1 Carbon Materials for Catalytic Applications 3 rd October 2009

2 Carbon materials as Catalyst support DMFCs – Energy sources – Mobile and stationary applications Feasible electrodes – Pt/C Crucial issues for commercialization – Activity and stability of the electrocatalysts Improvement in activity and stability via tuning carbon support properties Characteristics of ideal carbon support  high surface area – high dispersion  high graphitization – electrical conductivity and corrosion resistance  suitable pore structure – reduce transfer constraints Carbon blacks such as Vulcan XC 72 R are suitable supports for Pt  Draw backs - amorphous nature (low graphitization) and presence of micropores - electrochemical corrosion and diffusion constraints Objective of the present work – To improve the degree of graphitization and to induce mesoporosity in the Ketjen black carbon support Qi et al., Carbon, article in press

3 Improving the graphitization and porosity of Ketjen black Ketjen Black (KB) Co(NO 3 ) 2 6H 2 O and Ni(NO 3 ) 2 6H 2 O 1:2:2 (wt. ratio of carbon and nitrates) Co(NO 3 ) 2 6H 2 O and Ni(NO 3 ) 2 6H 2 O ethanol ultrasonic stirring for 20 min rotating evaporation at 60  C Carbon and metal nitrate composite Heating; 900  C; N 2 atm., 3 h 5 M HCl; stirring; 100  C; 6 h Oven drying; 100  C; 8 h Treated Ketjen Black (T-KB) XRD, EDX, TEM, BET sorptometry Preparation of C-KB

4 Enhancement in the degree of graphitization of KB – XRD analysis XRD pattern of KB heat-treated at 900  C with cobalt and nickel nitrates before acid treatment XRD patterns of KB (a), C-KB (b) and T-KB (c) Diffraction peaks at 2  values of 44.3, 51.8 and 76.2 characteristic of Ni and Co metal particles with FCC structure The (002) diffraction peak in the case of T-KB is sharper and also intense Co, Ni particles served as nuclei for the grown of graphitic carbon structure

5 Chemical composition and Morphology of T-KB (treated Ketjen Black) No trace of Ni or Co Transformation of carbon spheres to carbon cages T-KB KB

6 Enhancement of mesoporosity of KB – BET sorptometry N 2 adsorption and desorption isotherms of KB (a), C-KB (b), T-KB (c) and the corresponding pore size distributioncurves (d)

7 SampleS BET (m 2 /g)V macro (cc/g)V meso (cc/g)Pore diamter (nm) KB C-KB T-KB Comparison of textural properties of KB, C-KB and T-KB carbon blacks

8 Counter electrode – Pt-foil Reference electrode – saturated calomel electrode Working electrode – a thin porous coating glassy carbon disk electrode with a dia of 5 mm Electrode fabrication 5 mg of sample 1 ml ethanol; 50  L of Nafion Ultrasonication, 30 min Homogeneous ink 25  L ink spread on glassy carbon surface Pt/C- glassy carbon electrode Electrochemical studies Preparation of electro catalyst – Polyol method KB or T-KB H 2 PtCl 6 6H 2 O Ethylene glycol Homogeneous slurry 130  C, 3 h Black mixture Filtered; dried in vaccum oven 40 wt.% Pt/KB or T-KB

9 Stability of T-KB towards electrochemical oxidation – Cyclic voltammetry CV curves of KB (a) and T-KB (b) at different time intervals during electrochemical oxidation in 0.5 mol L -1 H 2 SO 4 solution at a scan rate of 20 mV s -1 the electrodes were held at a constant potential of 1.2 V for 4, 8 and 20 h and then the CVs were recorded Increased peak current was attributed to the surface oxide formation Peak current density of KB increases significantly where as the peak current density of T-KB increases only slightly T-KB is more resistant to electrochemical oxidation than KB

