ORIGINAL PAPER Tailoring the electrochemical charge storage properties of carbonaceous support by redox properties of heteropoly acids: where does the synergy come from? Danica Bajuk-Bogdanović 1 & Ivanka Holclajtner-Antunović 1 & Zoran Jovanović 2,3,4 & Željko Mravik 2 & Jugoslav Krstić 5 & Snežana Uskoković-Marković 6 & Milica Vujković 1 Received: 4 June 2019 /Revised: 7 August 2019 /Accepted: 13 August 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The synergistic effects between two Keggin-type heteropoly acids (HPAs) and carbon surface were examined and elucidated. An improved high rate capability (and potential high capacitor electrode for supercapacitors) of the hybrid materials, obtained by anchoring of α-dodecamolybdophosphoric (MoPA), α-dodecatungstophosphoric (WPA), and their mixture to activated carbon (AC), was achieved through the different mechanism of interaction. In order to elaborate this, a detailed analysis of AC-HPA composites has been performed by scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, temperature- programmed desorption (TPD), Fourier-transform infrared spectroscopy (FTIR), micro Raman spectroscopy, and zeta potential measurements. The zeta potential measurements revealed positive charge of carbon surface thus indicating attractive interactions with negatively charged Keggin anion. The surface analysis has shown that WPA spontaneously reduces the carbon surface, while interaction with MoPA leads to its oxidation. As the consequence of the tailoring of the functional groups at carbon surface through HPAs’ action, the distortion of cyclic voltammograms (CVs) decreased in the following order: AC-MoPA, AC-MoPA- WPA, and AC-WPA. A prominent rectangular shape of AC-WPA, even at an extremely high scan rate of 400 mVs -1 , was measured, which is rarely demonstrated for carbon-based composites. By applying the theory of electrode potentials, the HPA- AC synergistic effect was explained and discussed in terms of charge storage improvement of HPA-modified carbon. Keywords Heteropoly acid . Activated carbon . Oxygen functional groups . Composite materials . Supercapacitor Introduction Nowadays, an integral part to providing sustainable solutions for the growing global energy crisis is the electrochemical devices that enable highly efficient and rapid power delivery. Traditionally, the most common materials used for this pur- pose were carbon-based. However, increasing demand for higher energy density has highlighted the importance of com- posites. Among many, the composites of polyoxometalates (POMs) and carbon materials have shown promising proper- ties, coming not only from individual components but also from the synergy of their interaction. This is significantly in- fluenced by various molecular structures and sizes, i.e., versa- tility of physicochemical properties of POMs. POMs are char- acterized with fast reversible multielectron transfer processes but suffer from low specific surface and high solubility of some of POM in oxygen-containing solvents. Hence, their anchoring to conductive support is important for harnessing their full potential. Many reports illustrate that the synergy of Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-019-04369-4) contains supplementary material, which is available to authorized users. * Danica Bajuk-Bogdanović danabb@ffh.bg.ac.rs 1 Faculty of Physical Chemistry, University of Belgrade, PO Box 47, 11158, Belgrade, Serbia 2 Laboratory of Physics, Vinča Institute of Nuclear Sciences, University of Belgrade, PO Box 522, 11001, Belgrade, Serbia 3 Center of Excellence for Hydrogen and Renewable Energy (CONVINCE), Vinča Institute of Nuclear Sciences, University of Belgrade, PO Box 522, 11001, Belgrade, Serbia 4 Advanced Materials Department, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 5 Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000, Belgrade, Serbia 6 Faculty of Pharmacy, University of Belgrade, PO Box 146, 11221, Belgrade, Serbia Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-019-04369-4