Photoelectrochemical Hydrogen Evolution Using Dye-Sensitised Nickel Oxide
Photoelectrochemical Hydrogen Evolution Using Dye-Sensitised Nickel Oxide
Environmental effects and photocatalyst design considerations
Photoelectrocatalysis offers a way to generate hydrogen and oxygen from water under ambient light. Here, a series of hydrogen evolving photocatalysts based on a ruthenium(II) bipyridyl sensitiser covalently linked to platinum or palladium catalytic centres were adsorbed onto mesoporous nickel oxide and tested for hydrogen evolution in a photoelectrochemical half-cell. The electrolyte buffer was varied and certain catalysts performed better at pH 7 than pH 3 (for example, PC3 with photocurrent density = 8 μA cm–2), which is encouraging for coupling with an oxygen evolving photoanode in tandem water splitting devices. The molecular catalysts were surprisingly robust when integrated into devices, but the overall performance appears to be limited by rapid recombination at the photocatalyst|NiO interface. Our findings provide further insight towards basic design principles for hydrogen evolving photoelectrochemical systems and guidelines for further development.
The global effort to produce solar fuels by means of molecular photocatalysis continues to intensify following the establishment of basic design principles in the 1970s and 1980s (1–5). Many of the challenges encountered at the time remain relevant today, namely molecular systems for photocatalytic water oxidation or reduction often rely on a sacrificial electron source to drive the thermodynamically demanding multielectron reaction, most of which are not environmentally benign or renewable (6). However, a direct photocatalytic system would avoid the fabrication and systems costs required with photovoltaics coupled to electrolysis. Tremendous effort has been expended in an attempt to understand photocatalytic processes in solution either as a bimolecular process for homogeneous catalysis or on a semiconductor surface in a heterogeneous system (7–11). Over the years a vast catalogue of new molecular catalysts and sensitisers has been generated, a prototypic example being the well-known ruthenium water oxidation catalyst reported by Meyer et al., the “blue dimer” (12). Recent efforts have produced catalysts based on earth abundant metals such as iron and cobalt which typically underperform compared to prior systems, leaving the economic and environmental considerations for moving away from precious metal systems (platinum, palladium, rhodium or ruthenium) under some debate (13–15).
Sensitisers based on the thoroughly understood ruthenium(II) trisbipyridyl complex remain popular as a starting point for new avenues of research (16–19). More recent efforts have focused on covalently linking the catalyst and sensitiser to optimise charge separation (20). The fundamental principle of photocatalyst design where donor and acceptor are joined by a suitable covalent bridge is elegant and inspired by the photosynthetic reaction centre of green plants, but challenging to optimise in a working photochemical reactor, even where the molecular systems are relatively simple (21, 22). To date, practical implementations of molecular photocatalytic systems are scarce, despite the variety of systems proposed. Surface and solvent parameters introduce a large set of variables for optimising a photocatalytic reactor. At this point there are now an abundance of molecules and materials but developing a mechanistic understanding of such systems to aid design is still an ongoing process (23).
Over the past decade, research into dye-sensitised solar cells (DSSCs) has converged with heterogeneous photocatalysis (1, 24–27). The potential advantages include high atom efficiency for the catalyst, low cost assembly through solution-based processing, and devices that operate under ambient conditions. Dye-sensitised photocathodes have been constructed in photoelectrochemical devices to reduce protons to hydrogen, usually under a small applied bias for a half-cell in a photocatalytic system (28–31). These can be coupled to photoanodes which provide the electrons for proton reduction as a byproduct of water oxidation in a tandem device (31). The tandem system, where both anode and cathode are decorated with photocatalysts, removes the need for sacrificial reagents and enables a sustainable system (32). These dye-sensitised photoelectrochemical cells overcome many of the disadvantages of molecular systems in solution, while taking advantage of the catalogue of sensitisers and catalysts already available. Light absorption, charge transport and catalysis are separated between three tuneable components (33–35). Attention then turns to the interfaces between these components, for example, how photocatalysts attach and behave on the semiconductor surface, which requires considerable optimisation.
