Carbon-supported Pd-Co bimetallic nanoparticles as electrocatalysts for the oxygen reduction reaction

Carbon-supported Pd-Co bimetallic nanoparticle electrocatalysts of different Pd/Co atomic ratios were prepared by a modified polyol reduction. Electrocatalytic activities of the catalysts for the oxygen reduction reaction (ORR) have been investigated based on the porous rotating disk and disk-ring electrode techniques. As-prepared Pd-Co bimetallic nanoparticles evidence a single-phase fcc disordered structure, and the mean particle size is found to decrease with increase in Co content. A typical TEM image of the Pd₂Co/C catalyst, heat-treated at 500 °C, reveals a mean particle diameter is ca. 8.3 nm with a relatively narrow size distribution. For synthesized Pd-Co catalysts, the highest catalytic activity for the ORR, when supported on carbon (i.e., Pd-Co/C) was found for a Pd:Co atomic ratio of 2:1 and heat treatment at ca. 500 °C, corresponding to a Pd–Pd mean interatomic distance of ca. 0.273 nm. Kinetic analysis based on the rotating disk and disk-ring electrode measurements reveals that the ORR on Pd-Co/C catalysts undergoes a four-electron process in forming water. Because the Pd-Co/C catalyst is inactive for the adsorption and oxidation of methanol, it may function as a methanol-tolerant ORR catalyst in a direct methanol fuel cell.     

Mesoporous platinum nanosponges as electrocatalysts for the oxygen reduction reaction in an acidic electrolyte

Mesoporous Pt nanosponges of high activity were successfully synthesized for the oxygen reduction reaction (ORR). These porous nanosponges of 48.7, 53.7, 62.8, and 77 nm in size were synthesized by changing the concentration of polyvinylpyrrolidone electrolyte in the electrochemical synthesis. The electrochemically active surface areas (EASs) of the 48.7-, 53.7-, 62.8-, and 77-nm Pt nanosponges used toward H-adsorption and H-desorption were 162.84, 137.76, 116.71, and 103.83 m²/g, respectively. These measured EASs of these nanosponges were larger than the EAS contributed by the Pt nanoparticles of 76.69 m²/g. As for the ORR, electrochemical measurement and Koutecky-Levich plots showed that the kinetic current densities catalyzed by the 48.7-, 53.7-, 62.8-, and 77-nm Pt nanosponges at 0.7 V (vs. Ag/AgCl) were 0.488, 0.483, 0.450, and 0.370 mA cm⁻², respectively; and the 48.7-nm Pt nanosponges had high reducing activity in the ORR. A size-dependent activity was found. As a reveal of the results of a rotating ring-disk electrode experiment, the catalysis of the ORR by the 48.7-nm Pt nanosponges occurred through the 4-electron pathway, and the efficiency of H₂O production was approximately 99.4%.     

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.     

Bio-inspired highly active catalysts for oxygen reduction reaction in alkaline electrolyte

A novel non-platinum oxygen reduction reaction (ORR) catalyst was synthesised by the pyrolysis of carbon-supported vitamin B12 under ammonia atmosphere. The resultant catalyst was characterised by transmission electron microscopy (TEM), scanning TEM (STEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses. Results demonstrated that the catalyst had a spherical structure. XPS revealed that the nitrogen configuration was changed after pyrolysis, and nitrogen species played a key role in catalysing the ORR. The catalyst exhibited an enhanced ORR activity than commercial 20% Pt/C in alkaline media. The catalyst had an electron transfer number of 3.9, which was very close to the ideal theoretical value of 4. Moreover, the catalyst displayed superior methanol tolerance to Pt/C in alkaline medium, demonstrating its potential application as a cost-effective catalyst for direct methanol alkaline fuel cells.     

