Porous Microreactor Chip for Photocatalytic Seawater Splitting over 300 Hours at Atmospheric Pressure

Abstract

Photocatalytic seawater splitting is an attractive way for producing green hydrogen. Significant progresses have been made recently in catalytic efficiencies, but the activity of catalysts can only maintain stable for about 10 h. Here, we develop a vacancy-engineered Ag3PO4/CdS porous microreactor chip photocatalyst, operating in seawater with a performance stability exceeding 300 h. This is achieved by the establishment of both catalytic selectivity for impurity ions and tailored interactions between vacancies and sulfur species. Efficient transport of carriers with strong redox ability is ensured by forming a heterojunction within a space charge region, where the visualization of potential distribution confirms the key design concept of our chip. Moreover, the separation of oxidation and reduction reactions in space inhibits the reverse recombination, making the chip capable of working at atmospheric pressure. Consequently, in the presence of Pt co-catalysts, a high solar-to-hydrogen efficiency of 0.81% can be achieved in the whole durability test. When using a fully solar-driven 256 cm2 hydrogen production prototype, a H2 evolution rate of 68.01 mmol h−1 m−2 can be achieved under outdoor insolation. Our findings provide a novel approach to achieve high selectivity, and demonstrate an efficient and scalable prototype suitable for practical solar H2 production.

1. Introduction

Using inexhaustible and widely available renewable energy to provide long-term clean fuel for mankind is an invaluable quest. The Honda-Fujishima effect [1], proposed in early 1970s for water splitting via the photoelectrochemical (PEC) approach [2–6], offers a promising solution for the above-mentioned pursuit. Aiming to readily convert solar energy and water into chemical energy, extensive researches have been carried out, with a focus on bias-free unassisted photocatalytic overall water splitting (OWS) [7–14]. Such a technology can produce H2 and O2 at a molar ratio of 2:1 in one-step solar-to-chemical energy conversion. Photocatalytic OWS has achieved an inspiring solar-to-hydrogen (STH) efficiency exceeding 1% [15], with the major challenges being the rapid recombination of photogenerated carriers during transport and the conflict between the broad visible-light absorption and strong redox ability [16, 17]. Besides, the feedstock for most photocatalytic OWS systems is pure water [18, 19]. Abundant seawater is a promising substitute for increasingly scarce freshwater resources, but the bottleneck issue is to suppress or even avoid the sharp deactivation of photocatalysts induced by the high salinity of seawater.

Limited by the disorder kinetic behavior of photogenerated carriers, the stability of early reported half-reaction systems in seawater was mostly less than or equal to 6 h [18]. Coupling two individual photocatalysts to form a Z-scheme heterojunction [20–22] has recently been proposed to generate both H2 and O2. The Z-scheme configuration is superior in addressing the low photocatalytic activity, but has so far not been possible to achieve a cycle that lasts for tens of hours [23] due to the undesirable competing reactions of Cl species. Very recently, Mi et al. reported a series of work with InGaN/GaN nanowires (NWs) as the major photocatalysts [24, 25]. When assisted with a Rh/Cr2O3/Co3O4 cocatalyst, the STH efficiency was as high as 6.6%, but only tested for 10 h artificial seawater splitting. The efficiency improvement is probably due to both the high reaction temperature at 70 °C and the good lattice-matched structure.