In this contribution we focus on optimising the proton reduction half reaction, generating hydrogen by means of new integrated photocatalysts adsorbed onto the surface of a transparent p-type semiconductor, nickel(II) oxide. A proof-of-concept for this approach was reported by us recently for two supramolecular photocatalysts based on a bipyridyl ruthenium photosensitiser coupled to either a platinum or palladium catalytic centre via a terpyridine or triazole bridging ligand (36). In this follow up paper, we explore a series of new photocatalysts with structural modifications made to the bridge and the catalytic centre, and evaluate the impact these have on the photoelectrocatalytic reduction of protons (Figure 1). Consideration is given as to how the photocatalysts bind to the electrode surface and interact with the surrounding environment. Device testing in a photoelectrochemical cell is used to determine photocurrent and hydrogen produced as a result, and stability is evaluated over prolonged periods of illumination. The photophysical and electrochemical properties of the photocatalysts are examined to develop a mechanistic understanding of the system.
2.1 Optical Properties
The preparation and characterisation of mesoporous nickel oxide cathodes has been described previously (37). Adsorption of the photocatalysts onto the mesoporous nickel oxide films on NSG TECTM 15 conductive glass from acetonitrile solutions was recorded by ultraviolet-visible (UV-vis) spectroscopy. Normally in titania-based systems the ester group is hydrolysed prior to adsorption or the titania is treated with base to promote binding (38), but for nickel oxide we have found that these ester-functionalised systems bind as well as the carboxylic acid derivatives. To calculate the dye loading, we assumed that the molar absorption of the dyes did not change significantly on the nickel oxide surface compared to the dye in solution. In dry acetonitrile, all five dyes produce steady-state absorption spectra with characteristic metal to ligand charge transfer (MLCT) bands from ruthenium to the diethyl ester bipyridyl ligands (39). When immobilised on nickel oxide films, the overall trend is towards a broader, red-shifted spectrum. While we do not have a model regarding what causes this spectral shift, it indicates that the dye interacts with the nickel oxide surface through the ester anchoring groups. This spectral broadening is typical for dyes adsorbed on metal oxide surfaces, and is usually observed when carboxylic acid anchoring groups are used and it is possibly caused by deprotonation of the acid on binding (40, 41). In this case, where ester anchoring groups have been used it could be due to hydrolysis or, possibly, the result of overlap from several different orientations of the catalyst on the nickel oxide surface, including some aggregates (42). The steady state absorption spectra are shown in Figure 2 and the data are summarised in Table I.
2.2 Transient Absorption Spectroscopy
Transient absorption spectroscopy was used to probe the mechanism of electron transfer following excitation with visible light. In dye-sensitised photoelectrocatalytic devices, unlike many homogeneous photocatalytic systems which rely on long-lived excited states, the dye injects charge to the semiconductor rapidly upon excitation and subsequently returns to the ground state by transferring charge to the catalyst. Providing that these processes are more rapid, dye degradation is avoided, and the device is both efficient, as recombination is reduced, and leads to stability within the system.
The transient absorption spectra obtained following pulsed photolysis (λexc = 470 nm) of PC5 in acetonitrile solution and PC5 adsorbed on nickel oxide are shown in Figure 3. A ground state bleach occurs within the pulse following excitation together with a broad, weak absorption extending from 500 nm to 700 nm, and a strong absorption together with a sharp transient absorption band at ca. 395 nm. These features do not decrease in intensity during the 3 ns window of the experiment, which is consistent with the 3MLCT states formed on excitation of a ruthenium diimine chromophore. When the dye was adsorbed on nickel oxide, transient absorption bands were also formed within the time resolution of the experiment, however, the spectral features were broadened, and the signal was more intense in the red region of the spectrum compared to the blue. Based on our previous work, the spectral shape is consistent with the formation of a charge-separated state (36). The transient absorption bands, with peak maxima at ca. 395 nm and 580 nm, and the ground state bleach, decayed on a similar timescale (τ = ca. 250 ps) and the ground state was recovered in ca. 1 ns. The results of the transient absorption spectroscopy are consistent with the rapid transfer of an electron from the valence band of nickel oxide to the photocatalyst, rather than stepwise excitation followed by charge-transfer. Possibly, the MLCT states on the diethyl [2,2’-bipyridine]‐4,4’‐dicarboxylate are destabilised on binding, promoting charge-transfer towards the bridging ligand and the catalytic centre. Recombination between the reduced photocatalysts and the hole remaining in nickel oxide occurs rapidly.
