Polymeric carbon nitride-based photocathodes for visible light-driven selective reduction of oxygen to hydrogen peroxide☆
Graphical Abstract
Introduction
Polymeric carbon nitrides (PCN) are sustainable, tunable, non-toxic and chemically stable materials [1], [2], [3], [4], that have been utilized in the construction of various photocatalytic systems for a wide range of light-driven reactions, including hydrogen evolution [5], [6], [7], [8], [9], [10], water oxidation [6], [11], [12], CO2 reduction [13], [14], organic pollutant degradation [15], [16], [17], or selective chemical conversions [18], [19], [20], [21]. Notably, a vast majority of the studies on PCN-based photocatalytic systems encompassed investigations of suspensions of PCN powders, whereas studies of photoelectrocatalytic systems utilizing PCN-based photoelectrodes are much less frequent. In this context, it is important to realize that – apart from some drawbacks (e.g., increased installation costs due to photoelectrode and reactor fabrication) – there are several advantages associated with carrying out photoelectrocatalytic reactions in well-designed photoelectrochemical cells. From a technological point of view, it is preferable to have the products of the oxidation and reduction reactions separated in the two compartments of the photoelectrochemical cell. Furthermore, the (photo)anodic and (photo)cathodic half-cells can be first optimized separately, to be – eventually – implemented into a tandem photoelectrochemical device. Finally, from a more fundamental and scientific point of view, photoelectrocatalytic cells allow for studying light-driven conversions also in the absence of any additional (sacrificial) oxidizing and reducing agents which are typically used in photocatalytic studies using suspensions. Indeed, it is one of the many significant scientific contributions of Professor Detlef Bahnemann that he has repeatedly pointed out how the activity of many photocatalytic systems is often dictated by the reactivity of the sacrificial reagents, rather than by the intrinsic charge-separation dynamics and kinetics of photocatalyzed redox reactions under investigation [22], [23]. Studies of truly photoelectrocatalytic systems without any additional reducing and oxidizing agents are therefore of paramount importance.
However, the fabrication of PCN-based photoelectrodes is rather challenging, mainly due to poor adhesion of PCN to conductive substrates [24], [25], [26], [27] and low conductivity of PCN that hinders the transport of photogenerated charge carriers into the external circuit [28]. The development of effective PCN-based photoanodes has been enabled mainly by two approaches. Either conventional [26], [27] or ionic carbon nitride [29], [30] films were directly deposited onto conductive substrates, or PCN was deposited onto porous metal oxide (e.g., TiO2 or ITO) films acting as a scaffold and an effective n-type electron collector [11], [12], [31], [32], [33], [34], [35], [36]. In a similar vein, PCN-based photocathodes have been fabricated either as pristine [37] or biopolymer-activated films [38], or in combination with p-type semiconductors (e.g., CuGaSe2 [39], CuI [40] or NiO [41], [42]) acting as effective hole collectors.
One of the most attractive reductive conversions that is rather effectively photocatalyzed by PCN materials is the two-electron reduction of dioxygen to hydrogen peroxide [43]. H2O2 represents a highly valuable commodity chemical that is being used as a versatile and environmentally benign oxidizing agent in a number of important industrial processes. Although photocatalytic H2O2 production at PCN from pure water and oxygen has been reported [44], typically the presence of additional sacrificial electron donors (e.g., aliphatic or aromatic alcohols) is practically indispensable in order to achieve reasonable reaction rates [18], [21], [44], [45], [46], [47]. The key mechanistic steps proposed in the literature are the protonation of the heptazine nitrogen sites enabled by fast oxidation of the electron donor [48] and the activation of O2 by photogenerated electrons, enabled by the very negative quasi-Fermi level of electrons in PCN (typically ca. − 0.7 V vs. RHE [34]), with subsequent formation of the intermediate heptazine-bound endoperoxide species [18], [44]. While photocathodes for efficient H2O2 production from O2 based, for example, on epindolidione deposited on gold electrodes [49] or porphyrin-sensitized nickel oxide films [50] have been reported recently, there are – to the best of our knowledge – no PCN-based photocathodes for selective light-driven reduction of O2 to H2O2 reported so far.
Herein, we report for the first time a multicomponent hybrid photocathode based on PCN for selective reduction of O2 to H2O2 under visible light irradiation (420 nm LED), whereby the desired H2O2 product is determined in-situ using microelectrodes. The optimized photocathode architecture comprises PCN deposited on a porous NiOx film acting as a scaffold for effective extraction and collection of photogenerated holes. A detailed analysis of various photoelectrode configurations shows that the presence of PCN is crucial for obtaining H2O2 as a product, and provides evidence for the beneficial effect of a TiO2 interlayer on the overall photoelectrocatalytic performance.
Section snippets
Materials
Fluorine-doped tin oxide (FTO) Pilkington TEC glass was purchased from the XOP company (XOP Glass, Castellón Spain). For rinsing deionized water was used. For the preparation of the electrodes nickel(II) chloride hexahydrate (NiCl2∙6H2O, 99.9 %) and titanium tetraisopropoxide were purchased from Sigma Aldrich as well as 2-propanol, ethanol (99.95 %) from VWR Chemicals and polyethylene glycol (PEG) 10,000 from Alfa Aesar. Urea, boric acid (99.8 %) and hydrochloric acid (37 %) were provided by
Results and discussion
In order to fabricate mechanically stable PCN-based photocathodes, we used porous NiOx films on FTO as a scaffold for subsequent deposition of PCN using chemical vapor deposition from urea decomposition products [33]. Nickel oxide is well established as a large bandgap (∼ 3.3 eV) p-type semiconductor that can be used as a hole collector and/or hole transport layer in dye-sensitized photoelectrochemical cells [50], [51], [58], [59]. Since the intrinsic conductivity of PCN is typically very low
Conclusion
A multicomponent hybrid photocathode based on PCN that is capable of selective reduction of O2 to H2O2 under visible light irradiation (420 nm LED) is reported for the first time. The optimized photocathode architecture comprises PCN deposited on a porous NiOx film acting as a scaffold for effective extraction and collection of photogenerated holes, whereby a TiO2 interlayer between NiOx and PCN is found to exert a beneficial effect on the overall photoelectrocatalytic performance. The latter
CRediT authorship contribution statement
Hanna Braun: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Dariusz Mitoraj: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Joanna Kuncewicz: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Andreas Hellmann: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Mohamed M. Elnagar: Investigation, Formal analysis, Writing – original
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was funded by the Deutsche Forschungsgemeinschaft (DFG – Projektnummer BE 5102/5-1, JA 1072/27-1, and 364549901– TRR 234 CataLight [Projects A5, B6, B10, C4] and by the National Science Centre, Poland (Solar-Driven Chemistry, 2019/01/Y/ST5/00027). The authors also acknowledge support by the state of Baden-Württemberg and the DFG through Grant no. INST 40/574-1 FUGG.
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Cited by (2)
Editorial of special issue for Detlef W. Bahnemann on the occasion of his 70th birthday
2023, Applied Catalysis A: GeneralCarbon Nitrides in Photoelectrochemistry: State of the Art and Perspectives Beyond Water Splitting
2023, ACS Applied Energy Materials
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This article is an appreciation of the contributions to the field of photocatalysis made by Professor Detlef Bahnemann.