Internet of Things devices – wireless sensors and actuators – require long-term off-grid power sources that would be cheap, and at the same time have a small footprint with low environmental impact throughout their life cycle. Such an off-grid power source could be realized with a photosupercapacitor: A solar cell is integrated with an electrochemical double-layer capacitor (EDLC) into a single monolithic device using a three-electrode design, where the two components share a common electrode.¹ The solar cell converts ambient light into an electric current and charges the integrated supercapacitor via the common electrode, which in turn powers the external device. The supercapacitor, on the other hand, acts as a buffer, mediating between an intermittent light source and the load of the device. In terms of footprint and hence energy and power density, integrated monolithic photosupercapacitors have a clear advantage in comparison with simply wiring a solar cell to a supercapacitor as storage unit. When compared to batteries or pseudocapacitors that involve slow solid-state diffusion reactions and/or surface redox processes, EDLCs are particularly suited for the integration with solar cells into photosupercapacitors as they are able to operate efficiently in fast fluctuating environments where rapid on/off cycles are required without the need to supply a fixed voltage onset.²

Owing to their large specific surface area, good electrical conductivity, and electrochemical stability, mesoporous nitrogen-doped carbon nanospheres (MPNCs) show a great promise for being implemented as an electrode material for supercapacitors.^3,4 To synthesize MPNCs we used a hard-templating strategy based on the polymerization and self-assembly of aniline in the presence of SiO₂ nanoparticles (7 nm), which led to the formation of spherical SiO₂/Polyaniline composites. Their carbonization and template removal resulted in highly monodisperse, 140 nm-diameter spherical MPNCs with a large specific surface area (825 m² g^-1) and defined mesopores (7 nm). The MPNCs are easily dispersible and could be processed by doctor-blading in homogeneous 3D-percolated electrodes. Benefiting from the well-defined mesoporous network and the highly accessible electrochemical surface area, our MPNC-based gel-electrolyte freestanding EDLC delivered large energy and power densities and a high capacitance (400 F g^-1 at 1 A g^-1) with high (95 %) coulombic efficiency.

To achieve an energy-autonomous self-powered system, we combined the MPNC-based EDLCs with halide-perovskite-based solar cells in a monolithic three-electrode fashion. As the solar cell we used a p-i-n perovskite solar cell with a FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ (high bandgap) absorber and large photosensitive area (1 cm²) delivering a high open-short circuit voltage (V_OC) of 1.08 V and a short-circuit current (J_SC) of 17.9 mA/cm². To facilitate the assembly process and to address the adverse sensitivity of the perovskite layer to the electrolyte solvent, we optimized the solar cell layer sequence and termination. At the same time, we used a semi-solid gel electrolyte for the EDLC, which minimizes the contact of the perovskite layer with the electrolyte solvent. This allowed us to obtain a free-standing integrated photosupercapacitor without the necessity of any encapsulation. This reduces the overall device footprint and cost, and simplifies the assembly. Benefiting from the high efficiency of the solar cell and the excellent performance of the EDLC, the integrated hybrid photosupercapacitor demonstrated fast (< 5 s) photocharging up to 1 V under 1 sun illumination and discharge through the EDLC terminals, proving that the solar energy was harvested, converted, and stored. The photosupercapacitor delivered 4.27 μWh/cm² at a power density of 0.29 mW/cm² and 1.68 μWh/cm² at 2.2mW/cm² with an areal capacitance of 31 mF/cm². This resulted in an unprecedented overall electrochemical energy conversion efficiency of 11.5 %.⁵

The strategy for photosupercapacitor fabrication presented here was extended to the integration of MPNC-based EDLCs with other types of solar cells, depending on the intended application. The achieved results pave the way towards the development of off-grid powered energy-autonomous devices.

1 Q. Zeng, Y. Lai, L. Jiang, F. Liu, X. Hao, L. Wang and M. A. Green, Adv. Energy Mater., 2020, 10, 1–30.

2 F. Béguin, V. Presser, A. Balducci and E. Frackowiak, Adv. Mater., 2014, 26, 2219–2251.

3 J. Melke, R. Schuster, S. Möbus, T. Jurzinsky, P. Elsässer, A. Heilemann and A. Fischer, Carbon N. Y., 2019, 146, 44–59.

4 J. Melke, J. Martin, M. Bruns, P. Hu, A. Scho, A. Fischer, S. M. Isaza, F. Fink and P. Elsa, ACS Appl. Energy Mater, 2020, 3, 11627.

5 T. Berestok, C. Diestel, N. Ortlieb, J. Buettner, J. Matthews, P. S. C. Schulze, J. C. Goldschmidt, S. W. Glunz and A. Fischer, Sol. RRL, 2021, 5, 1–13.