This fast recombination is likely to be the major limitation to the performance of the photoelectrocatalytic devices (28). However, previous studies on nickel oxide from the groups of Hammarström and Papanikolas have highlighted how the bias applied to the film affects the recombination kinetics (43, 44). Under the range of potentials studied here, we would expect recombination to be slowed down by several orders of magnitude compared to the lifetime determined from the spectroscopic measurements performed on dry films.
The photocathodes were tested in a three-electrode setup with a platinised fluorine doped tin oxide (FTO) counter electrode and a silver/silver chloride reference electrode. The pH of the aqueous electrolyte and composition of the buffer was varied, and different applied potentials (Eappl) were tested to find the optimum reaction condition for each photocatalyst. The electrolyte solutions used were: pH 3 potassium hydrogen phthalate buffer (0.1 M), pH 5 acetate buffer (0.2 M) or (2-morpholino)ethanesulfonic acid buffer (0.1 M), pH 7 potassium phosphate buffer (0.1 M). The range of Eappl was chosen to be less than the conduction band edge of titania (VCB in V vs. SCE = –0.40 – 0.06 × pH) (45) to mimic the conditions in a tandem device. Within this range, applied potentials were chosen where the photocurrents were maximised and were most stable, whilst avoiding significant changes to the background current.
Photocurrent was generated under 1 sun, AM 1.5 illumination, initially with chopped light (30 s intervals) followed by uninterrupted illumination. Photocurrent values were evaluated against dark current obtained after a period of equilibration in a sealed dark box. Control experiments confirmed that a system comprising two platinum-FTO electrodes acted as a simple electrolyser under an applied voltage of –0.3 V vs. Ag/AgCl in pH 3 phthalate buffer solution. A second control experiment comprised a system of a nickel oxide working electrode lacking a sensitiser with a platinum-FTO counter electrode. Both systems generated stable current but were not sensitive to incident light, thus we can exclude the possibility of direct (> band gap) excitation of nickel oxide leading to photocurrent or hydrogen evolution.
Figure 4 displays the photocurrents generated under equivalent conditions (pH 3–7, Eappl = –0.2 V vs. Ag/AgCl) for the full series of photocatalysts, a representative example of the many datasets. Longer experiments demonstrating the stability of the current over 3600 s are provided in Figure S2 in the Supplementary Information accompanying the online version of this article. All photocatalysts produced current under chopped light, as expected. A characteristic of all the chronoamperometry experiments was an initial spike of current due to either an accumulation of charge on the electrode surface or the consumption of oxygen within the pores. This emphasises the need for an equilibration period at the beginning of each experiment to obtain a true baseline value for light and dark currents. It also indicates that there may be a mass transport limitation within the system. Typical photocurrent densities for these small-scale devices (active nickel oxide area 0.79 cm–2) are in the μA cm–2 region. Notably, all samples were stable under chopped light illumination over the initial testing period of approximately 10 min. After chopped light illumination the experiment was continued under steady illumination for 60 min.
