Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 ydrgen perxide electrchemistry n platinum: twards understanding the xygen reductin reactin mechanism I. Katsunars, W.B. Schneider, J.C. Meier, U. Benedikt, P.U. Biedermann, A.A. Auer and K.J.J. Mayrhfer Supprting infrmatin Thermdynamic cnsideratins The thermdynamic analysis f is based n the electrchemical Reactins (1)-(3) and the nn-electrchemical bimlecular disprprtinatin Reactin (4). The crrespnding equatins fr the reactin free energies and the Nernst equatins are summarized in Table S1. Frmally the fur electrn reductin f xygen, Reactin (1), crrespnds t the sum f the Reactins () and (3). ence, the reactin free energy f (1) is equivalent t the cmbined reactin free energies standard ptential is given by E / / / /, and the 1 E 1 E (using = n). The / / disprprtinatin Reactin (4) can be frmally derived as the sum f Reactin (3) and the reversed Reactin (). Accrdingly the reactin free energy is. Equatin (8) defines an equilibrium / / cncentratin fr f 0.86710 18 ml l 1 at p = 1 atm. At higher cncentratins is unstable with respect t disprprtinatin int xygen and water.
Table S1: Reactins, ibbs free energies and Nernst equatins Reactin ibbs free energy Nernst equatin + 4e + 4 + (1) 4 / / ln ( 4 / / / / = 474.3 kj ml 1 ) 4 / / ln 4 F E E (7) ( / E +1.9 V) 1 + e + + () / / ln ( / / = 134.1 kj ml 1 ) / / ln F E E (5) ( / E +0.695 V) 1 + e + + (3) ln / / ( / / = 340. kj ml 1 ) / / ln F E E (6) ( / E +1.763 V) 1 + (4) ln (8) / / = 06.1 kj ml 1 ) Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 Figure S1. Equilibrium ptentials fr hydrgen perxide xidatin (red), and reductin (green) as a functin f cncentratin at 1 atm. Fr cmparisn, the equilibrium ptential fr direct xygen reductin (blue) and the equilibrium cncentratin fr nn-electrchemical frmatin/decmpsitin (purple) are als shwn. Equilibrium lines at an xygen pressure f 0.001 atm are dashed. The equilibrium ptentials fr the PRR as functin f the cncentratin are shwn in Figure S1. The ptentials are expressed with respect t the reversible hydrgen electrde (RE) ptential. The diagram is calculated fr p values < 11, where is undissciated (pka = 11.63) and an xygen partial pressure f 1 bar. The effect f the xygen pressure is indicated by dashed lines, which crrespnd t p = 0.001 bar. The equilibrium lines f all fur Reactins (1)-(4) intersect in ne pint because the reactins are frmally interrelated (see abve). The intersectin pint crrespnds t a state where the thermdynamic driving frce fr all reactins vanishes. The Nernst ptential fr the fur electrn reductin f xygen t water is independent f the cncentratin and hence appears as a hrizntal line in Figure
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 S1. It separates the regins f xygen reductin at ptentials belw +1.9 V RE (at p = 1 atm) frm the regins f xygen evlutin abve the equilibrium ptential. Figure S1 shws that at ptentials where the xygen reductin prceeds (<0.9 V), significant amunts f hydrgen perxide (>10 7 ml l 1 ) may be frmed by Reactin (). Furthermre, the driving frce fr reductin f is high. ence the thermdynamic analysis shws that under these cnditins the xygen reductin may prceed by the perx -mechanism via a shrt-lived intermediate. If is added t the electrlyte with cncentratins >10 18 ml l 1 it is metastable. At ptentials between the equilibrium lines fr perxide reductin (Equatin (6)) and perxide xidatin (Equatin (5)), reductin as well as xidatin f are thermdynamically favrable. At lwer ptentials (belw the red line in Figure S1), nly the reductin f has a strng thermdynamic driving frce. In summary, is stable nly at very lw cncentratins and may be prduced in significant amunts either at relatively high ptentials by xidatin f water, r at lw ptentials by reductin f xygen, prvided decmpsitin reactins are kinetically hindered. At intermediate ptentials (arund 1. V) and cncentratins > 10 18 ml l 1, is unstable with respect t xidatin t xygen, reductin t water and disprprtinatin. Cyclic vltammetry at different hydrgen perxide cncentratin The PRR was studied in different cncentratins in the regin 1-0x10-3 M in a 0.1 M Cl 4 supprting electrlyte. The backgrund-crrected cyclic vltammgrams (nt depicted) scale with the cncentratin f in the whle ptential regin under study; as a cnsequence, the plts f the dimensinless currents vs. the ptential (i/i L vs. E) cincide fr all cncentratins (see Fig. S), as it is expected fr diffusin limited prcesses. Minr differences that are bserved in the transitin regin, particularly clse t the diffusin-limited currents, are attributed t uncmpensated resistance which was less than Ω. Nte that the measured current in this regin fr the highest cncentratin is in the rder f 10 ma.
