Catalytic Influence of Commercial Ru, Rh, Pt, and Pd ( 0,1 atomic percent) Intercalated in Graphite on the Hydrogen Evolution Reaction Joel Fournier, Piotr K. Wrona, *'a Andrzej Lasia,* Robert Lacasse, Jean-Marc Lalancette, Hugues Menard Department de Chimie, Universite de Sherbrooke, Sherbrooke, Quebec, Canada JIK 2R1 Louis Brossard* Institut de Recherche d'Hydro-Quebec, 1800 Montee Ste-Julie, Varennes, Quebec, Canada J3X 1S1 ABSTRACT Graphite electrodes intercalated with various metals (Ni, Co, Pd, Pt, Rh, and Ru) have been bonded with an inorganic polymer LaPO4 and used as cathodes in the hydrogen evolution reactions (HER) in 1M KOH. Four of the most active electrodes showed very good mechanical and electrochemical stability. The overvoltage of the HER at j = 0.10 A cm -2 decreased in the following order: -525 mV, for pure graphite and -254, -137, -103, and -58 mV, for the Pd/C, Rh/C, Pt/C, and Ru/C electrodes, respectively. The kinetics of the HER for two electrodes (Ru/C and Pd/C) were measured with the use of an ac impedance technique. In both cases, the HER proceeds via the Volmer-Heyrovsky mechanism. The results obtained proved that the Pd/C electrode had a larger active surface area than the Ru/C one did. The hydrogen evolution reaction (HER) has been investi- gated extensively in the literature. I-~ This reaction may be schematically described as composed of three elementary steps H20 + M + e = MHaas + OH- kl, k_l Volmer reaction [1] H20 + MHad~+ e = M + H2 + OH- ks, k_2 Heyrovsky reaction [2] 2MH~d~= 2M + Ha k3, k-3 Tafel reaction [3] where k represents the rate constant of the forward (+) and backward (-) processes. Water reduction with hydrogen adsorption (reaction 1) is followed by electrochemical (re- action 2) and/or chemical (reaction 3) desorption. The rates (v) of these processes are described by the following equa- tions v~ = k~(1 - {~)exp (-a~f~) - k_~8 exp [(1 - a~)/~)] [4] v2 = k28 exp (-affT1) - k_2(1 - 0) exp [(1 - a2)f~)] [5] v~ = k~} 2 - k_~(1 - 8)a [6] where a stands for the transfer coefficient, 8 for the surface coverage of the electrode by adsorbed hydrogen, and f = F/RT. Early studies by Bockris, Cenway and Parsons 6 showed that the rate of the HER reaches a maximum with platinum group metals, with nickel-based electrodes also being very active electrocatalysts. For hydrogen evolution in alkaline media, 7-9 the Ni-based electrode can be made by various methods such as sintering, vacuum or plasma-sprayed de- position, electrodeposition, or high-temperature deposi- tion. m~ High surface area nickel electrodes may also be obtained by using a tridimensional inorganic polymer of aluminum or lanthanum phosphate (AlPO4 or LaPO4), which cements the metallic particles. 172~ The electrodes are more stable in the aqueous alkaline solution when made with LaPO4 rather than A1PO4, since the former is less solu- ble in such solutions,a~ This new process is particularly at- tractive because cementation can be obtained without a reducing atmosphere (in this case only the presence of inert gas is required). Very active metallic powders such as platinum shall not be used, since these materials are expensive compared to nickel. However, these materials may be interesting with * Electrochemical Society Active Member. Present address: Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. regard to the HER if they are considerably dispersed by their intercalation into graphite. 21 Intercalation of alkali metals, chlorine, and bromine into graphite has been long known, 22whereas successful inter- calation of transition metals is relatively recent. ~3-~ Vari- ous active metals (Pt, Rh, Ru, Pd) may be intercalated into graphite by vapor or solvent techniques. Since graphite is very loosely bonded, paste electrodes of the Adams type, 2~ in which various hydrocarbons are used as binding sub- stances, would have a tendency to disintegrate under a large flow of hydrogen when graphite is used as the cathode for alkaline water electrolysis. In this laboratory, graphite powder electrodes were made with lanthanum phosphate as a binding material, resulting in strongly bonded graphite particles. In the present study, the electrochemical behavior of graphite powder bonded by polymerized LaPO~ is investi- gated for the HER in 1M KOH solution at 25~ Commer- cially prepared ruthenium, palladium, platinum, and rhodium intercalated into graphite (from the correspond- ing chlorides with the bulk content of the transition metals being -0.1 atom percent (a/o) were examined mainly. The electrodes with ruthenium at j -- 0.10 Acm -~ in 1M solution of KOH, at 25~ showed hydrogen overvoltage as low as -58 mV, i.e. much lower than the electrodes prepared from the nickel powder. Since the stability of these electrodes was also good, practical applications of these electrodes are attractive for water electrolysis or the electrohydrogena- tion of organic compounds in alkaline aqueous solutions. Experimental Preparation of electrodes.--Natural graphite or com- mercially available graphite with intercalated metals or metal chlorides were used as starting materials (Graphimets, Alfa Products). The latter compounds were reduced to remove chlorides. The C/LaPO4 and M/C/LaPO 4 electrodes were prepared with graphite containing interca- lated metals: Ni (2 a/o), Co (1 a/o), and Pt (0.1 a/o), Pd (0.1 a/o), Ru (0.1 a/o) and Rh (0.1 a/o). Lanthanum phosphate is prepared by combining acid lanthanum phosphate and lanthanum hydroxide, as de- scribed by Dumont et al. 2o Two reactions are involved in the formation of acid phos- phate lanthanum La(H2PO4)3 and its subsequent transfor- mation into LaPO4 1/2 LaaO3 + 3H~PO4 --> La(H2PO~)3 + 3/2 H20 [7] La(H2PQ)3 + 2La(OH)3 --> 3LaPO4 + 6H20 [8] 2372 J. Electrochem. Soc., Vol. 139, No. 9, September 1992 9 The Electrochemical Society, Inc.