Unravelling the role of interface of CuOx-TiO2 hybrid metal oxide in enhancement of oxygen reduction reaction performance

Energy security and clean energy are of increasing importance given the current scenario. Hence fuel cells and metal-air batteries are gaining significant attention in recent times. However, the major problem associated with these technologies is the sluggish nature of oxygen reduction reaction (ORR) and the expense associated with the catalyst. This study reports nitrogen-doped carbon-supported CuO_x/TiO₂-based heterostructure (CuO_x/TiO₂-10 wt%/NC-800) as an efficient, durable, and low-cost ORR catalyst. CuO_x/TiO₂-10 wt%/NC-800 catalyst is synthesized using a simple hydrothermal method. The ORR experiments are performed in an alkaline medium (0.1 M KOH). The catalyst has excellent ORR activity with onset potential and half-wave potential of 0.90 V and 0.80 V vs. RHE, respectively. It also shows outstanding limiting current density of 5.80 mA cm^?2. The catalyst offers excellent stability (with 92% current retention after 24 h) and durability (only 25 mV half-wave potential shift after 2000 CV cycles). The catalyst follows the 4e^? transfer process (n = 3.94) with rather nominal amount of hydrogen peroxide formation (3.5%). The enhanced ORR activity is attributed to the strong synergistic effect between CuO_x/TiO₂ and NC. The unique structure formation also helps to get excellent electronic conductivity, large surface area, appropriate porosity, and desirable interface charge transfer between CuO_x and TiO₂.     

Enhanced electrocatalytic performance of Cu0.02S0.01/rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature

Graphene, is a carbon allotrope, which is widely used as a substrate for various catalysts due to its interesting physicochemical properties. In the present study, graphene oxide sheets were prepared from graphite, then, the graphene oxide surface was modified by a low-temperature method using sulfur and copper atoms to obtain pseudo-enzyme Cu/S/Graphene prosthetic group. The current density passing through Cu/S/Graphene catalyst was four times higher than that passing through graphite. The novel copper-based catalyst had an extraordinary performance for oxygen reduction reaction (ORR) due to the unique bio-inspired and stoichiometric structure. The results of Raman and Dispersive X-ray spectroscopy confirmed the presence of ultra-low content of copper (2%) and sulfur (1%) atoms on the graphene surface. Thermogravimetric analysis indicated a strong interaction between nanoparticles and graphene layers. The number of electrons transferred for ORR varied from 3.98 to 4.16 in a wide range of over-potentials indicating an effective 4-electron pathway form O₂ to H₂O. The Tafel slopes indicated insignificant amount of formed copper oxide on the catalyst surface. The catalyst showed excellent electrochemical durability and its half-wave potential (E_1/2) was exhibited a negative shift only 8.2 mV after 10000 cycles.     

Enhanced electrocatalytic performance of NiOx@MnOx@graphene for oxygen reduction and evolution reactions

Oxygen electrodes for oxygen evolution reaction and oxygen reduction reaction were intensively investigated due to high overpotential required to drive the four-electron process. A NiOx@MnOx@G nanostructure supported homogenously on graphene nanosheets through an easy and scalable self-assembly method was studied. The NiOx@MnOx@G exhibited a nanostructured NiO_x nanocrystalline with size around 2.3 nm and an amorphous MnO_x with controllable thickness. The 25% NiOx@MnOx@G showed remarkable activity for oxygen reduction reaction with a 4-electron process, the half-wave potential was 50 mV, but the stability of 25% NiOx@MnOx@G was better than Pt/C-JM. NiOx@MnOx@G nanostructure exhibited significantly better activity for oxygen evolution reaction compared with MnO_x, which can demonstrate that NiO_x could tune the activity of surface amorphous MnO_x and dramatically increased oxygen evolution reaction activity. NiOx@MnOx@G is demonstrated with superior oxygen catalysts performance for reversible oxygen evolution and oxygen reduction reaction, due to the synergistic effect of NiO_x and amorphous MnO_x.     

Zeolitic-imidazolate-framework-derived Fe-NC catalysts towards efficient oxygen reduction reaction

The development of non-precious metal catalysts to replace scarce and expensive Pt-based catalysts is critical for oxygen reduction reactions (ORR), where zeolitic-imidazolate-framework-derived (ZIF-derived) iron-based electrocatalysts hold a promising prospect. Herein, Fe₃O₄ was used as Fe source, and ZIF-8 was used as C and N source to prepare Fe-NC catalysts. Specifically, the half-wave potential (E_1/2) of the Fe-NC reached 0.90 V, which was higher than commercial Pt/C catalysts (0.87 V), and the overpotential of OER reached 327 mV. In addition, the power density tested in Zn-air batteries upped to 129.59 mW cm⁻², surpassing that of the Pt/C (108.93 mW cm⁻²). The superior performance was attributed to the effective introduction of Fe, the large specific surface area (851.6 m² g⁻¹), relatively regular porous structure and the high degree of graphitization.     

