ACS NORM Kinetic Study of Formic Acid Oxidation using PtRu-CNT and PtBi-CNT Kenichi Shimizu; I. Frank Cheng; Clive Yen; Byounghoon Yoon; Chien M. Wai Dept. Chemistry University of Idaho, Moscow, ID

2 Direct Formic Acid Fuel Cell Largely available in nature (Renewable). Largely available in nature (Renewable). Higher fuel concentration than DMFC. Higher fuel concentration than DMFC. Up to ~20 M HCOOH vs. up to ~2 M CH 3 OH Up to ~20 M HCOOH vs. up to ~2 M CH 3 OH Less fuel crossover than methanol. Less fuel crossover than methanol. Higher theoretical cell potential. Higher theoretical cell potential V for DFAFC, 1.2 V for DMFC 1.45 V for DFAFC, 1.2 V for DMFC Kang, S.; et al.; J. Phys. Chem. B, 2006, 110, Rice, C.; et al.; J. Power Sources, 2003, 115,

3 Formic acid oxidation as a part of methanol oxidation process Cao, D.; et al. J. Phys. Chem. 2005, 109,

4 HCOOH  CO 2 + 2H + + 2e - E 0 = V NHE CO ads + H 2 O CO ads + OH ads  CO 2 +H + +e - H 2 O  OH abs + H + + e - Rxn 1 Rxn 2 Formic acid oxidation

5 Binary catalysts and their effects Bi-functional effect (Active secondary catalyst). Bi-functional effect (Active secondary catalyst). Pt, PtPd, PtRu Pt, PtPd, PtRu Third body effect (Inert secondary catalyst). Third body effect (Inert secondary catalyst). PtBi, PtPb, PtAu PtBi, PtPb, PtAu Catalyst support is either Pt itself or carbon black. Inert to oxidation of small organic solvent. Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369 Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, Conway, B.E.; et al. Zeitschrift für physikalische Chemie Neue Folge 1978, 112,

6 Bi-functional Effect Active secondary metal catalyst such as Ru. Dissociative absorption of CO onto Pt. 1. Pt-CH 3 OH ads  Pt-CO ads + 4H + + 4e - Or (Pt + HCOOH  Pt-CO ads + H 2 O) Absorption of OH onto Ru through dissociation of H 2 O. 2. H 2 O + Ru  Ru-OH ads + H + +e - 2. Pt-CO ads + Ru-OH ads  Pt + Ru + CO 2 +H + + e - Secondary catalyst must have lower dissociation potential than Pt. (Ru has V lower reaction potential than Pt) Secondary catalyst must have lower dissociation potential than Pt. (Ru has V lower reaction potential than Pt) Rate determining step is 3. Rate determining step is 3. Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, Christensen, P.A. et al. J. Electroanal. Chem. 1993, 362,

7 Third Body Effect Dissociative absorption on Pt takes more than one active site Dissociative absorption on Pt takes more than one active site Pt + CH 3 OH  Pt-CH 2 OH + H +e - Pt + CH 3 OH  Pt-CH 2 OH + H +e - Pt-CH 2 OH  Pt 2 -CHOH + H + + e - Pt-CH 2 OH  Pt 2 -CHOH + H + + e - Pt 2 -CHOH  Pt 3 -COH + H + + e - Pt 2 -CHOH  Pt 3 -COH + H + + e - Pt 3 -COH  Pt-CO + 2Pt +H + + e - Pt 3 -COH  Pt-CO + 2Pt +H + + e - Catalytically inert catalyst, such as Bi, sterically hinders absorption of poisonous carbon species. Catalytically inert catalyst, such as Bi, sterically hinders absorption of poisonous carbon species. Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48,

8 How can overall reaction be improved. How can overall reaction be improved. Chen, X.-Y., et al., J. Angew. Chem., Int. Ed. 2006, 45, 981. Research question

9 PtRu and PtBi CNT Atomic ratio of Pt:Ru is 1:1.4. Atomic ratio of Pt:Bi is 1:1.6. Pt 42 Ru 58 CNTPt 38 Bi 62 CNT

10 Catalytic effect of PtRuCNT 1 M H 2 SO 4 1 M H 2 SO M HCOOH 0.1 M HCOOH Forward peaks are not resolved as well as using PtCNT. Forward peaks are not resolved as well as using PtCNT. Peak current was enhanced with PtRu CNT. Peak current was enhanced with PtRu CNT.

11(1)(2)(3)PtCNT 665 mV PtCB* PtRuCNT642N/A395 PtRuCB*654N/A424 CNT supported catalysts resulted in slightly lower oxidation potential. CNT supported catalysts resulted in slightly lower oxidation potential. Peak 2 and 3 might be due to the same reaction. Peak 2 and 3 might be due to the same reaction. (1) (2) (3) Summary of Peak Potentials *Carbon black supported Pt and PtRu from ETEK Pt CNT

12(1)(2)(3)(1)/(3)PtCNT PtCB* PtRuCNT0.52N/A PtRuCB*0.18N/A Higher reverse peak than forward peak may indicate sluggish kinetic activity for Pt CNT and PtCB. Higher reverse peak than forward peak may indicate sluggish kinetic activity for Pt CNT and PtCB. (1) (2) (3) Summary of Peak Currents A/mg Pt Pt CNT *Carbon black supported Pt and PtRu from ETEK

13 Influence of Temperature

14 Activation Energy for intermediate oxidation

15 Summary of Activation Energy E ac (kJ/mole) CI (90%) PtCB**20.4N/A Pt*** PtCB* PtCNT PtRuCB* PtRuCNT **Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, % confidence interval *Carbon black supported Pt and PtRu from ETEK

16 Summary of Activation Energy E ac (kJ/mole) CI (90%) PtCB**20.4N/A Pt*** PtCB* PtCNT PtRuCB* PtRuCNT Results are agreeable to the others. Results are agreeable to the others. Mean activation energy was the smallest with Pt CNT. Mean activation energy was the smallest with Pt CNT. **Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, % confidence interval *Carbon black supported Pt and PtRu from ETEK

17 Summary of PtRuCNT It is effective towards formic acid oxidation. (Highest peak current with PtRuCNT). It is effective towards formic acid oxidation. (Highest peak current with PtRuCNT). There is no significant difference in activation energies. There is no significant difference in activation energies. PtRuCNT has higher turn over rate. PtRuCNT has higher turn over rate.

