Photo-Fuel-Cell: Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a Photoelectrochemical cell
Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a Photoelectrochemical (PEC) cell is an attractive project with double environmental benefit: waste material can be consumed and solar radiation can be converted into useful forms of energy, such as electricity and hydrogen.
Production of hydrogen by degradation of organic substances in the presence of a photocatalyst can be distinguished into two major categories:
(1) Photocatalytic (PC) production of hydrogen By this term, we usually mean production of hydrogen by heterogeneous photocatalysis using powdered or supported, pure or combined photocatalysts. As shown in Fig.1A, the photocatalyst is excited by absorption of photons, which create electron-hole pairs. Holes oxidize the photodegradable substance R, either directly or through radical intermediates, typically OH·, which are very efficient hole scavengers. Oxidation liberates hydrogen ions, which can be reduced by photogenerated electrons producing molecular hydrogen. The photodegradable substance can simply be water itself, however, the product of oxidation of water is oxygen, which interacts with hydrogen regenerating water. Thus it is difficult to produce hydrogen by photocatalytic water splitting, since hydrogen and oxygen must be spatially separated. This can be only managed in a PEC cell, as it will be explained below. As a matter of fact, in order to detect photocatalytically produced hydrogen, it is necessary to apply anaerobic conditions. Furthermore, the rate of hydrogen production may increase by two orders of magnitude in the presence of a fuel than in pure water.
Figure 1. Schematic representation of the Photocatalytic (A) and the Photoelectrochemical (B) production of hydrogen. The black full circle in A represents a co-catalyst (usually, a noble metal nanoparticle). The co-catalyst scavenges photogenerated electrons. In B, (Photo)anode electrode is on the left side and cathode on the right side. Oxidation and reduction may only take place when the photogenerated electron-hole pairs possess the necessary oxidation/reduction potential. In B, production of hydrogen is illustrated by the example of ethanol employed as fuel. Oxidation of ethanol takes place at the anode electrode at high pH while reduction of water takes place at the cathode. Photoanode bears photocatalyst and cathode bears an electrocatalyst.
(2) Photoelectrochemical production of hydrogen In this case, hydrogen is produced in a PEC cell. The following three components are the main components of a PEC cell: (a) The anode electrode, which carries the photocatalyst and thus it is usually named “Photoanode”. When the photocatalyst is an n-type semiconductor, which is almost the exclusive case, the photoanode produces electrons, i.e. it is the negative electrode. Oxidation reactions take place at the photoanode; (b) The cathode electrode, which carries the electrocatalyst, i.e., a material, which facilitates transfer of electrons from the cathode to the liquid phase. Reductive interactions take place at the (dark) cathode, for example, reduction of hydrogen ions to molecular hydrogen; (c) The electrolyte, which is added in order to increase conductivity and define the pH. The photoelectrochemical production of hydrogen is schematically illustrated in Fig.1B. Photons are absorbed by the photoanode generating electron-hole pairs. Holes oxidize the photodegradable substance, as above, liberating hydrogen ions, which diffuse in the liquid phase. Electrons are channeled through an external circuit towards the cathode, where they reduce hydrogen ions producing hydrogen molecules. At high pH, as illustrated in Fig.1B, no hydrogen ions can be detected in solution but molecular hydrogen is produced by reduction of water. Thus production of hydrogen is accompanied by flow of electrons, i.e. an electric current, in the external circuit. Hydrogen, of course, is detected in the absence of oxygen. Otherwise, in its presence, hydrogen is retained regenerating water. Water-splitting in a PEC cell does lead to hydrogen production, since the oxidation site, i.e. the photoanode, is spatially separated from the reduction site (cathode), thus O2 and H2 can be easily separated.
The Photofuelcell In the presence of oxygen, for example, aerated liquid phase, no hydrogen can be detected. In that case, current can still flow in the external circuit of the PEC cell. Electrons arriving at the cathode reduce O2 (see below). Then the cell acts as a Photofuelcell (PFC). A PFC consumes an organic substance, i.e. the fuel, and utilizes light energy to produce electricity.
Figure 2. Schematic representation of an H-shaped PEC cell with a TiO2 anode and a Pt cathode, divided into two compartments by an ion-exchange membrane. The current flows through an external load of resistance R.
