Electrochemistry of Selected Phosphorus Oxoacids on a Bulk Pt Electrode. Tomas Bystron Martin Prokop Karel Bouzek

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1 Electrochemistry of Selected Phosphorus Oxoacids on a Bulk Pt Electrode Tomas Bystron Martin Prokop Karel Bouzek

High Temperature PEM Fuel Cell (HT PEM FC) Operation temperature 130-200 C Enhanced rate of electrode reactions Enhanced Pt catalyst resistance against CO poisoning Combined

2 High Temperature PEM Fuel Cell (HT PEM FC) Operation temperature C Enhanced rate of electrode reactions Enhanced Pt catalyst resistance against CO poisoning Combined heat & power systems H 3 PO 4 -doped polybenzimidazole type membrane Aggressive media Not (electro)chemically stable 2

3 Bulk Pt in 100% H 3 PO 4 vs. 0.5 M H 2 SO j / ma cm M H 2 SO 4 99 % H 3 PO CV, stationary polycryst. Pt electrode, 50 mv s -1, 25 C, N 2 saturated electrolyte, electrolyte composition stated in the figure inset. 3

4 Unknown oxidation peak Strongly adsorbing impurities 0.5 Typical PH 3 odor Pt-P, H 3 in-situ by confirmed by photoelectron spectroscopy H 3 PO 4 + H 2 on a Pt surface j / ma cm CV, stationary polycryst. Pt electrode, 100 % H 3 PO 4, N 2 saturated electrolyte, 200 mv s -1, 160 C. 4

5 Aim H 3 Pt-P PH C < 100 C Electrochemical behaviour of H 3 a H 3 PO 2 (structural similarities) on a bulk Pt electrode, T < 100 C 5

6 Thermodynamic stability of P compounds H 3 a H 3 PO 2 thermodynamically unstable in aqueous environment E-pH diagram for selected P compounds*, 25 C. *Atlas of Electrochemical Equilibria in Aqueous Solutions, M. Pourbaix. 6

7 Thermal stability H 3 + H 2 O H 3 PO 4 + H 2 t < 180 C, very slow 4 H 3 3 H 3 PO 4 + PH 3 t > 190 C H 3 PO 2 + H 2 O H 3 + H 2 t = C, slow 3 H 3 PO 2 2 H 3 + PH 3 t > 110 C 7

8 H 3 H 3 PO H e - H 3 + H 2 O E 298 K = V vs. MSE M H 2 SO M H M H 3 p + 4 mm H 3 p pa: H 3 H 3 PO 4, on Pt surface pb: H 3 H 3 PO 4, on PtO surface pa : H 3 H 3 PO 4, on Pt surface -1-2 p ' Overvoltage > 0.9 V CV, stationary polycryst. Pt electrode, 0.5M H 2 SO C, 50 mv s -1, polarisation starts at V in negative direction, N 2 saturated electrolyte, H 3 concentration stated in the figure. 8

9 H 3 PO 2 H H e - H 3 PO 2 + H 2 O E 298 K = V vs. MSE M H 2 SO M H 3 PO 2 p ' + 40 M H 3 PO mm H 3 PO 2 p p pg: H 3 PO 2 H 3 H 3 PO 4, on Pt surface pd: H 3 PO 2 H 3 H 3 PO 4, on PtO surface pa : H 3 H 3 PO 4, on Pt surface Overvoltage > 1.1 V CV, stationary polycryst. Pt electrode, 0.5M H 2 SO C, 50 mv s -1, polarisation starts at V in negative direction, N 2 saturated electrolyte, H 3 PO 2 concentration stated in the figure. 9

10 Hydrogen underpotential deposition on Pt Pt + H + + e - Pt-H M H 2 SO M H M H M H mm H mm H mm H CV, polycryst. Pt electrode, 0.5 M H 2 SO 4 + H 3, 50 mv s -1, 25 C, N 2 saturated electrolyte. 10

11 H 3 PO x adsorption on Pt surface Surface coverage of Pt: θ H3POx = Q H2SO4 Q H3POx Q H2SO H 3, 25 o C H 3, 70 o C H 3 PO 2, 25 o C Strong adsorption of H 3 PO 2 & H 3 on Pt surface H3POx H 3 PO 2, 70 o C Adsorption extent increases with temperature 0.3 H 3 PO 4, 25 o C H 3 PO 4, 70 o C H3PO4 0.1 log c H3POx 1 10 µmol dm -3 Adsorption isotherms of H 3 and H 3 PO 2 on Pt surface (mol m -3 ) in 0.5 M H 2 SO 4. Temperature and electrolyte composition stated in the figure inset log c H3PO4 Adsorption isotherms of H 3 PO 4 on Pt surface ( mol m -3 ) v 0.5 M H 2 SO 4. Temperature and electrolyte composition stated in the figure inset. 11

12 Tautomerism H 3 Active form HO HO P.. OH Inactive form HO O OH P H ΔG Taut < -60 kj mol -1 K H3 = H PO(OH) 2 P(OH) H 3 PO 2 Active form Inactive form HO HO P.. H HO O P H H K H3 PO 2 = H 2 PO(OH) H P(OH) Exothermic process 12

13 More on H 3 adsorption V -0.5 V -0.6 V H 3 oxidation peak charge dependent on lower vertex Change in adsorbed H 3 amount CV, polycryst. Pt, 0.5 M H 2 SO mm H 3, 70 C. Polarisation: V lower vertex V 0.84 V V), 50 mv s -1, N 2 saturated electrolyte, lower vertex stated in the figure. 13

