Polycrystalline p-WSe2 photoelectrochemical solar cells I. Dark electrochemical behavior

Solar Energy Materials 11 (1984) 75-83 North-Holland, Amsterdam

75

POLYCRYSTALLINE p-WSe 2 P H O T O E L E C T R O C H E M I C A L SOLAR CELLS I. Dark electrochemical behavior Livia STOICOVICIU and Rodica M. C A N D E A Institute for Isotopic and Molecular Technology, 3400 Cluj-Napoca 5, P.O.Box 700, Romania

Received in revised form 10 April 1984 The dark electrochemical behavior of polycrystallinep-WSe2 electrodes is investigated. The dominant role of tungsten bonds for both the equilibrium behavior of the electrode and the reactions that take place at different electrodepotentials(hydrogentungsten bronze formation and oxidation, hydrogenevolutionreaction, etc.) is analyzed,

1. Introduction

The influence of the strong anisotropy and the physichemical properties of the single crystal layered dichalcogenide materials on their photoelectrical and photoelectrochemical behavior has been extensively discussed in relation to their potential use as photoelectrodes in a photoelectrochemical solar cell [1-5]. It was shown that photo-reactions take place predominantly through .J_ c oriented surfaces [3,5] and that an increased amount of II c oriented surface or a surface with atomic defects drastically affects the performances of the PEC [3-5]. Therefore, it is expected that the polycrystalline photoelectrodes perform much worse than single crystals in a PEC [6,7]. However, the fact that the polycrystalline layered dichalcogenides tend to orient preferentially with the c axis along the pressing direction [8] and the possibility of chemically modifying the electrode surface [9-11] suggest practical ways of increasing the performance of the polycrystalline electrodes. From this point of view the dark electrochemical behavior of the electrode provides information relative to the electrochemical processes that can take place at the semiconductor/electrolyte interface. This is especially true for the PEC where biasing is required; the electrode potential strongly influences the chemical state of the surface and therefore the efficiency of the photoreactions. Problems related to the dark electrochemical behavior of the polycrystalline p-WSe 2 electrode and the implications for the reactions under illumination are central to this paper.

2. Experimental The polycrystalline WSe 2 was prepared by direct synthesis at 1253 K from the constituent elements in evacuated quartz ampoules. The p-WSe 2 electrodes were 0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

76

l.. Stmco~,/ciu, R.M. ('andea /' Polvccvstalhne p W S e : clectrode,~. I

produced by pressing the powder into 1 m m thick discs of 115 mm diameter at 300 K with a pressure of 1.1 × 109 N / m 2. The size of the crystallites at the surface of the discs was about 5 ~m and X-ray diffraction showed a 98% _1_c orientation. The volume density of these electrodes was 76% of the single crystal density. C o m m o n potentiostatic, potentiodynamic and galvanostatic methods were used for characterizing the electrochemical behavior of the electrodes in 1 N H2SO 4 electrolyte. The p-WSe 2 electrode potential (Ew) was measured relative to a saturated calomel electrode (SCE), and the counter electrode was a smooth Pt strip of 20 cm 2. The active area of the working electrode was 1 cm 2. The cell was kept at constant room temperature, but no special care was taken to remove the air from the cell.

