Theoretical and Experimental Studies on the Relationship between

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Theoretical and Experimental Studies on the Relationship between the Structures of Molybdenum Nitrides and Their Catalytic Activities toward the Oxygen Reduction Reaction Jing Qi,†,‡ Luhua Jiang,† Qian Jiang,†,‡ Suli Wang,† and Gongquan Sun*,† Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China ReceiVed: March 13, 2010; ReVised Manuscript ReceiVed: August 21, 2010

Two carbon-supported molybdenum nitrides, MoN/C and Mo2N/C, were prepared by varying the experimental conditions in NH3 atmosphere. The physical properties of the catalysts were characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The catalytic activities of the catalysts toward the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR) were characterized by potentiodynamic measurements. The results show that the MoN/C exhibits higher catalytic activity toward the ORR than the Mo2N/C. Both catalysts are inert to methanol. The results from the density functional theory calculation indicate that both the MoN and the Mo2N facilitate dissociation of oxygen molecules. The suitable geometric structure of the MoN and the preferred oxygen adsorption type on it contribute to the higher activity of the MoN/C toward the ORR. The calculation results agree well with the results from the experiments. 1. Introduction Direct methanol fuel cells (DMFCs) have been recognized as alternative power sources because of their advantages such as high specific energy density, easy handling of liquid fuels, and low emission. However, the high cost caused by the exclusive use of Pt and Pt-based catalysts remains to be an obstacle for commercialization of DMFCs. Employing non-Pt catalysts is one of the approaches to reduce catalyst cost. Several non-Pt catalysts, such as transition metal chalcogenides,1-3 transition metal oxides,4,5 transition metal macrocycles,6-8 and transition metal nitrides,9,10 have been explored in recent years. Among them, molybdenum nitride was reported to be an attractive catalyst toward the oxygen reduction reaction (ORR). Molybdenum nitride has two stable structures: one is hexagonal MoN, and another is face-centered cubic (fcc) Mo2N. Several precursors have been employed to synthesize molybdenum nitride, including MoCl5 reacting with NH311 or urea,12 or molybdates reacting with NH3.13 Even though the activity of molybdenum nitride is lower than that of Pt, it is promising as a cathode catalyst for DMFCs due to its being insensitive to methanol crossover from the anode.10 In this paper, to discover the relationship between the structures of molybdenum nitrides and their catalytic activities toward the ORR, two carbon-supported molybdenum nitrides were synthesized, namely, MoN/C and Mo2N/C, by varying experimental conditions. The catalytic activities of the MoN/C and Mo2N/C for the ORR and MOR were investigated in O2saturated 0.5 M HClO4 solution and N2-saturated 0.5 M HClO4 + 1 M CH3OH solution, respectively. The stability of the MoN/C and Mo2N/C in oxygen atmosphere was investigated by fast scanning in O2-saturated 0.5 M HClO4 solution. In addition, density functional theory (DFT) calculations were * To whom correspondence should be addressed. Phone/Fax: +86 411 8437 9063. E-mail: [email protected]. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

performed to gain insights on a molecular level into the relationship between the structures of molybdenum nitrides and their catalytic activities. 2. Experimental Section 2.1. Synthesis of MoN/C and Mo2N/C. The procedure to synthesize a MoN/C (16 wt % Mo in nominal) is as follows: a suspension of 500 mg of carbon black (Vulcan XC-72R, Carbot Inc.) in about 50 mL of deionized water was ultrasonically stirred for 1 h, followed by addition of 184 mg of ammonium molybdate ((NH4)6Mo7O24 · 4H2O) dissolved in a suitable amount of deionized water under magnetic stirring for 12 h. After evaporating the water from the suspension by a rotating evaporator at 70 °C, the resulting mixture was loaded in a ceramic boat and transferred to a tubular oven. The sample was then heated to 500 °C with a heating rate of 5 °C min-1 and kept at 500 °C for 3 h in N2 atmosphere. Afterward, the sample was cooled to room temperature in N2 atmosphere. The obtained sample was denoted as A. Sample A was divided into two parts. One part was used for XRD analysis to investigate its crystal structure, and the other was continued to be heated in NH3 atmosphere from room temperature to 500 °C with a rate of 5 °C min-1, and then to 700 °C with a rate of 2.5 °C min-1, and kept at 700 °C for 3 h. After that, the sample was cooled to room temperature still in NH3 atmosphere. Then the sample was kept in a mixture of O2/N2 (1% O2, v/v) for 12 h for further use. The Mo2N/C (16 wt % Mo in nominal) was synthesized with a similar procedure as for the MoN/C but without thermal treatment at 500 °C for 3 h in N2 atmosphere. 2.2. Structural Analysis. The crystal structures of MoN/C and Mo2N/C catalysts were determined using the powder X-ray diffraction (XRD) technique. The XRD patterns were recorded on a Rigaku D/max-2400 X-ray diffractometer using Cu KR radiation. The scan rate was 5° min-1 with an angular resolution of 0.02°. The tube voltage and tube current were maintained at 40 kV and 100 mA, respectively. Transmission electron

