The oxygen reduction reaction (ORR) plays a crucial role in electrochemical energy conversion and storage devices such as alkaline fuel cells and metal–air batteries. These systems, which could ...employ non-platinum catalysts for oxygen reduction, are cheaper and stable alternatives to their expensive counterparts like proton exchange membrane fuel cells (PEMFCs) working on platinum based catalysts. Various binary and ternary silver nanoalloys have been reported to act as efficient electrocatalysts for the ORR in alkaline fuel cells and batteries. Herein, we present a critical review on the recent advances made in silver nanoalloy electrocatalysts for the ORR in alkaline media. The mechanism of ORR on nanoalloys is described; the effect of structure and composition of various silver nanoalloys (including Ag–Cu, Ag–Pd, Ag–Au, Ag–Co etc. ) on their ORR activity and stability is discussed. The rational design of electrocatalysts in order to maximize the number of catalytically active sites on the surface of the electrocatalysts for the ORR is also reviewed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.
The exploration and rational design of cost-effective, highly active, and durable catalysts for oxygen electrochemical reaction is crucial to actualize the prospective technologies such as metal–air ...batteries and fuel cells. Herein manganese cobalt oxide nanoparticles anchored on carbon nanofibers and wrapped in a nitrogen-doped carbon shell (MCO/CNFs@NC) is successfully prepared. Benefiting from the synergistic effect between the core nanoparticles and nitrogen-doped carbon shell, MCO/CNFs@NC catalyst exhibits oxygen reduction reaction (ORR) activity with comparable onset potential (1.00 V vs RHE) and half-wave potential (0.76 V vs RHE) which is only about 40 mV lower than that of the state of art Pt/C catalyst. Furthermore, the MCO/CNFs@NC catalyst exceeds the Pt/C catalyst by a great margin in terms of stability in alkaline media. Additionally, MCO/CNFs@NC catalyst is strongly tolerant to methanol crossover, promising its applicability as cathode catalyst in alcohol fuel cells. Moreover, MCO/CNFs@NC catalyst exhibits the oxygen evolution reaction (OER) activity with low overpotential of 0.41 V at the current density of 10 mA cm–2 and ORR/OER potential gap (ΔE) as low as 0.88 V, suggesting its strong bifunctionality. The Zn–air battery based on MCO/CNFs@NC catalyst is found to deliver a specific capacity of 695 mA h g–1 Zn and an energy density of 778 W h kg–1 Zn at a current density of 20 mA cm–2. The mechanically rechargeable Zn–air battery based on MCO/CNFs@NC catalyst is also found to function continually by only reloading the consumed Zn anode and electrolyte. Furthermore, the electrically rechargeable battery based on MCO/CNFs@NC catalyst is found to function for more than 220 cycles with negligible loss of voltaic efficiency. Moreover, MCO/CNFs@NC is found to display a supercapacitive nature with a good discharge capacity of 478 F g–1 at a discharge current density of 1 A g–1.
Highly active electrocatalysts with a novel bimetallic arrangement of atoms for the oxygen reduction reaction (ORR) are vital for the commercialization of fuel cells. An accurate understanding of the ...origin of activity enhancement is essential for exploiting any novel bimetallic catalyst. In this work, the reaction activation energy, reaction free energy and half-wave potential of AgCu alloys for the oxygen reduction have been investigated through both theoretical and experimental methods. The reaction activation energies on the pure Ag, core-shell Ag/Ag3Cu and alloy Ag3Cu are 1.097, 0.341 and 1.317 eV, respectively. The ORR activity is improved on core-shell Ag/Ag3Cu but deteriorated on alloy Ag3Cu nanoparticles in terms of the energy barrier for the rate-determining step during the ORR. The working potentials of pure Ag, core-shell Ag/Ag3Cu and alloy Ag3Cu are predicted to be 0.737, 0.761 and 0.675 V, respectively, indicating that the core-shell Ag/Ag3Cu nanoparticles provide the highest working potential and the lowest overpotential, which is comparable to that of the Pt(111) facets. AgCu bimetallic catalysts were prepared through the pulsed laser deposition, where the core-shell AgCu catalysts showed greater ORR activity than the alloy AgCu catalysts, which is consistent with the density functional theory calculations. Results not only indicate that a core-shell atom order should be designed for AgCu bimetallic nanoparticles to enhance their ORR activity but also provide a fundamental insight into the reason behind the synergetic effects in bimetallic catalysts.