In some cases, characteristic spikes in the photocurrent transients were observed. For example, PC4 at pH 3 shows charging and discharging at the electrode-electrolyte interface under light on-off cycles. This may indicate that electrons are not being transferred to the catalyst or protons efficiently. For PC1, spikes are present when the light is switched on, followed by stabilisation of the photocurrent. In this case, charge accumulation at the electrode-electrolyte interface may arise from trapped holes, slow kinetics of proton reduction, slow charge-extraction (high transport resistance), fast charge recombination between the electrons in the catalyst, and holes in the nickel oxide or poor diffusion in the pores of the semiconductor. The smooth shape of the photocurrent vs. time trace for PC5 at pH 3, however, is consistent with catalysis and the evolution of hydrogen. As the applied bias was increased to –0.4 V vs. Ag/AgCl, the spikes decreased (except for PC4) and smoother transients were observed (Figure S4 in the Supplementary Information). This is consistent with filling of trap states at the nickel oxide surface.
The choice of buffer had a surprising impact on the photocurrent density and stability. At pH 3 and pH 7 (phthalate and phosphate buffers respectively) all samples produced stable photocurrent during the prolonged period of illumination. At pH 5, where an acetate buffer was employed, there was a steady decay of photocurrent consistent with degradation or desorption of the photocatalyst. This is in contrast to studies by Massin et al. who found the opposite trend for their dye-sensitised photoelectrocatalytic system with an organic dye, which was found to be stable in acetate buffer but unstable with phosphate buffer (46). In order to address the problems with photocatalyst instability in pH 5 acetate buffer an alternative non coordinating buffer was employed, 2-(N-morpholino)ethanesulfonic acid (MES). Use of this buffer has been previously shown to improve stability of similar photocatalytic systems (47). An example of the comparison between electrolytes for PC5 is summarized in Figure S5 in the Supplementary Information. For each of the photocatalysts, we observed that employing the MES buffer improved photocurrent stability compared to acetate buffer. PC1 and PC5 gave consistent photocurrent density regardless of the pH. PC2 consistently gave relatively low photocurrent density compared to the others. PC3 gave the highest photocurrent density at pH 7 but a lower photocurrent density was recorded at pH 3. For PC4 very little photocurrent was observed at pH 3 and pH 5, but relatively high photocurrent density was recorded at pH 7. It is encouraging that the photocurrent density did not drop at higher pH, because this would be compatible with a tandem device.
For each photocatalyst, hydrogen was detected by sampling the headspace and analysing it by gas chromatography. Table S1 in Supplementary Information summarises the results. Despite the differences in photocurrent density with buffer and Eappl described above, the most hydrogen was detected with pH 7 buffer and these results are shown in Table II.
While attempts were made to keep the sampling consistent, there was considerable error in the measurements due to bubbles forming on the electrode surface and quantifying hydrogen by syringe (>7% deviation from the mean average). The amount of hydrogen detected may also vary due to differences in hydrogen solubility in different electrolytes (48, 49), or as a side product from the photocatalyst degrading.
2.4 X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) data was recorded for the photocatalyst films before application in a photoelectrocatalysis cell under illumination with pH 3 in phthalate buffer and an applied bias of –0.2 V vs. Ag/AgCl. The nickel 2p peak remained unchanged so this was used to normalise the spectra to give a rough comparison of dye-loading. Figures 5 and 6 show the palladium and platinum regions, where relevant, for PC1 to PC5. There was no shift in binding energy observed for platinum or palladium on the electrode surface under our conditions, suggesting that the photocatalyst remains intact. A slight change in relative intensity for some of the photocatalysts (PC1 and PC4) suggests that there may be some desorption of the photocatalyst. Figure S8 in Supplementary Information shows that additional bands are present in the binding energy region 275–300 eV. These are assigned to C 1s and K 2p, which probably arises from the buffer. Their presence on the electrode surface makes the analysis of the ruthenium 3d band difficult.
