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 heat & power systems H 3 PO 4 -doped polybenzimidazole type membrane Aggressive media Not (electro)chemically stable http://www.intelligent-energy.com/technology/technology-faq/ 2
Bulk Pt in 100% H 3 PO 4 vs. 0.5 M H 2 SO 4 0.1 j / ma cm -2 0.0-0.1 0.5 M H 2 SO 4 99 % H 3 PO 4-0.5 0.0 0.5 1.0 CV, stationary polycryst. Pt electrode, 50 mv s -1, 25 C, N 2 saturated electrolyte, electrolyte composition stated in the figure inset. 3
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 -2 0.0-0.5-1.0-1.5-0.4 0.0 0.4 0.8 1.2 CV, stationary polycryst. Pt electrode, 100 % H 3 PO 4, N 2 saturated electrolyte, 200 mv s -1, 160 C. 4
Aim H 3 Pt-P PH 3 160 C < 100 C Electrochemical behaviour of H 3 a H 3 PO 2 (structural similarities) on a bulk Pt electrode, T < 100 C 5
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
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 = 30 90 C, slow 3 H 3 PO 2 2 H 3 + PH 3 t > 110 C 7
H 3 H 3 PO 4 + 2 H + + 2 e - H 3 + H 2 O E 298 K = -0.916 V vs. MSE 3 2 1 0 0.5 M H 2 SO 4 + 4 M H 3 + 40 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 ' -0.6-0.3 0.0 0.3 0.6 0.9 Overvoltage > 0.9 V CV, stationary polycryst. Pt electrode, 0.5M H 2 SO 4. 25 C, 50 mv s -1, polarisation starts at -0.25 V in negative direction, N 2 saturated electrolyte, H 3 concentration stated in the figure. 8
H 3 PO 2 H 3 + 2 H + + 2 e - H 3 PO 2 + H 2 O E 298 K = -1.139 V vs. MSE 3 2 1 0 0.5 M H 2 SO 4 + 4 M H 3 PO 2 p ' + 40 M H 3 PO 2 + 4 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 -1-2 -0.6-0.3 0.0 0.3 0.6 0.9 Overvoltage > 1.1 V CV, stationary polycryst. Pt electrode, 0.5M H 2 SO 4. 25 C, 50 mv s -1, polarisation starts at -0.25 V in negative direction, N 2 saturated electrolyte, H 3 PO 2 concentration stated in the figure. 9
Hydrogen underpotential deposition on Pt Pt + H + + e - Pt-H 6 0.5 M H 2 SO 4 + 1 M H 3 1 4 + 10 M H 3 + 100 M H 3 2 0 + 1 mm H 3 + 10 mm H 3 + 100 mm H 3 0-1 -2-0.6-0.4-0.2 0.0 0.2-2 0.4 0.6 0.8-0.7-0.6-0.5-0.4-0.3-0.2 CV, polycryst. Pt electrode, 0.5 M H 2 SO 4 + H 3, 50 mv s -1, 25 C, N 2 saturated electrolyte. 10
H 3 PO x adsorption on Pt surface Surface coverage of Pt: θ H3POx = Q H2SO4 Q H3POx Q H2SO4 1.0 0.8 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 0.6 0.4 H 3 PO 2, 70 o C Adsorption extent increases with temperature 0.3 H 3 PO 4, 25 o C 0.2 0.2 H 3 PO 4, 70 o C 0.0-4 -2 0 2 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. 0.0-2 0 2 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
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) 3 10 11 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) 2 10 12 Exothermic process 12
More on H 3 adsorption 5 4 3 2-0.4 V -0.5 V -0.6 V H 3 oxidation peak charge dependent on lower vertex Change in adsorbed H 3 amount 1 0-1 -0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 CV, polycryst. Pt, 0.5 M H 2 SO 4 + 1 mm H 3, 70 C. Polarisation: -0.25 V lower vertex -0.65 V 0.84 V -0.25 V), 50 mv s -1, N 2 saturated electrolyte, lower vertex stated in the figure. 13
