Organic Photocathodes Research Articles

In situ polymerised polyaniline films over multi-walled carbon nanotubes coatings for enhanced photoelectrochemical performance

Polyaniline (PANI), which is a π-conjugated polymer, has stood out as a promising candidate for dihydrogen generation in photoelectrochemical (PEC) cells owing to its suitable optoelectronic properties and excellent stability. Nevertheless, the PEC performance of this material is still far from adequate for technological applications. Herein, we have focused on improving the PEC performance of PANI films by combining this polymer with multi-walled carbon nanotubes (MWCNTs) layers. Specifically, we have assessed the PEC activity of the in situ polymerised PANI films onto FTO substrates coated with different amounts of MWCTNs for the hydrogen evolution reaction (HER). The FTO/MWCNTs/PANI films with the optimum deposited amount of MWCNTs (i.e., 30 μL) featured a 2.4-fold increase of the cathodic photocurrent density response for the HER compared to that of FTO/PANI film. Moreover, the FTO/(30 μL)-MWCNTs/PANI films exhibited good PEC stability, as observed by the relatively low cathodic photocurrent density decay of up to 7% during 3720 s of stability test. Based on joint analyses of physical and PEC characterisation, the improved PEC performance for the FTO/(30 μL)-MWCNTs/PANI films were assigned to the combined contribution of the following factors: (i) enhancement of light absorption; (ii) possible improvement of excitons dissociation; (iii) minimisation of the electron-hole recombination process; and (iv) facilitation of electrons transfer at the PANI|electrolyte interface for the HER. The enhancement of PEC performance achieved in this work for the fully organic photocathode composed of MWCNTs/PANI paves the way to invest in additional modifications seeking to further improve the dissociation of excitons and the interfacial transfer of charge carriers for the light-driven HER.

Dual-Photoelectrode Fuel Cell Based Self-Powered Sensor for a Picomole Level Pollutant: Using an In Situ Molecularly Imprinted p-Type Organic Photocathode.

Developing a dual-photoelectrode fuel cell based self-powered sensor (DPFC-SPS) with an ideal signal output capability and high sensitivity performance for the detection of environmental pollutant atrazine (ATZ) has an important value. In this work, the in situ molecularly imprinting functionalized p-type organic semiconductor polyterthiophene (MI-pTTh) is used as a photocathode to construct a DPFC-SPS toward the typical environmental pollutant ATZ for the first time. Due to its excellent photoactivity, higher stability, and superior oxygen reduction reaction activity, pTTh serves as the photocathode material for constructing a self-powered sensing platform with a stable signal output and high photoelectric activity. Based on the sensitive light-triggered large self-bias of the DPFC-SPS, the open circuit potential (EOCV) of the device reaches 1.21 V and the maximum power density (Pmax) reaches 121.5 μW·cm-2, which is much higher than most reported PFC-SPSs. Simultaneously, in situ molecularly imprinted (MI) functionization of pTTh can further endow it with specific recognition ability, helping the constructed SPS achieve high sensitivity, selectivity, and effective recognition of the important environmental pollutants ATZ in complex systems. It exhibits a broad linear relationship from 0.002 to 100 nM and a low detection limit (estimated by S/N > 3) of 0.21 pM toward ATZ. The mechanism of the binding kinetics of the MI-pTTh with the target ATZ is further studied via in situ infrared spectroscopy. This work provides theoretical guidance for sensing strategies using dual-photoelectrode devices and offers a rational device design for cost-effective electricity generation from renewable resources.

An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting

AbstractPhotoelectrochemical cells employing organic semiconductors (OS) are promising for solar‐to‐fuel conversion via water splitting. However, despite encouraging advances with the half reactions, complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near‐neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge‐selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm−2 at 0 V vs the reversible hydrogen electrode (VRHE) for solar water reduction with noticeable operational stability (retaining ≈90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large‐area (2.4 cm2) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar‐to‐hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two‐electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H2 and O2 production.

Sp-Carbon Incorporated Conductive Metal-Organic Framework as Photocathode for Photoelectrochemical Hydrogen Generation.

