Introducing Dynamic Electrochemistry into the Physical Chemistry Curriculum Hydrogen Adsorption at a Platinum Electrode Miles D. Koppang and Thomas A. Holme University of South Dakota, Vermillion, SD 57069 Electrochemical methods as applied to undermaduate physical chemistry are predominantly orientex toward equilibrium phenomena. These pursuits are intended as complementary information in support of thermodynamic eauilibrium discussions commonly found in first-semester undergraduate physical chemistry offerings. Occasional references to dynamic electrochemistry appear in some physical chemistry textbooks ( I ) ,but even then inclusion in the courses is decidedly uncommon, as implied by placement as the last chapter in the hook. While this traditional emphasis of electrochemistry a s a subset of thermodynamics is pervasive, it hides much of the physical insight available in conjunction with modem electrochemical practices. Althoueh . ~ h" v s i c a chemistrv l courses are nefariously overcrowded, dynamic electrochemical experiments can ~rovidea substantial method for discussion of traditional& covered topics, albeit from a slightly different perspective. The importance of the liquid-solid interface in chemical processes is relatively well-known, and yet experimental investigation of that interface is rarely afforded to undergraduate chemistry majors. Cyclic voltammetry (CV) represents one electrochemical method for such investigation whose inclusion in physical chemistry laboratory is highly appropriate. Molecular level understanding of solid surfaces can be inferred via CV techniques, leading to a novel complement to bulk descriptions of surface thermodynamics, for exumplt~.We will describe one such experiment as imolementrd in the Chemistm De~artmentat the Universit; of South Dakota, that ailowl for both dynamic and bulk investieation of surface adsomtion processes. The adsorption process selected for implementation into our ohvsical chemistrv laboratorv curriculum was electro" deposition of atomic hydrogen onto a clean polycrystalline platinum surface. Through the combined application of dynamic electrochemical techniques, most notably cyclic voltammetm. and surface s ~ e d r o s c o ~techniaues. ic . . this process has i;een thoroughly characterized and is very well defined (2).Review articles regarding the process have been published ( 3 , 4 ) .We selected this system because the surface chemistry is well defined and the adsorption process can be dyn&nically observed via cyclic voliamm&y. The details of the hvdrogen . - adsorption experiment which has been developed are presented herein, including results obtained by undergraduate physical chemistry studmti. The effrctiveness of dynamic electrochemical experiments for studying traditional physical chemistry toplcs such as the interface and surfucr structure and bind~ - solid-liouid - ~ ing is consiered in addition to the discussion of the adsorption experiment.
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Experimental Apparatus All cyclic voltammograms (CV's) are obtained with a Bioanalytical Systems (BAS) CV-27 potentiostat. The
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potentiostat provides linear ramping of applied potentials to a working electrode in a three-electrode cell configuration. Details on the theory of cyclic voltammetry, threeelectrode cell configurations and potentiostat design and operation can be found in articles from this Journal (5, 6 ) and in excellent reference texts (7, 8). The CVs are recorded with a Houston Instrument Model 200 XY recorder and subsequent area measurements performed manually with a Keuffel and Esser planimeter. Alternatively, area measurements could be performed bv a dieitizine scanner attached to a microc~mputer.c b m p 2 e r - c ~ n t r o l l e d potentiostats have become widely available and area measurements of voltammograms by the computer are yet another alternative approach. A BAS platinum disk working electrode (geometricarea = 0.020 cm2)is used in conjunction with a platinum wire auxiliary electrode and a BAS AgIAgCl reference electrode. The reference electrode may be selected from a