Pd-Ni electrocatalysts for efficient ethanol oxidation reaction in alkaline electrolyte

Pd_xNi_y/C catalysts with high ethanol oxidation reaction (EOR) activity in alkaline solution have been prepared through a solution phase-based nanocapsule method. XRD and TEM show Pd_xNi_y nanoparticles with a small average diameter (2.4-3.2 nm) and narrow size distribution (1-6 nm) were homogeneously dispersed on carbon black XC-72 support. The EOR onset potential on Pd₄Ni₅/C (−801 mV vs. Hg/HgO) was observed shifted 180 mV more negative than that of Pd/C. Its exchange current density was 33 times higher than that of Pd/C (41.3 × 10⁻⁷ A/cm²vs. 1.24 × 10⁻⁷ A/cm²). After a 10,000-s chronoamperometry test at −0.5 V (vs Hg/HgO), the EOR mass activity of Pd₂Ni₃/C survived at 1.71 mA/mg, while that of Pd/C had dropped to 0, indicating Pd_xNi_y/C catalysts have a better ’detoxification’ ability for EOR than Pd/C. We propose that surface Ni could promote refreshing Pd active sites, thus enhancing the overall ethanol oxidation kinetics. The nanocapsule method is able to not only control over the diameter and size distribution of Pd-Ni particles, but also facilitate the formation of more efficient contacts between Pd and Ni on the catalyst surface, which is the key to improving the EOR activity.     

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.     

Kinetics and mechanism for the oxygen reduction reaction on polycrystalline cobalt–palladium electrocatalysts in acid media

Cobalt–palladium (CoPd_x) bimetallic electrocatalysts of various compositions were prepared and their physical and electrochemical properties were examined. The nominal composition of the polycrystalline electrocatalysts was estimated by energy dispersive X-ray analysis (EDX) and the lattice parameters were evaluated using X-ray diffraction (XRD). Electrocatalytic activity of the CoPd_x electrodes for the oxygen reduction reaction was compared using cyclic voltammograms obtained in oxygen saturated, 0.5 M H₂SO₄. CoPd₃ was identified as having the highest activity, showing peak currents comparable to a polycrystalline Pt electrode under identical experimental conditions. The rotating disk electrode technique was used to collect kinetic data for the oxygen reduction on CoPd₃ and the activity of the binary electrocatalysts was evaluated. Kinetic and mechanistic aspects of the oxygen reduction reaction on CoPd₃ are discussed and specific ways in which the surface structures may be involved are proposed.     

Kinetics of oxygen reduction reaction on Corich core-Ptrich shell/C electrocatalysts

The nanostructured Co_{rich core}-Pt_{rich shell}/C electrocatalysts were prepared by combining the thermal decomposition and the chemical reduction methods. The particle size of homemade Co_{rich core}-Pt_{rich shell}/C analyzed by TEM was significantly greater than that of Pt grain size calculated from the XRD data due to the existence of Co in core. The mass activity and specific activity of oxygen reduction reaction (ORR) at the overpotential (η) of 0.1 V were 6.69 A g⁻¹ and 1.51 × 10⁻⁵ A cm⁻² for Pt/C, and 10.22 A g⁻¹ and 2.73 × 10⁻⁵ A cm⁻² for Co_{rich core}-Pt_{rich shell}/C in 0.5 M HClO₄ aqueous solution at 25 °C. The Tafel slopes of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C electrocatalysts were obtained as 64 and 67 mV dec⁻¹ at a lower η (50–100 mV), and 116 and 110 mV dec⁻¹ at a higher η (120–200 mV). The exchange current densities of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C evaluated based on the higher Tafel slope regions were 6.76 × 10⁻⁵ and 9.21 × 10⁻⁵ A cm⁻², respectively. The experimental results indicated that the ORR on Co_{rich core}-Pt_{rich shell}/C electrocatalyst in 0.5 M HClO₄ aqueous solution was a four electron transfer mechanism and first order with respect to the dissolved oxygen.     

