Article pubs.acs.org/cm
Mesoporous Chromium Nitride as High Performance Catalyst Support for Methanol Electrooxidation Minghui Yang,* Rohiverth Guarecuco, and Francis J. DiSalvo* Department of Chemistry, Cornell University, Ithaca, New York 14853-1301, United States ABSTRACT: A simple process for preparing mesoporous chromium nitride (CrN) by the ammonolysis of a bulk ternary oxide (K2Cr2O7) is reported. The products were characterized by Rietveld refinement of powder X-ray diffraction patterns, scanning electron microscopy (SEM), and nitrogen adsorption/desorption analysis. Pore sizes ranging from 10 to 20 nm are easily accessible. The conductivity of mesoporous CrN powder compressed at 35 bar is 54 S/cm. A Pt/CrN catalyst prepared from the mesoporous CrN shows a negative onset potential for methanol electrooxidation (0.20 V vs SCE) similar to that of Pt/C (0.22 V vs SCE). The electrochemically active specific surface area (ECSA) of the Pt/CrN catalyst (82 m2/g) was only slightly higher than that of Pt/C (75 m2/g). More importantly, the Pt/CrN catalyst demonstrates high tolerance to corrosion and is a candidate to replace carbon black, which is known to corrode under high potentials, as a support for fuel cell catalysts. This work provides an efficient method for preparing mesoporous metal nitrides that are promising supports for the oxidation of small organic molecules in fuel cells. KEYWORDS: mesoporous, transition metal nitride, CrN, fuel cell, methanol electrooxidation
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INTRODUCTION The low cost, industrial-scale availability, and high energy density of liquid methanol has made direct methanol fuel cells (DMFCs) a promising, clean-energy technology with the potential to power future portable electronic devices.1−3 Current studies propose that polymer electrolyte membrane fuel cells (PEMFCs) such as DMFCs use platinum-based or palladium-based catalysts on high surface area catalyst supports such as carbon black, carbon nanofibers, carbon nanotubes, porous carbon structures, porous silicon structures, and graphene oxide.4−8 While the need for high surface area catalyst supports is undisputed, the aforementioned supports may not be conducive to long-term fuel cell durability. Corrosion of carbon support materials by electrochemical oxidation in PEMFC operating environments and loss of electro-catalytic surface area at high temperatures (>80 °C) due to catalyst aggregation or separation of the catalyst from the catalyst support have already been reported.9,10 In addition, methanol oxidation on carbon-supported Pt catalysts (Pt/C) has been shown to cause the formation of adsorbed CO, which poisons the catalytically active sites, thereby reducing the overall fuel cell performance.11−14 Furthermore, Pt dissolution and redeposition (also called Ostwald ripening) may be more significant in DMFCs than other PEMFCs.15,16 It is important to ensure electronic conductivity between the supported electrocatalyst phase and the gas-diffusing layer in PEMFCs.17 Thus, an ideal catalyst support for PEMFCs has high electrical conductivity. Enhanced electrical conductivity has proven to be specifically beneficial for methanol electrooxidation.18 An © 2013 American Chemical Society
additional property that is especially efficacious in DMFCs is mixed electron/proton conductivity of the electrocatalyst.19 Furthermore, it has been reported that the electrical properties of nanoscale materials differ from their bulk counterparts: in nanoscale, meso- and microporous materials, conduction occurs within nanoscale particles, between adjacent nanoscale particles, and along the surfaces of the particles.20 Transition metal nitrides (TMNs) are ideal candidates for noble metal catalyst supports in DMFCs because they are highly electrically conductive (metallic), thermally stable with high melting points, electrochemically stable in fuel cell operating conditions, and exhibit exceptional hardness and corrosion resistance.21−23 However, in terms of fuel cell application, there are few publications dealing with metal nitrides as catalyst supports. The performance of a tungsten nitride supported on carbon black (W2N/C) as a non-noble electrocatalyst for the oxygen reduction reaction (ORR) in PEMFCs has been reported; this report concluded that the activity of the W2N/C catalyst toward the ORR was inferior to that of commercially available Pt/C catalysts already widely used in PEMFCs.24 Titanium nitride (TiN) seems to be the most widely studied, but it suffers from wear and corrosion in acidic environments.9,25,26 In contrast, chromium nitride (CrN) has high resistance to wear and corrosion.27,28 CrN has been Received: January 24, 2013 Revised: March 12, 2013 Published: April 4, 2013 1783
