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Photogenerated Hydrogen by Organic Catalytic Systems

Final Report Summary - PHOCS (Photogenerated Hydrogen by Organic Catalytic Systems)

Executive Summary:
The aim of the project “Photogenerated Hydrogen by Organic Catalytic Systems (PHOCS)” is the realization of a new concept, a photo-electrochemical system for hydrogen production based on hybrid organic/inorganic and organic/liquid interfaces, aiming at exploiting the advantages offered by both organic and inorganic technologies in synergy.
PHOCS intended to combine the visible-light absorption properties of organic semiconducting materials, and more specifically of conjugated polymers, together with the enhanced charge transport capabilities of nanostructured inorganic semiconductors. Organic materials may also provide per se a novel photo-electrochemical paradigm, offering optoelectronic properties hitherto unexplored in photocatalytic applications, and in particular: i) the opportunity to closely match, by proper chemical tailoring, the oxidation/reduction levels required for Water Splitting (WS) processes, ii) avoiding the use of expensive and rare sacrificial agents and noble metal-based catalysts; iii) enhanced visible-light absorption and low-cost processing technologies, thus overcoming the traditional limitations of inorganic technology.
A typical shortcoming of organic semiconductors is poor charge transport. On the other side, inorganic semiconductors can ensure better transport properties, here amplified by the supra-hierarchical organization of the electrode contacts. In order to increase the surface area available for photoelectrochemical reactions, to provide ohmic contact with the organic active layer, to exploit the three-dimensional control of the charge-transfer reactions and ultimately to increase the overall photo-electrochemical efficiency of the device, nanostrcutured electrodes have to be properly developed and integrated with the light sensitive layer within a photoelectrochemical cell. Finally, issues like environmental stability, catalytic functionality, efficiency of electron transfer processes and overall photo-electrochemical performance had to be addressed.

The concept proposed by PHOCS was fully accomplished, demonstrating for the first time that organic semiconductors can be reliably employed within a photoelectrochemical system for hydrogen production.
The feedback loop between science and technology was at the basis of the PHOCS project, and it ensured advances in both science (new phenomena, tools and techniques) and technology (new devices and systems design). Spectro-electrochemical characterization of organic/inorganic and organic/electrolytic solution interfaces wascontinuously performed, thus providing in-deep characterization of the system and allowing to continuously improve the performances of the devices.
Several architectures were realized, reaching in some cases state-of-the-art performances in terms of photocurrent density and onset potential values. The most efficient PHOCS device showed photocurrent density of 8 mA/cm2 @0V vs. RHE, exceeding by more than two orders of magnitude the initially targeted objective of 50 uA/cm2, an onset potential of 0.7 V, IPCE above 50%. Faradaic efficiency is 100% and ideal ratiometric power-saved figure of merit is equal to 1.21%. Improtantly, some PHOCS devices were demonstrated to work in aqueous saline solutions, at neutral pH, without use of precious catalysts. Moreover, all realized architectures are based on a range of fully up-scalable techniques, thus offering the perspective of industrial exploitation.

PHOCS outcomes demonstrate the reliability of the proposed approach, assessing for the first time the possibility to use hybrid organic/inorganic systmes for photoelectrochemical hydrogen production.

Project Context and Objectives:
When PHOCS activities started, organic semiconductors were almost completely unexplored for water splitting applications, and in particular for hydrogen production. PHOCS overall addressed a twofold objective:

(i) The basic description of the main physical and chemical phenomena taking place in the various cell layers interphases, through a combination of optoelectronic, spectroscopic and microscopy techniques. Characterization of organic/inorganic and organic/electrolytic solution interfaces was continuously required for the whole project duration; a continuous feedback between synthesis, nanostructuring and characterization activities was in fact mandatory, in order to obtain materials and structures that optimize photoelectrochemical hydrogen production.

(ii) Engineering and fabrication of a new hybrid organic/inorganic PEC cell, which represented a complete innovation in the field of water splitting. Final aim of PHOCS project was the realization of a scaled-up, 10x10 cm2, 1% solar-to-hydrogen energy conversion efficiency device, as a tangible first step towards the new “organic water splitting” technology. The final cell was expected to be able to produce energy stored as hydrogen, converting sunlight radiation energy through a fully photo-electrochemical process. Planned steps leading to the realization of a final device were: a) synthesis of electron donor organic polymers and fullerenes acceptors, properly designed from the energetic, catalytic and stability point of view; b) choice of suitable p-type inorganic materials to be structured and used as highly conductive electrodes; c) coupling of organic polymer/fullerene active layer with the engineered inorganic electrode, and materials and structure optimization in order to achieve the best possible energetic matching; d) device scaling for prototypical cell realization and optimization of its efficiency.

PHOCS expected output is thus a proof-of-concept photoelectrochemical device, based on comparative evaluation of different systems and identification of the most promising technological solution in terms of fabrication easiness, materials availability and production costs, device geometries, overall photon to hydrogen conversion efficiency (target objective: 1%), corrosion resistance, estimated final cost/mole, lifetime (target objective: 4 weeks under visible light irradiation, working in photovoltaic conditions).