Relevance of the nature of bimetallic PtAu nanoparticles as electrocatalysts for the oxygen reduction reaction in the presence of methanol

A series of carbon supported PtAu electrocatalysts has been prepared. The performance of the samples in the methanol oxidation reaction and in the oxygen reduction reaction has been investigated by means of electrochemical techniques. The combined process, oxygen reduction reaction in the presence of methanol, has also been studied by electrochemical methods and in a single-cell. Irrespective of the performance of the samples in the oxygen reduction reaction, the ones displaying poor activity in the methanol oxidation reaction are the optimum cathode electrocatalysts for direct methanol fuel cell applications. The role of Au was found to be dependent on the actual nature of the catalyst. When alloyed, the role of Au on the methanol oxidation reaction is negligible. This is the first time that Au is being proposed as a component of methanol resistant cathode electrocatalysts.     

Effects of carbon pretreatment for oxygen reduction in alkaline electrolyte

The effects of different media on carbon pretreatments for oxygen reduction in alkaline electrolyte without application of active electrocatalysts were examined. Low surface area Vulcan XC-72 and high surface area Ketjenblack EC-300 were subjected to aqueous acid (flouric or formic), gaseous (H₂, N₂ or CO₂) and thermal treatments at 600 or 900 °C. Though non-scrubbed air was used, as a result of which carbonate build-up was high and peroxide concentration increased due to the electrode reaction, some of the electrodes resulted in life-tests of more than 2000 h at 65 °C, 6 M KOH with a constant load of 50 mA cm⁻² and intermittent polarisations at higher current densities. BET-surface areas and pH changes of more than 60% and weight losses of up to 15% of the carbon blacks were observed after the pretreatment steps. Electrochemical characterisation of the carbons showed that pretreatment steps of the carbon blacks have a significant effect on the long-term stability and activity of the gas diffusion electrodes in alkaline electrolyte.     

Platinum nanoparticles decorated Nb2CTx MXene as an efficient dual functional catalyst for hydrogen evolution and oxygen reduction reaction

Here, a dual functional Nb₂CT_x@Pt nanocomposite has been synthesized by in situ reduction method. The Pt loading in the composite has been optimized to get minimum overpotential (141 mV at 10 mA/cm²) for hydrogen evolution reaction (HER) along with a promising Tafel slope of 46.3 mV/dec, while Pt/C shows an overpotential and Tafel slope of 104 mV and 32.4 mV/dec, respectively. The Pt mass activity for Nb₂CT_x@Pt3.8 composite at 100 mV overpotential was 3.44 A g^?1 while the Pt mass activity for conventional Pt/C was 0.7 A g^?1, which shows that the activity of Nb₂CT_x@Pt3.8 composite is approximately 5 times higher than Pt/C. In addition, the catalyst was found to be stable for continuous 500 cycles without any binder molecules. The oxygen reduction reaction (ORR) capability of the material was also evaluated and found that the catalyst exhibited a current density of ?4.28 mA/cm² in the diffusion limiting region in comparison with the current density of ?5.82 mA/cm² for Pt/C at 2600 revolutions per minute (RPM). The Pt mass activity of Nb₂CT_x@Pt3.8 composite for ORR is approximately 10 times higher than Pt/C. The Nb₂CT_x@Pt3.8 composite was able to reduce O₂ completely using the 4-electron pathway with very little peroxide production. From these results, the dual functionality of the Nb₂CT_x@Pt3.8 composite for both HER and ORR has been established.     

N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte

In this work, a nitrogen-doped graphene (NG) catalyst was prepared using a hydrothermal method with ammonia as the nitrogen precursor, which was followed by a freeze-dry process. The catalyst was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy. The bifunctional catalytic activities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were investigated using cyclic voltammetry in an alkaline electrolyte. The results indicate that nitrogen is successfully doped in the NG catalyst, and the catalyst has a loose structure that was produced during the freeze-dry process. The catalyst exhibits an excellent ORR activity with an onset potential of −0.08 V and a high OER activity with an obvious OER current at 0.7 V. The rotating-disk-electrode test results indicate that the ORR process catalyzed by the NG catalyst involves a mix of the two-electron and four-electron transfer pathways. This work preliminarily explores the bifunctional catalytic properties for the ORR and the OER of nitrogen-doped graphene materials in alkaline electrolyte.     