As with our previous studies with integrated photocatalysts based on a similar architecture, ester groups on the bipyridyl ligands serve as an anchoring site to nickel oxide (36). Our previous findings clearly suggested that there was sufficient interaction between the ester groups and the nickel oxide surface to provide a system resistant to dye desorption during photocatalysis (under suitable conditions). XPS results largely corroborate these claims. The XPS results show that the buffer salts can assemble on the electrode surface, and it is possible that there is competition between organic salts and the photocatalyst, leading to some desorption. We continue to favour ester substituted photocatalysts due to the simplified synthesis and purification of the dye. However, the diethoxy ester bipyridyl ligand serves two functions, both as the anchoring group to the nickel oxide surface and as an electron withdrawing ligand which stabilises the MLCT excited states. Locating the electron density close to the nickel oxide surface could increase the rate of charge-recombination, so the best photocatalysts should promote charge-transfer to the catalyst centre via the bridging ligand (20).
All the photocatalysts in this study produced photocurrent and hydrogen under each varied condition. Some photocatalysts perform better at pH 3, whereas others performed better at pH 7. The performance with pH 5 acetate buffer was the most consistent between different catalysts, but was generally lower than the other two buffers. Changing to MES buffer improved the performance at pH 5. Different authors report contrasting results when changing the pH of the electrolyte for analogous. The type of buffer affects the performance in two ways. Lower pH should increase the rate of hydrogen formation from the catalyst (50), however, the effect of pH on the valence band edge of the nickel oxide is a shift to more positive potentials as the H+ concentration increases (51). Further transient absorption spectroscopy studies are necessary to probe the effect of pH on the dynamics. It is expected that charge transfer from the nickel oxide to the photocatalyst would be slower at higher pH due to the lower valence band edge and, therefore, smaller driving force (52). Charge-recombination should occur in the Marcus inverted region and would be expected to be slower at higher pH, and increase the charge-collection efficiency of the device. However, in most kinetic studies, charge-recombination at the photosensitiser/nickel oxide interface appears to follow Marcus normal behaviour, accelerating with increasing driving force, probably due to recombination with more energetic intra-bandgap defect states (53, 54).
While this study shows that there are a variety of ways to improve the performance of dye-sensitised photoelectrochemical devices, including tuning the structure of the catalyst or the environment, the performance of dye-sensitised photocathodes based on nickel oxide still lags behind equivalent photoanode devices based on titania. Further improvements are necessary, for example, improving the porosity of the electrodes to facilitate mass transport. Additionally, bubble formation on the electrode surface leads to a drop in the photocurrent and good diffusion is necessary to prevent a build-up of OH– in the pores. At the same time, high surface area must be maintained to adsorb enough photocatalyst to absorb all the incident light. A limitation of the ruthenium bipyridyl chromophores is the low absorption coefficient (ɛ = 27,600–42,500 M–1 cm–1 for PC1–PC5). Organic photosensitisers have two to three times larger absorption coefficients, enabling thinner or more porous films to be used (33, 55, 56). Future considerations for dye-sensitised photoelectrochemical assemblies should also consider the prospect of engineering the thermodynamic driving force for charge recombination towards the Marcus inverted region (similar to titania-based systems). Ongoing work within our laboratory looks to find an alternative semiconductor to nickel oxide with a more typical band structure to address this (32, 57).
The present study describes a series of novel sensitiser-catalyst dyads that were tested in a photoelectrocatalytic half-cell set-up to determine the optimum conditions for water splitting. At a range of pH levels the dyads remained intact with a stable photocurrent, demonstrating their robustness. However, when using a non-coordinating buffer we saw an increase in photocurrent. All our systems produced hydrogen under a small applied bias. Under optimal conditions, Faradaic efficiencies up to 90% with turnover numbers of up to 33 in an hour were achieved, which are comparative to our previously reported systems. Transient absorption measurements confirmed that charge separation on the surface of a p-type semiconductor, nickel oxide, is very efficient when compared to the same system in dilute solution. While rapid charge injection facilitates reduction of the catalyst, thus aiding photocatalysis, recombination of the oxidised sensitiser with nickel oxide remains equally efficient. The insight from transient absorption data in particular highlights a major challenge for integrated molecular photocatalysts on a semiconductor interface and further work will seek to address this.