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microscopy (TEM) was carried out using a JEOL JEM-2000EX microscope operated at 100 kV. Energy-dispersive X-ray spectroscopy (EDX) analysis was carried out on a JSM-6360LV scanning electron microscope equipped with an Oxford INCA energy-dispersive X-ray spectroscope. The accelerating voltage was 20 kV. X-ray photoelectron spectroscopy (XPS) of the catalysts was acquired on a Kratos Analytical Amicus XPS instrument. All XPS measurements were made by use of a Mg KR X-ray source (hν ) 1253.6 eV) operated at 200 W (10 kV, 20 mA). The C 1s peak of the carbon (binding energy ) 284.6 eV) was taken as a reference in calculating the binding energies and accounting for the charging effects. The spectra were fitted and evaluated by the XPS Peak4.1 program, while the background was subtracted using a Shirley function. 2.3. Electrochemical Measurements. Electrochemical measurements were conducted in a three-electrode electrochemical cell on an EG&G model 273A potentiostat/galvanostat at room temperature. A Pt foil and saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. All potentials in this work were converted to the normal hydrogen electrode (NHE). A thin porous film on a glassy carbon (GC) disk with a diameter of 5 mm was used as the working electrode. Typically, 5 mg of the catalyst was ultrasonically suspended in 1 mL of ethanol and 50 µL of Nafion solution (5 wt %, Du Pont) for 30 min to form homogeneous ink. Then 25 µL of the ink was spread onto the surface of the GC electrode with a micropipet to form a uniform film. The ORR activity was tested in O2-saturated 0.5 M HClO4 solution. The rotation rate of the electrode was 1600 rpm, and the scan rate was 5 mV s-1. The short-term stabilities of the MoN/C and Mo2N/C in oxygen atmosphere were carried out by cyclic voltammetry (CV) in O2-saturated 0.5 M HClO4. The electrode potential was cycled 500 cycles between 0.24 and 1.14 V in O2-saturated 0.5 M HClO4 solution with a scan rate of 100 mV s-1, and then the ORR curves for the catalysts with a scan rate of 5 mV s-1 were recorded. The catalytic activities of the catalysts for the MOR were checked by linear sweep voltammetry (LSV) in N2saturated 0.5 M HClO4 + 1 M CH3OH. 2.4. DFT Calculation. DFT calculations were performed at the B3LYP level14,15 with the Gaussian 03 program.16 The Los Alamos LANL2DZ effective core pseudopotential (ECP)17,18 was adopted for the Mo atom and a basis set of 6-311G** was used for N and O atoms. For MoN and Mo2N clusters, geometry optimization was performed with different geometries and spin states. The stable geometries of MoN and Mo2N clusters were further considered for O2 adsorption studies. The frequency calculations were also performed to verify that the frequencies of all the geometries were positive. 3. Results and Discussion 3.1. Physical Characterization of the Molybdenum Nitrides. Figure 1 shows the XRD patterns of sample A and the as-synthesized carbon-supported molybdenum nitrides. To see clearly, the diffraction peak position and relative intensity of the standard patterns of MoO2 (JCPDS PDF 32-0671), MoN (JCPDS PDF 25-1367), Mo2N (JCPDS PDF 25-1366), and graphite (JCPDS PDF 75-1621) are also included in Figure 1. For the three samples, the diffraction peaks located at 26° and 43° are attributed to the (002) and (101) diffraction peaks of graphite (JCPDS PDF 75-1621). For sample A, shown in Figure 1a, the diffraction peaks at 26°, 36.9°, 53.3°, 60.1°, 66.6°, and 78.7° correspond to the (111), (211), (312), (031), (402), and (231) diffraction peaks of the monoclinic MoO2 (JCPDS PDF

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Figure 1. XRD patterns of (a) sample A, (b) MoN/C, and (c) Mo2N/C.