10 Stability of Pt/T-KB towards electrochemical corrosion – Cyclic voltammetry Effect of electrochemical cycling on ECSA of Pt/KB and Pt/T-KB  The CVs were recorded on Pt/KB and Pt/T-KB electrocatalysts in 0.5 M H 2 SO 4 repeadly between 0 and 1.2 V at a scan rate of 100 mVs -1  Electrochemical surface area was determined after different cycles, namely, 50, 200, 400, 600, 800, 1000, 1300, 1500, 1800, 2000 and 3000 cycles.  Loss of ECSA is less in the case of Pt/T-KB which is an indication of tolerance to Electrochemical corrosion which is a result of improved graphitization as a result of which Pt nanoparticles agglomeration is prevented

11 Morphology of Pt/KB or T-KB electro catalysts before and after electrochemical cycling TEM images of Pt/KB before (a) and after (b) electrochemical cycling, Pt/T-KB before (c) and after (d) electrochemical cycling After cycling, the carbon surface of KB became smoother as a result of corrosion and agglomeration T-KB showed hardly any change in the morphology

12 Histograms of the particle size distribution of Pt/KB before (a) and after (b) electrochemical cycling, Pt/T-KB before (c) and after (d) electrochemical cycling  After 3000 cycles, the particle size distribution became broader and the average particle size became larger in the case of Pt/KB electro catalyst  The average particle size of Pt in Pt/T-KB catalyst Increased only slightly  Agglomeration is Pronounced in the case of Pt/KB compared to Pt/T-KB

13 Performance of single cell DMFC Anode: 45 wt.% PtRu/C (Johnson Matthey Inc.) Cathode: 40 wt.% Pt/KB or T-KB Membrane (solid electrolyte) : Nafion  115 Fuel : 1 M CH 3 OH fed into the anode side (flow rate – 1 ml min-1) Oxidant : Oxygen of 0.2 MPa Active area of single cell – 2 x 2 cm 2 Performance of single cell DMFC The output cell voltage of the single cell with Pt/T-KB cathode decreases gently unlike the sharp drop in the cell voltage exhibited by Pt/KB cathode Moreover the maximum output power density is higher (163 mW/cm 2 ) for Pt/T-KB compared to Pt/KB (145 mW/cm 2 ) Tbe better performance of Pt/T-KB was attributed to the appropiate pore stucture of the T-KB support

14 Stability (durability) of Pt/KB and Pt/T-KB catalysts – A comparison Durability test of the DMFC single cell with 40 wt.% Pt/KB and 40 wt.% Pt/T-KB as cathode electro catalysts. Discharging current density: 100 mA cm 2 ; cell temperature: 75  C; anode: 1 mol L -1 CH 3 OH; cathode: O 2 at pressure of 0.2 MPa  Durability of the cell was evaluated at a current density of 100 mAcm -2  During the 66 h continuous test, the output voltage of the cell with Pt/T-KB decreased only from 0.55 to 0.43 V in contrast to the large and sharp decrease of the output voltage from 0.55 to 0.39 V  The improved stability of Pt/T-KB electrocatalyst is attributed to the better graphitization of T-KB support compared to the KB support

15 Summary:  KB was treated at 900  C in N 2 atm. in the presence of cobalt and nickel nitrates.  The Pt/T-KB electro catalyst exhibited better electrochemical activity and stability than the Pt/KB electro catalyst  The improvement in the activity and stability of Pt/T-KB is due to the more appropriate pore structure and the better graphitization of T-KB.  The more appropriate pore structure of T-KB facilitates the transfer of reactants, intermediates, and products.  The better graphitization of T-KB inhibits the electrochemical corrosion and improves the stability of the supported electro catalyst.  This treatment method provides an efficient route to improvethe graphitization degree and to optimize the pore structure of carbon blacks which could be used as promising carbon supports of electro catalyst

16 Carbon materials for the sorption of mercuric ions from aqueous chloride solutions Precursor for activated carbon: Casurina equisetifolia leaves K. Ranganathan, Carbon 41 (2003)

