The safety and feasibility of industrial electrochemical production of sodium chlorate, an important chemical in the pulp and paper industry, depend on the selectivity of the electrode processes. The cathodic reduction of anodic products is sufficiently suppressed in the current technology by the addition of chromium(VI) to the electrolyte, but due to the high toxicity of these compounds, alternative pathways are required to maintain high process efficiency. In this paper, we evaluate the electrochemical hydrogen evolution reaction kinetics and selectivity on thermally formed manganese oxide-coated titanium electrodes in hypochlorite and chlorate solutions. The morphology and phase composition of manganese oxide layers were varied via alteration of the annealing temperature during synthesis, as confirmed by scanning electron microscopy, X-ray diffraction, synchrotron radiation X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy measurements. As shown in mass spectroscopy coupled electrochemical measurements, the hydrogen evolution selectivity in hypochlorite and chlorate solutions is dictated by the phase composition of the coating. Importantly, a hydrogen evolution efficiency of above 95% was achieved with electrodes of optimized composition (annealing temperature, thickness) in hypochlorite solutions. Further, these electrode coatings are nontoxic and Earth-abundant, offering the possibility of a more sustainable chlorate production.

BACKGROUND: Sodium chlorate (NaClO3) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na2Cr2O7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na2Cr2O7 or reduce its concentration.

RESULTS: The Na2Cr2O7 concentration can be significantly reduced by using Na2MoO4 as an electrolyte co-additive. Na2MoO4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na2Cr2O7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H2 and O2 production rates. The results indicate that 3 μmol L−1 Na2Cr2O7 (contrary to current industrial 10–30 mmol L−1) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L−1 Na2MoO4 the cathode is activated while avoiding detrimental O2 generation from chemical and electrochemical reactions. Higher concentrations of Na2MoO4 led to increased oxygen production.

CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater. 

Identifying the sources of oxygen in the chlorate process is challenging due to the complex set of chemical and electrochemical reactions involved. Here, two types of electrodes have been investigated-Ti0.7Ru0.3Ox, and electrodes with aimed composition Ti0.34Ru0.3Sn0.3Sb0.06Ox, both compared with platinum anodes. The cell oxygen off-gas was analyzed employing mass spectrometry together with ex situ UV-vis spectroscopy to quantify the kinetic rate constants. Noteworthy is that the respective rates of oxygen formation from anodic and chemical reactions in the presence of hypochlorite are of the same magnitude. The addition of Sn and Sb doubled the surface area of the electrodes and decreased oxygen production when electrodes were used for the first time. However, rate constants for total oxygen production with reused electrodes follow the trend: homogeneous hypochlorite decomposition < TiRu < TiRuSnSb to the highest value obtained by Pt. The same trend is noticed for rate constants concerning the hypochlorite decomposition.

NiMH batteries are popular in the electromotive industry due to good rate capability, reliability, and environmental friendliness. Although the battery type is thoroughly investigated, studies of battery gas composition formed during a work cycle are few. The gas composition would be useful to understand the reactions occurring in the battery during cycling and could be used to optimize battery operation. 

In this study, two methods for investigating the internal NiMH battery gas phase composition during different charge/discharge cycles using mass spectrometer (MS) were developed. In the first method, the battery module was connected by a sampler system. In the second method, the battery was connected directly using a microcapillary, and the gas composition was continuously measured. In addition to the gas composition the voltage, pressure, and temperature of the battery were recorded. 

The biggest contributor in the measured gas phase was N2, followed by H2. A clear rising trend of H2 pressure as SOC increased was recorded, while O2 levels were low except around end of charge. Thus, the methods were found to be a reliable way of investigating NiMH gas composition without negatively affecting the battery.