No. of H 3 monolayers desorbed More on H 3 adsorption 2.5 ph (electrolyte) = 0.35 2.0 1.5 1.0 25 C 55 C 70 C HO HO P.. OH HO O P H OH 0.5 pka 1 = 7.4 pka 1 = 1.5 0.0-0.5-0.7-0.6-0.5-0.4-0.3-0.2 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 4 + 1 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
H 3 oxidation on PtO surface OCP / V vs. MSE 3 0.5 M H 2 SO 4 2 1 0 + 4 M H 3 + 40 M H 3 p + 4 mm H 3 p 0.50 0.25-1 p ' -2-0.6-0.3 0.0 0.3 0.6 0.9 Pt Surface oxidation Pt + H 2 O PtO + 2 H + + 2 e- PtO dissolution PtO + 2 H + Pt 2+ + H 2 O H 3 chemically oxidised by PtO 0.00-0.25-0.50 0.5 M H 2 SO 4 + 1 mm H 3 + 1 mm H 3 po 30 s 0 1 2 3 4 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
H 3 6 0.87 V pa pb pa: on Pt 4 2 pa' H 3 + H 2 O H 3 PO 4 + 2 H + + 2 e - pb: on PtO 0 Pt + H 2 O PtO + 2 H + + 2 e- -2-0.6-0.3 0.0 0.3 0.6 0.9 CV, stationary polycryst. Pt electrode, 0.5M H 2 SO 4 + 40mM H 3, 25 C, 50 mv s -1, polarisation starts from -0.25 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 4 + 2 H + + 2 e - 16
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
Thank you Financial support of FCH JU in the framework of DEMSTACK (No. 325368) a MŠMT ČR (No. 7HX13002) is greatly acknowledged.
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 - 0 2 H 3 E H4 P 2 O 6 /H 3 = -0.274 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 = -1.587 V vs. MSE 19
Elektrochemické chování H 4 P 2 O 6 1.6 0.8 0.5M H 2 SO 4 + 1 mmol L -1 Na 2 H 2 P 2 O 6 + 10 mmol L -1 Na 2 H 2 P 2 O 6 Neadsorbuje se na Pt 0.0 Oxidace na PtO povrchu -0.8-1.6-0.6-0.3 0.0 0.3 0.6 0.9 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
Oxidace H 3 12 10 8 0.2 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 2 0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 T / C c H3PO3 / mmol dm -3 0.2 1 5 Potenciostatické voltametrické křivky, polykryst. Pt, 25 C, 0.5 M H 2 SO 4, elektrolyt nasycen N 2, koncentrace H 3 uvedena v grafu. 25 55 70 0.77 0.57 0.53 0.79 0.74 0.67 0.95 0.75 0.68 21
Pokrytí Pt povrchu H 3 při -0,65 V 0.8 Trojitá Langmuirova isoterma : 3 krystalické roviny Pt povrchu pokryti povrchu H 3 0.6 0.4 0.2 0.0 25 C experiment 25 C fit 70 C experiment 70 C fit -4.5-3.0-1.5 0.0 1.5 3.0 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
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1 1 5 s at -0.25 V 900 s at -0.54 V 0-1 0-1 -2 25 o C 70 o C -2-0.6-0.3 0.0 0.3 0.6 0.9-0.6-0.3 0.0 0.3 0.6 0.9 24
Elektrochemické chování H 4 P 2 O 6 2 1.6 0.8 0.5M H 2 SO 4 + 1 mmol L -1 Na 2 H 2 P 2 O 6 + 10 mmol L -1 Na 2 H 2 P 2 O 6 1 0.0 0-0.8-1 H 2 SO 4, 25 C H 3 PO 4, 25 C -1.6-0.6-0.3 0.0 0.3 0.6 0.9-2 H 2 SO 4, 70 C H 3 PO 4, 70 C -0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 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