Artificial light-driven ion pump for photoelectric energy conversion

Abstract

Biological light-driven ion pumps move ions against a concentration gradient to create a membrane potential, thus converting sunlight energy directly into an osmotic potential. Here, we describe an artificial light-driven ion pump system in which a carbon nitride nanotube membrane can drive ions thermodynamically uphill against an up to 5000-fold concentration gradient by illumination. The separation of electrons and holes in the membrane under illumination results in a transmembrane potential which is thought to be the foundation for the pumping phenomenon. When used for harvesting solar energy, a sustained open circuit voltage of 550 mV and a current density of 2.4 μA/cm² can reliably be generated, which can be further scaled up through series and parallel circuits of multiple membranes. The ion transport based photovoltaic system proposed here offers a roadmap for the development of devices by using simple, cheap, and stable polymeric carbon nitride.

Fig. 1

Light-induced ion pump based on carbon nitride nanotube membrane. a Schematic of the light-induced ion pump, which can pump ions transport against a concentration gradient. b Optical image (scale bar 0.2 cm), SEM image (scale bar 500 nm) of CNNM, and typical opening view of nanotube (scale bar 50 nm). The TEM images of a single nanotube (scale bar 50 nm), the enlarged crystalline pore wall section (scale bar 5 nm), and electron diffraction. c The typical current–voltage curves before and after light (143 mW/cm²) irradiation at 100-fold (C_H = 0.01 M; C_L = 0.0001 M) KCl concentration gradient. After irradiation, the zero-volt current changed from positive to negative value. The inset shows the light absorbance of carbon nitride nanotube and powder. d Measured cyclic constant zero-volt current with the alternating illumination at 100-fold KCl concentration gradient. e Measured open-circuit voltage across the CNNM before and after illumination in 100-fold KCl concentration gradient

Fig. 2

High-performance artificial ion pump. a Zero-volt current as a function of light density from 0 to 380 mW/cm². Only the light density stronger than 74 mW/cm² can change the direction of ionic current at 100-fold KCl concentration gradient. b Zero-volt current as a function of concentration gradient from 1 fold to 10,000 fold. The CNNM-based ion pump can realize ions “uphill” transport process at up to 5000-fold concentration gradient. c Zero-volt current as a function of monochromatic light (blue: 405 nm; green: 515 nm; yellow: 590 nm) at 100-fold KCl concentration gradient. The ionic current is consistent with the light absorbance. Error bars represent standard deviations from five independent experiments

Fig. 3

Origin of ion pump phenomenon. a Schematic of surface charge distribution on the nanotube before illumination, in which condition low-density negative charge is homogeneously distributed on the nanotube. b Equivalent circuit of a, the concentration gradient potential (V_CG) is the only ionic current source. c Negative charged molecular structure and photo-induced separation of electrons and holes of carbon nitride. d Schematic of surface charge distribution on the nanotube after illumination, in which condition the separation of electrons and holes results in the heterogeneous charge distribution. e Equivalent circuit of d, transmembrane potential (V_CNNM) provides a contrary potential with V_CG. f Zeta-potential of the CNNM shows the negative charged surface before illumination. g The voltage difference between illuminated and non-illuminated sides shows transmembrane potential. Error bars represent standard deviations of three independent experiments. h The calculated electrical potential profile along the axis of the nanotube when integrating 100-fold concentration gradient based on the two models illustrated in a and d, which are based on the 500-nm-long nanotube with 30-nm-diameter. The potential of low concentration side (C_L) is preset as 0 mV

Fig. 4

Photoelectric energy conversion system based on ion pumping in symmetric electrolyte. The electrolyte is 0.001 M KCl. a, b Open circuit voltage (a) and current density (b) generated by light-induced ions transport. c The generated power can be output to external circuit and supply an electronic load. The output power density reaches its peak value of 1.2 mW/m² at resistance of ∼400 kΩ. d Current–voltage curves of two individual CNNM and their series and parallel connections. Inset: circuit diagram. CNNM 1: 46 mW/cm²; CNNM 2: 143 mW/cm². e Light with different power density (A side: 143 mW/cm²; B side: 46 mW/cm²) from different direction resulted in alternating current

Fig. 5

Light intensity dependent photovoltage and photocurrent. a, b Photo-induced voltage (a) and ionic current (b) as a function of the light intensity from 0 to 380 mW/cm². Each illumination process continues 50 s