Overall water splitting based on particulate photocatalysts is an easily constructed and cost-effective technology for the conversion of abundant solar energy into clean and renewable hydrogen energy on a large scale. This promising technology can be achieved in a one-step excitation system using a single photocatalyst or via a Z-scheme process based on a pair of photocatalysts. Ideally, such photocatalysis will proceed with charge separation and transport unaffected by recombination and trapping, and surface catalytic processes will not involve undesirable reactions. This review summarizes the basics of overall water splitting via both one-step excitation and Z-scheme processes, with a focus on standard methods of determining photocatalytic performance. Various surface engineering strategies applied to photocatalysts, such as cocatalyst loading, surface morphology control, surface modification and surface phase junctions, have been developed to allow efficient one-step excitation overall water splitting. In addition, numerous visible-light-responsive photocatalysts have been successfully utilized as H2-evolution or O2-evolution photocatalysts in Z-scheme overall water splitting. Prototype particulate immobilization systems with photocatalytic performances comparable to or drastically higher than those of particle suspension systems suggest the exciting possibility of the large-scale production of low-cost renewable solar hydrogen.

Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting

Photocatalytic water splitting using particulate semiconductor materials has been studied as a simple means of hydrogen production. However, there are still many obstacles to the development of complete, practical and renewable solar hydrogen production processes. This review discusses particulate photocatalyst systems intended for large-scale solar hydrogen production via water splitting, focusing on their current status and potential impact. The cost and efficiency targets for solar-to-fuel conversion on a practical scale are also reviewed, based on the maximum allowable cost of solar hydrogen production systems, which has been estimated to be US$102 m–2, at most. Particulate photocatalyst material design principles are discussed, using efficient oxide photocatalysts as examples. Approaches to constructing photocatalytic reactors extensible to large areas are also introduced. Finally, challenges related to the development of efficient and inexpensive photocatalyst systems and potentially useful analytical methods are outlined. Photocatalytic water splitting with particulate semiconductors represents a promising strategy for the generation of hydrogen as a solar fuel. This review covers recent advances in the development of reaction systems and photocatalysts towards the scale-up of this technology, emphasizing at the same time the challenges to overcome.

Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts

Nature uses organic molecules for light harvesting and photosynthesis, but most man-made water splitting catalysts are inorganic semiconductors. Organic photocatalysts, while attractive because of their synthetic tunability, tend to have low quantum efficiencies for water splitting. Here we present a crystalline covalent organic framework (COF) based on a benzo-bis(benzothiophene sulfone) moiety that shows a much higher activity for photochemical hydrogen evolution than its amorphous or semicrystalline counterparts. The COF is stable under long-term visible irradiation and shows steady photochemical hydrogen evolution with a sacrificial electron donor for at least 50 hours. We attribute the high quantum efficiency of fused-sulfone-COF to its crystallinity, its strong visible light absorption, and its wettable, hydrophilic 3.2 nm mesopores. These pores allow the framework to be dye-sensitized, leading to a further 61% enhancement in the hydrogen evolution rate up to 16.3 mmol g−1 h−1. The COF also retained its photocatalytic activity when cast as a thin film onto a support.

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Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water

Conjugated microporous polymers (CMPs) are a unique class of materials that combine extended π-conjugation with a permanently microporous skeleton. Since their discovery in 2007, CMPs have become established as an important subclass of porous materials. A wide range of synthetic building blocks and network-forming reactions offers an enormous variety of CMPs with different properties and structures. This has allowed CMPs to be developed for gas adsorption and separations, chemical adsorption and encapsulation, heterogeneous catalysis, photoredox catalysis, light emittance, sensing, energy storage, biological applications, and solar fuels production. Here we review the progress of CMP research since its beginnings and offer an outlook for where these materials might be headed in the future. We also compare the prospect for CMPs against the growing range of conjugated crystalline covalent organic frameworks (COFs).

Advances in Conjugated Microporous Polymers.