variety of possibilities but our choice of AgIAgC1 is convenient. Solutions A 1.0 M HzS04 solution is used as the electrolyte, prepared from concentrated H2SO4 (Mallinckmdt) and deionized water. A 1.0 M KI solution is also prepared with 1.0 M H2S04electrolyte serving as the solvent. The deionized water is generated by passing freshly distilled water through a Barnstead NANOpure system consisting of charcoal adsorption, ion-exchange, and micro-filtration columns in seauence. and the water hereafter will be referred to as nanopuie water. A resistivity value greater than 17.5 MR cm indicates sufficiently purified water. Nanopure water is collected in acid-washed glass containers and used for all solutions and electrode-cleaning processes. The solutions are stored in glass containers to avoid leaching of organics (i.e., plasticizers, etc.) from plastic containers. Trace amounts of organic contaminants foul the working Pt electrode and dramaticallv diminish the effectivenessof the experiment. Electrode Polishing The Pt working electrode is polished successively using various sizes of alumina powder (Buehler, 1.0 pm, 0.3 pm, and 0.05 wm diameter). The polishing procedure consists of preparing a n alumina slurry fromahmina powder and nanopure water on a clean Microcloth pad (BAS) attached to a glass plate. Beginning with 1.0 pm alumina, the electrode surface is polished firmly by hand using a circular motion. After five minutes the electrode is rinsed with nanopure water, immersed in electrolyte solution and electrochemically cleaned by cycling for 15min from +1.40V to -0.20 V. When the characteristics of a clean electrode are observed (vide infra) the electrode is removed, rinsed with water and polished in a similar manner on a different Microcloth pad with the next smaller sized alumina.
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Figure 1. Cyclic voltammogram for a polycrystalline Pt electrode in 1M H,S04. The sweeq rate was 50 mV s-' and the geometric electrode area = 0.020 cm . Results A typical cyclic voltammetric experiment consists of two phases. First, the working electrode's potential is cycled over a relatively wide voltage range in order to electrochemically clean the electrode's surface (accomplished through oxidatiodreduction cycling a t the surface). For the 1.0 M HzS04electrolyte, cycling from +1.40 to -0.20 V is sufficient to accomplish the desired cleaning. A typical CV from a clean P t surface is presented in ~ i & r e1:The electrochemistrvof Pt electrodes has been discussed in detail elsewhere "(2) and only the salient features of the voltammogram will be discussed herein. The region marked a in Figure 1is assigned to oxidation of the surface to ~ l a t i n u moxide. The sham increase in oxidative current (peak h) corresponds to Gatinum surface and water oxidkion. The s h a h symmktrical reduction wave (oeak .. c) is reduction of the platinum oxide surface to elemental platinum. The reduction waves d and e and their comp~ementaryoxidation waves d' and e' are due to adsor~tionand desomtion of H atoms on the clean. metallic sukace. Protons are reduced to hydrogen atom; which adsorb onto the platinum surface at t w o different types of adsor~tionsites vieldine stronelv adsorbed hvdrocren - - (H. .. peak i)andwea6y adsorbed hydrogen (H,, peak e). These adsorbed atoms can be oxidized back to protons (peaks d' and e') upon reversing the direction of the potential scan. The large increase in cathodic current (peak flis due to proton reduction to hydrogen gas. The dynamics of electrode surface cleaning can be seen by observing the increase in the size of the hydrogen adsorptioddesorption waves. Aclean surface is achieved when the voltammetric behavior no longer changes with successive scans or has reached a steady state. The cleaning process, which takes approximately 15 min, serves as an ideal time for making
Figure 2. Cyclic voltammograms for a polycrystalline Pt electrode in 1M H2S04after polishing.with (a)1.0 ym (b) 0.3ym and (c)0.05 ym alumina and electrochemical cleaning for 15 min from +t .40 to 4.20
V versus AgIAgCi. The sweep rate was 50 mV s-' and the geometric electrode area = 0.020 cm2.
