Electrocatalytic performance of lanthanum phosphate-bonded porous electrodes for the hydrogen evolution reaction in 30wt% alkaline solution

Int. J. Hydrogen Energy, Vol. 18, No. 9, pp. 719 725, 1993. Printed in Great Britain.

0360 3199/93 $6.00 + 0.00 Pergamon Press Ltd. © 1993 International Association for Hydrogen Energy.

ELECTROCATALYTIC P E R F O R M A N C E OF L A N T H A N U M PHOSPHATEB O N D E D POROUS ELECTRODES FOR THE H Y D R O G E N E V O L U T I O N REACTION IN 30wt% ALKALINE S O L U T I O N H.

DUMONT,*P. Los,*t H. MI~NARD,*L. BROSSARD,~B. SALVATO§and O. VITTORI§

*Universit6 de Sherbrooke, 2500 blvd Universit6, Sherbrooke, Qu6bec, Canada JIK 2R1 :~Institut de recherche d'Hydro-Qu6bec, 1800 mont6e Ste-Julie, Varennes, Qu6bec, Canada J3X 1S1 § Universit6 Claude-Bernard, Lyon I, 69622 Villeurbanne Cedex, France (Received for publication 29 January 1993) The electrocatalytic properties of lanthanum phosphate-bonded nickel powder (LPBN), LPBN ruthenium and LPBN-rhodium electrodes were investigated for the hydrogen evolution reaction (HER) in 30wt~o alkaline solution at 70°C. The kinetic parameters of the HER were determined by the steady-state polarization curve method. The influence of dissolved metallic impurities such as iron and heavy metals on the electrocatalytic activity of different types of LPBN electrodes was considered. Long-term performance tests were conducted in 30wt~ alkaline solutions at 70°C by applying a current density of 375 mA cm -2 during 210 h, followed by 24 h at open circuit potential, then reapplying the same current density. The effect of long-term experiments on the cathode morphology was investigated by SEM and X-ray fluorescence methods. Heavy metal deposition had only little effect on the highly porous LPBN, LPBN Ru, and LPBN Rh electrodes, especially on the most active of these, and a stable performance toward the HER was obtained. Abstract

INTRODUCTION Hydrogen is an important raw material for many industrial processes and, at the same time, is considered as one of the fuels of the future. According to Semenenko [1], the amount of hydrogen produced by electrochemical methods equals approximately 1~o of the overall production (50-60 million tonnes per year). One very promising electrochemical technology is advanced alkaline water electrolysis (AWE), which has the most immediate widescale commercial potential application of all currently investigated technologies [2, 3]. Two electrolytes are commonly used: chlor-alkali, which contains 10-30Wt~o N a O H at 80°C, and industrial alkaline electrolyte, containing 30wt~o nonpurified K O H in the 70-90°C temperature range. It should be pointed out that the maximum conductivity of the electrolyte for A W E is reached for about 30Wt~o K O H water solution [4], while the total cell voltage decreases with temperature from 25 ° to 90°C. Nickel or nickel-based alloys are the most frequently used anodic and cathodic materials for industrial water electrolysis. The development of inexpensive, highly active and stable electrocatalysts is still one of the major issues for the improvement of the A W E and great efforts

tOn leave from Department of Pharmacy, Medical University, 50-139 Wroclaw, Poland

are therefore being made to find new electroactive electrodes [4-22]. In our laboratory, a new lanthanum phosphate binding material was used to obtain highly electroactive and mechanically stable nickel powder based (LPBN) electrodes [21]. The chemical deposition of small amounts of rhodium or ruthenium on the nickel particles (subsequently bonded through polymerization of the lanthanum phosphate [7, 8]) was seen to greatly improve the performance of cathodes in 1M alkaline solution at 25°C. The present paper deals with the hydrogen evolution reaction (HER) on LPBN, L P B N - r u t h e n i u m and L P B N rhodium electrodes under the experimental conditions prevailing in industrial water electrolyzers, i.e. a 30wt~o alkaline solution containing ~0.5 ppm of dissolved iron at 70°C. As far as the presence of iron is concerned, the deposition of iron impurities on cathodes during water electrolysis may be envisaged, with possibly some effects on the deactivation rate [9, 12, 23-25].

