Electrochemical CO2 reduction is a key reaction for CO2 conversion to valuable fuels and chemicals. Because of the high stability of the CO2 molecule, a catalyst is typically required to minimize the energy input and improve reaction rates needed for device level commercialization. In this paper, we report a nanostructured Zn dendrite catalyst that is able to electrochemically reduce CO2 to CO in an aqueous bicarbonate electrolyte with greatly enhanced properties. The catalytic activity is over an order of magnitude higher than that of bulk Zn counterparts, with a CO faradaic efficiency around three-fold higher. The stability of the Zn electrode under realistic CO2 electrolysis conditions was explored using scanning electron microscopy and in situ/operando Xray absorption spectroscopy techniques. The results clearly demonstrate that nanostructured and bulk Zn catalysts are structurally stable at potentials more negative than -0.7 V vs. RHE, while severe chemical oxidation occurs at more positive potentials.
A robust and efficient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-of-the-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.
The sluggish kinetics of methanol
oxidation reaction (MOR) is a
major barrier to the commercialization of direct methanol fuel cells
(DMFCs). In this work, we report a facile synthesis of platinum–ruthenium
nanotubes (PtRuNTs) and platinum–ruthenium-coated copper nanowires
(PtRu/CuNWs) by galvanic displacement reaction using copper nanowires
as a template. The PtRu compositional effect on MOR is investigated;
the optimum Pt/Ru bulk atomic ratio is about 4 and surface atomic
ratio about 1 for both PtRuNTs and PtRu/CuNWs. Enhanced specific MOR
activities are observed on both PtRuNTs and PtRu/CuNWs compared with
the benchmark commercial carbon-supported PtRu catalyst (PtRu/C, Hispec
12100). X-ray photoelectron spectroscopy (XPS) reveals a larger extent
of electron transfer from Ru to Pt on PtRu/CuNWs, which may lead to
a modification of the d-band center of Pt and consequently a weaker
bonding of CO (the poisoning intermediate) on Pt and a higher MOR
activity on PtRu/CuNWs.
The Na content of (Ag,Cu)(In,Ga)Se2 films was cyclically adjusted using a novel method involving cycles of water rinsing at 60 °C followed by heating in air at 200 °C to remove Na and evaporation of NaF to re-introduce Na back into the film. The low temperatures and short heating times ensure that Na is removed only from grain boundaries while leaving grain interiors unaffected. Cross-grain conductivity and Seebeck coefficient were measured during this removal procedure and both measurements decreased when Na was removed and both recovered upon the re-addition of Na, consistent with an increase in compensating donor defects in the absence of Na. These results demonstrate that Na reversibly affects the electrical properties of grain boundaries. We propose that Na reversibly passivates donor-like defects such as InCu double donors at grain boundaries.
The incorporation of sodium from sodium fluoride in single-crystal CuInSe2 (CIS) is investigated to provide insight into the intra-granular aspects of sodium incorporation in CIS-based thin films. Sodium was incorporated by evaporating NaF onto two CIS crystals of varying compositions and defect structures followed by heating under vacuum. Diffusion profiles show a near-surface reaction before a deeper diffusion zone which follows a complementary error function, confirming Na diffusion into the crystals. Transmission electron microscopy analysis indicates that dislocations do not control the diffusion process. The activation energy of diffusion is ∼0.7 eV for both crystals. This low activation energy suggests that Na diffusion occurs rapidly through the bulk at temperatures as low as 300 °C and helps explain the uniform Na concentration often observed in grain interiors of polycrystalline Cu(InGa)Se2 thin films.
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