Metal-organic frameworks (MOFs) have attracted increasing interest for broad applications in catalysis and gas separation due to their high porosity. However, the insulating feature and the limited active sites hindered MOFs as photocathode active materials for application in photoelectrocatalytic hydrogen generation. Herein, we develop a layered conductive two-dimensional conjugated MOF (2D c-MOF) comprising sp-carbon active sites based on arylene-ethynylene macrocycle ligand via CuO4 linking, named as Cu3 HHAE2 . This sp-carbon 2D c-MOF displays apparent semiconducting behavior and broad light absorption till the near-infrared band (1600 nm). Due to the abundant acetylene units, the Cu3 HHAE2 could act as the first case of MOF photocathode for photoelectrochemical (PEC) hydrogen generation and presents a record hydrogen-evolution photocurrent density of ≈260 μA cm-2 at 0 V vs. reversible hydrogen electrode among the structurally-defined cocatalyst-free organic photocathodes.

Organic Photocathode Supported by Copper Nanosheets Array for Overall Water Splitting.

The Z-scheme overall solar water splitting is a mimic of natural photosynthesis to convert solar energy into chemical energy. Since the energy levels of most organic semiconductors match well with the hydrogen evolution potential, they have great application prospects as photocathodes in Z-scheme photoelectrochemical systems. However, due to the weak light absorption and difficult carrier separation, the photocurrent density and onset potential of organic photocathodes are still low. To solve these problems, we introduced a copper nanosheets array (Cu NSA) framework under organic layers to increase the surface reaction sites, improve the light absorption and enhance the distribution range of built-in electric field simultaneously. As a result, the photocurrent density and onset potential of poly(3-hexylthiophene) : [6,6]-phenyl-C61 -butyric acid (P3HT : PCBM) photocathode were enhanced significantly. The onset potential increased by 50 mV to 0.65 V vs. RHE, and the photocurrent density reached -1 mA cm-2 at 0 V vs. RHE, which was 18 times that of the sample without Cu NSA. The optimized photocathode was connected with titanium dioxide nanorods array photoanode in a tandem manner to realize the spontaneous overall water splitting. Without bias and co-catalyst, the photocurrent density was maintained at 110 μA cm-2 and the solar-to-fuel conversion efficiency was 0.14 % in neutral solution. These results provide a feasible method for optimizing the performance of organic photocathodes.

Organic π‐Conjugated Polymers as Photocathode Materials for Visible‐Light‐Enhanced Hydrogen and Hydrogen Peroxide Production from Water

AbstractThe photo(electro)chemical reduction of water and oxygen to produce hydrogen (water‐splitting) and hydrogen peroxide, respectively, are well developed with inorganic semiconductors. In contrast, organic π‐conjugated polymers, especially precisely synthesized ones, have been extensively studied as photoharvesting and charge‐separating materials in dry photoelectron‐conversion devices, such as organic photovoltaic cells. However, the use of conjugated polymers as photocathodes; i.e., photoelectrocalatalytic reduction‐active materials in direct contact with aqueous electrolytes, has been less explored. This review describes the fundamentals of the electrochemistry of water and oxygen reduction, as well as organic semiconductor functions for photocathode activity in water, along with figures of merit for organic photocathodes used to practically produce hydrogen and hydrogen peroxide. Recent research on conjugated polymers in this field is then surveyed after briefly referring to early trial‐and‐error investigations using conventional conjugated polymers. Basic and very simple designs of robust polythiophene films are demonstrated as examples of successful materials which act as both visible‐light harvesters and catalysts for hydrogen evolution and the production of hydrogen peroxide, and as counterparts in combination with conventional oxygen‐evolution catalysts. Perspectives for this emerging field are also briefly provided.