MoS2||CoP heterostructure loaded on N,P-doped carbon as an efficient trifunctional catalyst for oxygen reduction,oxygen evolution,and hydrogen evolution reaction

Exploring multifunctional electrocatalysts is crucial for the development of energy conversion and storage equipments, such as fuel cells, water splitting devices and zinc-air batteries. Herein, we provide a rational design whereby the cobalt phosphide particles are introduced into molybdenum sulfide nanosheets to form a heterostructure (MoS₂||CoP) through the ultrasonic method and calcination. Subsequently, N, P-doped carbon (NPC) is obtained synchronously. The as-prepared MoS₂||CoP/NPC demonstrates highly effective multifunctional catalytic performance for oxygen evolution and hydrogen evolution reaction at lower overpotential, as well as oxygen reduction reaction at high half-wave potential. What this reveals is higher power density and superior stability in zinc-air battery. The excellent electrocatalytic activity of MoS₂||CoP/NPC may be attributed to the presence of the MoS₂||CoP heterostructure, as well as N, P-doped carbon coupled with a high percentage of pyridinic-N. This work proposes a novel and facile strategy to prepare the heterostructure compound and serves as a good reference for constructing efficient and low-cost electrocatalysts.     

Hollow NH2-MIL-101@TA derived electrocatalyst for enhanced oxygen reduction reaction and oxygen evolution reaction

Non-precious metal-based electrocatalysts with excellent activity and stability are highly desired for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a tannic acid (TA) etching strategy is used to inhibit the metal aggregation and achieve muti-metal doping. The hollow NH₂-MIL-101@TA derived Fe–N–C catalyst exhibits superior ORR catalytic activity with an E_1/2 of 0.872 V and a maximum output power density of 123.4 mW cm⁻² in Zn-air battery. Since TA can easily chelate with metal ions, Fe/Co–N–C and Fe/Ni–N–C are also synthesized. Fe/Ni–N–C manifests exceptional bifunctional activity with an E_j = 10 of 1.67 V and a potential gap of 0.833 V between E_j = 10 and E_1/2 in alkaline electrolyte, which is 45 mV smaller than Pt/C–IrO₂. The improvement of ORR and OER performance of the catalysts via the simple TA etching and chelation method provides a novel strategy for the design and synthesis of efficient electrocatalysts.     

Interfacial interaction between NiMoP and NiFe-LDH to regulate the electronic structure toward high-efficiency electrocatalytic oxygen evolution reaction

Development of low-cost, high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is challenging, even though it is critical for the overall electrochemical splitting. Herein, we report a NiMoP@NiFe-LDH heterostructure electrode supported on nickel foam. The study shows that the electrocatalytic activity for the OER can be improved by coupling NiMoP and NiFe-LDH. The resulting NiMoP@NiFe-LDH heterostructure exhibited remarkable catalytic performance with an ultralow overpotential of only 299 mV at a current density of 150 mA cm^?2 and a Tafel slope of 23.3 mV dec^?1 in 1.0 M KOH solution. Electron transfer from NiFe-LDH to NiMoP at the nanointerface reduces the energy barrier of the catalytic process, thus improving the OER activity performance. Thus, high-efficiency electrocatalysts can be utilised by constructing heterojunctions to regulate the electronic structure at the interface of the electrocatalysts.     

Synergistic improvement of oxygen reduction reaction on gold/cerium-phosphate catalysts

The main challenge in fuel cells lies in improving slow oxygen reduction reaction (ORR) kinetics causing low conversion efficiencies. Here, we introduce the Au/CePO₄-binary nanocomposites as effective oxygen reduction catalysts in alkaline media. The ORR activity comparable with Pt is achieved through the serial 4-electron reduction pathway. The bi-functionality of CePO₄ is suggested to explain the remarkably enhanced activity on the Au/CePO₄ nanocomposites. Significantly, the own catalytic activity of CePO₄ for hydrogen peroxide is demonstrated, validating synergistic effects with Au for complete ORR.     

