High-performance potassium poly(heptazine imide) films for photoelectrochemical water splitting

Abstract

Photoelectrochemical (PEC) water splitting is an appealing approach by which to convert solar energy into hydrogen fuel. Polymeric semiconductors have recently attracted intense interest of many scientists for PEC water splitting. The crystallinity of polymer films is regarded as the main factor that determines the conversion efficiency. Herein, potassium poly(heptazine) imide (K-PHI) films with improved crystallinity were in situ prepared on a conductive substrate as a photoanode for solar-driven water splitting. A remarkable photocurrent density of ca. 0.80 mA cm^-2 was achieved under air mass 1.5 global illumination without the use of any sacrificial agent, a performance that is ca. 20 times higher than that of the photoanode in an amorphous state, and higher than those of other related polymeric photoanodes. The boosted performance can be attributed to improved charge transfer, which has been investigated using steady state and operando approaches. This work elucidates the pivotal importance of the crystallinity of conjugated polymer semiconductors for PEC water splitting and other advanced photocatalytic applications.

Fig. 1. (a) Illustration of the polymerization of K-PHI films on an FTO substrate. (b) Top view and (c) cross-sectional view of the K-PHI photoanode. TEM images of K-PHI from the (e) [1̄10] and (f) [002] planes with the corresponding schematic illustrations shown in (d). (g) TEM images of K-PHI recorded using HAADF techniques with EDS studies on (h) C, (i) K, and (j) N.

Fig. 2. (a) PXRD patterns of powder samples scraped from films of melon and K-PHI. (b) PXRD patterns of powder samples of melon and K-PHI prepared using the same approaches as for the photoanodes. (c) FTIR spectra of powder samples scraped from the melon and K-PHI films. (d) Raman spectra of the melon and K-PHI photoanodes.

Fig. 3. High-resolution XPS spectra of melon and XPS depth profiles of K-PHI. (a) K 2p and C 1s. (b) N 1s. (c) K 2p. (d) Sn 3d. (e) UV-DRS spectra of melon and K-PHI. (f) Calculated band structure and the corresponding projected density of states (PDOS) for K-PHI. Orbital diagrams of the (g) CBM and (h) VBM of K-PHI. The orbitals with cyan and yellow colors represent the positive and negative values of the molecular orbitals. The pink, brown, gray, and purple balls represent H, C, N, and K atoms, respectively.

Fig. 4. (a) LSV curves of the melon and K-PHI photoanodes under AM 1.5G illumination (solid curves) and in the absence of illumination (dash curves) in 1.0 M NaOH aqueous solution. (b) Photocurrent densities of the melon and K-PHI photoanodes in 1.0 M NaOH aqueous solution at 1.23 V vs. RHE. (c) IPCE plots of the melon and K-PHI photoanodes in 1.0 M NaOH aqueous solution at 1.23 V vs. RHE. (d) Evolution rates of oxygen and hydrogen gases by the K-PHI photoanode under an applied voltage of 1.23 V (vs. RHE).

Fig. 5. (a) The transient decay of the PL of melon and K-PHI. (b) Mott–Schottky plots of the EIS measurements of the melon and K-PHI photoanodes. (c) IMPS of the K-PHI photoanode. (d) Potential dependence of the rate constants K_t and K_r as a function of applied potential for the K-PHI photoanode.