PLATINIZED PLATINUM ELECTRODES A. M. FELTHAM AND M. SPIRO Department of Chemistry, Imperial College of Science and Technology, London S. W.7, England Received July 29, 1970 (Revised Manuscript Received October 23, 1970) Contents overcome this difficulty, Kurlbaum and Lummer3 recalled that platinum black can easily be precipitated from chloro- I. Introduction 177 11. Historical Origins 177 platinic acid solution by adding copper or lead, so that such 111. Electrode Kinetics and Mechanism 177 chemically prepared platinum black always contains some A. In the Absence of Lead 178 copper or lead as well. Accordingly, they tried adding a small B. In the Presence of Lead 180 amount of copper sulfate, to the extent of about 1 of the C. Coulombic Efficiency 180 chloroplatinic acid present, to the plating solution and found D. Substrate Pretreatment 180 that this regularly produced very good platinum black de- 1V. Electrodeposition 181 posits. Even better results were achieved by adding a small A. Deposit Appearance 181 quantity of lead acetate. Their final recipe3 was 1 part of B. Deposit Growth 183 chloroplatinic acid and 0.008 part of lead acetate to 30 parts C. Constitution of Lead-Containing Deposits 184 D. The Effect of Alternating Current 185 of water, the electrolysis being carried out with a potential V. Surface Area 185 difference of 4 V between the (platinum) working electrodes A. Electrochemical Determination 186 which gave a current density of 30 mA cm-2 at the cathode. B. Dependence on Plating Conditions 187 It is this recipe which was shortly afterwards adopted by C. Reproducibility 191 Kohlrausch4 for making platinum black electrodes for elec- D. Decrease with Time 191 trochemical purposes. Kohlrausch, to whom this preparation VI. Survey of Recommended Platinizing Procedures 193 has at times been incorrectly attributed,8 stated clearly, but without quoting any literature sources, that the method was that of Lummer and Kurlbaum. Only in a later book6 is ref 3 1. Introduction specifically referred to. That this historic recipe is quite sound The platinized platinum electrode is the most widely used is shown by the results of more modern research which is type of electrode. It makes a regular appearance in conduc- discussed in the following sections. The main modification tance cells, forms the basis of the hydrogen reference electrode, desirable is a reduction of the lead acetate concentration by a and is unsurpassed as an electrocatalyst in fuel cells. Yet the factor of approximately 5. mechanism of the formation of the platinum deposit has been In the original bolometric study the platinum compound investigated only recently, and information on the properties was referred to21* as “Platinchlorid,” but both Kohlrausch’ of the electrode (its appearance, structure, area, and repro- and more recent booksstg have pointed out that this term was ducibility), and how these are affected by its method of prepa- commonly applied to chloroplatinic acid. This loose nomen- ration, is widely scattered and not well known. The main factors clature has caused some confusion in the later literature and in are critically assessed in this review. many present-day undergraduate laboratory manuals. Even PtC14 may mean1O*lichloroplatinic acid unless a special prep- 11. Historical Origins aration’ of platinic chloride is given. It is an accident of history that the original recipes for 111. Electrode Kinetics and Mechanism platinum black electrodeposition were devised, not for electro- chemical purposes, but in connection with the measurement of The electrodeposition of platinum from chloroplatinic acid radiation. When radiation falls on a black metallic strip the solution involves three couples. temperature of the strip rises, and the resulting increase in PtIVCla?-+ 2e- +PtTIClla- + 2C1- (1) resistance can be determined with a Wheatstone bridge ar- PtI1Cl4’+ + 2e- e Pt + 4C1- (2) rangement. Early models of such devices, known as bolometers, made use ofi platinum strips blackened with soot by means of a petroleum flame. The irreproducibility of this method of (3) F. Kurlbaum and 0. Lummer, Verh. Phys. Ges. Berlin, 14, 56 (1895). blackening led Lummer and Kurlbaum2 to try instead the (4) F. Kohlrausch, Ann. Phys. Chem., 60,315 (1897). electrodeposition of platinum black, since composition of the (5) G. Jones and D. M. Bollinger, J. Amer. Chem. Soc., 57,280 (1935). plating solution, current, voltage, and time can be exactly (6) F. Kohlrausch and .L. .Holborn, “Das Leitvermogen der Elektro- lyte,” B. G. Teubner, Leipzig, 1898, p 9. defined and controlled. But there was one drawback: platinum (7) F. Kohlrausch, Ann. Phys. Chem., 63,423 (1897). black electrodeposited from pure chloroplatinic acid solutions (8) A. 1: Vogel, ‘‘6 Textbook of Macro and Semimicro Qualitative did not adhere properly to the electrode. In attempting to Inorganic Analysis, 4th ed, Longmans, Green and Co., London, 1954, p 581. (9) “Platinum,” International Nickel Ltd., London, p 11. (1) 0.Lummer and F. Kurlbaum, Ann. Phys. Chem., 46,204 (1892). (10) G. Hevesy and T. Somiya, Z.Phys. Chem., 171.41 (1934). (2) 0. Lummer and F. Kurlbaum, Sitzungsber. Kgl. Preuss. Akud. (11) A. Slygin and A. Frumkin, Acta Physicochim. URSS, 3, 791 Wiss., 229 (1894). (1935). 