transfer pathway is achieved by the formation of NiBMP/MnBMP-Pd[P(Ph3)4] active sites. This work indicates such metalloporphyrin-Pd[P(Ph3)4] active sites on MWCNTs have significantly positive influence on electrocatalytic ORR systems and provides facile and mild strategy for designing highly efficient ORR electrocatalysts with ultra-low loading precious metal.

Atomically Dispersed Cu Nanostructures with pH-Universal Electrocatalytic Oxygen Reduction Activity for Zn–Air Batteries

Atomically Dispersed Cu Nanostructures with pH-Universal Electrocatalytic Oxygen Reduction Activity for Zn–Air Batteries

Cu@Pt/CNT catalysts for oxygen reduction prepared by a facile two-step synthesis: Chemical vs. electrochemical leaching

Cu@Pt/CNT electrocatalysts were prepared by a facile two-step impregnation process. Pt nanoparticles supported on CNTs were prepared by microwave-assisted polyol reduction followed by impregnation with a copper precursor. The final catalysts were obtained by thermal alloying at different temperatures; namely 600, 800, 1000 °C. The alloying extent between Pt and Cu was found to increase significantly with the alloying temperature as revealed from XRD analysis, as does the particle size obtained from TEM. Electrochemical dealloying was performed with the three different Cu@Pt/CNT catalysts, aiming to form a core shell structure. The formation of a Pt enriched surface upon dealloying was confirmed electrochemically by cyclic voltammtery and CO stripping. The electrocatalytic activity for oxygen reduction of the electrochemically dealloyed catalysts depends on the alloying temperature where the catalyst alloyed at 800 °C showed the highest surface specific activity, which is explained in view of the high Cu content in the alloy core compared to the sample alloyed at 600 °C. Alloying at 1000 °C and chemical/electrochemical leaching leads to porous structures with high activity, albeit lower than that of the 800 °C sample. Notably these porous particles provide high activity also with a low copper content in the core. In general, the dealloyed Cu@Pt/CNT catalysts showed higher catalytic activity both in terms of surface specific activity (SSA) and mass specific activity (MSA) than their Pt/CNT counterpart.

Improved Ionic Transport Using a Novel Semiconductor Co0.6Mn0.4Fe0.4Al1.6O4 and Its Heterostructure with Zinc Oxide for Electrolyte Membrane in LT-CFCs

Improving the ionic conductivity and slow oxygen reduction electro-catalytic activity of reactions occurring at low operating temperature would do wonders for the widespread use of low-operating temperature ceramic fuel cells (LT-CFCs; 450–550 °C). In this work, we present a novel semiconductor heterostructure composite made of a spinel-like structure of Co0.6Mn0.4Fe0.4Al1.6O4 (CMFA) and ZnO, which functions as an effective electrolyte membrane for solid oxide fuel cells. For enhanced fuel cell performance at sub-optimal temperatures, the CMFA–ZnO heterostructure composite was developed. We have shown that a button-sized SOFC fueled by H2 and ambient air can provide 835 mW/cm2 of power and 2216 mA/cm2 of current at 550 °C, possibly functioning down to 450 °C. In addition, the oxygen vacancy formation energy and activation energy of the CMFA–ZnO heterostructure composite is lower than those of the individual CMFA and ZnO, facilitating ion transit. The improved ionic conduction of the CMFA–ZnO heterostructure composite was investigated using several transmission and spectroscopic measures, including X-ray diffraction, photoelectron, and UV–visible spectroscopy, and density functional theory (DFT) calculations. These findings suggest that the heterostructure approach is practical for LT-SOFCs.

Pyridinic nitrogen induced compressed bilayer graphene for oxygen reduction reaction

Despite the emergence of nitrogen-doped graphene as a noble-metal free electrocatalyst for oxygen reduction reaction, its participation in the electrochemical conversion mechanism is not well-established. In the present study, functionalities of the nitrogen species on the oxygen reduction activity of bilayer graphene were investigated by combining atom-specific X-ray spectroscopy, Raman spectroscopy, and density functional theory calculations with electrochemical activity tests in alkaline media. Among various nitrogen species, pyridinic nitrogen as the dominant species improved the electrochemical activity of bilayer graphene, which was followed by graphene bilayers doped with graphitic nitrogen in majority. Polarization curves revealed a significantly high electrocatalytic oxygen reduction activity of the nitrogen-doped bilayer graphene where the pyridinic nitrogen was the major dopant. This improved activity was confirmed by the lowest overpotential and Tafel slope (78.9 mV/dec). The enhanced interaction of graphene bilayers doped with pyridinic nitrogen is shown to be the main reason for this improvement.

