The Rate of the Photoelectrochemical Generation of Hydrogen at p-Type Semiconductors

The current -potent ia l relations with and without i l lumination, quantum eff iciency-wavelength relations at several potentials, the flatband potentials, the transient behavior, and the stabili ty of seven ptype semiconductors, i.e., ZnTe, CdTe, GaAs, InP, GaP, SiC, and St, have been measured in 1N NaOH and 1N H2SO~. The position of the photocurrent-potent ia l relations are re la ted to the flatband potential and the energy gap of the semiconductor. The existence of the max imum in quantum e~nciency-wavelength relation is analyzed by considering surface recombination. The stabili ty and the transient behavior are analyzed. Photoelectrochemical production of hydrogen was envisaged by Fuj i sh ima and Honda in 1972 (1). To obtain hydrogen, ei ther a pH gradient or an external power source in the cell was required (2-4). However , homogenizat ion of the solution would inevi tably occur on prolonged functioning in such an arrangement . The lack of need for single crystals in the photoelect rochemical approach to energy conversion (5, 6) gives the prospect of favorable economics in pure ly photoelectrochemical hydrogen production from water. The pr imary aim is the development of a suitable cathode, so that l ight may be directed both onto the cathode and anode, with the objective of obtaining stable photoelectrolysis in a cell with a uniform pH. A previously reported photocathode is unstable (7). We report investigations concerning the stability and efficiency of certain new photocathodes. Experimental Apparatus.--The photoelectrochemical cell is shown in Fig. 1. Stopcocks and taps were Teflon. To avoid contact of the metal used to form an ohmic contact with the solution, the back face and side of the electrode were covered with epoxy resin. To minimize contact of this wi th the solution, a Teflon electrode holder was used. The absence of a leak was verified by the small magni tude of the dark current. All photoelectrode areas were 0.125 cm 2. A PAR Model 173 potent iostat /galvanostat , wi th a Model 176 current -potent ia l converter, was used to control the potential. The electrode potential was swept by a Wenking SMP 69 potential stepping motor control. The current -potent ia l relat ionship was recorded by a Hewle t t -Packard Model 7004B X-Y recorder. The t ime dependence of the photocurrent was recorded by means of a Hitachi QD25 recorder. A 900W xenon lamp (Canrad-Hanovia 538C1) was used as a light source and a Ja r re l l -Ash quar t e r -me te r grating monochrometer (Cat. no. 82-410) was employed to obtain monochromat ic light. An IR absorbing filter (Oriel G-776-7100) was placed between the cell and the l ight house, when current -potent ia l measurements were carried out wi thout the monochrometer . However, a quartz lens (d ---5 cm, f ---5 cm) was employed to concentrate the light on the electrode surface when the photocurrent was measured under monochromat ic light. In this case, two long pass filters (Oriel G-7723900 and Oriel G-772-5400) were used with an IR absorbing filter to el iminate second-order diffraction. The conditions used in this respect were: 3000 ~ 5000A, IR * E l e c t r o c h e m i c a l Society Active M e m b e r . 1 P r e s e n t a d d r e s s : Mi t sub i sh i P e t r o c h e m i c a l C o m p a n y , L imi t ed , Ami , I b a r a k i , J a p a n . Key w o r d s : h y d r o g e n p r o d u c t i o n , p h o t o e l e c t r o c h e m i c a l r eac t ion , p-type semiconductors, photocathodes. absorbing filter only; 5000 ~ 7000A, IR absorbing filter 4G-772-3900 filter; 7000 ,-~ 7500A, IR absorbing filter § G-772-5400 filter; 7500A, G-772-5400 filter only. The intensi ty of l ight was measured by means of a Hewle t t -Packard Model 8334 radiant flux meter with ei ther a 8334A radian flux detector or a Carl Zeiss vacuum thermocouple (VT Q3/A) with a Kei th ley 149 mil l imicrovol tmeter . The error in re la t ive intensi ty measurement was < 5%. However , absolute intensi ty measurements had an uncer ta inty of • 20% error. The electrochemical cell and the optical system were set up on an optical bench. Impedance measurement -The cell for impedance measurements had a working electrode surrounded by a cylindrical platinized pla t inum counterelectrode, apparent area 60 cm 2. Hydrogen gas was passed into the solution before and during measurement . The direct method was employed (8, 9). The circuit contained a dry cell (6V) as a d-c source and the potential was controlled by a t entu rn variable resistor