Silver nanoparticles supported on cobalt and nitrogen embedded reduced graphene oxide, Ag/Co-NGr, are synthesized by one-step hydrothermal route with remarkable catalytic activity for oxygen ...reduction reaction (ORR). As-synthesized electrocatalyst exhibits half-wave potential (0.82 V) comparable to commercial Pt/C (0.85 V), specific activity (0.45 mA cm−2) better than commercial Pt/C (0.35 mA cm−2) along with superior stability in alkaline environment (≈95% activity retention after 5000s compared to 80% for Pt/C). Moreover, Ag/Co-NGr is highly tolerant to methanol poisoning during ORR and delivers an excellent specific capacity of 789 mAh.g−1Zn with energy density of 947 Wh. kg−1 at a current density of 20 mA cm−2 in a Zn-air battery. For the first time, it is proven that particle size refinement and electronic perturbation of Ag nanoparticles take place due to metal-support interactions between Ag and Co/NGr. d-band center of Ag in Ag/Co-NGr upshifts toward the Fermi level with respect to the Ag/NGr as a result of charge transfer between Ag and Co/NGr. The superior catalytic activity and excellent stability of Ag/Co-NGr is attributed to the structural and electronic modification of Ag nanoparticles by Co and N elements on graphene.
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•Ag nanoparticles on Co, N embedded graphene are prepared by hydrothermal route.•Ag develops into crystalline morphology while cobalt forms amorphous oxide.•As-synthesized material show superior catalytic activity and stability during ORR.•Particle size refinement and d-band center shift of Ag improves its ORR activity.•Ag/Co-NGr is methanol tolerant and functions with stable profile in Zn-air battery.
The electrocatalytic activity of Pt‐based alloys exhibits a strong dependence on their electronic structures, but a relationship between electronic structure and oxygen reduction reaction (ORR) ...activity in Ag‐based alloys is still not clear. Here, a vapor deposition based approach is reported for the preparation of Ag75M25 (M = Cu, Co, Fe, and In) and AgxCu100−x (x = 0, 25, 45, 50, 55, 75, 90, and 100) nanocatalysts and their electronic structures are determined by valence band spectra. The relationship of the d‐band center and ORR activity exhibits volcano‐shape behaviors, where the maximum catalytic activity is obtained for Ag75Cu25 alloys. The ORR enhancement of Ag75Cu25 alloys originates from the 0.12 eV upshift in d‐band center relative to pure Ag, which is different from the downshift in the d‐band center in Pt‐based alloys. The activity trend for these Ag75M25 alloys is in the order of Ag75Cu25 > Ag75Fe25 > Ag75Co25. These results provide an insight to understand the activity and stability enhancement of Ag75Cu25 and Ag50Cu50 catalysts by alloying.
The oxygen reduction reaction (ORR) activity trends of binary silver alloy nanocatalysts are discussed, which are highly dependent on the d‐band center of valence band spectrum. The results present that the upshift d‐band center is beneficial for ORR and the optimal Ag50Cu50 catalyst (0.1 eV) shows high activity and durability for ORR in alkaline media, which is comparable to the commercial Pt/C‐20% catalyst.
Development of highly active and stable Pt‐free oxygen reduction reaction catalysts from earth‐abundant elements remains a grand challenge for highly demanded metal–air batteries. Ag‐based alloys ...have many advantages over platinum group catalysts due to their low cost, high stability, and acceptable oxygen reduction reaction (ORR) performance in alkaline solutions. Nevertheless, compared to commercial Pt/C‐20%, their catalytic activity still cannot meet the demand of commercialization. In this study, a kind of catalysts screening strategy on AgxCu100−x nanoalloys is reported, containing the surface modification method, studies of activity enhancement mechanism, and applied research on zinc–air batteries. The results exhibit that the role of selective dealloying (DE) or galvanic displacement (GD) is limited by the “parting limitation”, and this “parting limitation” determines the surface topography, position of d‐band center, and ORR performance of AgxCu100−x alloys. The GD‐Ag55Cu45 and DE‐Ag25Cu75 catalysts alloys present excellent ORR performance that is comparable to Pt/C‐20%. The relationship between electronic perturbation and specific activity demonstrates that positive shift of the d‐band center (≈0.12 eV, relative to Ag) for GD‐Ag55Cu45 is beneficial for ORR, which is contrary to Pt‐based alloys (negative shift, ≈0.1 eV). Meanwhile, extensive electrochemical and electronic structure characterization indicates that the high work function of GD‐Ag55Cu45 (4.8 eV) is the reason behind their excellent durability for zinc–air batteries.
