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An unsupported Cu-Pt core-shell catalyst is prepared by a transmetalation reaction between copper and Pt 2+ ions, and a Cu-Pt bimetallic alloy catalyst by a simultaneous reduction reaction. Both catalysts are subjected to electrochemical leaching without further treatment and their electrochemical characteristics and ORR activities are compared to that of a standard Pt black catalyst. Potential cycling in 0.1 M HClO 4 and 0.5 M H 2 SO 4 shows that the core-shell catalyst is highly stable and the electrochemical features show that it is purely Pt on the catalyst surface. The electrochemical surface area of the Cu-Pt core-shell catalyst is much higher than that of the Pt black. When the Pt loading on the electrode is increased with Pt black catalyst to match its geometric electrochemical surface area (cm 2 /electrode area (cm 2 )) with that of Cu-Pt core-shell catalyst, the activity of the latter is still higher. The mass activities of Pt black, Cu-Pt binary alloy, and Cu-Pt core-shell catalysts measured using a rotating disk electrode are 0.053, 0.153 and 0.262 A mg −1 Pt , and their respective specific activities are 197, 496, and 710 μA cm −2 Pt .

Oxygen reduction reaction (ORR) is investigated on bulk PdO-based catalysts (oxides of Pd and Pd3Co) in oxygen-saturated 0.1 M HClO4 to establish the role of surface oxides and adsorbed hydrogen in the activity and product selectivity (H2O/H2O2). The initial voltammetric features suggest that the oxides are inactive toward ORR. The evolution of the ORR voltammograms and potential-dependent H2O2 generation features on the PdO catalyst suggest gradual and parallel in situ reduction of the bulk PdO phase below ∼0.4 V in the hydrogen underpotential deposition (Hupd) region; the reduction of the bulk PdO catalyst is confirmed from the X-ray photoelectron spectra (XPS) and X-ray diffraction (XRD) patterns. The potential-dependent H2O2 generation features originate due to the presence of surface oxides and adsorbed hydrogen; this is further confirmed using halide ions (Cl(-) and Br(-)) and peroxide as the external impurities.

Layered Sn-based chalcogenides and heterostructures are widely used in batteries and photocatalysis, but its utilizations in a supercapacitor is limited by its structural instability and low conductivity. Here, SnS x thin films are directly and conformally deposited on a three-dimensional (3D) Ni-foam (NF) substrate by atomic layer deposition (ALD), using tetrakis(dimethylamino)tin [TDMASn, ((CH 3 ) 2 N) 4 Sn] and H 2 S that serves as an electrode for supercapacitor without any additional treatment. Two kinds of ALD-SnS x films grown at 160 °C and 180 °C are investigated systematically by X-ray diffractometry, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM). All of the characterization results indicate that the films deposited at 160 °C and 180 °C predominantly consist of hexagonal structured-SnS 2 and orthorhombic-SnS phases, respectively. Moreover, the high-resolution TEM analyses (HRTEM) reveals the (001) oriented polycrystalline hexagonal-SnS 2 layered structure for the films grown at 160 °C. The double layer capacitance with the composite electrode of SnS x @NF grown at 160 °C is higher than that of SnS x @NF at 180 °C, while pseudocapacitive Faradaic reactions are evident for both SnS x @NF electrodes. The superior performance as an electrode is directly linked to the layered structure of SnS 2 . Further, the optimal thickness of ALD-SnS x thin film is found to be 60 nm for the composite electrode of SnS x @NF grown at 160 °C by controlling the number of ALD cycles. The optimized SnS x @NF electrode delivers an areal capacitance of 805.5 mF/cm 2 at a current density of 0.5 mA/cm 2 and excellent cyclic stability over 5000 charge/discharge cycles.

Carbon-supported Pd and Pd3Co catalysts have been electrochemically characterized in 0.1 M HClO4 solution and we found that both catalysts were unstable. On repeated potential cycling, the electrochemical surface area of the catalysts decreases and the oxygen reduction reaction (ORR) activity suffers. To stabilize surface Pd atoms of both Pd and Pd3Co catalysts, we deposited Pt using adsorbed hydrogen on the catalytically active Pd sites. The Pt : Pd ratio of Pt-coated Pd and Pt-coated Pd3Co catalysts suggests half-a-monolayer coverage of Pt (two hydrogen atoms required for reducing a Pt(2+) ion). The Pt : Pd ratio of Pt-coated Pd3Co catalyst obtained from the simple geometrical hard sphere model, energy-dispersive X-ray spectroscopy (EDS) line scan and bulk EDS agrees very well with that calculated from the hydrogen desorption (H(des)) charge of Pd3Co. At the same time, the Pt : Pd ratio of Pt-coated Pd calculated from the H(des) charge of Pd catalyst is significantly lower than the ratio obtained from the other methods. Thus, the Pt : Pd ratio of the Pt-coated Pd catalyst estimated from the H(des) region of Pd is an underestimation of the composition. This suggests that Pd forms an electrochemically inactive species from the H(upd) region itself and Co in Pd3Co seems to stabilize Pd against oxidation by delaying the formation of electrochemically inactive species to higher potentials above the H(upd) region. The voltammograms along with the peroxide formation characteristics of the catalysts support the above observations. The deposited Pt on the surface of the Pd and Pd3Co catalysts masks active Pd sites from the electrochemical environment and even partial coverage with Pt improves the stability and ORR activity of the catalysts when compared to that of the respective Pt-free counterparts.

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Future realization of a hydrogen-based economy requires a high-surface-area, low-cost, and robust electrocatalyst for the hydrogen evolution reaction (HER). In this study, the MoN x thin layer is synthesized on to a high-surface-area three-dimensional (3D) nickel foam (NF) substrate using atomic layer deposition (ALD) for HER catalysis. MoN x is grown on NF by the sequential exposure of Mo­(CO)6 and NH3 at 225 °C. The thickness of the thin film is controlled by varying the number of ALD cycles to maximize the HER performance of the MoN x /NF composite catalyst. The scanning electron microscopy and transmission electron microscopy (TEM) images of MoN x /NF highlight that ALD facilitates uniform and conformal coating. TEM analysis highlights that the MoN x film is predominantly amorphous with the nanocrystalline MoN grains (4 nm) dispersed throughout it. Moreover, the high-resolution (HR)-TEM analysis shows a rough surface of the MoN x film with an overall composition of Mo0.59N0.41. X-ray photoelectron spectroscopy depth-profile analysis reveals that oxygen contamination is concentrated at the surface because of surface oxidation of the MoN x film under ambient conditions. The HER activity of MoN x is evaluated under acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) conditions. In an acidic electrolyte, the sample prepared with 700 ALD cycles exhibits significant HER activity and a low overpotential (η) of 148 mV at 10 mA cm–2. Under an alkaline condition, it achieves 10 mA cm–2 with η of 125 mV for MoN x /NF (700 cycles). In both electrolytes, the MoN x thin film exhibits enhanced activity and stability because of the uniform and conformal coating on NF. Thus, this study facilitates the development of a large-area 3D freestanding catalyst for efficient electrochemical water-splitting, which may have commercial applicability.

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