Can polyoxometalates enhance the capacitance and energy density of activated carbon in organic electrolyte supercapacitors?

Polyoxometalates (POMs) have been shown to work as faradaic additives to activated carbon (AC) in acidic aqueous electrolytes. Yet, their use in organic media allows not only for added capacity but also higher voltage. Here we show that the tetraethylammonium derivative of phosphotungstate [PW12O40]3− (PW12) can be homogeneously distributed throughout the pores of activated carbon (AC) in organic solvents such as N,N’-dimethylformamide (DMF) and demonstrate the use of this hybrid electrode material in an organic electrolyte (1 M TEABF4 in acetonitrile) supercapacitor. Our results show the efficient electroactivity of the PW12 cluster even in the absence of protons, providing a higher voltage than aqueous electrolytes and fast and reversible redox activity. The hybrid material shows a combination of double-layer (AC) and redox (PW12) capacities leading to an increase (36%) in volumetric capacitance with respect to pristine AC in the same organic electrolyte (1 M TEABF4 in acetonitrile). Remarkably, we were able to quantify this increase as coming predominantly from non-diffusion-limited processes thanks to the utterly dispersed nature of POMs. Moreover, the hybrid material delivers a good rate capability and excellent cycle stability (93% retention of the initial capacitance after 10,000 cycles). This study has a profound significance on improving capacitance of carbon-based materials in organic electrolytes.Partial funding from Ministry of Science, Innovation and Universities (MCIU), the State Research Agency (AEI) and the European Regional Development Fund (FEDER) (grant RTI2018-099826-B-I00) and AGAUR (2017 SGR 00870) are gratefully acknowledged. ICN2 is funded by the CERCA programme / Generalitat de Catalunya, and it is supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV-2017-0706). J.J Z. acknowledges his scholarship (No. 201806370211) under China Scholarship Council. This work has been carried out within the framework of doctoral program (PhD) of Material Science (Department of Physics) of Universitat Autònoma de Barcelona (UAB).Peer reviewe