32-0671), respectively. No other diffraction peaks are observed in Figure 1a, indicating that only MoO2 phase is included in sample A. In Figure 1b, the diffraction peaks at 31.9°, 36.2°, 49.0°, 65.1°, and 74.5° correspond to the (002), (200), (202), (220), and (222) diffraction peaks of the hexagonal MoN, respectively, based on the standard data (JCPDS PDF 25-1367) and refs 10 and 12. In Figure 1c, in addition to the diffraction peaks of graphite, the diffraction peaks at 37°, 43.4°, 63.1°, and 75.7° are attributed to the (111), (200), (220), and (311) diffraction peaks of the face-centered cubic (fcc) Mo2N, respectively, according to the standard data (JCPDS PDF 251366) and refs 9 and 12. No evidence of any impurities, such as metallic molybdenum, molybdenum carbide, molybdenum oxide, or other molybdenum nitride, is found in the XRD patterns of the MoN/C and Mo2N/C. XRD analysis suggests that two different carbon-supported molybdenum nitrides (MoN/C and Mo2N/C) were prepared by controlling the experimental conditions. The broadening of the diffraction peaks indicates that the crystal size in both samples is on a nanometer scale. The average particle size of the MoN/C and Mo2N/C calculated by Scherrer’s equation is about 6 and 4 nm, respectively. The TEM images of the MoN/C and Mo2N/C are shown in Figure 2a and 2b. It can be seen from Figure 2b that except for carbon spheres of around 20-40 nm in diameter, the sphere Mo2N particles with an average particle size of around 5 nm are deposited on the carbon support; in contrast, the particles of MoN are in needle shape with a length of around 8 nm. For comparison, the TEM image of the MoO2/C (sample A, see section 2.1) is shown in Figure 2c. A distinct morphology, namely, coil-shaped particles, from carbon spheres was observed in this sample, which should be MoO2 particles since no other species except for MoO2 and carbon phases is found in the XRD pattern. By comparing Figure 2a and 2c, it can be concluded that the coil-shaped MoO2 particles evolved into the needleshaped Mo2N particles during the heat treatment of sample A in NH3 atmosphere. The SEM images and EDX results for the MoN/C and Mo2N/C are shown in Figures 3 and 4. It can be seen from the

Structures of Molybdenum Nitrides and Their Catalytic Activities

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Figure 2. TEM images of (a) MoN/C, (b) Mo2N/C, and (c) MoO2/C.

Figure 3. SEM image and EDX results for MoN/C.

EDX spectra that Mo and C elements can be clearly detected in both samples. Moreover, N element is also found in both samples; however, it is only a qualitative analysis but not quantitative due to the influence from the nearby signal from C element. To investigate the distribution of Mo and N elements in the two catalysts, the EDX mapping of Mo and N elements in the two catalysts is shown in Figures 3 and 4. It can be seen that both Mo and N elements are distributed uniformly on the carbon support for both the MoN/C and the Mo2N/C. Figure 5 shows the XPS spectra of the Mo3d, Mo3p, and N1s levels for the MoN/C and Mo2N/C. The Mo core is spin-orbit split to 3d5/2 (232.7 eV) and 3d3/2 (235.8 eV). The results on the MoN/C and Mo2N/C indicate formation of nitride phases, which are characterized by a Mo3d peak at about 228.7 eV (Moδ+, 0 < δ < 4) and a binding energy for the N1s signal (about 397.8 eV).19,20 Considering the overlap of the Mo3P3/2 spectra with the N1s spectra and the possible aerial surface oxidation of the molybdenum nitrides, only qualitative analysis was done. 3.2. Activity Measurements of the Molybdenum Nitrides toward the ORR. The potentiodynamic measurements of the ORR were performed in 0.5 M HClO4 solution saturated with

