Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges.

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Highly Stable Photo-Assisted Zinc-Ion Batteries via Regulated Photo-Induced Proton Transfer.

Photo-assisted ion batteries utilize light to boost capacity but face cycling instability due to complex charge/ion transfer under illumination. This study identified photo-induced proton transfer (photo-induced PT) as a significant process in photo-(dis)charging of widely-used V2O5-based zinc-ion batteries, contributing to enhanced capacity under illumination but jeopardizing photo-stability. Photo-induced PT occurs at 100 ps after photo-excitation, inducing rapid proton extraction into V2O5 photoelectrode. This process creates a proton-deficient microenvironment on surface, leading to repetitive cathode dissolution and anode corrosion in each cycle. Enabling the intercalated protons from photo-induced PT to be reversibly employed in charge-discharge processes via the anode-alloying strategy achieves high photo-stability for the battery. Consequently, a ~54 % capacity enhancement was achieved in a V2O5-based zinc-ion battery under illumination, with ~90 % capacity retention after 4000 cycles. This extends the photo-stability record by 10 times. This study offers promising advancements in energy storage by addressing instability issues in photo-assisted ion batteries.

Insight on Shallow Trap States-Introduced Photocathodic Performance in n-Type Polymer Photocatalysts.

Graphitic carbon nitride (g-C3N4) is a robust organic semiconductor photocatalyst with proven H2 evolution ability. However, its application in a photoelectrochemical system as a photocathode for H2 production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C3N4 photocathodes in recent years, factors affecting an n-type semiconductor's properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C3N4 photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C-OH terminal groups. As compared to the g-C3N4 photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C3N4 is 5 times longer than that observed for g-C3N4. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around -10 µA/cm2 at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott-Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C3N4, thus further demonstrating g-C3N4 can be both a photoanode and a photocathode candidate.

A Nanojunction Polymer Photoelectrode for Efficient Charge Transport and Separation.

A metal-free photoanode nanojunction architecture, composed of B-doped carbon nitride nanolayer and bulk carbon nitride, was fabricated by a one-step approach. This type of nanojunction (s-BCN) overcomes a few intrinsic drawbacks of carbon nitride film (severe bulk charge recombination and slow charge transfer). The top layer of the nanojunction has a depth of ca. 100 nm and the bottom layer is ca. 900 nm. The nanojunction photoanode results into a 10-fold higher photocurrent than bulk graphitic carbon nitride (G-CN) photoanode, with a record photocurrent density of 103.2 µA cm-2 at 1.23 V vs. RHE under one sun irradiation and an extremely high incident photon-to-current efficiency (IPCE) of ca. 10 % at 400 nm. Electrochemical impedance spectroscopy, Mott-Schottky plots, and intensity-modulated photocurrent spectroscopy show that such enhancement is mainly due to the mitigated deep trap states, a more than 10 times faster charge transfer rate and nearly three times higher conductivity due to the nanojunction architecture.

Heavily Doped Carbon Nitride Nanocrystal Promotes Visible-Near-Infrared Photosynthesis of Hydrogen Peroxide with Near-Unit Photon Utilization.

Direct photosynthesis of hydrogen peroxide (H2O2) from water and oxygen represents an intriguing alternative to the current indirect process involving the reduction and oxidation of quinones. However, limited light utilization and sluggish charge transfer largely impede overall photocatalytic efficiency. Herein, we present a heavily doped carbon nitride (CNKLi) nanocrystal for efficient and selective photoproduction of H2O2 via a two-electron oxygen reduction reaction (ORR) pathway. CNKLi induces metal-to-ligand charge transfer (MLCT) and electron trapping, which broadens the light absorption to the visible-near-infrared (vis-NIR) spectrum and prolongs the photoelectron lifetime to the microsecond time scale with an exceptional charge diffusion length of ∼1200 nm. Near-unit photoutilization with an apparent quantum yield (AQY) of 100% for H2O2 generation is achieved below 420 nm. Impressively, CNKLi exhibits an appreciable AQY of 16% at 700 nm, which reaches the absorption capacity (∼16%), thus suggesting a near-unit photon utilization <700 nm. In situ characterization and theoretical calculations reveal the facilitated charge transfer from K+ to the heptazine ring skeleton. These findings provide an approach to improve the photosynthetic efficiency of direct H2O2 preparation in the vis-NIR region and expand applications for driving kinetically slow and technologically desirable oxidations or high-value chemical generation.

Realizing Unassisted Photo-Charging of Zinc-Air Batteries by Anisotropic Charge Separation in Photoelectrodes.

Solar rechargeable zinc-air battery is a promising approach for capturing and storing intermittent solar energy through photoelectrochemical reactions. However, unassisted photo-charging of zinc-air batteries is challenging due to suboptimal carrier accumulation on photoelectrodes, resulting in sluggish reaction kinetics. Here, unassisted photo-charging of zinc-air battery is achieved by investigating anisotropic photogenerated charge separation on a series of representative semiconductors (ZnIn2 S4 , TiO2 , and In2 O3 ), among which the exceptional anisotropic charge separation on a ZnIn2 S4 photoelectrode is revealed based on anisotropic charge diffusion capabilities. The charge separation is facet-dependent, which is observed using Kelvin probe force microscopy, verifying a cause-and-effect relationship between the photo-charge accumulation on photoelectrodes and their photo-charging performance in zinc-air batteries. This work achieves an unassisted photo-charging current density of 1.9 mA cm-2 with a light-to-chemical energy conversion efficiency of 1.45%, highlighting the importance of anisotropic semiconductors for unassisted photo-charging of zinc-air batteries via efficient photogenerated charge separation.