Nonfullerene Bulk Heterojunction‐Based Photocathodes for Highly Efficient Solar Hydrogen Production in Acidic and Neutral Solutions

AbstractHighly efficient photocathode with onset potential beyond 0.6 V is very desirable in order to construct photoelectrochemical (PEC) device for stand‐alone overall water splitting. In this work, it is reported a nonfullerene PBDB‐T:ITIC bulk heterojunction (BHJ) based photocathode delivering unprecedented positive onset potential of 0.8 V and optimal photocurrent density of −11.7 mA cm−2 at 0 V in pH 1 solution. The yielded high ideal ratiometric power‐saved efficiency (Φsaved,ideal) of 4.1% is a record among the previous organic photocathodes. When the electrolyte pH moves to neutral, the Φsaved,ideal is still to be 3.2%, yet being twice more than the currently reported highest value. Pairing with TiOx, the hole transport layer of CuOx with more efficient charge carrier transportation and hole extraction ability excels MoO3 and NiOx, contributing to the highly efficient solar hydrogen production in both acidic and neutral solutions. Premixing the BHJ blend precursor with an anti‐oxidative agent is proved to be a beneficial strategy to improve the PEC stability by suppressing the photo‐oxidation degradation of organic polymer materials. Subsequently a conceptual PEC cell comprising of tandem CuOx/BHJ/TiOx/Pt photocathode and BiVO4/NiFeOx photoanode with a high solar‐to‐hydrogen efficiency up to 1.1% is fabricated in neutral solution without assistance of extra bias.

Molecular Engineering of Conjugated Acetylenic Polymers for Efficient Cocatalyst‐free Photoelectrochemical Water Reduction

AbstractConjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene‐ to thiophene‐based. The polarized thiophene‐based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on‐surface Glaser polycondensation, as‐fabricated poly(2,5‐diethynylthieno[3,2‐b]thiophene) on commercial Cu foam exhibits a record H2‐evolution photocurrent density of 370 μA cm−2 at 0.3 V vs. reversible hydrogen electrode among current cocatalyst‐free organic photocathodes (1–100 μA cm−2). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high‐activity organic photoelectrodes.

Molecular Engineering of Conjugated Acetylenic Polymers for Efficient Cocatalyst-free Photoelectrochemical Water Reduction.

Conjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene- to thiophene-based. The polarized thiophene-based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on-surface Glaser polycondensation, as-fabricated poly(2,5-diethynylthieno[3,2-b]thiophene) on commercial Cu foam exhibits a record H2 -evolution photocurrent density of 370 μA cm-2 at 0.3 V vs. reversible hydrogen electrode among current cocatalyst-free organic photocathodes (1-100 μA cm-2 ). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high-activity organic photoelectrodes.

Poly(1,4-Diethynylbenzene) Gradient Homojunction with Enhanced Charge Carrier Separation for Photoelectrochemical Water Reduction.

As appealing photoelectrode materials for photoeletrochemical hydrogen evolution reaction (PEC HER), conjugated polymers still show poor PEC HER performance as a result of their serious recombination of photogenerated electrons and holes. Herein, a novel design of gradient homojunction is demonstrated by controlled copolymerization of 1,4-diethynylbenzene (DEB) and 1,3,5-triethynylbenzene (TEB). The as-built gradient distribution of TEB monomer in poly(1,4-diethynylbenzene) (pDEB) leads to continuous band bending engineering, which constitutes a gradient homojunction. Under AM 1.5G irradiation and in 0.1 m Na2 SO4 aqueous solution, the as-fabricated pDEB gradient homojunction exhibits a charge separation efficiency of 0.27% at 0.3 V versus reversible hydrogen electrode (RHE), which is 3.4 and 1.7 times higher than those for pure pDEB and the traditionally designed pDEB homojunction. As a result, the photocurrent of the pDEB gradient homojunction unprecedentedly reaches 55 µA cm-2 at 0.3 V versus RHE, which is much higher than 19 µA cm-2 for pure pDEB, 32 µA cm-2 for the pDEB homojunction, and state-of-the-art organic photocathodes, e.g., g-C3 N4 (≈1-32 µA cm-2 ). This work opens up a new window for the design of gradient homojunctions and will advance the exploration of high-performance organic photoelectrodes.