O2 at room temperature and is shown in Figure 6. The electrode was kept at a rotation rate of 1600 rpm and a scan rate of 5 mV s-1. As a background, Vulcan XC-72R (denoted as C) and MoO2/C were also included. It can be seen from Figure 6 that almost no ORR current is produced on Vulcan XC-72R. On the MoO2/C, a little ORR current is observed. In contrast, the ORR currents on both molybdenum nitrides are significant. The ORR onset potential for the MoN/C is 0.75 V, which is about 50 mV more positive than that for the Mo2N/C (0.7 V). This seems contradictory to those reported in refs 9 (Mo2N) and 10 (MoN), where MoN/C is 120 mV more negative than that for Mo2N/C. However, it should be noticed that the catalyst loadings on GC electrodes in refs 9 and 10 are different, i.e., the catalyst loading in ref 10 is only approximately 1/3 of that in ref 9 and 2/3 of that in the present work. The ORR activity depends significantly on catalyst loadings as reported in ref 21. The lower catalyst loading in this work compared with that in ref 9 also provides an explanation for the lower current densities of both the MoN/C and the Mo2N/C than that of the Mo2N/C in ref 9. The significant difference in the ORR activities between the

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Figure 4. SEM image and EDX results for Mo2N/C.

Figure 5. XPS spectra of the Mo3d level for (a) MoN/C and (b) Mo2N/C and the N1s and Mo3p levels for (a′) MoN/C and (b′) Mo2N/C.

MoN/C and the Mo2N/C is probably relative with their specific structures. We will discuss it in detail later in the DFT section. To measure the stability of the MoN/C and Mo2N/C in oxygen atmosphere, CV measurement was carried out in O2-saturated 0.5 M HClO4 solution at room temperature cycled between 0.24

and 1.14 V for 500 cycles at a scan rate of 100 mV s-1. Figure 7 shows potentiodynamic measurements of the ORR currents before and after the 500-cycle scanning for the Mo2N/C (a) and MoN/C (b). It can be seen from Figure 7a that the ORR curves for the Mo2N/C almost overlap, indicating the good electrochemical stability of this sample. Similarly, the activity of the


Figure 6. ORR curves for MoN/C, Mo2N/C, XC-72R (denoted as C), and MoO2/C.

Figure 7. ORR curves for (a) Mo2N/C and (b) MoN/C before and after being cycled 500 cycles between 0.24 and 1.14 V in 0.5 M HClO4 solution saturated with O2. Rotation rate: 1600 rpm. Scan rate: 5 mV s-1. Disk area: 0.196 cm2. The currents were normalized to the geometric area of the electrode (0.196 cm2).

MoN/C remains constant but decays slightly when the potential is more negative than 0.45 V after 500 cycles. As a cathode catalyst for a DMFC, the insensitivity to methanol crossover from anode to cathode is very important. In order to test the sensitivity of the MoN/C and Mo2N/C catalysts toward the MOR, LSV measurements in N2-saturated 0.5 M HClO4 containing 1 M CH3OH were carried out. The results are shown in Figure 8. For comparison, the MOR on a


Figure 8. LSV curves for MoN/C, Mo2N/C, and Pt/C-JM in 0.5 M HClO4 + 1 M CH3OH solution saturated with N2. Scan rate: 5 mV s-1. Disk area: 0.196 cm2. The ORR currents were normalized to the geometric area of the electrode (0.196 cm2).