Trimetallic PtNiCo nanoflowers as efficient electrocatalysts towards oxygen reduction reaction

Nanostructures of PtNiCo alloy have been prepared using a simple solvothermal process followed by annealing at higher temperature and studied for electrochemical oxygen reduction reaction (ORR) kinetics. PtNiCo/C catalyst has demonstrated an interesting trend of enhancement in the ORR activity along with long-term durability. The specific activity of 2.47 mA cm^?2 for PtNiCo-16h/C (PtNiCo/C prepared at reaction time of 16 h) is ～12 times higher than that of Pt/C (0.2 mA cm^?2). Further, X-ray diffraction, transmission electron microscopy and X-ray photo electron spectroscopy studies have been carried out systematically to understand the phase formation, morphology along with surface defects and elemental analysis respectively. The durability of the catalyst was evaluated over 10,000 potential cycles using standard triangular potential scan in the lifetime regime. Interestingly, after 10k durability cycles, PtNiCo-16h/C electrocatalyst showed enhanced ORR activity (32% higher activity; Im@10k cycles = 0.716 A mg_Pt^?1) and stability compared to commercial Pt/C signifying the retention of Ni and Co due to higher lattice contraction in PtNiCo alloy electrocatalyst.     

Enhanced durability of Pt/C catalyst by coating carbon black with silica for oxygen reduction reaction

A platinum electrocatalyst was presented for oxygen reduction reaction that the durability to potential cycling was enhanced. It was synthesized by coating carbon black with a silica layer, followed by Pt deposition on it. To investigate the durability of the electrocatalyst, two accelerated degradation testing protocols were carried out. Carbon corrosion and platinum metal degradation properties were evaluated under the potential cycling between 1.0 and 1.5 V and between 0.6 and 0.95 V, respectively. Silica-coated catalysts (Pt/CB-SiO₂) showed better stabilities compared to the commercial Pt/C catalyst under both of the two protocols. Commercial Pt/C catalyst initially had better mass activity than silica-coated catalysts but it became similar after the potential cycling of carbon corrosion. TEM showed the platinum particle aggregation and particle density decrease especially for the commercial catalyst by the potential cycling. The silica coating prevents carbon corrosion by blocking the carbon support from direct contact with the oxygen source and preventing the effect of oxygen spillover from the platinum to carbon during the potential cycling between 1.0 and 1.5 V. It also alleviates platinum dissolution in reverse scans by reducing the formation of Pt oxide during potential cycling in between 0.6 and 0.95 V. The results suggest the coating of carbon support can enhance the durability of Pt/C catalyst to the potential cycling.     

Freestanding palladium nanonetworks electrocatalyst for oxygen reduction reaction in fuel cells

Still it's a main challenge to design of highly efficient electrocatalysts to reduce the high overpotential of the oxygen reduction reaction (ORR). The 1 dimensional (1D) palladium nanonetworks (Pd-Net) can be a promising alternative to platinum (Pt)-based electrocatalyst for ORR. In this study, the Pd-Net electrocatalysts have been synthesized via a simple wet-chemical method with the assistance of cetyltrimethylammonium bromide (CTAB) and zinc precursor. Further investigation indicates that the thickness of Pd-Net can be regulated by simply changing the molar ratio of CTAB and the 5 ± 0.1 nm is proven as an efficient ORR electrocatalyst without any support material. The freestanding 1D Pd-Net has shown 2.2 and 3.6-fold higher electrochemical surface area than that of commercially available Pt/C and homemade Pd nanoparticles (PdNPs) catalysts, respectively. As a result, it provides a higher density of ORR active sites and facilitated the electron transport. The Pd-Net catalyst shows 2.1 and 4.1 times higher mass activity and 1.3 and 3.1 higher specific activity at 0.85 V (vs. RHE) with better ORR kinetics than that of Pt/C and PdNPs, respectively. Additionally, the Pd-Net catalyst has been demonstrated a significant tolerance to the anodic fuels (i.e. methanol) and enhanced durability than the Pt/C and PdNPs catalysts for ORR.     

Phosphorus-doped hierarchical porous carbon as efficient metal-free electrocatalysts for oxygen reduction reaction

Recently, fuel cells and metal-air batteries have attracted extensive attentions. Researching and developing non-noble metal catalyst with high electrocatalytic activity and low cost is one of the important challenges for these energy storage and conversion devices. In this study, phosphorus doped hierarchical porous carbon (P-HPC) has been firstly synthesized via a hard template method. The prepared PHPC possesses a unique porous structure which consists of micropores, mesopores and macropores simultaneously. The electrocatalytic activity of the PHPC toward ORR in KOH solution has been studied and compared with the ordinary structured phosphorus doped carbon (PC) and the commercial Pt/C by means of rotating ring-disk electrode (RRDE) technique. The prepared PHPC exhibits an excellent electrocatalytic performance toward ORR in terms of the electrocatalytic activity, the reaction kinetics, the durability and the methanol tolerance. And the high electrocatalytic activity and durability of PHPC could be attributed to the special hierarchical porous structure. This research demonstrates that the rational design of the microstructures for catalyst plays significant roles in improving the catalytic activity for the ORR.     

