PdP/WO3 multi-functional catalyst with high activity and stability for direct liquid fuel cells (DLFCs)

Direct liquid fuel cells are energy conversion devices which utilize formate and methanol as fuels. These systems are relieved of the problem of H2transport and storage, making them highly desirable for various practical applications. However, the low stability and activity of carbon supported catalysts such as Pt/C both in the anode and cathode is a critical hindering factor towards their further development. As a practical solution to overcome this issue, in this work, we report on the development of phosphorus-doped palladium (PdP) nanoparticle-supported tungsten oxide (WO3) nanorods (PdP/WO3) as a versatile multifunctional catalyst for facilitating the oxidation of formate and methanol in the anode and the oxygen reduction reaction (ORR) in the cathode. Strong metal-support interactions and electronic modifications incurred by the doped phosphorus help this system to achieve desirable properties to enable it to effectively function both for the anode and cathode applications. PdP/WO3showed 16-times higher mass activity compared to Pt/C even after 3000 start/stop cycles for the ORR. For formate and methanol oxidation, PdP/WO3exhibited current densities of 0.50 and 0.734 A mgPd−1, respectively, outperforming thestate-of-the-artcatalysts. With these bifunctional features, PdP/WO3stands out as a potential system to be used as an anode and cathode catalyst in direct liquid fuel cells, all the while offering an opportunity for the development of carbon-free electrocatalysts

Zirconium-Substituted Cobalt Ferrite Nanoparticle Supported N‑doped Reduced Graphene Oxide as an Efficient Bifunctional Electrocatalyst for Rechargeable Zn–Air Battery

Solvothermal synthesis of zirconium-substituted cobalt ferrite nanoparticles was accomplished by the introduction of zirconium (Zr) in the spinel matrix to obtain a cost-effective and robust electrocatalyst that does not use noble metals. A variation in the cobalt ferrite structure CoFe_2–<i>x</i>Zr_<i>x</i>O₄ with Zr (0.1–0.4) substitution has significantly altered the overpotential for the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), leading to an optimum composition of CFZr(0.3). The incorporation of the foreign Zr⁴⁺ ion in the cobalt ferrite spinel lattices has effectively enhanced the oxygen evolution reaction (OER) activity in comparison to the parent cobalt ferrite (CF) nanocrystals. However, a nominal change in the ORR current density has been observed due to Zr incorporation. For the OER, the Zr-substituted catalyst has shown a 40 mV negative shift in the overpotential in comparison with the CF nanoparticles at 10 mA/cm² current density. Interestingly, the in situ grafting of Zr-substituted cobalt ferrite nanoparticles over N-doped reduced graphene oxide (CFZr(0.3)/N-rGO) results in remarkably enhanced performance during the ORR and moderately favored the OER with an overall potential difference (Δ<i><i>E</i></i>) of 0.840 V. The enhanced bifunctional electrocatalytic activity of the material is crucial for the fabrication of high-performance rechargeable Zn–air batteries (ZABs). The prepared catalyst exhibited an overpotential of 80 mV for the ORR in comparison with the state-of-the-art (Pt/C) catalyst and an overpotential of 340 mV at 10 mA/cm² current density for the OER from the standard value (1.23 V vs RHE). This potential bifunctional electrocatalyst has been employed as an electrode material for the fabrication of a primary ZAB, where it exhibited discharge capacities of 727 and 730 mAh/g at current densities of 20 and 30 mA/cm², respectively, under ambient conditions. The notable performance of the catalyst as a bifunctional material is observed during the cycling of the rechargeable ZAB. The prepared catalyst showed an increase of 200 mV in the overall operating overpotential after cycling for 10 cycles at 15 mA/cm² in comparison to the 350 mV increase shown by the Pt/C catalyst

Cu–Pt Nanocage with 3‑D Electrocatalytic Surface as an Efficient Oxygen Reduction Electrocatalyst for a Primary Zn–Air Battery