177 178 Chemical Reviews, 1971, Vol. 71, No. 2 A. M. Feltham and M. Spiro PtIVCle* + 4e- =+= Pt + 6C1- (3) century after the process itself had been introduced.2-40eThat The electrode potentials have been measured under a variety of PtC142-participated was shown by two main lines of evidence. conditions, and the results are summarized in Table I. Those First, the current-voltage curves, of which an example is given in Figure 1, exhibited not one but several waves, the last steep rise being caused by hydrogen evolution. The quan- Table I titative concordance between the curves obtained by 21 Electrode Potentials of the Three Pertinent and laterzz,23 workers is relatively poor, partly because the Platinum Couples (See Text) compositions and temperatures of the plating solutions varied Temp, Supporting and also as a result of the different speeds with which the Couple 'C electrolyte EO, Va Ref curves were drawn. The slower the experiment, the greater 1 25 None 0.77 & 0.05 12 the area of the platinized platinum deposit, and so the lower 1 25 None 0.68 13b the real current density. Grube and Reinhardt's resultsz0are 1 25 1 MHCl 0.72b 25 not atypical and encompass a wider range of conditions than 1 25 1 M NaC104 0. 72d 25 most: their initial deposition potentials with 0.1 M HzPtCls 1 50 2.5 MHCl 0.74 21 at 18°C were 0.45 V in the absence of HC1 and 0.12 V in 5M 1 60 None 0.745c 20 HC1 and, at 0.54 and 0.36 V, respectively. Comparison 2 25 None 0.75 12 60°, with Table I shows that the initial electrode process (almost 2 25 None 0.73 13b 2 25 1 M NaC104 0.78 25 certainly reaction 1) is irreversible and this, as has already 2 60 None 0.78jc 20 been pointed out, is not unexpected. The shift to more anodic 3 25 None 0.76" 12 deposition potentials as the temperature is raised demonstrates 3 25 None 0.705' 13b the anticipated increase in exchange current density. 3 25 1 M NaC104 0.74b 25 The second reason for pointing to the participation of 3 60 None 0.76y 20 PtCL2- came from the analysis of the electrode products after various times. Initially there was preferential reduction to a All potentials in this review are given on the NHE scale, unless stated otherwise. Values given on the SCE scale have been converted PtC142-, as shown also by the fact that the solution color by using E(SCE) = 0.245 V at 25". Originally measured US. a 1 M turned from lemon yellow to dark red, to be followed by the NaCl calomel electrode (NaCE). The cell SCE/l M HClO4iNaCE deposition of platinum metal. This change with time was had an emf of 65 mV; hence E(NaCE) = 0.310 V if the large liquid particularly pronounced at higher temperatures. 2o junction potential is ignored. c Measured us. SCE at 18" with a KCl salt bridge. dcalculated from the corresponding values for Another third of a century passed before more quantitative couples 2 and 3. *Calculated from the corresponding values for information appeared with the advent of sophisticated elec- couples 1 and 2. trochemical techniques. The most useful of these to date has been thin-layer voltammetry24 where the peak currents are a few microamperes only and the amount of platinum deposited of Goldberg and Heplerl2are the selected best standard values. during the course of each run is manageably small. 19 Lau and In general, the emf's of these couples lack reproducibility.13a H~bbard~~applied slow voltage sweeps to a cell 26 pm thick This is not unexpected; reactions 1-3 involve the breaking of containing either PtC16'- or PtC1d2- solutions, and fitted the several Pt-Cl bonds and must therefore be kinetically slow resulting current-voltage curves to theoretical rate equations. and possess low exchange current densities. The derived rate constants k are listed in Table 11. In agree- Reference is occasionally made to the extent to which PtCle2- hydrolyzes in solution. The hydrolysis kinetics have been investigated,14-17 and the reaction has been found to be Table II catalyzed by a product of the hydrolysis,16 light,1e18 and Rate Parameters" for the Reduction of PtClP to PtC1d2- platinum black.16 In fact, most plating solutions are acid (kl, alnlo) and of PtC14Z- to Platinum (k2, CUZJZ~O) (usually HCl is added), and the evidence strongly suggests at 25OC from Ref 25 that under these conditions the major species present is [NaCII, PtCls2-.