Sol−Gel Processing of Water‐Soluble Carbon Nitride Enables High‐Performance Photoanodes**

Abstract

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based solÀ gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The solÀ gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectro-chemical (PEC) performance in alcohol reforming and highly selective (~100 %) conversions with very high photocurrents (> 0.25 mA cm À 2 under 2 sun) down to < 0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from solÀ gel processing of watersoluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

“…-0.32 V vs. RHE, which makes the injection of an electron from the radical to the conduction band edge of PHI-DB/S450 (-0.63 V vs. RHE) rather unlikely. [10] Our pulsed LED induced photocurrent transients (Figure 1c) suggest that a pre-illumination time of ~10 min is sufficient for the system to reach optimum performance. This was further confirmed from measurement of the photocurrents during pre-illumination using chronoamperometry (Figure S1), where we can observe that the photocurrent continues to increase and plateaus within a period of 7-10 mins of pre-illumination.…”

“…[2c] However, such negative effects of electron accumulation in our PHI-DB/S450 photoanodes are clearly outbalanced by the positive effect of trap passivation, as evidenced by enhanced photocurrent generation (Figure 1c) which is superior to that typically encountered in conventional (non-ionic) PCN materials. In this context, we speculate that, due to the distinct nature of the ionic PHI-based materials and the highly porous nature of our PHI-DB/S450 photoanodes [10] (as compared to a dense analogue that shows much lower photocurrents, Figure S5), the negative charge of the accumulated trapped electrons in the PHI photoanodes can be effectively screened by cations (K + , Na + , H + ) from the film and from the solution, which may at least partially eliminate the detrimental effects of charge accumulation. We find that electron accumulation during pre-illumination is reversible, shown in Figure S6.…”

“…The structural, optical and photoelectrochemical properties of our porous PHI photoanodes (PHI-DB/S450) were reported in detail in Ref. [10] . While the PHI photoanodes did not oxidize water, the photocurrents during oxidation of alcohols, such as methanol, were among the highest observed for PCN-based photoanodes.…”

“…[10] Our PHI photoanodes exhibited selective (~100% Faradaic efficiency) photoelectrocatalytic conversion of various alcohols, standing out, in particular, by their very low photocurrent onset potential and exceptionally high fill factor, thus permitting effective photoreforming of alcohols to hydrogen even without any applied electric bias. [10] Herein, we provide, for the first time, in operando investigations of photoelectrochemical processes in the highly stable, binder-free PHI photoanodes. We employ transient photocurrent (TPC) and spectroelectrochemical photo-induced absorption spectroscopy (PIAS) measurements that enable us to directly observe photogenerated charge dynamics under operando conditions and correlate these with the concomitantly measured photocurrent.…”

“…Polymeric carbon nitride (PCN) materials represent one of the most vigorously studied class of low-cost, tunable and highly stable polymeric photocatalysts, with potential applications in light-driven hydrogen production, selective oxidations of organic substrates and other useful chemical transformations. [1] The fundamental photophysical properties and processes (photoexcitation, recombination, charge separation, photoluminescence) that govern the performance of PCN-based photocatalysts have been intensely studied, [2] revealing many aspects related to the electron transfer within and between different PCN units, [2f] charge separation [2a, b] and recombination [2c] processes, in addition to providing insight into the hole transfer to electron donors. [2e] Recently, there has been increasing interest in poly(heptazine imide) (PHI)-based materials, a class of (semi)crystalline ionic PCNs comprising stacks of two-dimensional heptazine-based networks and alkali metal cations and/or protons.…”