Cu–Pt nanocage (CuPt-NC) intermetallic structures have been prepared by an in situ galvanic displacement reaction. The structures are found to be well organized within the framework demarcated with distinguishing arms, having clear edges and corners with a size of ∼20 nm. The unique nanocage structure possessing large specific surface area and better structural integrity helps to achieve improved electrochemical oxygen reduction reaction activity and stability in alkaline solution in comparison to the commercially available 20 wt % Pt/C. CuPt-NC shows 50 mV positive onset potential shift with significantly higher limiting current in comparison to Pt/C. Interestingly, CuPt-NC has shown 2.9- and 2.5-fold improved mass activity and specific activity, respectively, for ORR at 0.9 V vs RHE in comparison to Pt/C. Moreover, the stability of CuPt-NC has been tested by an accelerated durability test under alkaline conditions. CuPt-NC has been subsequently utilized as the air electrode in a primary Zn–air battery and is found to possess 1.30- and 1.34-fold improved power density and current density at 1 V, respectively, in comparison to the state-of-the-art Pt/C catalyst. In addition, CuPt-NC has shown several hours of constant discharge stability at 20 mA cm^–2 with a specific capacity of 560 mAh g_Zn^–1 and energy density of 728 Wh kg_Zn^–1 in the primary Zn–air battery system

Electrochemical preparation of nitrogen-doped graphene quantum dots and their size-dependent electrocatalytic activity for oxygen reduction

Here we report a remarkable transformation of nitrogen-doped multiwalled carbon nanotubes (MWCNTs) to size selective nitrogen-doped graphene quantum dots (N-GQDs) by a two-step electrochemical method. The sizes of the N-GQDs strongly depend on the applied anodic potential, moreover increasing potential resulted in a smaller size of N-GQDs. These N-GQDs display many unusual size-dependant optoelectronic (blue emission) and electrocatalytic (oxygen reduction) properties. The presence of N dopants in the carbon framework not only causes faster unzipping of MWCNTs but also provides more low activation energy site for enhancing the electrocatalytic activity for technologically daunting reactions like oxygen reduction. The smaller size of N-GQDs has shown better performance as compared to the large N-GQDs. Interestingly, N-GQDs-3 (size = 2.5 ± 0.3 nm, onset potential = 0.75 V) show a 30-mV higher positive onset potential shift compared to that of N-GQDs-2 (size = 4.7 ± 0.3 nm, onset potential = 0.72 V) and 70 mV than that of N-GQDs-1 (size = 7.2 ± 0.3, onset potential = 0.68 V) for oxygen reduction reaction (ORR) in a liquid phase. These result in the size-dependent electrocatalytic activity of N-GQDs for ORR as illustrated by the smaller sized N-GQDs (2.5 ± 0.3 nm) undoubtedly promising metal-free electrocatalysts for fuel cell application

Pt-Anchored-Zirconium Phosphate Nanoplates as High-Durable Carbon-Free Oxygen Reduction Reaction Electrocatalyst for PEM Fuel Cell Applications

Commercially available platinum-supported carbon (Pt/C) catalysts are the most widely used oxygen reduction reaction (ORR) electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs). However, inadequate active triple-phase boundary formation and carbon oxidation in Pt/C during PEMFC operation shorten its lifetime and efficiency. In this direction, a new class of carbon-free electrocatalysts for ORR is prepared by dispersing Pt nanoparticles on ZrP (Zirconium phosphates) nanoplates. In one case (ZrP@Pt), the Pt nanoparticles are found to be closely distributed and completely covering the ZrP nanoplates, whereas in the second case (Pt/ZrP), the Pt nanoparticles selectively restrict dispersion along the edges of the support. ZrP as the support displays an intrinsic proton conductivity of ≈0.5 × 10−4 S cm−1 at 70 °C, with an activation energy (Ea) of 0.19 eV. Pt/ZrP shows better durability after 3000 start-stop cycles. The mass activity of Pt/ZrP is increased by 4.6 times compared to Pt/C, which exhibits a loss in mass activity by 1.37 times. The single-cell level validation of ZrP@Pt, Pt/ZrP, and Pt/C as the electrocatalysts in PEMFC at an operating potential of 0.60 V shows the achievable current densities of 0.600, 0.890, and 0.890 A cm−2