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We thank the Leverhulme Trust for a project grant RGS108374, The North East Centre for Energy Materials EP/R021503/1; Science and Technology Facilities Council (STFC) for access to the Central Laser Facility (CLF) ULTRA facility for transient spectroscopy; NEXUS XPS facility for conducting the XPS measurements. Laura O’Reilly thanks the Irish Research Council for financial support and Martin Kaufmann gratefully acknowledges support by the Project “HYLANTIC” – EAPA_204/2016 which is co‐financed by the European Regional Development Fund in the framework of the Interreg Atlantic programme. Data supporting this publication is openly available under an ‘Open Data Commons Open Database License’. Additional metadata are available (58).
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Abigail Seddon is currently a PhD student in the Energy Materials Group at School of Natural and Environmental Sciences, Newcastle University, UK, working on applications of polyoxometalates. Previously, she graduated from Newcastle University with an MChem in Chemistry with an Industrial Placement year at AstraZeneca. Her research interests include artificial photosynthesis, photovoltaics and inorganic synthesis.
Joshua Karlsson graduated from the University of York, UK, with a BSc in Chemistry and Imperial College London, UK, with an MRes in Green Chemistry in 2013 and 2014 respectively, before undertaking a PhD at Newcastle University in 2015. His doctoral studies covered a wide range of topics on the molecular photophysics of organic dyes pertinent to solar cells and medical imaging. His expertise lies in photocatalysis and detailed interrogation of excited state properties for organic and inorganic chromophores using UV-vis absorption, fluorescence and time-resolved optical spectroscopy.
Libby Gibson joined Newcastle University as a Lecturer in Physical Chemistry in 2014 and was promoted to Reader in Energy Materials in 2018. Prior to her current role, she held a University of Nottingham Anne McLaren Research Fellowship and a Royal Society Dorothy Hodgkin Research Fellowship. She obtained her PhD in 2007 from the University of York, supervised by Robin Perutz FRS and Anne-Kathrin Duhme-Klair. Research in her group focuses on solar cell and solar fuel devices that function at a molecular level and challenge the conventional solid-state photovoltaic technologies. Her current European Research Council (ERC)-funded project focuses on developing transparent p-type semiconductors for tandem solar cells and artificial photosynthesis.
Laura O’Reilly graduated from Dublin City University, with a BSc in Chemistry and Pharmaceutical Science in 2014, and subsequently undertook a PhD at Dublin City University. Her doctoral research focused on the design and synthesis of novel photocatalysts for hydrogen generation. During her PhD programme she used a range of spectroscopic techniques including UV-vis absorption, time-resolved UV-vis and time resolved infrared spectroscopy to probe the photophysical properties of the photocatalysts.
Martin Kaufmann is currently a MaREI postdoctoral researcher within the School of Chemical Sciences at Dublin City University. Prior to joining Mary Pryce’s research group in Dublin, Martin did his PhD in organic chemistry at the Friedrich Schiller University Jena in Germany. His research interests are the syntheses of heterocyclic functional dyes, transition metal complexes and their application in the photocatalytic hydrogen generation.
Han Vos is Emeritus Professor of Inorganic Chemistry at Dublin City University. His research interests are in the design of supramolecular systems containing transition metal complexes. Of particular interest are the synthesis, photophysical and electrochemical properties of dinuclear and polymeric ruthenium and osmium polypyridyl complexes both in solution and when immobilised on solid substrates, and their application in energy sources.
Mary Pryce joined Dublin City University, Ireland, in 1997 as a Lecturer in Inorganic Chemistry. Prior to joining the School of Chemical Sciences, she was employed as a postdoctoral Fellow at the University of Milan, Italy. In 1995, she obtained her PhD from Dublin City University in the area of organometallic photochemistry. Current research projects within the group focus on designing new materials (polymers, organometallic compounds or organic dyes) for energy applications such as hydrogen generation, or CO2 conversion. Another aspect of research focuses on antimicrobial materials. Central to both of these research areas is understanding the photophysical properties using time resolved techniques.