(Invited) The Benefits of Nanoscale Metal Oxide Films to Solar Fuels Research

Though the field of photoelectrochemical (PEC) solar fuel production focuses on a broad variety of materials, metal oxides are probably the most commonly employed materials in state-of-the-art photoelectrodes. Metal oxides such as bismuth vanadate and hematite are currently intensely studied as photoanode materials in our group and by the PEC water oxidation community.1-4 Stable metal oxides such as TiO2 have been successfully employed as protection layers in, for example, Cu2O photocathodes and are now advancing the field of organic photocathodes for hydrogen evolution.5, 6 Especially metal oxides grown by atomic layer deposition (ALD) offer many benefits to solar fuel research such as i) precise growth control on the atomic scale ii) conformal and pinhole-free coatings and ii) formation of high-quality interfaces. In our studies of recombination losses, we benefit from high-quality ALD hematite films and are able to gain new insights from electrochemical impedance spectroscopy that will be reported here. Furthermore, we could demonstrate stable organic photocathodes for hydrogen evolution and will report on new organic photocathode designs employing ALD TiO2 electron selective layers.6 Finally, I will discuss the importance of ALD surface treatments in the science of CO2 reduction.7 Le Formal, F.; Pastor, E.; Tilley, S. D.; Mesa, C. A.; Pendlebury, S. R.; Gratzel, M.; Durrant, J. R. Rate Law Analysis of Water Oxidation on a Hematite Surface. Journal of the American Chemical Society 2015, 137, 6629-6637.Mesa, C. A.; Kafizas, A.; Francas, L.; Pendlebury, S. R.; Pastor, E.; Ma, Y. M.; Le Formal, F.; Mayer, M. T.; Gratzel, M.; Durrant, J. R. Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes. Journal of the American Chemical Society 2017, 139, 11537-11543.Ma, Y. M.; Mesa, C. A.; Pastor, E.; Kafizas, A.; Francas, L.; Le Formal, F.; Pendlebury, S. R.; Durrant, J. R. Rate Law Analysis of Water Oxidation and Hole Scavenging on a Bivo4 Photoanode. Acs Energy Letters 2016, 1, 618-623.Ma, Y. M.; Kafizas, A.; Pendlebury, S. R.; Le Formal, F.; Durrant, J. R. Photoinduced Absorption Spectroscopy of Copi on Bivo4: The Function of Copi During Water Oxidation. Advanced Functional Materials 2016, 26, 4951-4960.Luo, J. S.; Steier, L.; Son, M. K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2o Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Letters 2016, 16, 1848-1857.Steier, L.; Bellani, S.; Rojas, H. C.; Pan, L. F.; Laitinen, M.; Sajavaara, T.; Di Fonzo, F.; Gratzel, M.; Antognazza, M. R.; Mayer, M. T. Stabilizing Organic Photocathodes by Low-Temperature Atomic Layer Deposition of Tio2. Sustainable Energy & Fuels 2017, 1, 1915-1920.Schreier, M.; Heroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J. S.; Mayer, M. T.; Luo, J. S.; Gratzel, M. Solar Conversion of Co2 to Co Using Earth-Abundant Electrocatalysts Prepared by Atomic Layer Modification of Cuo. Nature Energy 2017, 2.

Rational design of a neutral pH functional and stable organic photocathode.

In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.

Graphene-Based Hole-Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the workhorse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs RHE (J0 V vs RHE) of up to 8 mA cm–2. The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron- (ESL) and hole- (HSL) selective layers. While TiO2 and its substoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation under hydrogen evolution reaction (HER) working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost the efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J0 V vs RHE of 6.01 mA cm–2, onset potential (Vo) of 0.6 V vs RHE, ratiometric power-saved efficiency (φsaved) of 1.11%, and operational activity of 20 h in 0.5 M H2SO4 solution. Moreover, the designed photocathodes are effectively working in different pH environments ranging from acidic to basic. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the scalability of our all-solution-processed approach by fabricating a large-area (∼9 cm2) photocathode on flexible substrate, achieving a remarkable cathodic J0 V vs RHE of 2.8 mA cm–2, Vo of 0.45 V vs RHE, and φsaved of 0.31%. This is the first demonstration of highly efficient rr-P3HT:PCBM flexible photocathodes based on low-cost and solution-processed manufacturing techniques.

Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes.

