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• Perchlorates in environment rise concerns, sensitive determination techniques needed. • Analysis and critical evaluation of published electrochemical perchlorate sensors. • Voltammetric methods superior over „classical" potentiometric ISE. • Electrochemical impedance and capacitance sensors - the lowest detection limit. • Capability of electrochemical sensors to replace ion/mass chromatography shown. Growing concerns about adverse effects of perchlorate (ClO 4 −) together with its relatively frequent occurrence in the environment demand sensitive analytical methods. It is also anticipated that ClO 4 − sensors will find their application in future extra-terrestrial missions. Ion chromatography (IC) and mass spectroscopy (MS) provide sufficiently low detection limit (LOD) but lack transportability and field analysis possibility that are provided by most electrochemical analytical methods. On the other side, commercially available potentiometric ion-selective electrodes (ISEs) cannot compete with IC or MS in terms of sensitivity. To answer the question whether (and how) such drawback can be overcome, in this paper electrochemical methods of ClO 4 − determination are introduced and evaluated. Starting with potentiometric ISEs which can theoretically reach LOD suitable for assessment of WHO-set limits of ClO 4 −, it is further shown that ISEs operating in voltammetric mode are superior in terms of sensitivity. Next section covers impedimetric and capacitance ClO 4 − sensors, concluding that they can reach detection limits of IC or MS while retaining their advantages. In further sections related issues are discussed including biosensing or direct electrochemical reduction of perchlorate as well as diversity of (nano)materials applicable for perchlorate sensors preparation. This critical review thus shows that electrochemical ClO 4 − sensors can reach sensitivity of standard methods while retaining their portability and ease of operation. [ABSTRACT FROM AUTHOR]

The oxygen-evolution reaction (OER) is pivotal in many energy-conversion technologies as it is an important counter reaction to others that convert stable chemicals to higher-value products using electrochemistry. The local microenvironment and pH for the anode OER can vary from acidic to neutral to alkaline depending on the system being explored, making definitive mechanistic insights difficult. In this paper, we couple experiments, first-principles calculations based on density functional theory, microkinetics, and transport modeling to explore the entire pH range of the OER. At low current densities, neutral pH values unexpectedly perform better than the acidic and alkaline conditions, and this trend is reversed at higher current densities (> 20 mA cm−2). Using multiscale modeling, this switch is rationalized by a change from a dual-reaction mechanism to a single rate-determining step. The model also shows how the alkaline reaction rates dominate in the middle to high pH range. Furthermore, we explore that the local pH for near-neutral conditions is much different (e.g., 2.4 at the reaction surface vs. 9 in the bulk) than the pH extremes, demonstrating the criticality that transport phenomena plays in kinetic activity. [ABSTRACT FROM AUTHOR]

• Electrochemical behavior of Co(III)-catalyzed gold thiosulfate leaching was studied. • Co(NH 3) 6 3+ showed higher coulombic efficiencies of gold dissolution than Cu(NH 3) 4 2+. • Co(NH 3) 6 3+ achieved continuous gold leaching with much reduced S 2 O 3 2− consumption. • Stable mixed ligand complexes of Co2+ with ammonia and thiosulfate were formed. • The cycle between Co(NH 3) 6 3+ and Co(NH 3) x (S 2 O 3)0 (x = 4, 5) played the catalytic role. Thiosulfate is a green gold lixiviant that has promise to replace cyanide at the gold mining industry. Hexaamminecobalt(III) (Co(NH 3) 6 3+) is effective in catalyzing the thiosulfate leaching of gold from ores, but the electrochemical behavior and catalytic leaching mechanisms remain unclear. This paper reports an in-depth investigation into the electrochemical behavior of gold leaching in ammoniacal thiosulfate solutions catalyzed by Co(NH 3) 6 3+. The electrochemical results suggested that Co(NH 3) 6 3+ is more efficient than Cu(NH 3) 4 2+ in catalyzing the ammoniacal thiosulfate leaching of gold. Compared with Cu(NH 3) 4 2+, the coulombic efficiency of gold dissolution (Q Au / Q T) under the Co(NH 3) 6 3+ catalysis is much higher. In the presence of Co(NH 3) 6 3+, the Q Au / Q T was nearly 100% at 150–250 mV and higher than 78.9% at 250–350 mV; at 250 mV, the Q Au / Q T was approximately 100% in the initial electrolysis of 300 s and remained basically unchanged at around 75% after 1200 s. Leaching results further showed that the catalysis of Co(NH 3) 6 3+ not only achieved a continuous leaching of gold with time, but also significantly decreased the consumption of thiosulfate from 36.5% (for Cu(NH 3) 4 2+) to 20.9% after leaching 24 h. In the ammoniacal thiosulfate solution, Co(NH 3) 6 3+ is likely reduced to mixed ligand Co(II) complexes of Co(NH 3) x (S 2 O 3)0 (x = 4, 5), which are readily oxidized back to Co(NH 3) 6 3+. Thus, an improved catalytic mechanism of gold leaching is proposed that the redox cycle between Co(NH 3) 6 3+ and Co(NH 3) x (S 2 O 3)0 (x = 4, 5) catalyzes the leaching of gold with ammoniacal thiosulfate. [Display omitted] [ABSTRACT FROM AUTHOR]