A platform guideline for designing Ag–Cu electrocatalysts is reported. The screened Ag–Cu electrocatalysts exhibit not only high oxygen reduction reaction (ORR) activity, but also superior stability. The ORR performance and working mechanism is investigated, and the application in zinc–air batteries is demonstrated, promising great potential for portable electronic devices.
Highly active electrocatalysts with good long term stability are vital for the commercialization of metal air batteries and alkaline fuel cells which involve the oxygen reduction reaction (ORR) at ...the cathode end. Herein, we developed a pulsed laser deposition (PLD) technique for the precise fabrication of silver-copper metallic glass (AgCu-MG) electrocatalysts. This PLD technique provides excellent control over the surface microtopography along with high flexibility for the deposition of different compositions of silver-copper metallic glass electrocatalysts onto nickel foam. Among all investigated Ag-based catalysts, AgCu-MG catalysts exhibit high electrocatalytic activity with a half-wave potential of 0.67 V (vs. RHE) which can be in situ enhanced by dealloying treatment in N sub(2) saturated 0.1 M KOH solution. In situ dealloying of the AgCu-MG provides exceptional ORR catalytic activity with a half-wave potential of 0.78 V (vs. RHE) at 1600 rpm, which is comparable to 0.81 V (vs. RHE) of commercial Pt/C-20%. The AgCu-MG electrocatalyst showed excellent long-term stability in rechargeable zinc-air batteries. After 1000 charge-discharge cycles at 20 mA cm super(-2), the discharge voltage of batteries was stable at 1.0 V demonstrating the potential application of AgCu-MG as an alternative to Pt/C-20% in zinc-air batteries and alkaline fuel cells.
This research presents an experimental study of punching shear strength of reactive powder concrete (RPC) square and trapezoidal flat slabs. Reactive powder concrete is an ultra-high strength and ...high ductility composite materials in form of a super plasticized cement mixture with silica fume and steel fibers. Six reduced scale reinforced concrete slab specimens divided into two groups (square and trapezoidal slabs) were casted and tested in this study. Each group consists of three specimens which are identical in size and shape but contains different percentages of steel fibers (0, 0.5 and 1) % of total volume. Results indicated that, punching shear strength increases by about (62.5 and 100) % in square slabs and is about (8.3 and 41.7) % in trapezoidal slabs containing 0.5% and 1% of steel fibers respectively.
Highly active electrocatalysts with a novel bimetallic arrangement of atoms for the oxygen reduction reaction (ORR) are vital for the commercialization of fuel cells. An accurate understanding of the ...origin of activity enhancement is essential for exploiting any novel bimetallic catalyst. In this work, the reaction activation energy, reaction free energy and half-wave potential of AgCu alloys for the oxygen reduction have been investigated through both theoretical and experimental methods. The reaction activation energies on the pure Ag, core-shell Ag/Ag
3
Cu and alloy Ag
3
Cu are 1.097, 0.341 and 1.317 eV, respectively. The ORR activity is improved on core-shell Ag/Ag
3
Cu but deteriorated on alloy Ag
3
Cu nanoparticles in terms of the energy barrier for the rate-determining step during the ORR. The working potentials of pure Ag, core-shell Ag/Ag
3
Cu and alloy Ag
3
Cu are predicted to be 0.737, 0.761 and 0.675 V, respectively, indicating that the core-shell Ag/Ag
3
Cu nanoparticles provide the highest working potential and the lowest overpotential, which is comparable to that of the Pt(111) facets. AgCu bimetallic catalysts were prepared through the pulsed laser deposition, where the core-shell AgCu catalysts showed greater ORR activity than the alloy AgCu catalysts, which is consistent with the density functional theory calculations. Results not only indicate that a core-shell atom order should be designed for AgCu bimetallic nanoparticles to enhance their ORR activity but also provide a fundamental insight into the reason behind the synergetic effects in bimetallic catalysts.
Highly active electrocatalysts with a novel bimetallic arrangement of atoms for the oxygen reduction reaction (ORR) are vital for the commercialization of fuel cells.
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