dynamic observations and relating them to the chemistry occurring a t the electrode surface. The second phase of the voltammetric experiment focuses more closely on the adsorptioddesorption waves for hydrogen atoms. Once the electrode is cleaned, it remains pristine long enough (without further cleaning)to decrease the scan limits for the CV. Following reduction of platinum oxide to platinum, the scan is stopped (Ehold = +020 V versus AgIAgC1)and the limits are changed to +0.20 V to -0.20 V. This reduction of limits allows for an increase in sensitivity of both the current and voltage measurement with a resultant improvement in the resolution of the experiment. A CV is then recorded and used by the students to measure the amount of charge associated with the adsomtioddesorption process. ~ x a m ~ lof e sthree different C ~ S are shown in Figure 2. The voltammogram labeled a was recorded after polishing with 1.0 Fm alumina; b, with 0.3 ym alumina; and c, with 0.05 pm alumina. The relationship of alumina particle size and mamitude of the adsomtio&desorption waves will be dism-sed later. The sharp features of the hydrogen adsorptioddesorption waves are greatly dependent on the pristine conditions of the electrochemical cell and electrolvte. If contaminants are present. the resolution of the peaks diminishes and the ma'gnitudd of the waves quickly decreases with successive scans. To further demonstrate the nature of poisoning of the electrode surface, the system can be intentionally contaminated with dilute KI solution. Iodide adsorbs as an iodine atom onto a clean Pt surface and blocks it for adsorption by other species (9,10). This demonstration first electrochemically cleans an electrode until hydrogen adsorptionldesomtion waves reach a steadv state. The scan is stopped at t0.2 V while n c a n ~ n gin the negative direction (following Pt reduction but prior to proton reduction lrefer Volume 69 Number 9 September 1992
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tion number in deciding which species present is likely to be the one reduced or oxidized at a given potential. As an example, when pointing out the oxidation waves (region a and peak b in Fig. 11, some students suggested that the reaction must involve the ~02' ion until prompted to give such an answer further consideration, and the oxidation state of the S atom is deduced. It is clear in the cleaning Nn that something physically must happen to change the surface and allow for the hydrogen adsorption waves subsequently observed. The evidence for the atomic level mechanism is present in the CV as the formation and reduction of Pt-0 (reeion a and oeak c..resoec. lively, in Fig 11.Thin process provides an opportunity todiscuss surface bindmg processes and how the existence of other atoms can hmder, or in the case of I- (see Flg. 3A,. completely inhibit certain surface processes. The source of the current recorded by the system is another facet useful for discussion while CVs are being recorded. In oarticular. ~~, notine that the reduction of H* reauires one electran per hydro,& atom will be useful when'dircussing the ultnnate data processing to he carried out for the expenment. There are two resolvable hydrogen adsorption waves. This feature sueeests adsorbs in more than one en..., that hvdroeen " viranment. Discussing the possihilitirs for explainmg thro ubrrlvation represents one key point at which surface bmding can be dwcursed. It is straightforward m note that nne hydrogen is bound more strongly than the other, but suggesting reasons is less obvious. Ultimately this point allows for the discussion of just what the electrode surface looks like from an atomic viewpoint. Finally, there is strong evidence that the different adsorotions corres~ondto interactwns with differentplanes ofthe PIsurface; apecrfically the Pu 100, nnd Prtlll,, the stronger mteract>onoccurring mth Pt1100, r2,.
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Figure 3. (A) Cyclic voltammogram recorded for a polycrystalline Pt electrode in fresh 1.0 M H,SO,foilowing pretreatment in 1.0 M KI. (6) Cyclic voltammogram of the same iodine-pretreated Pt electrode from an initial potential of +0.20 V and initial cathodic sweep direction. The sweep rates for A and Bwere 50 mV s' and the geometric electrode area = 0.020 cm2.
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to Fie. 11). -..the cell is disconnected and the working electrode is removed. The electrode is then placed in a-1.0 M KI solution for one minute, removed and rinsed with copious amounts of nanopure water and returned to a fresh, uncontaminated 1.0 M HzSOd electrolyte. Using an initial potential of +0.20 V, the