Here, we have developed an organic photocathode for water reduction to H2 , delivering more than 1 mA cm-2 at 0 V versus RHE and above 3 mA cm-2 at -0.5 V versus RHE with moderate stability under neutral pH conditions. The initial competitive reduction of water to H2 and ZnO to metallic Zn is responsible for the dynamic behaviour of both photocurrent and Faradaic efficiency of the device, which reaches 100 % Faradaic efficiency after 90 min operation. In any case, outstanding stable H2 flow of approximately 2 μmol h-1 is measured over 1 h at 0 V versus RHE and at neutral pH, after equilibrium between the Zn2+ /Zn0 concentration in the AZO film is reached. This achievement opens new avenues for the development of allsolution-processed organic photoelectrochemical cells for the solar generation of H2 from sea water.

All solution-processed organic photocathodes with increased efficiency and stability via the tuning of the hole-extracting layer

This study correlates the work function of the hole-extracting layer and the efficiency of photocathodes for the photo-production of hydrogen.

Toward Stable Solar Hydrogen Generation Using Organic Photoelectrochemical Cells

Organic photoactive materials are promising candidates for the generation of solar fuels in terms of efficiency and cost. However, their low stability in aqueous media constitutes a serious problem for technological deployment. Here we present organic photocathodes for the generation of hydrogen in aqueous media with outstanding stability. The device design relies on the use of water-resistant selective contacts, which protect a P3HT:PCBM photoactive layer. An insoluble cross-linked PEDOT:PSS hole-selective layer avoids delamination of the film, and an electron-selective TiOx layer in contact with the aqueous solution electrically communicates the organic layer with the hydrogen-evolving catalyst (Pt). We developed a novel method for the synthesis of the TiOx layer compatible with low-temperature conditions. Tuning the thickness of the TiOx/Pt layer leads to a trade-off between the achievable photocurrent (∼1 mAcm–2) and the stability of the photocathode (stable hydrogen generation of 1.5 μmol h–1 cm–2 fo...

Investigation of New Organic Photocathodes in Vacuum for Position Sensitive Gaseous Detectors

Position sensitive gaseous detectors with reflective photocathodes provide an attractive solution for photon localization over very large sensitive areas, under intense magnetic fields mainly due to their ease of construction. We report Quantum efficiency measurement of two new organic semiconductor photocathodes PEDOT/PSS and TECEB in spectral region 190-250nm, which are proposed for use in photosensitive gaseous detectors. Photocathodes were prepared by using resistive thermal vacuum evaporation technique. QE were measured in vacuum at about 10 -4 Torr and both the photocathodes exhibit non-negligible quantum efficiency up to 230nm.

Organic UV photocathodes for gaseous particle detectors

We report on the result of search for organic photocathodes, to be used in multiplicative particle detectors, as alternatives to TMAE. We review the measurements we performed in recent years, in the wavelength range of 150–220 nm. The quantum efficiencies are given relative to TMAE and CsI. Some ferrocene derivatives exhibit relatively high quantum efficiency.

Liquid and solid organic photocathodes

We have investigated the possibility of creating photocathodes for gaseous detectors with a high sensitivity in the photon spectral region between 105 and 300 nm. Metal cathodes covered with liquid or solid organic layers, such as tetrakis (dimethylamine)ethylene (TMAE) and tetramethyl-p-phenylenediamine (TMPD) and solutions of these substances, were studied using two different experimental setups: a proportional wire chamber and a single-wire counter. There effects were observed. First, a thin film of absorbed vapours led to the efficiency of the metallic cathode being increased by more than one order of magnitude at photon wavelengths up to λ ≈ 400 nm. Secondly, a thick layer of liquid strongly increased the cathode efficiency Q for radiations of λ < 270 nm: e.g. with TMAE we obtained a yield of about 0.5%, for λ = 235 nm. Thirdly, we found that some solid layers, such as neopentane+TMAE, give a maximum efficiency Q ≈ 3% at λ = 235 nm. The quantum yields of liquid and solid photocathodes increase with increasing applied electric field. Some applications of the liquid and solid photocathodes are discussed.