Composition-tunable PtCu porous nanowires as highly active and durable catalyst for oxygen reduction reaction

To accelerate the commercialization of fuel cells, many efforts have been made to develope highly active and durable Pt-based catalyst for oxygen reduction reaction (ORR). Herein, PtCu porous nanowires (PNWs) with controllable composition are synthesized through an ultrasound-assisted galvanic replacement reaction. The porous structure, surface strain, and electronic property of PtCu PNWs are optimized by tuning composition, which can improve activity for ORR. Electrochemical tests reveal that the mass activity of Pt_0.5Cu_0.5 PNWs (Pt/Cu atomic ratio of 1:1) reaches 0.80 A mg_Pt^?1, which is about 5 times higher than that of the commercial Pt/C catalyst. Notably, the improved activity of the porous nanowire catalyst is also confirmed in the single-cell test. In addition, the large contact area with the carrier and internal interconnection structure of Pt_0.5Cu_0.5 PNWs enables them to exhibit much better durability than the commercial Pt/C catalyst and Pt_0.5Cu_0.5 nanotubes in accelerated durability test.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

Carbon supported Pt-Y electrocatalysts for the oxygen reduction reaction

Carbon supported Pt₃Y (Pt₃Y/C) and PtY (PtY/C) were investigated as oxygen reduction reaction (ORR) catalysts. After synthesis via reduction by NaBH₄, the alloy catalysts exhibited 10-20% higher mass activity (mA mg_Pt⁻¹) than comparably synthesized Pt/C catalyst. The specific activity (μA cm_Pt⁻²) was 23 and 65% higher for the Pt₃Y/C and PtY/C catalysts, respectively, compared to Pt/C. After annealing at 900 °C under a reducing atmosphere, Pt₃Y/C-900 and PtY/C-900 catalysts showed improved ORR activity; the Pt/C and Pt/C-900 (Pt/C catalyst annealed at 900 °C) catalysts exhibited specific activities of 334 and 393 μA cm_Pt⁻², respectively, while those of the Pt₃Y/C-900 and PtY/C-900 catalysts were 492 and 1050 μA cm_Pt⁻², respectively. X-ray diffraction results revealed that both the Pt₃Y/C and PtY/C catalysts have a fcc Pt structure with slight Y doping. After annealing, XRD showed that more Y was incorporated into the Pt structure in the Pt₃Y/C-900 catalyst, while the PtY/C-900 catalyst remained unchanged. Although these results suggested that the high ORR activity of the PtY/C-900 catalyst did not originate from Pt-Y alloy formation, it is clear that the Pt-Y system is a promising ORR catalyst which merits further investigation.     

Ultrafine iron oxide nanoparticles supported on N-doped carbon black as an oxygen reduction reaction catalyst

An oxygen reduction reaction (ORR) catalyst comprising ultrafine iron oxide nanoparticles supported on N-doped Vulcan carbon (FeO_1.4/N-C) was prepared via a two-step method. X-ray photoelectron spectroscopy revealed the iron oxide nanoparticles comprised Fe₂O₃ and FeO phases with a combined average oxidation state of 2.8. The FeO_1.4/N-C catalyst produced an ORR onset potential of −0.056 V and a half-wave potential of −0.190 V in alkaline media, which was comparable to that of commercial Pt/C catalyst. Moreover, FeO_1.4/N-C had higher methanol tolerance than Pt/C catalyst and thus affords a promising non-precious metal ORR catalyst for fuel cells.     

