The effect of some sulfur-containing additives on the initial cathode overpotential during copper electrodeposition
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Relationships developed earlier (1) for the dependence of the rate of appearance and propagation of dendrites on the properties of the system and overpotential, have been used to develop a quantitative theory of induction period and critical overpotential of dendritic growth. It has been shown that the thermodynamic concept of critical overpotential is applicable to metals of high exchange current density only. The limitations in the appearance of dendrites in metals of low exchange current density are of kinetic character and can be represented by a kinetically defined critical overpotential. The theory has been verified experimentally by following the yield of dendritic deposit of zinc from alkaline zincate solutions as a function of concentration of depositing ions, of overpotential, and of time of deposition.
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Oxygen evolution reaction (OER) involves multiple electron-transfer processes, resulting in a high activation barrier. Developing catalysts with low overpotential and high intrinsic activity toward OER is critical but challenging. Here we report a major advancement in decreasing the overpotential for oxygen evolution reaction. Ni foam-supported Fe-doped β-Ni(OH)2 nanosheets achieve an overpotential of 219 mV at the geometric current density of 10 mA cm–2. To our knowledge, this is the best value reported for Ni- or Fe hydroxide-based OER catalysts. In addition, the catalyst yields a current density of 6.25 mA cm–2 at the overpotential of 300 mV when it is normalized to the electrochemical surface area of the catalyst. This intrinsic catalytic activity is also better than the values reported for most state-of-the-art OER catalysts at the same overpotential.
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An association of Cu with sulfide in aerobic natural waters has been attributed to these components' coexistence in clusters of sizes intermediate between mononuclear complexes and colloidal particles. This hypothesis is investigated here. Copper sulfide solid phases display size-related voltammetric behavior at Hg electrodes. Suspensions of copper sulfide powders held at accumulation potentials of 0 to −0.2 V (vs Ag/AgCl) produce voltammetric peaks near −0.15, −0.65, and −0.95 V during subsequent cathodic scans. The first two peaks arise from electrochemically generated Cu-oxyhydroxides and HgS; the −0.95 V peak arises from reduction of sorbed copper sulfide particles. Nanoparticles of radius ∼10-8 m produce the third peak even without stirring or accumulation. Still smaller analytes give only the first two peaks. Published evidence alleging production of subnanometer copper sulfide clusters during titrations of Cu2+ and HS- was not reproduced when sulfide oxidation was avoided. Instead, such titrations apparently generate nanoparticles. The titration stoichiometry is 1/1, consistent with previous descriptions of this process: Cu2+ + HS- → 1/2Cu2S·S0 (brown sol) → CuS (green sol). Titrating Cu2+ into organic-rich (muscilaginous) Adriatic Sea water, which contains 10-7 M natural thiols and sulfide, produces solid products. In the future, voltammetry might prove useful for studying semiconductor sulfide nanoparticles in nature.
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Copper sulfide
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Cobalt sulfide
Cadmium sulfide
Selected area diffraction
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In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe2. Solution phase deposition of 2H WSe2 nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M H2SO4 with an overpotential at −10 mA/cm2 of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerwein's salt ([Et3O][BF4]) results in a reduction in overpotential by approximately 130 mV at −10 mA/cm2. Second, we observe a decrease in overpotential of approximately 200–300 mV when the TMDC electrode is exposed to H+, Li+, Na+, or K+ ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li+ activated WSe2 NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of −10 mA/cm2. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement.
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The promise and challenge of electrochemical mitigation of CO2 calls for innovations on both catalyst and reactor levels. In this work, enabled by our high-performance and earth-abundant CO2 electroreduction catalyst materials, we developed alkaline microflow electrolytic cells for energy-efficient, selective, fast, and durable CO2 conversion to CO and HCOO–. With a cobalt phthalocyanine-based cathode catalyst, the CO-selective cell starts to operate at a 0.26 V overpotential and reaches a Faradaic efficiency of 94% and a partial current density of 31 mA/cm2 at a 0.56 V overpotential. With a SnO2-based cathode catalyst, the HCOO–-selective cell starts to operate at a 0.76 V overpotential and reaches a Faradaic efficiency of 82% and a partial current density of 113 mA/cm2 at a 1.36 V overpotential. In contrast to previous studies, we found that the overpotential reduction from using the alkaline electrolyte is mostly contributed by a pH gradient near the cathode surface.
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