3. Results and discussion Unlike single crystal p-WSe 2 electrodes, the polycrystalline ones show dark currents for values of Ew between - 0 . 8 and 1 V. Typical current-potential curves are shown in fig. 1, where Ew was potentiostatically varied from the equilibrium value through the range of H 2 evolution and back again at a scanning rate of 100 m V / m i n . Depending on the history of the electrode, different values for the stationary potential were found, ranging from 176 to 381 mV. Although shifts of the 1 - E w curves are seen, the plateau around - 4 0 0 mV followed by H 2 evolution is maintained. When electrode potentials were scanned only in the cathodic current region, the value of the E w ( I = 0) shifted toward more negative values and became reproducibile after it reached - 380 mV. When the anodic scanning was extended to the 02 evolution region, the curves in fig. 2 were obtained. The polycrystalline WSe2, although preferentially oriented, has a complex electronic surface structure with unsaturated bonds in both W and Se [3]. It has already been established that for single crystal WSe 2 the anodic reaction products are HESeO 4 and W TM, and the cathodic reaction products are HE, HESe and lower oxidation states of W [12]. On the other hand, it has been shown that by anodically biasing W in acid solution, a passive WO 3 film is obtained [13], and Sinclet et al. found hydrogen tungsten bronze on cathodically biased WO 3 [14]. It is also known that stoichiometric or nonstoichiometric WO 3 catalyzes the adsorption and oxidation of H E but is a bad catalyst for the 02 evolution reaction [15,16]. Tungsten diselenide does not catalyze the H 2 evolution reaction [4] and is stable in concentrated acids. Consequently, the curves in fig. 2 can be interpreted as follows: (i) Reactions in the cathodic direction include reduction of the adsorbed oxygen ( E w ~ 350 mV), hydrogen tungsten bronze formation in two steps [12] and the H 2 evolution reaction (Ew ~< 400 mV). (ii) Reactions in the anodic direction include the hydrogen tungsten bronze oxidation in two steps. We believe that the broken W bonds are used in forming WO 3 layer which is probably nonstoichiometric. In order to check this we performed stationary measurements o n WSe 2. at constant electrode potential and constant current and cyclic voltammetry on W. In the cathodic region, for E w less than - 3 0 0 mV, the stationary values of the

L. Stoicooiciu, R.M. Candea / Polycrystalline p- WSe2 electrodes. I

77

current were the same as the ones found in the reproducible dynamical runs (scanning E w ( I = 0) to - 6 0 0 mV). Thus, we can conclude that, under cathodic biasing, the state of the surface changes toward bronze formation on which, subsequently, H 2 evolution takes place. This can also be inferred from fig. 3 where I ( t ) at E w = - 3 5 0 mV is shown for a fresh electrode. After the surface layer is formed (t > 100 min), when the electrode potential is made more negative, the current becomes stationary with no further need of electrical energy for bronze formation. For E w values of - 3 5 0 , - 5 5 0 , - 7 0 0 and - 9 0 0 mV, the gas in the cell was collected and analysed by mass spectrometry ( M A T 311 spectrometer). These results are given in table 1. The measured a m o u n t s of H 2 are in all cases smaller than the a m o u n t s calculated corresponding to the evolved oxygen concentration. Thus, for E w less than - 350 mV, there is an electrochemical process which consumes H 2- Because no H2Se was detected in the evolved gas within the 10 p p m error limit, the missing

I (mA/cm 2)

- 600 3' ~,~/ \.~ /

1'

t

/

/

400

J / / ~~--.~2 /I 00, 0 ~ ~ . ~ L..(mV,SCE) ,/ / ' . / " 1 /' "

~5

~6

I

-3

Fig. 1. Cyclic voltammetry on WSe2 electrodes in INH2SO4 (sweep rate 100 mV/min): 1-1' first cycle, 2-2' reproducible cycle, 3-3' cathodic reproducible cycle.

78

L. Stoicoviciu, R.M. Candea / Polvcoestalline p- WSe: electrodes, l

I (mMcm2)F 5

-600

200 -t~00

/

10

/ /

600

,v_1__,scel 800

400

_1 ¸

-2

-5 Fig. 2. Cyclic voltammetry on WSe 2 electrodes in 1NH2SO 4 (sweep rate 100 mV/min).

Table 1 Compositions for the gas collected after constant potential operation Potential a)

(voi%)

H 2 consumed

(mV)

Hz

02

N2

H 2Se

(%)

-350 -550 -7~ -900

1.0±0.0 15.9±0.3 57.8±1.2 41.8±0.8

42.8±0.9 41.5±0.8 33.8±0.7 31.3±0.6

57.2±1.1 42.6±0.8 8.5±0.2 26.9±0.5

-

98 67 9 13

a) The constant potential was reached by sweeping the potential first to I = 0, then to - 600 mV, back to l ~ 0, and then to the constant potential value.

L Stoicoviciu, R.M. Candea / Polycrystallinep- WSe, electrodes. I

79

ImA[cm21 10 OB

06

O~

02

~0

I

I

I

I

I

40

60

~0

100

120

I~0

160

t(min)

Fig. 3. Potentiostatic curve for W S e 2 a t E w = - 350 mV relative to SCE.

imA ~2 -0~ /

/

Ew(V,SCE)

I

t-6 I f-B

.-10

I'

I

1

Fig. 4. Cyclic voltammetry on W wire electrode in 1 N H2SO 4 (sweep rate 200 m V / m i n ) : 1-1' first cycle, 2 second cycle, 3-3' cathodic reproducible cycle.