Specific scientific and technical objectives are listed below:
(1) Synthesis of organic conjugated polymers and chemically modified fullerenes.
(2) Morphological and electronic engineering of the inorganic electrode through nanostructuring aiming at the best possible compromise between active surface area, polymers infiltration and efficient charge transfer and transport. Particular attention should be devoted to the interface with the organic materials aiming at an ideal ohmic contact by proper band engineering of the inorganic component. All oxide (transparent or semi-transparent) and metal-oxide (opaque) material systems should be considered.
(3) Definition of the energetic diagram of the inorganic electrode/organic active layer/aqueous electrolyte cell (M7).
(4) Full opto-electronic characterization of the interfaces phenomena (organic/inorganic and organic/electrolyte), in terms of charge transfer and charge transport.
(5) Realization of a large area cell (active area approx. 10 x 10 cm2) as a proof-of-concept photoelectrochemical device.

Project Results:
The main concept proposed by PHOCS, namely the realization of hybrid organic/inorganic devices for photo-electrochemical water splitting and hydrogen production, was accomplished, fully demonstrating its reliability and huge application potential.
First of all, we demonstrated that conjugated polymers can efficiently work as light-absorbing materials within a photo-electrochemical cell, thus in direct contact with aqueous and/or non aqueous electrolytes. In such conditions the organic semiconductors fully preserve their peculiar optoelectronic properties, and in particular their capability to efficiently generate electrical charges upon visible light excitation. The main phenomena occurring at the interface between organic polymers and electrolytes were described in detail for the first time, by making recourse to a combination of different spectroscopic and microscopic techniques.
As an intermediate project result, it was demonstrated that conjugated polymers can efficiently promote oxygen reduction reactions, thus offering the possibility to be exploited as light-absorbing, functional materials for the realization of highly sensitive, photo-electrochemical oxygen sensors.
Instead, the intrinsic catalytic activity towards hydrogen reduction of all tested organic materials, including newly synthesized electron donors and electron acceptors, was reported to be quite low, giving rise to photocurrent density of less than 50 uA/cm2. This outcome is certainly interesting from a fundamental point of view, since the catalytic activity of organic semiconductors was assessed and fully demonstrated here for the first time in a direct way, by measuring the evolved gas. Despite the photocurrent value matched the technical objective targeted by the project, however, it was considered to be of no interest for practical applications.
The detailed knowledge of the hybrid system developed in parallel within the project represented the necessary, key milestone for the subsequent, successful implementation of the device, based on the coupling between the organic component with inorganic electrodes. In particular, consortium partners recognized the importance of developing proper charge selective layers, both electron- and hole-selective (ESL and HSL, respectively), in order to efficiently extract charges photogenerated within the light-sensitive polymer layer. It was clearly established that one of the main bottlenecks limiting device performances and stability is not represented by the polymer layer itself, but by the possibility to control interfacial phenomena. Multiple strategies were carried out in parallel to efficiently couple multifunctional inorganic electrodes to light sensitive semiconductors, including: three-dimensional hierarchical fluorine-doped tin oxide (FTO) nanostructures; high work function transition metal oxide nanostructured 3D electrodes; nanostructured electrodes based on vertically aligned molybdenum tri-oxide and tungsten trioxide; cuprous iodide nanostructured layers. Different fabrication techniques were employed and engineered, including Pulsed Laser Deposition, Atomic Layer Deposition, Electrochemical Anodization, Solution-based processes. Device performances were carefully evaluated, considering as prominent figures of merit the photocurrent density (PC), the open circuit potential (OCP), the internal photo-conversion efficiency (IPCE) and the overall Faradaic efficiency. Many of the architectures realized in the framework of the PHOCS project show state-of-the-art performances in the use of polymers for photo-electrochemical applications, and in some cases are comparable to fully-inorganic, photocathodes reported in literature.
The first device overcoming the 1 mA/cm2 photocurrent threshold was the one with the following architecture: MoO3, grown by sputtering on top of an FTO substrate and used as an HSL, polymer blend composed by region-regular poly-hexylthiophene doped with fullerene acceptor (P3HT:PCBM) deposited by spin coating, Titanium Oxide (TiOx) deposited by Pulsed Laser Deposition and used as ESL, and platinum used as a catalyst, deposited by sputtering. In this case, a photocurrent density of up to 3 mA/cm2 @0V vs. RHE, at pH 1, and an Open Circuit Potential (OCP) of about 0.67 V were observed. Internal photo-conversion efficiency (IPCE) exceeded 20%. Interestingly, hydrogen evolution was measured by gas-chromatography, and 100% faradaic efficiency was demonstrated. Unfortunately, the overall performance and the temporal stability of this device are strongly affected by proton intercalation processes occurring within the HSL layer, thus requiring a pre-treatment of the device. Having acknowledged the HSL as the key element for the successful realization of an efficient photocathode, we focused our efforts on the use of other materials, able to efficiently collect photo-generated holes and to substitute MoO3. We screened and optimized several possibilities, coming to a final PHOCS photocathode architecture capable of generating 8 mA/cm2 @0V vs. RHE. The initially targeted objective of 50uA/cm2 was thus exceeded by more than 2 orders of magnitude. Faradaic efficiency was estimated to be 100%, and ideal ratiometric power-saved figure of merit equal to 1.21%. Theoretical photoconversion efficiency approaches 10%. Moreover, this architecture was demonstrated to be prone to implementation in an all-solution processed device, to coupling with a non-precious catalyst, to be able to work in different pH conditions and in saline water electrolyte.
The technical objective established for temporal stability was not achieved by the most performing photocathode realized within PHOCS, showing operational stability of less than one hour. Alternative PHOCS devices architectures were thus developed in parallel, aiming at improving both temporal and environmental stability. The most stable system exhibited a 50% decrease of the initial performance after as long as 10 hours, which, while not fulfilling the targeted project objective, still it can be considered as a milestone in the field. Importantly, all PHOCS’ devices were based on a range of fully up-scalable techniques, thus offering an industrial exploitation perspective to the project.
Obtained photocurrent density for the up-scaled device (25 cm2 area) amounts at almost 1 mA/cm2, a value lower than the maximum photocurrent obtained for the lab-scale device, but still well beyond the targeted, final technical objective of the project, with a theoretical photoconversion efficiency of more than 1%. Notably, our up-scaled photocathode was demonstrated to work even in saline, sea-like water, at neutral pH.

