Unassisted Switchable Dual-Photoelectrode Devices Utilizing p-n Carbon Quantum Dots as "Semiconductor Electrolytes": Optimization Between H2O2 and Solar Electricity Production

Abstract

Switchable self-driven photoelectrochemical (PEC) devices are developed to boost H₂O₂ or electricity generation under visible-light illumination, in which p-n type carbon quantum dots (N-CQDs) is applied as conceptually-new "semiconductor electrolytes". The N-CQDs contains N-dopants, and both negatively- and positively-charged surface groups. This allows N-CQDs to act as the electrolyte and to interact with both a BiVO₄ photoanode and a Cu₂O photocathode. In a two-compartment cell with a separating membrane, N-CQDs can dynamically form p-n heterojunctions with the photoanode or the photocathode, facilitating charge separation. In this setup, the fine-tuned electronic structure of N-CQDs promotes the two-electron reactions with water or O₂ to produce H₂O₂, achieving a rate of 28 µm min^-1 and Faradic efficiency exceeding 80%. Switching into a one-compartment cell, N-CQDs promotes four-electron charge transfer and stabilizes the photoelectrodes, giving electricity output for over 120 h. This control over electron transfer, selectivity, and durability cannot be achieved using traditional electrolytes.

Keywords: H2O2 production; dual‐photoelectrode; electricity production; electrolyte optimization; p‐n carbon dots.

Figure 1

Characterization of the N‐CQDs. a) TEM spectra of N‐CQDs. b) Raman spectrum of N‐CQDs. High‐resolution c) C1s, d) N1s, and e) O1s XPS spectra of N‐CQDs. f) Schematic synthesis and proposed structure of N‐CQDs. g) Mott–Schottky plots of N‐CQDs showing the co‐existence of n‐ and p‐type conductivities. h) The band levels (left) and electron cloud distributions (right) of the LUMO and the HOMO from the DFT model of N‐CQDs. i) Energy band diagrams for p‐n junctions of N‐CQDs with BiVO₄ and Cu₂O respectively, and the band locations of BiVO₄ in KHCO₃ solution and Cu₂O in citrate‐phosphate buffer solution. V _fn denotes the Fermi level corresponding to the n‐type semiconductor behavior of N‐CQDs, and V _fp represents the Fermi level associated with its p‐type semiconductor behavior. Both are taken from the intercept with the x‐axis in g).

Figure 2

a,b) The ORR LSV and H₂O₂ generation of a Cu₂O photocathode (2 cm²) in N‐CQDs or buffer solution in a three‐electrode configuration. c,d) The WOR LSV and H₂O₂ generation of a BiVO₄ photoanode (2 cm²) in N‐CQDs, KHCO₃ or precursor solution in a three‐electrode configuration. In (c) the PEC region starts at 0.32 V and the electrochemical (EC) region at 1.8 V given by the onset potential for N‐CQDs in light and dark, respectively.

Figure 3

a) LSV intersections of photoanodic and photocathodic systems in a three‐electrode configuration. b) Schematic diagrams for the membrane‐separated two‐compartment cell and one‐compartment cell. c–g) LSV curves with chopped light irradiation, long‐time PEC operation, the sum of H₂O₂, FE generation, and IPCE on the photoanode and photocathode (6 cm² electrode area) in different electrolytes and different cell setups.

Figure 4

The J–V curve for the one‐compartment cell and calculated power density curve in a) N‐CQDs solution under air or O₂‐saturated conditions, and b) in different O₂‐saturated electrolyte solutions. c) The image for lightening a LED light by the light‐driven one‐compartment cell in N‐CQDs solution.

Figure 5

a) OCVD of the separated two‐compartment cell in different solutions. b) Mechanisms for the two‐compartment cell in N‐CQDs solutions. c,d) LSV curves and corresponding K‐L plots of the two‐compartment cell in N‐CQDs or KHCO₃ buffer solutions respectively. e) Mott–Schottky plot and locations of the reference CQDs. f) The i‐t curves for photoanodic systems or photocathodic systems respectively in N‐CQDs solution or reference CQDs solution.

Figure 6

a) Fitted Nyquist plots of the EIS spectra for the one‐compartment cell in different solutions. b) SEM images of BiVO₄ electrodes before (top left) and in different solutions after PEC operation (20 h) in a one‐compartment cell. c,d) EPR spectra of the BiVO₄, N‐CQDs, and Cu₂O modified FTO plate in DMSO or water respectively with DMPO as trapping agent. Note: The loose contact of components lead to the unavoidable formation of p‐n junctions between BiVO₄ and N‐CQDs, therefore, holes were abstracted by N‐CQDs, and a minor signal ascribed to the DMPO‐C adduct was observed (α_N = 16.2 G, α_H = 23.3 G). (e) Mechanism of the one‐compartment cell in N‐CQDs solution. f,g) Potential application scenarios of the one‐compartment cell as a light indicator.