Polymeric carbon nitride-based photocathodes for visible light-driven selective reduction of oxygen to hydrogen peroxide

Highlights

• A multicomponent hybrid photocathode based on polymeric carbon nitride (PCN) is reported.

• The photocathode is capable of selective reduction of dioxygen to H₂O₂ under visible light irradiation (420 nm LED).

• The results confirm that PCN is intrinsically catalytically active in reduction of O₂ to H₂O₂.

Abstract

Polymeric carbon nitrides (PCN) are sustainable, tunable, non-toxic and chemically stable materials that represent highly promising heterogeneous photocatalysts for light-driven hydrogen peroxide production via selective reduction of dioxygen. However, most of the studies on photocatalytic H₂O₂ production using PCN-based photocatalysts reported so far have used PCN powder suspensions and have been carried out in the presence of additional (sacrificial) electron donors, such as aliphatic or aromatic alcohols. Herein, we report the first multicomponent hybrid photocathode based on PCN that is capable of selective reduction of dioxygen to H₂O₂ under visible light irradiation (420 nm LED). A comparative analysis of various photocathode architectures is carried out using electronic absorption spectroscopy, surface photovoltage spectroscopy, open-circuit photopotential spectroscopy, and photocurrent measurements, including in-situ detection of formed H₂O₂ using microelectrodes. Notably, the ability of PCN-based photocathodes to catalyze the light-driven reduction of O₂ to H₂O₂ in the absence of any additional electron donor is unambiguously demonstrated. Our study thus highlights the intrinsic nature of the photocatalytic activity of PCN in H₂O₂ production, and paves the way for the development of further PCN-based photocathodes in which PCN could be coupled with more effective light absorbers to increase the overall performance.

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 NiO_x 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.