The main target of this work was to investigate the influence of the hardeners on the properties of the prepared supercapacitors named geopolymer cements. The supercapacitors were obtained by mixing an aluminosilicate source (metakaolin) and hardeners (sodium waterglass or phosphoric acid solution). The obtained supercapacitors were characterized by measuring the compressive strengths, infrared spectroscopy and scanning electron microscope. The electrochemical properties of the prepared supercapacitors were carried out by cyclic voltammetry and electrochemical impedance spectroscopy. The obtained results show that the compressive strengths of geopolymer cements from sodium waterglass and phosphoric acid solution are 43.50 and 66.12 MPa, respectively. The micrograph images indicate that both supercapacitors are compact, homogeneous and denser matrix. It appears that the supercapacitors from acidic medium have a lower internal resistance (45 kΩ), i.e. higher ionic conductivity, whereas, the one from alkaline medium indicates the higher value of the internal resistance (450 kΩ). It was typically found that the geopolymer cements from acidic medium have a higher electrical conductivity making it a potential electrode material for structural supercapacitor compared to the one obtained in alkaline medium. Graphic abstract

In this work, the plant alkaloid Voacangine, isolated from the bark of Tabernaemontana contorta, was examined as a corrosion inhibitor for steel in 1 M hydrochloric acid. The study was carried out using a comparative approach where several independent methods (spectroscopic solution analysis, electrochemical impedance, potentiodynamic polarisation and mass loss measurements) were performed in order to strengthen the validity of the results. All the findings revealed to be in close agreement with each other. Voacangine was determined to be a mixed corrosion inhibitor and to closely follow Langmuir-type adsorption onto active corrosion sites on the metal surface. The maximal inhibition efficiency was 90% which is lower than other comparable species. This deficiency was ascribed to the presence of a basic nitrogen group causing a degree of steric hindrance for the adsorption.