14 No. of H 3 monolayers desorbed More on H 3 adsorption 2.5 ph (electrolyte) = C 55 C 70 C HO HO P.. OH HO O P H OH 0.5 pka 1 = 7.4 pka 1 = Lower vertex potential / V Adsorption in protonated form E pzc (Pt 0,5 M H 2 SO 4 ) Desorbed amount of H 3 vs. lower vertex potential, 0.5 M H 2 SO mm H 3, Electrolyte temperature stated in the figure. H 3 adsorbed amount exceeds monolayer Formation of H 4 P 2 O 5 (ads.), H 5 P 3 O 7 (ads.)? 14

15 H 3 oxidation on PtO surface OCP / V vs. MSE M H 2 SO M H M H 3 p + 4 mm H 3 p p ' Pt Surface oxidation Pt + H 2 O PtO + 2 H e- PtO dissolution PtO + 2 H + Pt 2+ + H 2 O H 3 chemically oxidised by PtO M H 2 SO mm H mm H 3 po 30 s log t / s Development of OCP in time, polycryst. Pt electrode, (E = 0.6 V, t = 6 s) 0.5 M H 2 SO 4, 25 C, N 2 saturated electrolyte. H 3 + PtO H 3 PO 4 + Pt 15

16 H V pa pb pa: on Pt 4 2 pa' H 3 + H 2 O H 3 PO H e - pb: on PtO 0 Pt + H 2 O PtO + 2 H e CV, stationary polycryst. Pt electrode, 0.5M H 2 SO mM H 3, 25 C, 50 mv s -1, polarisation starts from V in negative direction, N 2 sturated electrolyte. H 3 + PtO H 3 PO 4 + Pt pa : on Pt H 3 + H 2 O H 3 PO H e - 16

17 H 3 (H 3 PO 2 ) Conclusion Adsorption on Pt electrode (blocking of HT PEM FC anode?) Potential dependent on potential, electrolyte composition Anodic oxidation on Pt electrode, large overpotential 1 V Chemical oxidation of H 3 by PtO Tautomeric equilibria Conditions relevant for HT PEM FC ( 100 % H 3 PO 4, 160 C) 17

18 Thank you Financial support of FCH JU in the framework of DEMSTACK (No ) a MŠMT ČR (No. 7HX13002) is greatly acknowledged.

19 Vlastnosti H 4 P 2 O 6 H 4 P 2 O 6 + H 2 O H 3 + H 3 PO 4 zvýšená teplota, nízké ph H 4 P 2 O 6 + 2H + + 2e H 3 E H4 P 2 O 6 /H 3 = V vs. MSE 2 H 3 PO 4 + 2H + + 2e - 0 H 4 P 2 O 6 + H 2 O E H3 PO 4 /H 4 P 2 O 6 = V vs. MSE 19

20 Elektrochemické chování H 4 P 2 O M H 2 SO mmol L -1 Na 2 H 2 P 2 O mmol L -1 Na 2 H 2 P 2 O 6 Neadsorbuje se na Pt 0.0 Oxidace na PtO povrchu Disproporcionace CV, stacionární polykryst. Pt elektroda, 0.5 M H 2 SO 4, 25 C, elektrolyt nasycen N 2, složení elektrolytu vedeno v grafu. 20

21 Oxidace H mmol L -1 H 3 1 mmol L -1 H 3 5 mmol L -1 H 3 I corr = II lim I lim I 6 4 Vliv adsorpce H 3 na kinetiku oxidace T / C c H3PO3 / mmol dm Potenciostatické voltametrické křivky, polykryst. Pt, 25 C, 0.5 M H 2 SO 4, elektrolyt nasycen N 2, koncentrace H 3 uvedena v grafu

22 Pokrytí Pt povrchu H 3 při -0,65 V 0.8 Trojitá Langmuirova isoterma : 3 krystalické roviny Pt povrchu pokryti povrchu H C experiment 25 C fit 70 C experiment 70 C fit log c θ = K ads,1 c K ads,1 c + 1 θ max,1 + K ads,2 c K ads,2 c + 1 θ max,2 + K ads,3 c K ads,3 c + 1 θ max,3 θ max,1 + θ max,2 + θ max,3 = 1 K ads,1 - rovnovážná konstanta adsorpce θ max,1 - max. stupeň pokrytí kryst. roviny Pt c koncentrace H 3 22

23 23

24 1 1 5 s at V 900 s at V o C 70 o C

25 Elektrochemické chování H 4 P 2 O M H 2 SO mmol L -1 Na 2 H 2 P 2 O mmol L -1 Na 2 H 2 P 2 O H 2 SO 4, 25 C H 3 PO 4, 25 C H 2 SO 4, 70 C H 3 PO 4, 70 C CV, stacionární polykryst. Pt elektroda, 0.5 M H 2 SO 4, 25 C, elektrolyt nasycen N 2, složení elektrolytu vedeno v grafu. CV, stacionární polykryst. Pt elektroda, 1 mm H 4 P 2 O 6, elektrolyt nasycen N 2, složení a teplota elektrolytu vedeny v grafu. Neadsorbuje se na Pt Oxidace na PtO povrchu Zpomalení oxidace v přítomnosti H 3 PO 4 (adsorpce fosforečnanu) 25

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