80

L. Stoicouwiu, R.M. Candea / Po(vc~'stallme p- W S e , electrodes. I

hydrogen had to have been consumed either in bronze formation or in WO3 reduction to inferior forms. The last reaction is unlikely due to the high stability of WO 3.

The voltammograms at a scanning rate of 200 m V / m i n presented in fig. 4 were measured on W wire ( ~ = 0.5 ram) which had been washed in 12 M HC1. In the cathodic region the curve is strikingly similar to the voltammograms obtained for WSe 2 (fig. 2). The same plateau appears around - 4 0 0 mV, the H 2 evolution potential is the same and E w ( I -- 0) stabilizes around - 4 0 0 mV for both materials. Conversely, the anodic region of the curve is completely different for W and WSe 2. The equilibrium potential for W S e 2 , in 1 N H2SO 4 becomes stable immediately after contacting the electrolyte, and its value depends on the pretreatment of the electrode surface. Argon bubbling does not influence Eeq but H 2 or 02 bubbling slowly shifts Eeq toward either more negative or more positive values, respectively. The value of Eeq decreases with decreasing p H for both W S e 2 and W. In light of these findings and taking into account the work of E1 Wakkad et al. [17] concerning the electrochemical behavior of the W electrode, we conclude that on W S e 2 as well as on W, the same reaction is responsible for the equilibrium potential: W 2 0 5 --1- H 2 0

~ 2WO 3 + 2H + + 2 e - .

(1)

This reaction is shifted favorably to inferior or superior oxide formation by the presence of H 2 or 02, respectively. An example of a chronopotentiometric curve is given in fig. 5 for I c a t h = Ianod = 500

EmV

Ik I=1 ial= O,5mA

600 500 400

I

300 200

I]

,

,

100

-100 10 ~]~

~0 tCm~

0

I

-200 -300

I

-~00 -500 Fig. 5. Chronopotentiometric curves on WSe 2 electrodes, I = 4-0.5 m A / c m 2.

8!

L. Stoicoviciu, R.M. Candea / Polycrystalline p - W S e 2 electrodes. I

~A. Plateau I appears only if in the anodic branch the 02 evolution plateau (III) had been reached previously. Plateau I ' is affected by the presence of region III. Plateau II depends on the state of the electrode at equilibrium and is related to plateau II'. The transition time ~" was determined for each plateau using the classical method described in ref. [18]. In fig. 6, E(t) is plotted as a function of ln(~- - t) for plateau I I ' and straight line behavior is observed. This is consistent with the following equation: E(t)

= E o +

RT

ln(~" - t )

(2)

OiZ

which has been derived for the oxidation of H dissolved in a finite layer at Pd [19].

E(t) is the value of the potential for a point 0 < t < ~" on the plateau and E o is a constant proportional to the rate constant for the reaction corresponding to the plateau. This similar behavior leads us to conclude that the H E and 0 2 involved in the electrode reaction are distributed over a finite thin layer at the surface of the electrode. The transition times and the data obtained with (2) lead to the following conclusions: Process I is reversible ( a = 1 / 2 ) and corresponds to a reaction with 4 electrons. This is most likely the reduction of the adsorbed 0 2, although the reaction SeO3H 2 + 4H + + 4 e - ~ Se + 3 H 2 0

(3)

is not excluded if SeO3H 2 is present in the solution [20]. Plateau II corresponds to the reverse process of I I ' because the transition times are equal and the parameters of the line E(t)=f(ln(~" - t)) are equal and opposite in sign. In this charge transfer process only one electron is involved ( a = 0.75).

E (,}mV(SCE} 200

150

1 O0

4

t~5

5

55

6

65

"l'tn (~;-t) Fig. 6. E(t) vs. In(~"- t) for the chronopotentiometdc data from fig. 5, plateau If'.