Enhancement of the electrocatalytic activity for the oxygen reduction reaction of boron-doped reduced graphene oxide via ultrasonic treatment

Commercial polymer electrolyte membrane fuel cells have relied on scares Platinum to catalyse the kinetically sluggish oxygen reduction reaction occurring at their anodes. Over the last decade organic materials, frequently based on graphitic structures have been demonstrated as promising alternative electrocatalysts to the noble metals. Researchers typically utilize ultrasonic treatment as part of the synthesis procedure to achieve homogeneous dispersion of graphitic carbon prior to. Herein we investigate the implications of the structural and compositional changes induced by the ultrasonication treatment on boron-doped reduced graphene oxide for oxygen reduction reaction. It is shown that ultrasonication pre-treatment prior to the boron doping and reduction of graphene oxide via hydrothermal process step leads to the increase of both substitutional B and electrocatalytic surface area, with associated reduction of average pore size diameter, leading to a significant improvement in the oxygen reduction reaction performance, with respect to the non-ultrasonicated material. It is proposed that the higher degree of substitutional doping of boron is a result of formation of the additional epoxy functionalities on graphitic planes, which act as a doping site for boric acid.     

Enhancing oxygen reduction reaction performance in acidic media via bimetal Fe and Cr synergistic effects

Currently, it is a great challenge to improve the stability of Fe-based oxygen reduction reaction (ORR) catalysts in acidic medium, because the Fenton reaction between Fe-based catalyst and H₂O₂ will reduce the stability of the catalyst. In this study, Fe, Cr and nitrogen-doped carbon (FeCr-N-C) was synthesized via co-impregnation of biomass walnut shells with metal precursor solutions. The FeCr-N-C catalyst has an onset potential of 0.88 V vs. RHE and outperforms the Fe-N-C catalyst in acidic media. Moreover, FeCr-N-C shows negligible activity decay (ΔE_1/2 = 14 mV) in 0.1 M HClO₄ after 20 000 cycles. The experimental results proved that the bimetal synergism can produce low yield of H₂O₂ (<2%), which might attribute to variations of local electronic structure. The reactive oxygen species of the catalyst were analyzed by Ultraviolet-visible (UV-Vis) absorption spectra. It was proved that the presence of bimetal inhibited the Fenton reaction between Fe²⁺/Fe³⁺ and H₂O₂, and further improved the stability of the catalysts. Hence, this study proposes an efficient strategy to facilitate the practical application of Fe-based catalysts in acidic media.     

Controllable synthesis of three-dimensional nitrogen-doped graphene as a high performance electrocatalyst for oxygen reduction reaction

Three-dimensional nitrogen-doped graphene (3D-NG@SiO₂) is prepared by pyrolyzing poly (o-phenylenediamine) (POPD) with high nitrogen content. POPD is prepared via an in situ chemical oxidation polymerization of o-phenylenediamine (OPD) in acetic acid with silica colloid as templates. The optimum parameter is OPD:SiO₂ = 1:2, pyrolysis @ 900 °C. SEM and TEM images show the wrinkled and porous graphene structures. Raman spectra indicate that 3D-NG@SiO₂ consists of 4–6 layers graphene. N₂ adsorption–desorption isotherms reveal that the pore size distributions mainly centralize at 5–10 nm. XRD illustrates the amorphous structure. XPS analysis shows that the nitrogen content is 3.6% and nitrogen mainly exists in the form of pyridinic nitrogen and pyrrolic nitrogen. The oxygen reduction reaction (ORR) performance investigated using a rotating disk electrode shows that the initial potential of 3D-NG@SiO₂ is 0.08 V (vs. Hg/HgO). The electron transfer number is 3.92 @ ?0.3 V (vs. Hg/HgO), indicating that 3D-NG@SiO₂ mainly occurs via a four-electron process. The oxygen reduction current density decreases by 21% after 60 h in the chronoamperometry test. The CVs manifests a current density loss of 0.16 mA cm^?2 after scanning for 5000 cycles. The high activity and durability indicate the promising potential of 3D-NG@SiO₂ as ORR catalysts.     

In situ synthesis of CoFe2O4 nanoparticles embedded in N-doped carbon nanotubes for efficient electrocatalytic oxygen reduction reaction

The development of electrocatalyst possessing superior oxygen reduction reaction (ORR) activity is highly desirable due to the low sluggish kinetics at the cathode of fuel cell. Here, CoFe₂O₄ nanoparticles embedded in N-doped carbon nanotubes electrocatalyst (denoted as CoFe₂O₄-NC) is synthesized via polymerization of pyrrole, absorbing metal ion and annealing under Ar/NH₃ atmosphere. By in situ integrating the catalytically active CoFe₂O₄ nanoparticles with the N-doped carbon nanotubes and enhancing electrical conductivity, the as-obtained electrocatalyst exhibits excellent ORR activity and long-term stability with a half-wave potential of 0.86 V and 10 h continuous cycling, outperforming the reported similar catalysts. This work opens a new path for the expansion of low cost and efficient ORR electrocatalysts to substitute Pt-based metals for energy storage and conversion devices.