Plasma-Tuned nitrogen vacancy graphitic carbon nitride sphere for efficient photocatalytic H2O2 production.

Graphitic carbon nitride (CN) is a promising photocatalyst for sustainable energy conversion. Meanwhile, N vacancies are useful for H2O2 generation; however, they are hard to control. In this study, the N vacancy CN sphere (NVCNS) is synthesized by H2 plasma treatment to tune the NV. The as-synthesized NVCNS exhibits an efficient and stable photocatalytic H2O2 yield of 4413.1 µmol gcat-1h-1, which is 2.5 and 4.6 times higher than that of CNS (1766.4 µmol gcat-1h-1) and bulk CN (956.6 µmol gcat-1h-1), respectively, using a Xe lamp with an intensity of 100 mWcm-2. In particular, the charges recombination rate is remarkably reduced by introducing N defect state, promoting electron accumulation and O2 adsorption, through theoretical calculation and experiments. Furthermore, the NV creates abundant unsaturated sites and induces strong interlayer interactions, leading to effective electronic excitation and the promotion of charge transport.

Light-Fueled Hydrogel Actuators with Controlled Deformation and Photocatalytic Activity.

Hydrogel actuators have shown great promise in underwater robotic applications as they can generate controllable shape transformations upon stimulation due to their ability to absorb and release water reversibly. Herein, a photoresponsive anisotropic hydrogel actuator is developed from poly(N-isopropylacrylamide) (PNIPAM) and gold-decorated carbon nitride (Au/g-C3 N4 ) nanoparticles. Carbon nitride nanoparticles endow hydrogel actuators with photocatalytic properties, while their reorientation and mobility driven by the electrical field provide anisotropic properties to the surrounding network. A variety of light-fueled soft robotic functionalities including controllable and programmable shape-change, gripping, and locomotion is elicited. A responsive flower-like photocatalytic reactor is also fabricated, for water splitting, which maximizes its energy-harvesting efficiency, that is, hydrogen generation rate of 1061.82 µmol g-1 h-1 , and the apparent quantum yield of 8.55% at 400 nm, by facing its light-receiving area adaptively towards the light. The synergy between photoactive and photocatalytic properties of this hydrogel portrays a new perspective for the design of underwater robotic and photocatalytic devices.

Ru and RuOx decorated carbon nitride for efficient ammonia photosynthesis.

Photocatalytic ammonia synthesis is a promising strategy for sustainable development compared to the energy-intensive industrial Haber-Bosch approach. Herein, a ternary heterostructure that consists of ruthenium species and carbon nitride (C3N4) was rationally explored for ammonia photosynthesis. Compared to the small ammonia yield from the g-C3N4 and Ru/g-C3N4 system, the Ru/RuO2/g-C3N4 system represents 6 times higher activity with excellent stability under full-spectrum irradiation. Such an enhancement is not only due to efficient transfer of electrons and holes to Ru and RuO2, respectively, facilitating both the reduction and oxidation reaction, but also taking advantage of Ru for N[triple bond, length as m-dash]N activation.

The role of carbon dots - derived underlayer in hematite photoanodes.

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

Key factors affecting photoelectrochemical performance of g-C3N4 polymer films.

We investigated the relationship between crystallinity, deep trap states and PEC performance of g-C3N4 photoelectrodes. Long-lived charge carriers were present in the more poorly crystalline samples, due to deeper trap states, which inversely correlated with photoelectrochemical performance. The charge diffusion length in a compact g-C3N4 film was determined to be ca. 1000 nm.

Tunable Covalent Triazine-Based Frameworks (CTF-0) for Visible-Light-Driven Hydrogen and Oxygen Generation from Water Splitting.

Covalent triazine-based frameworks (CTFs), a group of semiconductive polymers, have been identified for photocatalytic water splitting recently. Their adjustable band gap and facile processing offer great potential for discovery and development. Here, we present a series of CTF-0 materials fabricated by two different approaches, a microwave-assisted synthesis and an ionothermal method, for water splitting driven by visible-light irradiation. The material (CTF-0-M2) synthesized by microwave technology shows a high photocatalytic activity for hydrogen evolution (up to 7010 µmol h-1 g-1), which is 7 times higher than another (CTF-0-I) prepared by conventional ionothermal trimerization under identical photocatalytic conditions. This leads to a high turnover number (TON) of 726 with respect to the platinum cocatalyst after seven cycles under visible light. We attribute this to the narrowed band gap, the most negative conduction band, and the rapid photogenerated charge separation and transfer. On the other hand, the material prepared by the ionothermal method is the most efficient one for oxygen evolution. CTF-0-I initially produces ca. 6 times greater volumes of oxygen gas than CTF-0-M2 under identical experimental conditions. CTF-0-I presents an apparent quantum efficiency (AQY) of 5.2% at 420 nm for oxygen production without any cocatalyst. The activity for water oxidation exceeds that of most reported CTFs due to a large driving force for oxidation and a large number of active sites. Our findings indicate that the band positions and the interlayer stacking structures of CTF-0 were modulated by varying synthesis conditions. These modulations impact the optical and redox properties, resulting in an enhanced performance for photocatalytic hydrogen and oxygen evolution, confirmed by first-principles calculations.