David A. Vermaas

Photo-assisted water splitting with bipolar membrane induced pH gradients for practical solar fuel devices

Different pH requirements for a cathode and an anode result in a non-optimal performance for practical solar fuel systems. We present for the first time a photo-assisted water splitting device using a bipolar membrane, which allows a cathode to operate in an acidic electrolyte while the photoanode is in alkaline conditions. The bipolar membrane dissociates water into H+ and OH−, which is consumed for hydrogen evolution at the cathode and oxygen evolution at the anode, respectively. The introduction of such a bipolar membrane for solar fuel systems provides ultimate freedom for combining different (photo)cathodes and -anodes. This paper shows that photo-assisted water splitting at both extreme pH gradients (0–14) as well as mild pH gradients (0–7) yields current densities of 2–2.5 mA cm−2 using a BiVO4 photoanode and a bipolar membrane. The membrane potentials are within 30 mV of the theoretical electrochemical potential for low current densities. The pH gradient is maintained for 4 days of continuous operation and electrolyte analysis shows that salt cross-over is minimal. The stable operation of the bipolar membrane in extreme and mild pH gradients, at negligible loss, contributes to a sustainable and practically feasible solar fuel device with existing photoactive electrodes operating at different pH.

Near-complete suppression of surface losses and total internal quantum efficiency in BiVO4 photoanodes

Bismuth vanadate (BiVO4) is one of the most efficient light absorbing metal oxides for solar water splitting. BiVO4 photoanodes immersed in an electrolyte in an open circuit configuration and exposed to simulated solar illumination for prolonged time achieve superior photoelectrochemical (PEC) activity. This photocharging (PC) effect is capable of almost completely overcoming the surface and bulk limitations of BiVO4. Herein we show that alkaline conditions favor the PC effect; specifically BiVO4 photoanodes subjected to PC treatment at pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO4 of 4.3 mA cm−2 @ 1.23 VRHE, an outstandingly low onset potential of 0.25 VRHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95% respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in situ XAS study suggest that electronic, structural and chemical properties of the bulk of these films remain unchanged during the PC treatment. However, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO4 is enhanced by surface reaction pathways of the semiconductor–liquid junction, which is directly correlated with the electrochemical environment in which it is modified.

Chapter 8:Applications of Bipolar Membranes for Electrochemical and Photoelectrochemical Water Splitting

In designing electrochemical or photoelectrical water splitting systems, the choice of electrolyte and possible membrane separators has a major interplay with the catalytic activity and overall performance of the water splitting system. To facilitate the optimal pH in both the catholyte and anolyte, a bipolar membrane has been introduced recently. Bipolar membranes dissociate water into H+ and OH−, where the H+ is supplied to the cathode and OH− supplied to the anode, and thereby balancing the consumption of these ions in the hydrogen and oxygen evolution, respectively. As a result, the pH in the catholyte and anolyte can be chosen and maintained independently. This chapter discusses the use of both monopolar membranes and bipolar membranes in (photo)electrochemical water splitting systems and provides an overview of the research in this field, including the principle of water dissociation, fabrication of bipolar membranes and use of bipolar membranes in other applications.

Assisted reverse electrodialysis : principles, mechanisms, and potential

Although seawater reverse osmosis (RO) is nearing its thermodynamic minimum energy limit, it is still an energy-intensive process, requiring 2–3 kWh/m³ at a recovery of 50%. Pre-desalination of the seawater by reverse electrodialysis (RED), using an impaired water source, can further decrease this energy demand by producing energy and reducing the seawater concentration. However, RED is hampered by the initial high resistance of the fresh water source, resulting in a high required membrane area (i.e., high investment costs). In this paper, a new process is presented that can overcome this initial resistance and decrease the RED investment cost without the need for additional infrastructure: assisted RED (ARED). In ARED, a small potential difference is applied in the direction of the natural salinity gradient, increasing the ionic transport rate and rapidly decreasing the initial diluate resistance. This decreasing resistance is shown to outweigh any negative effects caused by, for example, concentration polarization, resulting in a process that is more efficient than theoretically expected. As this effect is mainly important at low diluate concentrations (up to 0.1 M), ARED is proposed as a first step in an economic and energy efficient (A)RED-RO hybrid process.Seawater desalination: assisting reverse osmosisCoupling reverse osmosis with assisted reverse electrodialysis can reduce the cost of seawater desalination. While reverse osmosis currently accounts for more than 60% of our worldwide seawater desalination capacity, this crucial process operates at a high energy demand. A reverse electrodialysis (RED) pre-treatment of seawater, diluted with a waste water stream, reduces the energy demand by producing energy and by reducing the concentration of the seawater subjected to reverse osmosis. Nonetheless, low transport rates in RED require high-membrane surface areas, making the costs impractical. A team led by Marjolein Vanoppen at Gent University in Belgium design an assisted RED pre-treatment process, where a small potential difference is applied in the direction of the salinity gradient, increasing the ionic transport rate and decreasing the required membrane surface area, thus offering a more economically viable alternative.