commercial 40 wt % Pt/C catalyst (Johnson Matthey Inc., denoted as Pt/C-JM) was also plotted in Figure 8. An obvious methanol oxidation peak is observed on the Pt/C-JM, indicating that Pt/C-JM is active to the MOR. In contrast, no methanol oxidation peak is observed for both the MoN/C and the Mo2N/C catalysts. This means that both the MoN and the Mo2N are inert to the MOR. Pt-based noble metals are active toward oxidation of small organic molecules, including the MOR. Mo could provide OH species to accelerate the removal of COad species dissociated from methanol molecules; however, it is unable to adsorb methanol molecules itself as reported in refs 22 and 23. Therefore, MoN/C and Mo2N/C, when used as cathode ORR catalysts for a DMFC, could avoid the negative effect of the methanol crossover due to their insensitivity toward methanol. 3.3. DFT Calculation. To gain insights on a molecular level into the relationship between the structures of molybdenum nitrides and their catalytic activities, DFT calculations were carried out on molybdenum nitride clusters. The cluster approximation approach, in which a two-atom MoN cluster and a three-atom Mo2N cluster, was chosen for simulation. The stable geometries of MoN and Mo2N clusters were further considered for O2 adsorption studies. Three types of O2 adsorption (i.e., Griffiths, Pauling, and Bridge model24) on the stable geometries of MoN and Mo2N clusters were performed. To include all three O2-adsorption types for MoN/C, a four-atom MoN cluster was also added to include the bridge adsorption of O2 on it. All stable geometric structures of MoN, Mo2N, and the corresponding O2adsorbed clusters are shown in Figure 9. The structural parameters, including electronic energy (Ecluster), bond length, and Mulliken charge carried by each atom, are shown in Table 1. As shown in Figure 9, the two-atom MoN cluster has only one geometric structure (denoted as A) and the corresponding O2-adsorbed cluster is denoted as A-O2. The four-atom MoN cluster has two possible stable structures (denoted as B and C), and the corresponding O2-adsorbed clusters are denoted as B-O2 and C-O2, respectively. The Mo2N cluster has four possible stable structures (denoted as D, E, F, and G), and the corresponding O2-adsorbed clusters are denoted as D-O2, E-O2, F-O2, and G-O2, respectively. It is found that the D-O2, E-O2, and F-O2 clusters have the same geometric structure, while the G-O2 cluster is different from the above three clusters. As shown in Table 1, in all MoN and Mo2N clusters, Mo is positively charged while N is negatively charged, indicating charge transfer from the Mo to the N atom. In addition, in all

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Figure 9. Calculated stable geometric structures of MoN, Mo2N, and the corresponding O2-adsorbed clusters.

O2-adsorbed MoN and Mo2N clusters, the Mo atom is more positively charged while the O atom is more negatively charged. This is reasonable since the O atom is more electronegative than the Mo atom. Moreover, it should be noted that the negative charge carried by the N atom in the MoN cluster almost remained constant before and after O2 adsorption. The same trend is also found for the Mo2N cluster and the O2-adsorbed Mo2N clusters. This might be an implication that, during O2 adsorption, the negative charge carried by the O atom is primarily transferred from the Mo atom while the N atom is nearly inert. Elongation of the O-O bond is a sign of dissociation of the O2 molecule. As shown in Table 1, the O-O bond lengths in all O2-adsorbed MoN and Mo2N clusters are above 1.4 Å, longer than that of the O2 molecule (1.21 Å), indicating that both MoN and Mo2N facilitate dissociation of the O2 molecule. From the DFT calculation results, it is known that three optimized geometric structures exist for MoN-O2 clusters, i.e., A-O2, B-O2, and C-O2 clusters with corresponding O-O bond lengths of 1.43, 1.46, and 1.43 Å, respectively; in contrast, four optimized geometric structures exist for Mo2N-O2 clusters, i.e.,

D-O2, E-O2, F-O2, and G-O2 clusters with corresponding O-O bond lengths of 1.41, 1.41, 1.41, and 1.47 Å, respectively. It is hard to conclude which is more suitable for O2 dissociation based on this information. Furthermore, considering the adsorption types of the seven kinds of clusters, except on cluster A, O2 molecules adsorb in a bridge type on all other clusters. From this point of view, the Griffiths-type adsorption of O2 doubles the efficiency of Mo assuming other conditions are the same. In a real catalyst, the MoN/C is more efficient by adsorbing more O2 molecules with a same loading of Mo. This might explain the better activity of the MoN than the Mo2N. In summary, both the suitable geometric structure of MoN and the preferred oxygen adsorption type on it contribute to the higher activity of the MoN/C toward the ORR. Further analysis is based on frontier orbital theory (FOT). The half-occupied 2π* antibonding orbital for the ground state of O2 is considered to be both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), so the HOMO and LUMO energies of O2 should have the same value. The LUMO energy of O2 is lower than the HOMO energies of MoN and Mo2N. Therefore, charge transfer



TABLE 1: Calculated Structure Parameters for MoN, Mo2N, O2, and the Corresponding O2-Adsorbed Clusters bond length(Å) cluster