17 Characteristics of actiated carbon materials produced from Casurina equisetifolia

18 Effect of agitation time on the Hg (II) sorption Fig. Effect of agitation time on the removal of Hg (II) from aqueous solution Conditions: Hg (II) conc. = 20 mgL-1; Carbon dose = 0.1 g per 100 mL Conc. Of NaCl = 0.1 M pH of initial solution = 5.5 Temperature = 30  C  SLC, ZLC, SHC and ZHC removed 7, 11,16.2 and 15 mg Hg (II) per g of carbon  Equilibration time – 6 h

19 Evaluation of equilibrium adsorption data using Langmuir adsorption model Langmuir isotherms for adsorption of Hg(II) ions from aqueous solution. Conditions: dosage of carbon = 0.1 g per 100 mL; concentration of NaCl = 0.1 M; Initial solution pH = 5.5; temperature = 30  C

20 Effect of carbon dose on the sorption of Hg (II) Effect of carbon dosage on removal of Hg(II) from aqueous solution Conditions: Hg(II) = 20 mg l -1 ; concentration of NaCl = 0.1 M; initial solution pH = 5.5; Temperature = 30  C.  Increase in carbon dosage increased percent removal of Hg (II)  Maximum removal – 99.9 % with 150 mg of SHC and 200 mg of ZHC from 100 mL of aqueous solution of 20 mgl -1 Hg (II)

21 Effect of pH of Hg (II) solution on the sorption of mercuric ions Effect of pH on removal of Hg (II) from aqueous solution Conditions: Hg (II) = 20 mgl -1 Conc. of NaCl = 0.1 M Carbon dosage = 100 mg per 100 ml Temperature = 30  C Solution pH governs the solute speciation, chemical nature of the protolyzable surface species and surface charge of the adsorbent Presence of Cl - ions makes a strong complex with Hg; species such as Hg(OH) 2, HgCl 2, HgCl 3 - and HgCl 4 2- will be present At 0.1 M NaCl conc. predominent species that are present are: Hg(OH) 2, HgCl 2, HgCl 3 - and HgCl 4 2- Increase of pH resulted in the decrease in Hg (II) sorption At low pH the the conc. of anionic species is increased At low pH the more positively charged carbon surface attracts the negatively charged species and so the adsorption is high

22 Effect of NaCl concentration on the sorption of Hg (II) Effect of concentration of NaCl on the removal of Hg (II) From aqueous solution Conditions: Hg (II) = 20 mgl -1 ; Dosage of carbon = 100 mg per 100 ml; Initial solution pH = 5.5 Temperature = 30  C The adsorption process is influenced by the presence of complexing ions More over, the chloralkali industry effluents, that causes 25 % mercury Pollution, contains nearly gl -1 of Cl - Increase in chloride ion conc. decreased the adsorption considerably Maximum removal in the absence of NaCl is due to the presence of Hg(OH) 2 species whose adsorption is Better compared to HgCl 2. At higher Cl - concentration, stable Hg-Cl complexes were formed which are poorly adsorbed o the carbon surface  Washing with 2 % solution of sodium sulphide in 1 % NaOH solution is effective for regeneration

23 Summary: o Waste leaves of Casurina equisetifolia serve as a low-cost source material for the preparation of powdered activated carbon o The carbon materials produced were used for the adsorption of mercury from aqueous solution o The carbon materials treated with sulphuric acid (SHC) and ZnCl 2 (ZHC) at a high temperature (825  C) were more effective for the removal of Hg (II) compared to the samples prepared at low temperature (425  C) o The Langmuir adsorption capacities of SHC and ZHC were and mgg -1 respectively o 99.9 % of mercury from 100 mL of 20 mgL -1 of Hg (II) was achieved by 150 and 200 mg of carbons SHC and ZHC respectively o Sulphuric acid treated high temperature carbon is more effective than ZnCl 2 – treated high temperature carbon o In the presence of chloride ions low Hg (II) adsorption was observed