Elecmchmuca Acta, Vol 37. No 8 pi 145X1459.1992 Printed ID Gterc Bntmn THE HYDROGEN EVOLUTION REACTION IN ALKALINEMEDIUMON NICKEL MODIFIED WITH W04= OR MoO,~0. SAVADOGO DCpartement de m&tllurgie et de g&tie des mat&faux &cole pofytechnique de Montr6al C.P . 6079 Succ “A” Mont&l, Qc E3C 3A7 Canada ABSTRACT Nickel was modtfied at the surface from alkalme solution contamrng dtfferent concentrattons of WO,% or MoG4= It was shown that improvement in the overpotential (n) and in the exchange current density of the hydrogen evolution reaction m KOH 3M on the electrode depends on the nature of the heteropolyaad of the modtticetion bath. The best electrocatalyttc parameters were obtatned on electrodes mod&d with 4 gI’ of W042mor 2 gP* of MOO,” 1. INTBODUCI’ION The work done to date relevant to the analysts of the hydrogen evolution reaction (h e.r ) on elcctromodilled electrodes has been summarized bnefly elsewhere [l and reference therein] The basic concepts of the Brewer theory of mtermetalltc phases have been described [2-4] and typmal tssues ansmg from the elcctrocatalyttc acttvtty achmved as a function of the composition of hypo-hyperdclectronrc transition metals have been presented [S] It has been shown that the bulk properties of these intermetallic phases determine their electrocatalyttc acttwy for the hydrogen evolutton reaction (h e r ) Thb reaction has also been performed wtth Chevrel-type cluster compounds [6j The re-mtxed cluster compounds were found to be the best electrodes for the h e r. (Both intermetallic systems of hype-hyperd-electronic metal combinattons and cluster materials were prepared thermally) In prevtous papers [7-H]. we have shown that electrodeposrted mckel electrodes that have been modified by heteropolyactds (HPAs) provide good electroattalyttc properties for the her in an aad medium Thts report deals wtth the hydrogen evolwon reactton in baste medtum at 300 K of nickel zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA moddied wrth different concentrattons of W042-or Mo042-. 2. EXPERIMENTAL The modtflcation of NI electrodes was performed under galvanostatic condtttons A constant current density of -0 5 A. cme2was applied to the electrode immersed m a denvatton bath composed of IM KOH and dtfferent concentrattons of W04’- or Mo042- respecttvely Dunng the modtficatton process, the m~msurcd potential changes The potential increases very quickly towards anodtc values at the begmnmg of the process and then stabihzes (from -0.900 to -0 800 volt E CE ) The modification time was determmed after the stabtlmation of the activation current and found to be 30 minutes. Concentrattons of 0 5.10.2 0,4 0 or 8 0 grams of W042or Mo042-per htre of base electrolyte were used The activated nickel was called rcspecttvely by Ni(AxW) or Ni(AxMo) and with x varying from 0 5 gi-’ to 16 gl-* The cathode polarization of the modified electrodes was measured using a PAR (Model 273) potentiostat 3 RESULTS AND DISCUSSION Fig. 1 shows typical experimental quasi-steady state tB corrected polarization curves obtained under potentiodynamlc condttions at a low sweep rate of 2 mV S* for Nt(AxMo) m KOH 3M. Analogous results are obtained with electrodes activated with W042eg. Ni(AxW). The results of the electrocatalyttc parameters of these electrodes for the hydrogen evoluuon reaction m KOH 3M can be summarmed as follows The exchange current density and the overvoltage of the activated niche1 electrode changes with MOO,%concentrations whereas their Tafel slope was constant (- 140 mv/ decade) 1457 A stgntftcant decrease of the overvoltage and an Important mcrcase of the exchange current denstty were observed (Table 1) The maxtmum value of te was obtamed wrth Nt(A4Mo) The mevalues of the h e r on the electrodes modtfled wrth W042-are greater than the lo values of the h e r on non acttvated nxltel (figure 2). The maxtmum values of I,, are obtamed for Nt(A4W) The h e r overvoltage changes wtth [WO4*J (see Rg 3) Aner modtficatton wrth drffercnt concentrations of MoOk or WO,*; the modtficatton wtth MOO,*has a htgher exchange current denstty than that mod&d wrth the other species (see fig 2 and table 1) Hi a0 b0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA NI(A0 5th) 0 Wi(A1 MO) . WI d ONI -0 7 -0.6 -0 5 -04 -0.3 -02 -0 1 00 7) (volt) FIN. 1 . Quasi-steady state polartzatton curves for the h e.r. obtained under potenttodynamtc condtttons at a low sweep rate of 2 mVs-t in KOH 3M at 3OOK.The results were referenced to the apparent geometnc surface area (2 cm*) of the electrodes. TABLE1 i,, (mA cm-*) and /u/ at-0 1 A.cm-* in KOH 3M at 306 K for NI acttvated at the surface with different concentrations of MoO4” Electrode ‘0 @cm-*) (*05x10-4) /u&Jolt) NI (A0 5Mo) Ni (AlMo) Ni (A2 MO) 10x10-~ 40x loa 60x 10’ 60x lo” Ni (A4Mo) 8.0 x 10-4 058 042 038 020 032 Nl (+ 005) 0010. 7 5 : 0001 ~~~~~~~ 5 OOOE-6, -1 Ftg 2 /IL 3 7 11 [woi- ] / 9 I-’ 15 19 Varration of the exchange current density of the hydrogen evolution reaction (lo) tn KOH 3M at 366 K of mckel acttvated at the surface wtth [W042] Short Commurucatton zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ 1459 The XPS (X-Ray photoelectron spectroscopy) results showed that MoO,~’ ts detected on modtlled surface wtth MoO,~ and WO, ts detected on electrode mod&d wtth W04*- These results are of course in agreement wth the Pourbatx diagram [12] From the results obtatned on the surface chemical composttton and then electrocatalyttc properttes of the mod&d electrodes, tt may be concluded that the electrocatalyttc properues can be correlated to the chemical composition of the modified electrode surface On the other hand, tt may be seen from table 1 and fig 2 and 3 that the electrocatalyttc properties for the h e r of nickel acttvated wrth MOO,**are better electrocatalysts than ntckel mod&d wtth WO,*- The main charactertstta of these films (thakness, morphology, phases, ;;I::: 0 $ ) are still under study sso 0 200, -1 3 7 11 [wo;- ]/ 15 19 grl Ftg. 3. Vanation of the hydrogen overvoltage /n/at 0 1 A. cm-* tn KOH 3M at 300 K of nickel activated wtth rW8Z-l ACKNOWLEDGEMENTS The author gratefully acknowledges the Natural Sctences and Engineedng Research Council of Canada and ‘Le programme actions structurantes des fonds FCAR du Gouvemement du Quebec” for their flnanaal support. REFERENCES 1 0 Savadogo and S L.&esque, J Appl Electrochem ,21.457 (1991) 2. MM Jaksic, Electrochtm, Acta, a 3 MM Jakstc, Int J Hydrogen Energy, Vol a 4 MM Jakstc, J Mol Catalysts, j& 161 (1986) 1539 (1984) 727 (1987) 5 M M Jakstc, Matenals Chemistry and Phystcs, 22.1 (1989) 6 N Alonso-Vant6, B Schubert and I-I. Tnbutscb, Mater& 7 0 Savadogo and E Fo.get, Int. J Hydrogen Energy, (in press) 8 0 Savadogo and D L Ptron, Int. 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