Alkali Halide Boost of Carbon Nitride for Photocatalytic H₂ Evolution in Seawater

Abstract

Photocatalytic H2 evolution (PHE) from extremely abundant seawater resources is an ideal way to secure sustainable H2 for humanity, but the saline in seawater easily competitively absorbs the active sites and poisons the catalyst. Herein, a series of low-cost alkali halide (NaI, KI, RbI, CsI, CsBr, and CsCl), analogous to the saline in natural seawater, was selected to modify carbon nitride (MX-CN) through one-step facile pyrolysis with the assistance of water. MX-CN possesses a large amount of negative charges, which could inhibit anion absorption, to some extent, preventing chloride corrosion. Importantly, it can greatly boost the electron transfer between MX-CN and triethanolamine (TEOA) (sacrificial agent) because the alkali cation in seawater can coordinate with TEOA, and easily come in contact with MX-CN through alkali-cation exchange and electrostatic attraction. Benefiting from it, the PHE performance in seawater is 200 times better than that of original CN in deionized water above, and the apparent quantum efficiency of MX-CN (CsI-CN) under 420 nm light irradiation comes to 72% in seawater, the highest value reported for seawater thus far. This work provides a new research direction for engineering the electron transfer pathway between the photocatalyst and sacrificial agent (e.g., pollutant) in natural seawater.

A comparison study of sodium ion- and potassium ion-modified graphitic carbon nitride for photocatalytic hydrogen evolution

“…Here, the peaks located at 707 cm À1 and 725 cm À1 are assigned as the breathing modes of the triazine rings. 18,25,44 Xu et al reported the appearance of a peak at 706 cm À1 for pristine CN, whereas a peak at 726 cm À1 appeared for Cs + -modied CN. 44 According to Fig.…”

“…10,11 There are several ways to intercalate Na + or K + into GCN including the calcination of a mixture of carbon nitride precursors (cyanamide, melamine, etc.) with alkali halide, [11][12][13][14][15][16][17][18][19][20][21][22] a mixture of the prepared GCN and alkali halide, [23][24][25][26][27] a mixture of carbon nitride precursors and alkali metal hydroxide, [28][29][30][31] and a mixture of the prepared GCN and alkali metal hydroxide. 6,[32][33][34][35] Most of the studies revealed that for one or several factors, alkali metal ion doping in GCN can help increase its surface specic area, 6,11,19,20,23,29 promote its crystallinity, 15,17,22,27 enhance its light absorption, 13,[15][16][17]19,21,22,27 reduce its bandgap,…”

“…[15][16][17]19,21,23,29 To the best of our knowledge, only a few reports explored a series of alkali metal elementdoped GCN. 6,11,15,25 To fully understand the effect of alkali metal ions on GCN, a systematic examination of alkali halide-modied GCN is still needed. In this study, we comparatively investigated Na-doped and K-doped GCN by heating a mixture of melamine and NaCl or KCl, and then washing off the redundant ions, respectively.…”

A review of sodium chloride-based electrolytes and materials for electrochemical energy technology

“…The electrons in the former come from an external circuit, 26,206,207 while the electrons in the latter come from the separation of electrons and holes triggered by light irradiation on a photocatalyst. [208][209][210] Cl À in NaCl solution can be oxidized into Cl 2 and OCl À at the anode under acidic and basic conditions, instead of O 2 evolution by the oxidation of water. 211,212 Although Cl 2 and OCl À produced on a large scale by the electrolysis of brine are the valuable intermediate products in the chlor-alkali industry for disinfection and bleaching, 185,211,213 their toxicity and heavy corrosiveness present a major environmental problem during hydrogen production by water splitting.…”

Overcoming Electron Transfer Efficiency Bottlenecks for Hydrogen Production in Highly Crystalline Carbon Nitride‐Based Materials

“…A similar effect was reported by Wu and co-workers to alkali halide salts in seawater, and the author attributed the promoting alkali effect to the coordination to the TEOA, thus facilitating its contact with the carbon nitride-based material. [21] As the oxidation step takes place with a faster rate constant (k 2 ), it is not the limiting step for the hydrogen evolution reaction, consequently, it is very unlikely that the alkali promoting effect previously reported is related to the alkali cation coordination to the TEOA molecule.…”