Cobalt selenide electrocatalyst supported by nitrogen-doped carbon and its stable activity toward oxygen reduction reaction

We have prepared durable catalysts of CoSe₂/N-carbon using low-cost raw materials, measured their activities, peroxide yields, stabilities in reducing molecular oxygen, and characterized their crystalline phases and morphology. CoSe₂/N-carbon is featured with an active support, N-carbon, which by itself shows high stability as evidenced in its small activity decay. After 1000 CV cycles, the half-wave potential (E_1/2) of N-carbon decreases from 0.667 V to 0.636 V in 0.5 M H₂SO₄. Loading of CoSe₂ enhances the activity of N-carbon, when the samples were synthesized above 385 °C and formulated with the Se/Co ratio higher than 10. The higher activity is attributed to the pyrite phase of CoSe₂. But the stability of pyrite CoSe₂ is less than that of N-carbon. Corrosion during the stability test exposes the active sites of underlying N-carbon, which sustains the catalyst activity. Consequently the E_1/2 value of the active CoSe₂/N-carbon decreases moderately, from 0.711 V to 0.644 V after 1000 CV cycles. In contrast, the E_1/2 value of CoSe₂/C descends much more, from 0.681 V to 0.475 V.     

Recent innovations of silk-derived electrocatalysts for hydrogen evolution reaction,oxygen evolution reaction and oxygen reduction reaction

Numerous researches have proved that heteroatom-doping, especially N-doping, is able to enhance the electrocatalytic performance for carbon materials toward water electrolysis and oxygen reduction reaction. Hence, the production of N-doped carbon materials from cheap and earth-abundant precursor resources such as biomass materials has great potential in the application of these fields. Among various biomass precursors, silk, a N-rich protein, is an ideal candidate for the synthesis of N-doped carbon. Meanwhile, without addition of chemical N-rich precursors during preparation, N-doped carbon derived from silk is clean and easy to synthesize but possesses superb performance. Silk derived catalysts can exhibit overpotential of 61 mV @ 10 mA/cm² (49 mV @ 10 mA/cm² of commercial 20%Pt/C for comparison) and Tafel slope of 89 mV/dec towards hydrogen evolution reaction. To date, great progress has been made in the application of silk-derived catalysts to electrocatalysts, but there exists few summary work. Herein, we summarized recent progress on silk-derived electrocatalysts, focusing on their preparation process and their application on water electrolysis and oxygen reduction reactions. This review provides consolidated accounts of silk-derived catalysts and highlights ideas of their preparation and their performance.     

Catalyst-free growth of large scale nitrogen-doped carbon spheres as efficient electrocatalysts for oxygen reduction in alkaline medium

Nitrogen-doped carbon spheres (NCS) are synthesized by directly pyrolyzing a nebulized solution of xylene and ethylenediamine via a spray pyrolysis method, without using a catalyst. X-ray photoelectron spectroscopy (XPS) measurements confirm that the NCS only contain C, N and O. The electrocatalytic performances show that the NCS exhibit a high catalytic activity, long-term stability, and an excellent methanol tolerance for the oxygen reduction reaction (ORR) in an alkaline medium. This study successfully develops a new non-precious metal catalyst, which exhibits an excellent electrocatalytic performance, using a simple, cost-effective and scalable method. It also provides an increased fundamental understanding in the origins of ORR activity enhancements using N-doped carbon materials.     

Pd nanoparticles supported on WO3/C hybrid material as catalyst for oxygen reduction reaction

Pd nanoparticles supported on WO₃/C hybrid material have been developed as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells. The resultant Pd–WO₃/C catalyst has an ORR activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst prepared with the same method. Based on the physical and electrochemical characterizations, the improvement in the catalytic performance may be attributed to the small particle sizes and uniform dispersion of Pd on the WO₃/C, the strong interaction between Pd and WO₃ and the formation of hydrogen tungsten bronze which effectively promote the direct 4-electron pathway of the ORR at Pd.     