EXPERIMENTAL The ruthenium and rhodium chemical deposition on nickel particles (Inco 255) was carried out using the procedure given in previous papers [7, 8]. The electrodes were prepared by pressing under vacuum a mixture of La(H2PO4) 3 and La(OH)3 with nickel powder, either 719

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H. DUMONT et al.

pure or covered by ruthenium or rhodium. The mould diameter was equal to 1.3 cm. Nickel foil was introduced into the pellet to ensure good electrical contact. Polymerization was achieved by heating the pellets at temperatures ranging from 300 ° to 600°C for 3 h under a flow of argon. The polymerization process was in accordance with a previously developed procedure I-7, 8, 21, 24]. The electrodes were coated on one face and on the sides with Epofix resin (Struers) to obtain a geometric surface area of 1.33 cm 2. The electrode material contents consisted of 0.8, 1.5 or 3.5wt~o in rhodium and 1, 2 or 3.5Wt~o in ruthenium compared with 2Wt~o for the binder; nickel made up the difference. Barnstead nanopure water with a 17.5 Mf~ resistance was used to prepare the K O H solution ( B D H certified ACS reagent grade). The iron and silver content of the solutions was determined by I C P AES (induced coupled p l a s m a - - a t o m i c emission spectroscopy): for a 30Wt~o K O H B D H solution, the iron content was ~0.5 ppm. For the H E R investigation, a polysulfone electrochemical cell was used with a nickel grid counter electrode with a geometric surface area of 40 cm 2 and an external H g / H g O / K O H 1M reference electrode immersed in 30wt~ K O H maintained at room temperature. The cell is described in detail elsewhere [12, 14, 20, 23]. The cathodic polarization curves were obtained by decreasing the applied current galvanostatically from 250 to 0.01 mA cm -2 after 30 min under a cathodic current of 250 mA cm -2. The electrode potentials were corrected for the ohmic drop, which was determined by the current-interruption and a.c. impedance techniques, which gave similar results [8, 9]. The steady-state and a.c. measurements were made by a galvanostat-potentiostat

PAR 273 and an E G & G 5208 lock-in analyzer controlled by a C o m m o d o r e PC2 microcomputer. The influence of the Rh and Ru content in the L P B N electrodes and the effect of the polymerization temperature on the kinetic parameters of the H E R were considered. The deactivation was investigated on porous Ni-based cathodes. RESULTS AND DISCUSSION Prior to the long-term performance characterization, the effect of the polymerization temperature of the L P B N electrode on the H E R in 30wt~o K O H solution at 70°C with ~0.5 ppm dissolved iron and silver was investigated. The lower the polymerization temperature, the better the electrocatalytic activity for the HER. Since the rate-determining step of the H E R is a surface reaction, this behavior is linked to a decrease of the real surface area as polymerization temperature rises [8, 9]. The effect of the polymerization temperature on the electrocatalytic activity at 250 mA crn- 2 is generally the same for the LPBN, L P B N - R h and L P B N - R u electrodes and the highest electrocatalytic activity for all electrodes corresponds to a polymerization temperature of 300°C. The best electrocatalytic activity at 250 mA cm -2 (30Wt~o, 70°C) was found for the lanthanum phosphate-bonded electrode with lwt~o and 3.5Wt~o Ru (Table 1). It is worth noting that, as shown in Table 1, there are no significant differences in performance between the most active L P B N - R u and L P B N - R h electrodes in 1M and 30Wt~o K O H solutions at 25 ° and 70°C, respectively. Figure 1 shows the overpotentials with time for the long-term experiments, while the kinetic parameters of the H E R on L P B N - R u , L P B N - R h and L P B N electrodes are summarized in Table 2. N o significant change