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

A Self-Healable, Highly Stretchable, and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing

Direct water splitting into H2 and O2 using photocatalysts by harnessing sunlight is very appealing to produce storable chemical fuels. Conjugated polymers, which have tunable molecular structures and optoelectronic properties, are promising alternatives to inorganic semiconductors for water splitting. Unfortunately, conjugated polymers that are able to efficiently split pure water under visible light (400 nm) via a four-electron pathway have not been previously reported. This study demonstrates that 1,3-diyne-linked conjugated microporous polymer nanosheets (CMPNs) prepared by oxidative coupling of terminal alkynes such as 1,3,5-tris-(4-ethynylphenyl)-benzene (TEPB) and 1,3,5-triethynylbenzene (TEB) can act as highly efficient photocatalysts for splitting pure water (pH ≈ 7) into stoichiometric amounts of H2 and O2 under visible light. The apparent quantum efficiencies at 420 nm are 10.3% and 7.6% for CMPNs synthesized from TEPB and TEB, respectively; the measured solar-to-hydrogen conversion efficiency using the full solar spectrum can reach 0.6%, surpassing photosynthetic plants in converting solar energy to biomass (globally average ≈0.10%). First-principles calculations reveal that photocatalytic H2 and O2 evolution reactions are energetically feasible for CMPNs under visible light irradiation. The findings suggest that organic polymers hold great potential for stable and scalable solar-fuel generation.

Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light.

Converting solar energy into storable and transportable chemical fuels using artificial photosynthetic systems can provide an alternative route to the current unsustainable use of fossil fuels, addressing the worldwide energy crisis and environmental issues. Recently, semiconducting polymers have emerged as a very promising class of photocatalysts for water splitting as their electronic and structural properties can be conveniently controlled and systematically designed at a molecular level. Among the various polymer photocatalysts that are reported so far, 2D polymer nanosheets are particularly interesting and gaining more attention. The 2D planar structure offers unique features such as high surface area, abundant surface active sites, efficient charge separation, and facile formation of heterostructures. The design and synthesis of 2D polymer nanosheets have greatly advanced the research in photocatalytic overall water splitting. Here, recent advances in developing photocatalysts based on 2D polymer nanosheets for photocatalytic overall water splitting are highlighted. Specifically, the existing approaches to tune their electronic structures and surface active sites for photocatalysis are discussed. Future opportunities and challenges for developing 2D polymers for photocatalytic overall water splitting are also included.

2D Polymers as Emerging Materials for Photocatalytic Overall Water Splitting.

Metal-free polymer photocatalysts have shown great promise for photocatalytic H2 O2 production via two-electron reduction of molecular O2 . The other half-reaction, which is the two-electron oxidation of water, still remains elusive toward H2 O2 production. However, enabling this water oxidation pathway is critically important to improve the yield and maximize atom utilization efficiency. It is shown that introducing acetylene (CC) or diacetylene (CCCC) moieties into covalent triazine frameworks (CTFs) can remarkably promote photocatalytic H2 O2 production. This enhancement is inherent to the incorporated carbon-carbon triple bonds which are essential in modulating the electronic structures of CTFs and suppressing charge recombinations. Furthermore, the acetylene and diacetylene moieties can significantly reduce the energy associated with OH* formation and thus enable a new two-electron oxidation pathway toward H2 O2 production. The study unveils an important reaction pathway toward photocatalytic H2 O2 production, reflecting that precise control over the chemical structures of polymer photocatalysts is vital to achieve efficient solar-to-chemical energy conversion.

Acetylene and Diacetylene Functionalized Covalent Triazine Frameworks as Metal-Free Photocatalysts for Hydrogen Peroxide Production: A New Two-Electron Water Oxidation Pathway.

Physically transient electronics, a form of electronics that can physically disappear in a controllable manner, is very promising for emerging applications. Most of the transient processes reported so far only occur in aqueous solutions or biofluids, offering limited control over the triggering and degradation processes. We report novel moisture-triggered physically transient electronics, which exempt the needs of resorption solutions and can completely disappear within well-controlled time frames. The triggered transient process starts with the hydrolysis of the polyanhydride substrate in the presence of trace amounts of moisture in the air, a process that can generate products of corrosive organic acids to digest various inorganic electronic materials and components. Polyanhydride is the only example of polymer that undergoes surface erosion, a distinct feature that enables stable operation of the functional devices over a predefined time frame. Clear advantages of this novel triggered transience mode include that the lifetime of the devices can be precisely controlled by varying the moisture levels and changing the composition of the polymer substrate. The transience time scale can be tuned from days to weeks. Various transient devices, ranging from passive electronics (such as antenna, resistor, and capacitor) to active electronics (such as transistor, diodes, optoelectronics, and memories), and an integrated system as a platform demonstration have been developed to illustrate the concept and verify the feasibility of this design strategy.

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Ying Zhang

Papers

A Self-Healable, Highly Stretchable, and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing

Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light.

2D Polymers as Emerging Materials for Photocatalytic Overall Water Splitting.

Acetylene and Diacetylene Functionalized Covalent Triazine Frameworks as Metal-Free Photocatalysts for Hydrogen Peroxide Production: A New Two-Electron Water Oxidation Pathway.

Moisture-triggered physically transient electronics