Basic configuration of a PEC cell
The basic configuration of a PEC cell is schematically shown in Fig.1B and Fig.2. The cell comprises a photoanode that carries a semiconductor photocatalyst, a (dark) cathode that carries the electrocatalyst and the liquid phase that carries an electrolyte. Anode and cathode are connected through an external load. When the photocatalyst is an n-type semiconductor and the electrocatalyst is a noble metal, as it is the usual case, the photoanode acts as the negative electrode and the cathode as the positive electrode, i.e. electrons move from the anode to the cathode. The direction of the external current, of course, depends on the electric potential of each electrode. The electrochemistry of a PEC cell is fairly complicated. However, even an inexperienced experimenter can make a cell run by taking into account the following elementary considerations. When the cathode is in contact with an aqueous electrolyte at zero pH, its potential depends on the presence or not of oxygen. In the absence of oxygen, cathode behaves as a hydrogen electrode, the potential of which is conventionally taken equal to zero. In the presence of oxygen, the cathode behaves as an oxygen electrode. Its potential is then affected by the following reductive reactions
O2+4H++4e-®2H2O (+1.23 V) (1)
O2+2H++2e-®H2O2 (+0.68 V) (2)
This means that its value is between 0.68 and 1.23 Volts. The potential of the anode depends on the Fermi level of the semiconductor photocatalyst. In the case, for example, of titania, which is the most usual case, the conduction band at zero pH has a potential, which is slightly positive and when excited it becomes slightly negative. In the absence of oxygen, taking into account also the unavoidable losses, this potential difference is too weak to make the cell run spontaneously. Therefore, an external electric bias is required. By bias, is meant an external electric potential, which is added between the two electrodes, as in Fig.3, in order to increase the electromotive force driving electrons from the anode to the cathode.
Figure 3. Schematic representation of the polarity of an external electric bias for a PEC cell
Indeed, in such cases, various possibilities of additional bias have been proposed, the most notable ones being those, where the additional voltage is provided by renewable energy devices, like photovoltaic cells. In the presence of oxygen, no bias is required, since in that case a potential difference of several hundreds of mV between the two electrodes could be obtained without much of an effort. Thus the Photofuelcells, which function in the presence of oxygen, are spontaneously running devices. The above electrode potentials, as already said, correspond to zero pH value. At higher pH and at room temperature, the potential of both electrodes drops according to the following equation:
DV(Volts)= -0.059x(DpH) (3)
Thus if the pH increases by the same amount for the whole liquid phase of the cell, the variation of the potential of both electrodes is the same, so the difference between the electric potential of the two electrodes remains the same. If the pH of the electrolyte around the anode is basic and that around the cathode is acidic, then the potential difference between the two electrodes increases. In that case, we say that the cell functions under chemical bias. The potential difference between the two electrodes reflects on the measured open-circuit voltage Voc of the cell, which corresponds to infinite resistance R in Fig.2. Thus Voc can be much larger than 1.0 Volts, when chemical bias is applied. Chemical bias can be applied when the cell is structured into two compartments (as in Fig.2) communicating through an ion exchange membrane, for example, proton-transporting Nafion membrane. However, chemical bias is not a self-sustainable situation since chemical reagents must be continuously added in the two compartments to keep pH difference, which is otherwise eventually removed by ion transport through the membrane. Further chemical bias is offered to the system when a fuel, i.e. a sacrificial agent that retains photogenerated holes, is added to the cell. In that case, the consumption of holes increases the number of photogenerated free electrons that makes anode potential more electronegative. This electric-potential variation reflects on the increase of the Voc of the cell. Indeed, in a cell running with the same pH value in the anode and the cathode compartment, in the presence of ethanol or glycerol Voc becomes about 0.3 Volts larger. The above discussion shows that it is very easy to run a PFC both with and without fuel, simply by shining light on the photoanode. It is understood that this light must contain the appropriate wavelengths necessary to excite the photocatalyst.
Further information on the same subject can be found in the following review:
Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a Photoelectrochemical cell. The concept of the Photo-Fuel-Cell. A review of a re-emerging research field: Panagiotis Lianos, J.Hazardous Materials, 185(2011)575-590
Page updated August 2011