18 Catalytic Effect of PtBiCNT 1 M H 2 SO 4 1 M H 2 SO M HCOOH 0.1 M HCOOH Position of the forward peak of PtBiCNT was almost same as the backward peak. Position of the forward peak of PtBiCNT was almost same as the backward peak. PtBiCNT had lower current output than PtCNT. PtBiCNT had lower current output than PtCNT.

19 Summary of Peak Potentials Peak (1) was not observed for PtBi catalyst; no formation of Pt-CO. Peak (1) was not observed for PtBi catalyst; no formation of Pt-CO. (1)(2)(3) PtCNT665mV PtCB* PtRuCNT642N/A395 PtRuCB*654N/A424 PtBiCNTN/A (2) (1) (3) Pt CNT *Carbon black supported Pt and PtRu from ETEK

20 Summary of Peak Currents Low catalytic activity of PtBi may be attributed to the larger particle size. Low catalytic activity of PtBi may be attributed to the larger particle size. Better Efficiency than PtCNT and PtCB catalysts. Better Efficiency than PtCNT and PtCB catalysts. (1)(2)(3)(2)/(3) PtCNT PtCB* PtRuCNT0.52N/A0.40N/A PtRuCB*0.18N/A0.18N/A PtBiCNTN/A (2) (1) (3) Pt CNT *Carbon black supported Pt and PtRu from ETEK

21 Influence of Temperature Peak current leveled off above 23 °C. Peak current leveled off above 23 °C. *Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294. **Ronald, W.; et al.; J. Electrochem. Soc., 1984, 2369.

22 Summary of Activation Energy Bi secondary catalyst is not taking part of electro-oxidation process itself. Bi secondary catalyst is not taking part of electro-oxidation process itself. Restriction of Pt reaction site by Bi resulted in high activation energy. Restriction of Pt reaction site by Bi resulted in high activation energy. E ac (kJ/mole) CI (90%) PtCB**20.4N/A Pt*** PtCB* PtCNT PtRuCB* PtRuCNT PtBiCNT PtBi(III) *** **Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294. ***Ronald, W.; et al.; J. Electrochem. Soc., 1984, *Carbon black supported Pt and PtRu from ETEK

23 Summary of PtBiCNT Only peak associated with reaction 1 was observed. Only peak associated with reaction 1 was observed. Low catalytic activity (low peak current). Low catalytic activity (low peak current). Activation energy was significantly larger than Pt and PtRu electrocatalysts. Activation energy was significantly larger than Pt and PtRu electrocatalysts. Bi suppresses reaction 2. Bi suppresses reaction 2.

24 Tafel analysis Slope (mV/dec) PtCB*150 PtCNT 271 (140) PtCB*198 PtRuCNT61 PtRuCB*88 PtBiCNT331 Higher tafel slope indicated that the first electron transfer likely be the rate determining step. Higher tafel slope indicated that the first electron transfer likely be the rate determining step. *Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294. Maciá M.D.; et al.; J. Electroanal. Chem. 2003, , 25. *Carbon black supported Pt and PtRu from ETEK

25 Conclusion PtRu was possible to enhance formic acid oxidation through reaction 2. PtRu was possible to enhance formic acid oxidation through reaction 2. Binary catalyst with bi-functional effect, i.e. PtRuCNT, did not affect on the activation energy of formic acid oxidation. Binary catalyst with bi-functional effect, i.e. PtRuCNT, did not affect on the activation energy of formic acid oxidation. With PtBiCNT, major reaction was reaction 1, which had the lower oxidation potential than reaction 2. With PtBiCNT, major reaction was reaction 1, which had the lower oxidation potential than reaction 2. PtBiCNT catalyst did not improve the catalytic activity towards formic acid oxidation. PtBiCNT catalyst did not improve the catalytic activity towards formic acid oxidation. PtBiCNT caused large increase in activation energy indicating effective suppression of reaction 2. PtBiCNT caused large increase in activation energy indicating effective suppression of reaction 2.

26 Improvement of formic acid oxidation can be achieved by using catalyst with third body effect. Improvement of formic acid oxidation can be achieved by using catalyst with third body effect. We need to prepare more active PtBi CNT. We need to prepare more active PtBi CNT. Conclusion

27 Acknowledgement Dr. I. Frank Cheng Dr. I. Frank Cheng Chris Roske Chris Roske Dr. Chen M. Wai Dr. Chen M. Wai Dr. Byunghoon Yoon Dr. Byunghoon Yoon Dr. Clive H. Yen Dr. Clive H. Yen Dept of Chemistry at the University of Idaho Dept of Chemistry at the University of Idaho Financial support Electric Power Research Institute (EPRI) Innovative Small Grants Program Electric Power Research Institute (EPRI) Innovative Small Grants Program Dr. and Mrs. Renfrew Summer Scholarship Dr. and Mrs. Renfrew Summer Scholarship