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 Figure S. Dimensinless currents vs. the ptential (i/i L vs. E) (psitive scan) in different cncentratins f (in the regin between 1x10-3 M and 0x10-3 M ). The inset shws the diffusin-limited reductin current vs. the bulk cncentratin f hydrgen perxide. Supprting electrlyte: 0.1 M Cl 4. Rtatin rate: 1600 rpm. Scan rate: 0.1 V s 1. The diffusin limited current fr bth the xidatin and the reductin f depends linearly n its cncentratin, as it is expected frm the Levich equatin. Using this equatin and the slpe f the diagram f i L,c vs. cncentratin (see inset in Fig. S), the diffusin cefficient f in 0.1 M Cl 4 was calculated as 1.8x10 5 cm s 1. Electrlysis experiments The electrlysis experiments were perfrmed using the fllwing methdlgy, which is schematically depicted in Scheme S1: i) First, the electrde was plarized fr tw minutes at the apprpriate ptential, while it was rtated with a rate f 500 rpm. ii) Then, three cyclic vltammgrams were recrded between +0.3 and +1.7 V RE with a scan rate f 0. V s 1, whereas the rtatin rate was 400 rpm. iii) The first step was repeated again.
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 Scheme S1. Methdlgy fr the electrlysis experiments The cyclic vltammgrams in between the cnstant ptential steps were perfrmed fr tw reasns: (i) the surface was cleaned because accumulatin f impurities (especially at the lw ptentials) takes place rapidly due t the high rtatin rate and (ii) the cncentratin f culd be directly determined by the diffusin limited current fr the xidatin f during the psitive sweep in the 3rd cycle. The switching f the rtatin rate frm 500 rpm (in the cnstant ptential step) t 400 rpm (in the sweeping experiment) was necessary in rder t minimize the cntributin f the latter (which lasts in ttal 4 s fr 3 cycles) t the depletin f, nevertheless still being able t determine the cncentratin f hydrgen perxide frm the diffusinlimited current. It shuld be nted that fr the calculatin f the expected cncentratin vs. time using Levich and Faraday equatins, the rate f decmpsitin f during bth steps f the experiment (cnstant ptential and ptential sweep) was taken int accunt, cnsidering the time needed in the tw different steps and that tw different rtatin rates were applied. Mrever, the diffusin cefficient that we determined frm cyclic vltammetry (D = 1.8. 10 5 cm s 1 ) was used instead f any literature value. In the additinal experiment at pen circuit as mentined in the text, the current was cntrlled at zer minutes instead f hlding a cnstant ptential; all ther cnditins f the experiment were the same.
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 Experiment at pen circuit in 0.1 M Cl 4 + 0. 10 3 M electrlyte Figure S3 shws the cncentratin f hydrgen perxide vs. time in the experiment at pen circuit that was perfrmed in a 0.1 M Cl 4 + 0. 10 3 M electrlyte. The cmparisn with the slid line, which represents the expected cncentratin decay if the reactin rate is diffusin limited, shws that this is true even in this high cncentratin f perxide. The way this experiment was perfrmed, was the same as described abve. If the cncentratin decay is expressed in terms f dimensinless cncentratin (C(t)/C(0) vs. time, where C(t) is the cncentratin at time t and C(0) the initial cncentratin f hydrgen perxide), then all cncentratin prfiles cincide, regardless f the applied ptential r the initial cncentratin f. This reveals that the rate f decmpsitin is linearly prprtinal t the bulk cncentratin, which is characteristic fr a diffusin-cntrlled prcess. Figure S3. Cncentratin f vs. time during the experiment at pen circuit in 0.1 M Cl 4 + 0. 10 3 M. The slid line represents the expected cncentratin decay fr a diffusin limited reactin rate, using Levich and Faraday equatins as described in the paper. Cyclic vltammetry at different rtatin rates Figure S4 shws the backgrund-crrected cyclic vltammgrams in 0.1 M Cl 4 + 1. 10 3 M slutin at different rtatin rates f the platinum wrking electrde. The data used here are the same as thse shwn in Fig. 3(c). The current is prprtinal t the square rt f the rtatin rate in the whle ptential regin, even in the transitin regin. This indicates that the shape f the cyclic vltammgram des nt
Electrnic Supplementary Material (ESI) fr Physical Chemistry Chemical Physics This jurnal is The wner Scieties 01 change with the change f the diffusin layer thickness. In the inset f Fig. S4, the cyclic vltammgrams fr each rtatin rate are nrmalized t the crrespnding diffusinlimited xidatin current, where ne can see that the dimensinless cyclic vltammgrams cincide fr all rtatin rates. Figure S4. Cyclic vltammgrams at different rtatin rates in 0.1 M Cl 4 + 1. 10 3 M. The inset shws the cyclic vltammgrams fr each rtatin rate nrmalized t the crrespnding diffusin-limited xidatin current. References (1) are, J. P. In Standard ptentials in aqueus slutin; Bard, A. J.; Parsns, R.; Jrdan, J. Ed.; Marcel Dekker: New Yrk, 1985; p. 49. () Purbaix, M. Atlas f electrchemical equilibria in aqueus slutins; Pergamn Press: Brussels, 1966; p. 99.