NiOx nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.     

Hybrid palladium-ceria nanorod electrocatalysts applications in oxygen reduction and ethanol oxidation reactions in alkaline media

Fossil fuel alternatives are being increasingly studied, and alkaline direct ethanol fuel cells (ADEFC) have acquired importance, as to ethanol is a renewable fuel. In this context, the aims of the present study were to synthesize, characterize and evaluate electrocatalytic activity in oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) using hybrid electrocatalysts based on Pd nanoparticles and CeO₂ nanorods supported on carbon black for application in ADEFC. The highest OCV, maximum current and power densities obtained using Pd₁₅(CeO₂ NR)₁₀(Vn)₇₅ as the cathode and Pd₁₀(CeO₂ NR)₂₀(Vn)₇₀ as the anode were 1270 mV, 190 mA cm^?2 and 65 mW cm^?2, respectively. These interesting results are justified by the highest I_D/I_G ratio and ECSA, which suggest a high number of oxygenated species, defects and vacancies in these electrocatalysts and by the synergistic effect between CeO₂ NR and Pd nanoparticles. Therefore, these hybrid electrocatalysts are promising for ADEFC applications.     

High-yield, size-controlled synthesis of silver nanoplates and their applications as methanol-tolerant electrocatalysts in oxygen reduction reaction

A novel method for the synthesis of as-prepared Ag nanoplates in high yield and the control of their dimensions has been developed. In this method, hexadecyltrimethyl ammonium ions (CTA⁺) are used as a trace additive in a seed solution for blocking the seed surface to govern the growth direction on nanoplate in the growth pathway, leading to a high-yield production of the Ag nanoplates with mixed morphologies, mainly triangular nanoplates and nanodisks. The spectra of the obtained nanoplate solution showed a high-intensity peak attributed to the in-plane dipole resonance and a low-intensity peak at 400 nm. By decreasing the amount of CTA⁺, the mean edge length of triangular nanoplates could be changed from ∼78.7 nm-∼124.8 nm. The in-plane dipole resonance peak corresponding to change in the mean edge length shifted from 630 nm to 785 nm, respectively. The mean edge length of triangular nanoplates could also be controlled from 70 nm to 148 nm by decreasing the CTA⁺-adsorbed seed amount. To investigate the practical feasibility of application of the proposed method, the prepared nanoplates were used as a methanol-tolerant electrocatalyst in an oxygen reduction reaction (ORR). An analysis conducted using a rotating ring-disk electrode showed that these nanoplates have high activity towards the ORR and that the electron transfer numbers (n) were 3.85, 3.83, 3.81, and 2.94 for 70 nm, 124 nm, 148 nm nanoplates, and macroscopic Ag electrode, respectively. If the present of methanol, the corresponding n values of 3.82, 3.81, 3.78, and 2.30 were detected. Despite working in the methanol-tolerant solution, the prepared Ag nanoplates still exhibited high electroactivity and their ORR proceeded via an approaching 4-electron pathway.     

Nitrogen and sulfur co-doped graphene supported PdW alloys as highly active electrocatalysts for oxygen reduction reaction

Nitrogen and sulfur co-doped graphene (NSG) is prepared by a facile microwave irradiation method and palladium-tungsten (PdW) alloy nanoparticles are supported on the NSG substrate. Several techniques, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and scanning electrochemical microscopy etc. are used to characterize the physical and electrochemical properties of the as-prepared samples. It is found that the PdW alloy nanoparticles are uniformly dispersed on the surface of NSG and the electrochemical performance of PdW/NSG is much better than those of Pd/NSG and Pd/G. The reason for the improved electrochemistry performance of PdW/NSG is considered to be the strong interactions and synergetic effects between PdW nanoparticles and NSG.