Bifunctional electrocatalytic activity of two-dimensional multilayered vanadium carbide (MXene) for ORR and OER

MXenes are a new class of 2D materials that demonstrate effective bifunctional electrocatalytic activity for oxygen reduction (ORR) and evolution reactions (OER). We synthesized V2C (VC–HF) MXene from V2AlC (VAC) via etching using HF at low temperatures. The structure, composition, and morphology of VC-HF MXene were determined using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) analysis. The XPS analysis demonstrated that VC-HF contained a small amount of Al, suggesting the presence of unreacted VAC even after etching. With an impressive ΔE of ∼0.90 V (the potential difference between ORR and OER), the VC-HF MXene exhibited excellent ORR and OER catalytic activity. This electrocatalyst showed an Eonset of 0.87 V for the reduction of O2, which is remarkable compared to those of the reported bifunctional oxygen catalysts, and exhibited a 430 mV OER overpotential value (η10) at 10 mA cm−2. Besides higher catalytic activity, VC-HF MXene demonstrated good catalytic stability over 10000 s. This work suggests that VC-HF is a promising and efficient bifunctional catalyst for use in various oxygen-based electrochemical devices.

Electrochemical and in-situ spectroelectrochemical properties of novel (5-(tert-butyl)-2-((3,4-dicyanophenoxy)methyl)phenyl)methanolate substituted mononuclear metal phthalocyanines

This work involves the synthesis and characterization of metallic (Co, Fe, Mn, Ni, Zn) ball-type phthalocyanine precursors by using novel 4,4′-(((4-(tert-butyl)-1,2-phenylene)bis-(methylene))bis(oxy))diphthalonitrile starting compound. The classical oxo-bridge in phthalocyanines was altered with the -OCH2- bridge first time for these ball-type phthalocyanine precursors, and the effects of this change on the chemical, physical and spectral features of phthalocyanine complexes were investigated, in the study. The redox properties of phthalocyanines were researched by electrochemical and in situ spectroelectrochemical measurements on a Pt working electrode in non-aqueous medium. The color changes involved in the redox processes were observed via in situ electrocolorimetric techniques. Electrochemical and UV–Vis spectral measurements exhibited that the compounds had reversible and serial one-electron reduction and oxidation processes. Furthermore, they also had nitrile reduction processes because of opened nitrile groups located at peripheral tails of the phthalocyanine complexes. The plentiful redox conducts of the complexes such as Pc ring based and/or central redox active metal-based reduction and oxidation reactions at low potentials showed that these complexes can be used as functional materials. Because of the apparent spectral and net color changes, the complexes are suitable for utilization in electrochromic devices. Moreover, their rich redox features are a sign of their high electrocatalytic activity for oxygen reduction.

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single‐Atom Catalysts throughp‐nJunction Rectification

AbstractFine‐tuning single‐atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploitp‐type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeNx) and rectify their local charge density by ann‐type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing ann‐type gallium monosulfide that features a low work function generates a space‐charged region across the junction interface, and causes distortion of the FeN4moiety and spin‐state transition in the FeIIcenter. This catalyst shows an over two‐fold higher specific oxygen‐reduction activity than that of pristine FePc. We further employ three othern‐type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN4and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification.

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeNx ) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN4 moiety and spin-state transition in the FeII center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN4 and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.

Electrocatalytic oxygen reduction activity of AgCoCu oxides on reduced graphene oxide in alkaline media.

Silver-based electrocatalysts as promising substitutes for platinum materials for cathodic oxygen electroreduction have been extensively researched. Electrocatalytic enhancement of the Ag nanoarchitectonics can be obtained via support structures and amalgamating Ag with one or two additional metals. The work presented here deals with a facile microwave-assisted synthesis to produce bimetallic Ag-Cu and Ag-Co (1:1) oxide nanoparticles (NPs) and trimetallic AgCuCo (0.6:1.5:1.5, 2:1:1, and 6:1:1) oxide NPs supported on a reduced graphene oxide (rGO) matrix. Morphology, composition, and functional groups were methodically analysed using various microscopic and spectroscopic techniques. The as-prepared electrocatalysts were employed as cathode substrates for the oxygen reduction reaction (ORR) in alkaline medium. Varying the Ag fraction in copper cobalt oxide has a significant influence on the ORR activity. At a ratio of 2:1:1, AgCuCo oxide NPs on rGO displayed the best values for onset potential, half-wave potential, and limiting current density (Jk) of 0.94 V vs RHE, 0.78 V, and 3.6 mA·cm−2, respectively, with an electrochemical active surface area of 66.92 m2·g−1 and a mass activity of 40.55 mA·mg−1. The optimum electrocatalyst shows considerable electrochemical stability over 10,000 cycles in 0.1 M KOH solution.

