Composite proton-conducting polymer membranes for clean hydrogen production with solar light in a simple photoelectrochemical compartment cell
Highlights
► A simple two-compartment photoelectrochemical cell is introduced. ► No chemical or electrical bias is needed. ► New composite polymer membranes act as compartment separator. ► The cell is used for photocatalytic water splitting experiments. ► Hydrogen can be generated spatially separated from the oxidation products.
Introduction
Fujishima and Honda discovered photocatalytic water splitting on a TiO2 photoelectrode already in 1972 [1]. However, only recently the search on new photocatalytic materials and the development of novel reactor designs for efficient water splitting to produce hydrogen has been intensified tremendously [2], [3], [4], [5].
For the effective and economical generation of hydrogen as clean energy carrier one of the most important issues in an overall water splitting process is the spatial separation of generated oxygen and hydrogen in order to avoid the need of expensive gas purification/separation steps [6]. Nevertheless, the number of reports describing the simultaneous production and separation of oxygen and hydrogen in photocatalytic water splitting is very limited [7], [8], [9], [10], [11], [12], [13]. All the state-of-the-art two-compartment cells use standard Nafion membranes, for compartment separation and proton transport; their drawback, however, is that they rely on electrolyte solutions (e.g. NaOH and H2SO4) [8], [9], [10], [11] to apply a chemical bias between both half-cells. Furthermore, the proton-conducting membrane and the photocatalyst, which is in most cases TiO2 [14], [15], are spatially separated in the cells. For instance, Fujihara et al. used Pt electrodes and a suspension of Pt-loaded TiO2 as photocatalysts in the different parts of their reactor cell, which were connected via the Nafion membrane [7]. Kitano et al. and Selli et al. coated non-proton-conducting Ti substrates with TiO2 and Pt, and used this three-layered block as additional compartment separator besides Nafion [8], [9]. Antoniadou et al. deposited the TiO2 photoanode directly on an electron-conductive glass, but still used a separately positioned Pt wire as the cathode and an as well separate Nafion foil for proton transport between the compartments [10]. Sun et al. just recently used TiO2 nanotubes on Ti foil in a comparable setup, using H2SO4 and KOH electrolytes for a chemical bias [11].
Seger et al. reported recently a TiO2-Nafion-Pt membrane assembly combining directly photocatalytic activity and separation. The system worked without applied bias, but was only tested under pure UV light and required additionally the use of methanol as sacrificial agent [12]. A different approach was applied by Li et al., using a Ti mesh anodized with TiO2 nanotubes as anode, Pt/C as cathode, and an asbestos diaphragm for hydroxide diffusion in a similar setup like Seger et al. [13] However, they needed an applied electrical bias and KOH solution to produce hydrogen.
By using known proton exchange membranes (PEM) we constructed a very simple photoelectrochemical cell (PEC) with two chambers intended for separated production of oxygen and hydrogen. The PEM acts as both, the separator and the support for a carbon coated Degussa TiO2 P25 photoanode and a Pt cathode for preferred hydrogen evolution, which is in contrast to older reports, where photocatalysts were mixed into the polymer or coated with a polmer [16], [17]. Neither electrolyte solutions for applying a chemical bias nor an external electrical bias or sacrificial agents are needed in this cell; clean hydrogen can be formed directly from pure water in one step.
Several types of PEMs including commercial Nafion® and FKE® membranes (fumatech), our home-prepared sulfonated polyethersulfone (sPES) polymer membranes [18] and composite membranes consisting of sPES and sulfonated mesoporous Si-MCM-41 nanoparticles [18], [19] were evaluated in the PEC for hydrogen production.
The incorporation of sulfonated mesoporous Si-MCM-41 nanoparticles improves not only the proton conductivity, but also the water uptake and mechanical stability of polymer membranes [20], [21]. However, this is the first study exploring the use of such composite polymer membranes for photocatalytical applications. Here, we show that with these newly-developed composite membranes and our novel PEC design, the presented photocatalytic process exhibits hydrogen production yields superior to those using the commercial membranes.
Section snippets
Materials
The preparation of sulfonated polyethersulfone (sPES) polymer membranes and composite membranes containing sPES and sulfonated mesoporous Si-MCM-41 nanoparticles were described in detail in our earlier reports [18], [19]. In a typical preparation, firstly 25 wt.-% sPES polymer solutions were prepared using dimethylformamide (Sigma) as the solvent. Then 0–0.5 wt.-% of sulfonated mesoporous Si-MCM-41 nanoparticles were mixed with the polymer solutions at 60 °C for 4 h while stirring. Sonication
Results and discussion
Fig. 3 shows the hydrogen evolution rates with the new cell-type for five different membranes employed all coated with the same amounts of P25 TiO2 nanoparticles. Two of the membranes (Nafion® and FKE®) were commercially available systems, with the FKE® less-swelling upon water uptake than the Nafion® membrane and, thus, being dimensionally more stable. Both membranes can be regarded as benchmarks for our home-made sPES40 membrane as well as for the novel composite membranes containing
Conclusions
In summary, we have demonstrated photocatalytic hydrogen production under simulated sunlight using a simple two-compartment cell without any electrical or chemical bias, in which new composite polymer membranes played a triple role as proton conductor, compartment separator and support for coated electrodes and photocatalyst. Our composite polymer membranes sPES40,0.5%SN exhibited superior hydrogen generation performance, i.e., 82.4 μmol h−1 for P25 in water, and 104.5 μmol h−1 using diluted
Acknowledgments
This project was supported by a fellowship for R.M. via the Postdoc-program of the German Academic Exchange Service (DAAD) and Australian Research Council (through its Centers of Excellence grant and DP programs).
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2018, Solar Energy Materials and Solar CellsCitation Excerpt :Since the pioneering work by Fujishima and Honda in 1972 [10] on photolysis of water, most of the photoelectrochemical (PEC) cells described in literature so far utilize a similar design based on aqueous phase electrolytes [1–3,5–9]. Inspired by the reactor design of polymeric electrolyte membrane (PEM) electrolyzers/fuel cells [11], few groups have attempted to separate the two electrochemical half-reactions with a polymeric electrolyte membrane [12–26]. The PEM-PEC cell design is advantageous compared to PEC cells with liquid electrolyte since it is more robust and potentially more scalable, the polymeric membrane does not need replenishment during prolonged operation and also direct separation of the reaction products is possible during gas-phase photoelectrochemical experiments.
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