Ecluster (Hartree)

dN-Mo

A

-122.20

1.64

A-O2

-272.64

1.64

B

-244.47

1.66 1.66

B-O2

-394.92

1.65 1.65

C

-244.45

1.64 1.64

C-O2

-394.90

1.65 1.65

D

-189.60

1.81 1.81

D-O2

-340.15

1.78 1.97

E

-189.66

1.91 1.91

E-O2

-340.14

1.92 1.80

F

-189.66

1.91 1.76

F-O2

-340.15

1.78 1.97

G

-189.68

1.66

G-O2

-340.13

1.66

O2

-150.36

dMo-O

1.97 1.97

2.00 2.00

2.04 1.99

2.00 1.97

2.01 1.96

2.00 1.97

2.00 2.00

dO-O

1.43

1.46

1.43

1.41

1.41

1.41

1.47

EHOMO (Hartree)

ELUMO (Hartree)

-0.195

-0.107

-0.239

-0.140

-0.194

-0.118

-0.228

-0.117

-0.203

-0.116

-0.224

-0.139

-0.154

-0.093

-0.182

-0.099

-0.144

-0.128

-0.177

-0.104

-0.154

-0.091

-0.182

-0.099

-0.195

-0.096

-0.207

-0.117

-0.234

1.27

is believed to proceed from the HOMO of MoN and Mo2N to the LUMO of O2. This is consistent with the above Mulliken charge analysis in the DFT calculation that partial charge transfers from Mo to O. Figure 10 shows the LUMO energy of O2 and the HOMO energies of MoN and Mo2N clusters. The smaller the energy gap between the HOMO of molybdenum nitrides and the LUMO of O2, the easier electron transfer from the HOMO of molybdenum nitrides to the LUMO of O2. This

Mulliken charge Mo N Mo N O O Mo Mo N N Mo Mo N N O O Mo Mo N N Mo Mo N N O O Mo Mo N Mo Mo N O O Mo Mo N Mo Mo N O O Mo Mo N Mo Mo N O O Mo Mo N Mo Mo N O O

0.31 -0.31 1.03 -0.33 -0.35 -0.35 0.36 0.36 -0.36 -0.36 0.73 0.73 -0.34 -0.34 -0.39 -0.39 0.27 0.27 -0.27 -0.27 0.66 0.72 -0.30 -0.30 -0.36 0.42 0.28 0.28 -0.56 0.74 0.67 -0.63 -0.39 -0.38 0.31 0.31 -0.62 0.72 0.66 -0.62 -0.38 -0.38 0.34 0.33 -0.67 0.74 0.67 -0.63 -0.39 -0.38 0.12 0.22 -0.34 0.50 0.64 -0.35 -0.41 -0.39

electron transfer consequently increases the electron density of the LUMO of O2 and in turn results in the increase of the O-O bond length, which provides an explanation for the catalytic activities of the MoN/C and Mo2N/C toward the ORR. 4. Conclusions Two carbon-supported molybdenum nitrides, MoN/C and Mo2N/C, were prepared in NH3 atmosphere. The poteniody-

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Figure 10. LUMO energy of O2, and HOMO energies of MoN and Mo2N clusters.

namic measurements in O2-saturated 0.5 M HClO4 show that the MoN/C exhibits higher catalytic activity toward the ORR than the Mo2N/C. As ORR catalysts, both catalysts are insensitive to methanol, which avoids the negative effect from methanol crossover from the anode. DFT calculation results indicate that both the MoN and the Mo2N facilitate dissociation of the oxygen molecule. Both the suitable geometric structure of the MoN and the preferred oxygen adsorption type on it contribute to the higher activity of the MoN/C toward the ORR. Acknowledgment. This work was funded by the National Natural Science Foundation of China (20973169) and DICPSAIT Joint Project. Dr. Jiang appreciates the funding support by the DICP-100-talents Program. References and Notes (1) Ziegelbauer, J. M.; Murthi, V. S.; O’Laoire, C.; Guila, A. F.; Mukerjee, S. Electrochim. Acta 2008, 53, 5587–5596. (2) Serov, A. A.; Min, M.; Chai, G.; Han, S.; Kang, S.; Kwak, C. J. Power Sources 2008, 175, 175–182.

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