Table 1. Comparison of electrocatalytic activity of LPBN, LPBN-Rh and LPBN-Ru electrodes polymerized at 300°C for the HER in 1M in 30Wt~o KOH solutions at 25 ° and 70°C after 2 h electrolysis

Electrode composition Ni Ni Ni/Rh Ni/Rh Ni/Rh Ni/Rh Ni/Rh Ni/Rh Ni/Ru Ni/Ru Ni/Ru Ni/Ru

0.8~o 0.8~ 1.5~o 1.5~o 3.5~o 3.5~ 1.0~o 1.0~o 3.5~o 3.5~o

Concentration and temperature of electrolyte

Overpotential at 250 mA cm- 2 (mV)

Exchange-current density (mA cm- 2)

Tafel slope (mV dec- 1)

30~o-- 70°C 1 1M--25°C 2 30~o--70°C ~ 1M--25°C z 30~o--70°C 1 1M--25°C 2 30~o-- 70°C 1 1M--25°C 2 30~o-- 70°C 1 1M--25°C 3 30~o-- 70°C 1 1M--25°C 3

155 177 110 132 85 125 75* 77* 50* 64* 50* 40*

28 12 38 17 43 20

161 132 134 106 110 122

--

--

---

---

* Linear dependence of log/vs r/cannot be assumed for such low overpotential values. 1 This work. 2 From Ref. [8]. 3 From Ref. I-7].

LANTHANUM PHOSPHATE-BONDED POROUS ELECTRODES

q-. I

.

'C:

.

~ C"3

.

150

.

200

250

t, h

Fig. 1. HER overpotential vs time in 30wt~ KOH solution with 0.5 ppm iron at 70°C under: ( - - ) a current density of 375 mA cm-2 and (..... ) open circuit condition using: (O) LPBN-Rh (3.5Wt~o); (+) LPBN-Ru (lWt~o); (V) LPBN; and (D) LPBN-Ru (3.5Wt~o)electrodes.

in the electrocatalytic activity of the LPBN-Rh (3.5wt~o) electrodes was noticed during 210 h of electrolysis in 30wt~o of BDH K O H solution at 70°C at a current density of 375 mA cm -2. A strong deactivation was observed for the LPBN electrodes compared with a mild deactivation of the LPBN-Ru electrodes. As shown in Table 2, the increase in the overpotential at 250 mA cm-2 for the LPBN electrode is mostly due to the lower exchange current density and increase in the Tafel slope after 210 h of cathodic polarization. The SEM pictures in Fig. 2a reveal morphological changes in the electrode

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surface resulting from cathodic polarization. This transformation, combined with a shift in the open circuit potential in the cathodic direction ( ~ 30 mV), suggest that the LPBN electrodes are deactivated by the absorption of hydrogen into the nickel lattice during the HER [7, 11, 12, 14, 26]. A subsequent 25 h at open circuit potential together with 20 h electrolysis led to deactivation for all electrode materials, except in the case of the LPBN electrodes for which a recovery of lost efficiency because of desorption of the atomic hydrogen in the metal lattice and the dissolution of the iron deposit was observed [15, 23"]. In contrast with the LPBN cathodes, slight changes in morphology were observed for LPBN-Ru and LPBN-Rh electrodes after the HER. The surface analysis of the LPBN nickel powder electrodes by the energy dispersive X-ray fluorescence (XRF) method was carried out before electrolysis, after 24h and 210h of HER at 375mA cm -2 in KOH solutions. The surface concentration of iron and silver on the LPBN electrode increased substantially during electrolysis. The amount of iron detected increases from 0 before electrolysis to lwt~o after 24 h and 10wt~o after the next 186 h of galvanostatic testing. For LPBN-Ru and LPBN-Rh electrodes, the iron deposition rate is much smaller, because of the lower cathodic hydrogen overpotentials [13, 25], and the detected amount of iron reached only 0.15wt~ after 210 h of electrolysis. Further electrolysis (210 h of electrolysis, followed by 24 h at open circuit potential with 20 h of electrolysis) on the same electrodes had no influence on the electrode morphology, except for the LPBN electrode for which the above-mentioned extensive iron deposition occurred.