L. Stoicotqciu, R.M. Candea / Polvco,stalline p - W S e , electrodes. I

82

Processes III and I ' involve one electron each with transfer coefficients of 0.4 and 0.6, respectively. The significance of plateau II, III and I% I I ' could be the formation of hydrogen tungsten bronze [11,21]: W O 3 + n H + + ne

= HnWO3,

0 < n < 1

(4)

and its oxidation, respectively. Plateau I I I ' corresponds to the 0 2 evolution reaction which competes with the W O 3 formation reaction and with the Se dissolution process: Se + 3 H 2 0 -

4 e - ~ H2SeO 3 + 4 H ÷.

(5)

4. Conclusions (i) The polycrystalline p-WSe z electrodes have large dark currents in the range of potential in which for single crystal electrodes the dark current is zero ( - 0.8 V to 1 V relative to SCE). (ii) The equilibrium reaction and the electrochemical reactions which occur at different electrode potentials are determined by the presence of W with dangling b o n d s on the surface of the electrode. These bonds saturate yielding more or less stoichiometric W O 3. (iii) The state of the electrode surface changes with biasing, and therefore, one can expect the photoreactions to be influenced. This suggests another way of improving the performance of polycrystalline electrodes and will be discussed in a subsequent paper.

Acknowledgements Helpful discussions with Dr. R.V. Bucur are greatly appreciated, and the preparation of the polycrystalline p-WSe2 electrodes b y Dr. P. Stetiu is acknowledged. We also wish to thank Dr. N. Palibroda for the mass spectrometric gas analysis.

References [1] [2] [3] [4] [5l

H. Tributsch, Ber. Bunsenges. Phys. Chem. 81 (1977) 361. S.M. Ahmed and H. Gerischer, Electrochim. Acta 24 (1979) 705. W. Kautek, H. G-erischer and H. Trihutsch, Ber. Bunsenges. Phys. Chem. 83 (1979) 1000. W. Kautek, J. Gobrecht and H. Gerischer, Ber. Bunsenges. Phys. Chem. 84 (1980) 1034. H. Gerischer, ACS Syrup. Ser., No. 146, Photoeffects at Scmiconductor-Electrolyte Interfaces, ed. A. Nozik (American Chemical SOt., New York, 1981). [6] R.M. Candea and P. Stetiu, Solver Energy 29 (1982) 435; 2nd Symp. Modern Science and Energy (26-27 March 1981), Cluj-Napoca, Romania; Nat. Conf. Progress in Physics (22-24 Oct. 1981), Timisoara, Romania, vol. IV, p. 111. [7] D.S. Ginley, R.M. Biefeld, B.A. Parkinson and K. Keun-Kam, J. Electrochem. Soc. 129 (1982) 145.

L. Stoicoviciu, R.M. Candea /Polycrystallinep-WSe 2 electrodes. I

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[8] L. Bnxner, J. Electrochera. SOc. 110 (1963) 289. [9] B.A. Parkinson, T.E. Furtak, D. Canfield, K. Kan and G. Kleine, Disc. Faraday SOc. (Sept. 1980) 233. [10] L. Fornarini, F. Stirpe, B. Scrosati and G. Razzini, Solar Energy Mater. 5 (1981) 107. [11] G. Razzini, M. Lazari, L. Peraldo Bicelli, F. Levy, L. de Angelis, F. Galluzzi, E. Scafe, U Fornarini and B. Scrosati, J. Power Sources 6 (1981) 371. [12] H. Tributsch, Faraday Disc. Chem. Soc. 70 (1980) 189. [13] L. Young, Anodic Oxide Films (Academic Press, London, 1961). [14] G. Siclet, J. Chevrier, J. Lenoir and C. Eyrand, C.R. Acad. Sci. Paris 277C (1973) 227. [15] A.M. Alqui6, G. Cr6py and G. Lamy, C.R. Acad. Sci. Paris 275C (1972) 1471. [16] G. Feuiilad¢, J. Bonet and B. Chenaux, Eiectrochim. Acta 15 (1970) 1527. [17] S.E.S.EI Wakkad, H.A. Rizk and I.G. Ebaid, J. Phys. Chem. 59 (1955) 1004. [18] P. Delahay, New Instrumental Methods in Electrochemistry (Interscience, New York, 1954). [19] R.V. Bucur and I. Covaci, Electrochim. Acta 24 (1979) 1213. [20] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (Pergamon Press, Cebelcor, 1966). [21] B.S. Hobbs and A.C. Tseung, Nature 222 (1969) 556.