Efficient Photocatalytic Dissociation of Water into Hydrogen and Oxygen

Подпись: Figure 9.1 Schematic diagram [111] of illuminated monolithic photovoltaic- photoelectrochemical device for hydrogen production by water splitting, with 12.4% efficiency in converting solar light energy into chemical energy. In this cell, with no external connection, hydrogen bubbles appear on the illuminated surface at the right (cathode), and

One element in this category is a cell for splitting water to produce hydrogen with sunlight, called photocatalytic splitting of water. Direct photocatalytic water splitting is not easy to accomplish. Efficiency is needed in the several steps: absorption of photons to create electron-hole pairs, in their transport to the water interface, in dissociating the water, and in separating and collecting hydrogen. The large area needed for a useful amount of hydrogen production, stemming from the low energy density in sunlight, is a difficulty, which could be addressed with focusing mirrors or Fresnel lenses. We consider a device in Figure 9.1 [111] based on a tandem solar cell.

oxygen bubbles appear at the left (platinum anode). Mass spectroscopy revealed the evolved gases to be pure hydrogen and oxygen in ratio 2 to 1. In this device, electrons flow to the illuminated contact where they act to release hydrogen gas. This cell is built on the GaAs multijunction tandem solar cell technology as described in Chapter 7, and is clearly an expensive device.

This device [111] can be regarded as a water electrolysis cell in series with a tandem solar cell, to supply the needed voltage to split water. Crudely, one can think of the semiconductor layers as a (light-activated) battery, the right-hand cathode terminal is in the electrolyte and the left-hand terminal (“ohmic contact”) is insulated from the electrolyte, but connected to the platinum (foil or gauze) anode through the external ammeter. The electrolyte in the cell is 3 molar H2SO4, a strongly acidic solution conductive by H + ions, also known as solvated protons or “hydronium ions” H3O +.

Consider first the electrolytic cell aspects. The cathode was coated with a thin layer of platinum particles, in the nature of “platinum black,” by an electroplating procedure.

The cathode reaction (reduction) is
2H + (aq.) + 2e~ ! H2 (gas)

The anode reaction (oxidation) is

2H2O (liq.) ! O2 (gas) + 4H + (aq.)+4e“

Taking twice the first reaction and adding to the second reaction, we get 2H2O (liq.) ! O2 (gas) + 2H2 (gas)

The standard potential for this reaction is 1.23 V. To make the reaction work, a larger voltage is needed, the excesses being termed cathodic and anodic overvoltages.

Solar cell output voltages are limited by the bandgaps of the underlying semi­conductors, which are 1.83 eV on the right and 1.42 eV on the left, in Figure 9.1. The sum ofthese voltages provides an upper limit ofvoltage from the tandem cell, which well exceeds the voltage, nominally 1.23 V, to decompose water.

The photocurrent flows along the wire at the top of the diagram (ammeter was shown in the previous diagram) and the Fermi levels of the ohmic contact and the platinum electrode are aligned. So, the voltage developed in the semiconductor layers is dropped across the electrolye, driving the decomposition of water. In the two semiconductor layers, it is seen that the Fermi level is flat (no voltage drop) across the connecting “tunnel diode interconnect,” so that the photovoltages of the two junc­tions are additive. The light enters on the right, and the right-hand junction has the larger bandgap, so that lower energy photons that are not absorbed on the right pass to the left to the second junction where they are absorbed.

In such a tandem cell, the electrical current has to be the same in each junction, so that the photocurrent of the series tandem cell is less than the short-circuit current of the weaker junction. The voltages add directly, allowing a higher conversion effi­ciency than a single-junction photocell. The efficiency was calculated as the chemical energy in the resulting hydrogen divided by the radiation power input. The chemical energy was calculated from the measured current multiplied by the voltage 1.23 V that is related to the hydrogen molecule.

The efficiency ofwater splitting was calculated from curve 1, with no external voltage applied, as power-out divided by power-in, or 0.12A x 1.23 V/(1.190 W) = 0.124, for 1 cm2 (Figure 9.3). The light intensity was about 11 suns for this experiment. (The

Подпись: Ohmic Transparent Efficient Photocatalytic Dissociation of Water into Hydrogen and Oxygen

Efficient Photocatalytic Dissociation of Water into Hydrogen and OxygenИ-О’Н

contact ohmic contact electrolyte interface anode

Подпись: Figure 9.2 Schematic band diagram [111] (electron energy increasing vertically) of illuminated monolithic photovoltaic-photoelectrochemical device for hydrogen production by water splitting, with 12.4% efficiency. (This is the same device as shown in Figure 9.1, but the platinum anode has been moved to the right side ofthe picture.) In this device, electrons flow to the illuminated semiconductor-electrolyte interface, where they actto release hydrogen gas. Energy levels forwater reduction (release of hydrogen) and water Подпись: oxidation (release ofoxygen) are sketched on the right side ofdiagram. An alignment is needed between conduction band edge and water reduction level, at the semiconductor interface. Alignment is also needed between the lower water oxidation energy and a level providing holes from the anode. So, on the right, the metal Fermi level (dashed line on the right) has to be pushed down by 1.23eV + 2 g to allow a hole to be injected into the water (i.e., platinum anode accepts an electron from the water) and thus to release oxygen.

tunnel junction

device here described is expensive and not a candidate for any practical water splitting approach. In the next sections, we will find examples of cheaper tandem cell possibilities.) One can infer that adding an additional junction in the cell to make a three or more junction tandem cell could result in excess cell output voltage beyond that needed to release hydrogen. Such a cell might be configured to simultaneously split water and supply power to an external load.

9.3.1

Updated: October 27, 2015 — 12:10 pm