Dealloying-Derived Porous Spinel Oxide for Bifunctional Oxygen Electrocatalysis and Rechargeable Zinc-Air Batteries: Promotion of Activity Via Hereditary Al-Doping.

The large-scale fabrication of highly efficient and low-cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical to the development of rechargeable zinc-air batteries (ZABs). Herein, a scalable dealloying strategy was proposed to obtain hierarchically porous spinel-type oxide with minor hereditary Al doping. Benefiting from the well-structured porosity and native dopant, O-np-Ni5 Co10 (Al), namely Al-NiCo2 O4 , exhibited excellent electrocatalytic ORR and OER activities, giving a small potential gap of 0.71 V. The rechargeable ZAB with O-np-Ni5 Co10 (Al) as cathode catalyst delivered a high specific capacity of 757 mAh g-1 , a competitive peak power density of 142 mW cm-2 , and a long-term discharge-charge cycling stability. Furthermore, density functional theory calculations evidenced that appropriate Al doping into NiCo2 O4 could significantly reduce the Gibbs free energy difference to 1.71 eV. This work is expected to inspire the design of performance-oriented bifunctional electrocatalysts for wider applications in renewable energy systems.

A Mean‐Field Model for Oxygen Reduction Electrocatalytic Activity on High‐Entropy Alloys**

AbstractHigh‐entropy alloys (HEAs) represent near‐equimolar points in the middle of a vast composition space of multi‐metallic catalysts. Successful modeling of the catalytic activity of these complex materials allows to search this composition space for optimal catalysts. This study shows the effect of approximating the ligand effect of the surrounding atoms around an adsorption site with a mean‐field perturbation. Modeling the electrocatalytic activity of the oxygen reduction reaction on the quinary AgIrPdPtRu HEA, it is shown that the extent of such a mean‐field approximation is valid up to and including equimolar ternary alloys, corresponding to 60.3 % of the quinary composition space, by comparing to models that consider the ligand effect locally. When extrapolating to make predictions far from near‐equimolar compositions, such as for binary alloys, the mean field has been sufficiently perturbed to cause large discrepancies. Here, the local ligand models thus prove more useful for discovering optimal catalysts.

Electrochemical aptasensor for aflatoxin B1 detection using cerium dioxide nanoparticle supported on iron-porphyrinic metal–organic framework as signal probes

A novel electrochemical aptasensor was fabricated by using cerium oxide (CeO2) nanoparticle loaded spindle iron-porphyrinic metal–organic framework (PorMOF) as signal probe to detect aflatoxin B1 (AFB1). PorMOF with zirconium ion as the center ion and iron-porphyrin as linker was synthesized. CeO2 nanoparticle was supported on PorMOF to obtain CeO2/PorMOF. PorMOF and CeO2/PorMOF were characterized by scanning electron microscopy, and energy dispersive and X-ray diffraction spectroscopies. CeO2/PorMOF modified electrode showed enhanced electrocatalytic activity for oxygen reduction, and CeO2/PorMOF was functionalized with streptavidin by glutaraldehyde cross-linking to prepare traces of AFB1 aptasensor. A competitive aptasensing assay was applied for AFB1 detection. Firstly, AFB1 capture DNA (cDNA) was fixed on electrode by Au-S bond. Sequentially, AFB1 with biotin was hybridized with cDNA aptamer. Thirdly, after recognizing of biotin and streptavidin, CeO2/PorMOF traces was introduced into electrode to produce a strong electrochemical reduction signal of oxygen. In the presence of AFB1, AFB1 aptamer preferentially bound to AFB1, and the electrochemical signal decreased. The designed method had a wide detection range of 0.1 pM-10 nM, a detection limit as low as 30 fM (S/N = 3), and good selectivity, reproducibility and stability. The aptasensor had been successfully applied to detect AFB1 in peanut and milk samples.