Table 2. Tafel parameters during the long-term electrolysis tests for different types of lanthanum phosphate-bonded electrodes polymerized at 300°C in 30wt~ KOH solution at 70°C. Electrolysis time

2h 210 h Plus25 h at o.c.p Plus 20 h

LPBN-Rh (3.5wt~o) b

Io

~2so

b

Io

~250

-(90) 135 156

-(54) 59 59

63 63 81 92

-121 109 136

-35 30 35

50 100 98 114

Electrolysis time

2h 210 h Plus 25 h at o.c.p Plus20h

LPBN-Ru (lwt~)

LPBN

LPBN-Ru (3.5wt~o)

b

I0

17250

b

Io

~/2so

161 193 164 182

28 9 9 11.5

155 275 230 236

-126 140 116

42 52 36

50 93 94 92

b: Tafel slope (mV dec-1); Io: exchange current density (mA cm-2); ~/25o:overpotential at 250 mA cm -2 (mV); and o.c.p.: open circuit potential.

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Fig. 2. (a) SEM picture of the LPBN electrode surface: (i) before electrolysis; (ii) after 24 h electrolysis under a cathodic current density of 375 mA cm -2 in 30Wt~o K O H at 70°C.

LANTHANUM PHOSPHATE-BONDED POROUS ELECTRODES

Fig. 2. (b) Energy dispersive X-ray mapping of LPBN electrode surface after long-term electrolysis under a cathodic current density of 375 mA cm-2 in 30wt~o KOH at 70°C: (i) SEM picture; (ii) iron mapping of the sample of picture b (i).

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Fig. 2. (c). Energy dispersive X-ray mapping of LPBN electrode cross-section after long-term electrolysis under a cathodic current density of 375 mA cm-2 in 30wt~o KOH at 70°C: (i) iron mapping of the sample; (ii) nickel mapping of the same sample.

LANTHANUM PHOSPHATE-BONDED POROUS ELECTRODES Energy dispersive X-ray mapping of the surface (Fig. 2b) and the cross-section (Fig. 2c) of the same L P B N electrodes after 210 h of electrolysis shows a smooth iron layer deposited on the nickel particles. Only traces of iron were found in the pores of the electrodes (Fig. 2b and c). From these results, it is clear that rhodium or ruthenium coatings on the nickel particles form a protective layer against deactivation processes induced by hydride formation, and the rate of iron deposition is decreased since the H E R overpotentials at 250 mA c m - 2 are lower for these coatings. The deposition of iron is less detrimental for the increase in the HER overpotential than the hydride formation, which is not surprising, considering the large surface area of the L P B N electrodes.

CONCLUSION The electrocatalytic activity of L P B N - R u and L P B N - R h electrodes is characterized by high exchangecurrent density, low Tafel slopes and low overpotentials at 250 mA cm -z in 30wt% alkaline solutions at 70°C. The best performance was found for lanthanum phosphate-bonded nickel-ruthenium (1-3.5wt%) and nickel-rhodium (3.5wt%) electrodes polymerized at 300°C. The good performance of the L P B N - R u and L P B N - R h electrodes was observed during long-term stability tests in 30wt% alkaline solutions at 70°C and under a current density of 375 m A c m -2. O n the other hand, the L P B N electrode was strongly deactivated during 24 h of electrolysis in 30wt% K O H . The deposition of iron and heavy metals from concentrated alkaline solutions does not influence the electrocatalytic activity of large-surfacearea l a n t h a n u m phosphate-bonded electrodes.

Acknowledoements The Natural Science and Engineering Research Council of Canada, IREQ and the Qu6bec Government are acknowledged for their financial support. Thanks also go to Mr L. Timberg of Inco Metals Company for furnishing the characterized nickel powders.

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