Besides demonstrating a completely new concept in the field of photo-electrochemical cells, i.e. assessing for the first time the application potential of visible light-sensitive organic semiconductors, PHOCS activity led to the synthesis of novel materials, both organic and inorganic. Several three-dimensional nanostructures were designed, developed and usefully exploited in a wealth of different device architectures, being optimally coupled to their organic counter-part. New organic materials, both conjugated polymers and electron acceptor fullerenes, were also synthesized. In a parallel effort, newly synthesized organic semiconductors were coupled to molecular catalysts, thus opening the way to their use as chemical sensors in an electrocatalytic environment.
The overall activity was sustained by a strong effort in terms of fundamental physics, aimed at fully understanding the behavior of polymer/electrolyte and polymer/inorganic/electrolyte complex interfacial physics, a completely new field respect to the well-assessed organic photovoltaics, explored here for the first time in a systematic way.

Overall, PHOCS activities showed a fully unprecedented application potential of organic semiconductors in the fields of water splitting and solar fuels.

Potential Impact:
Organic semiconductors represent nowadays a well established technology in the optoelectronics field, having reached the necessary development for industrial (as in the case of light emitting devices) and pre-competitive (like organic solar cells and transistors) use. Surprisingly, before PHOCS, they were never considered for electrochemical applications, and in particualr for photoelectrochemical water splitting. PHOCS project contributed to create a completely new research field, demonstrating for the first time that organic materials can be usefully coupled to inorganic electrodes and used as the visible-light absorbing component within a photo-electrochemical cell. Having established that the hybrid, organic/inorganic device performances are comparable to some state-of-the-art, fully inorganic devices, further intensive investigation is expected in the forthcoming years, aimed at consolidating and boosting the main concepts proposed by PHOCS.
Still, an interdisciplinary research community must be created, including both organic chemists and experts in organic photovoltaics, on one side, and material scientists working on solar fuels, on the other side. For this reason, project partners put a lot of efforts in dissemination activities, presenting PHOCS main results to the two different communities, in the framework of internationally reknowned conferences (>30 contributions, the large majority as oral presentations. Out of these, 8 invited talks to world leading conferences). To the same goal, an international workshop was organized at the SOLAR FUELS15 Conference, entirely dedicated to promoting and disseminating PHOCS outcomes.
PHOCS results were recently published in high-impact, peer reviewed journals; several papers ( ca. 10) are currently under review or in preparation.

PHOCS outcomes go beyond the use of hybrid interfaces for hydrogen production. PHOCS activities provided new knowledge about physical and chemical phenomena occurring at the hybrid organic/electrolyte and organic/inorganic interfaces; new materials, both organic and inorganic; new device architectures. Organic/inorganic systems can be used also in other photo-electrochemical applications, for instance for carbon dioxide reduction. Newly synthesixed fullerenes have been proposed for promoting metal-free oxygen reduction reactions. Other related fields include photovoltaics, organic bioelectronics, photodetection in harsh environmental conditions, inorganic nanostructures processing techniques.

Consortium partners were also involved in several outreach initiatives targeted to the general public, in particular to scholar groups, and were often contacted by mass media for interviews and publication of articles in newspapers and websites.

A proof-of-concept device was finally realized, successfully fullfilling PHOCS targeted objectives in terms of photoconversion efficiency. The device clearly shows how organic/inorganic hybrid interfaces, realized by low-cost and easily up-scalable techniques, can efficiently promote hydrogen production by using visible light, saline water and non precious catalysts (see attached picture). This device will be employed to further exploit project results, and to contact potential stakeholders.

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