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microscopy (TEM) was performed with a FEI T12 Spirit TEM STEM. Nitrogen adsorption/desorption isotherms were measured at −196 °C using a Micromeritics ASAP 2020 system. The samples were degassed at 200 °C for 24 h on a vacuum line. Elemental analyses of nitrogen and oxygen content of the nitride samples were done with a LECO TC-600 analyzer using the inert gas fusion method. Nitrogen was detected as N2 by thermal conductivity, and oxygen was detected as CO2 by infrared detection. The apparatus was calibrated using Leco standard oxides; Si2N2O and TaN were used as nitrogen standards.34 A four-point probe measurement of conductivity of the compressed powders at a relatively low pressure of 35 bar was used to estimate the electrical conductivity of compressed mesoporous CrN powder. Electrochemical Measurements. Electrochemical measurements were done with a potentiostat/galvanostat (WaveNano USB Potentiostat) and a three-electrode test cell. Catalyst inks were prepared by ultrasonically dispersing the mixtures of 5 mg of catalyst (Pt/CrN or Pt/C), 1 mL of ethanol, and 50 μL of 5 wt % Nafion solution. Ten microliters of catalyst inks was spread on the carbon disk. A Pt foil was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. To determine the electrochemical stability of CrN, cyclic voltammograms of CrN were measured in 0.5 M H2SO4 at a scan rate of 20 mV s−1. Electrochemically active specific surface area (ECSA), which reflects intrinsic catalytic surface area, was determined for the Pt/CrN catalyst and a Pt/C catalyst by the preadsorbed CO (COad) stripping method.37,38 A flow of CO gas through a 0.5 M H2SO4 solution for 30 min was used to allow complete adsorption of CO onto the catalyst, while the working electrode was still at 0 mV vs SCE electrode. A flow of N2 gas through the solution was then used for 10 min to remove excess CO from the electrolyte. The CO stripping voltamogramms were measured by oxidation of the COad in the 0.5 M H2SO4 solution; the amount of COad was measured by integration of the COad stripping peak. To compare the electrocatalytic activities of Pt/CrN and Pt/C, cyclic voltammograms of Pt/CrN and Pt/C were measured in 1 M CH3OH + 0.5 M H2SO4 solution at a scan rate of 20 mV s−1. Chronoamperometry (with a potential fixed at 0.6 V for the measurements of polarization current vs time) was used to determine the longer term stability of the Pt/CrN catalyst.
used as a coating material for bipolar plates in fuel cells, which is an important component of PEMFC stacks.29,30 Various synthetic approaches to nanostructured metal nitrides have been reported including vapor deposition, nanopatterning, or other templating.31−33 We recently reported a simple route for preparing mesoporous, conducting nitrides from Zn, Cd, or K-containing ternary transition metal oxides.22,34,35 The reported nitride materials result from the condensation of atomic scale voids created by the evaporative loss of Zn or Cd, the replacement of 3 oxygen anions by 2 nitrogen anions, and, in most cases, the loss of oxygen to form water on the reduction of the transition metal. In the K case, the byproduct does not sublime away, but may be removed by washing with water. In this Article, we demonstrate a facile synthesis of mesoporous CrN with smaller pores and higher surface area than attainable using Zn- or Cd-containing precursor oxides from a potassium-containing oxide. We subsequently show that the CrN can be used as a Pt catalyst support for methanol electrooxidation, resulting in a Pt/CrN electrocatalyst that displays both higher electrocatalytic activity and higher corrosion resistance than the conventional Pt/C catalyst in PEMFCs.
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EXPERIMENTAL PROCEDURES
Synthesis. All chemicals used for the synthesis can be purchased commercially and have the highest possible purities (≥99.99%). Onehalf of a gram of K2Cr2O7 was placed in an alumina boat. This boat was then placed in a stainless steel tube with airtight, stainless steel end-caps that have welded valves and connections to input and output gas lines. All gases were purified to remove trace amounts of oxygen or water using pellet copper, nickel, palladium, and platinum on zeolite supports. The stainless steel tube was then placed in a split tube furnace, and the appropriate connections to the gas sources were made. An argon gas flow through the tube was used for 15 min to expel the air remaining in the tube before establishing the flow of ammonia gas (Anhydrous, Airgas) through the tube. The sample was then heated in the tube to 700 °C at 150 °C/h. After 8 h elapsed at 700 °C, the furnace power was turned off and the product was cooled to room temperature in ∼4 h under an ammonia gas flow. Before the stainless steel tube was taken out of the split tube furnace, an argon gas flow through the tube was used to expel the ammonia gas remaining in the tube. After the stainless steel tube was taken out, it was left on a lab bench for 24 h with one valve open so that the ammonolysis product could be exposed to air slowly. The CrN product was washed three times using deionized (DI) water, then dried in a glass desiccator under vacuum for 24 h. Pt nanoparticles supported on CrN and Vulcan XC-72 carbon were prepared by a polyol process with ethylene glycol (EG) as a reducing agent.36 The total Pt loading on each support was 20 wt %. An appropriate amount of H2PtCl6 (99.995%, Sigma-Aldrich) and 50 mg of mesoporous CrN (or C) were suspended in 50 mL of ethylene glycol solution. Next, the mixture was heated at 140 °C for 3 h. Subsequently, the suspension was filtered and washed with deionized water, and then dried at 80 °C for 6 h to obtain the Pt/CrN catalysts (or Pt/C). Analysis. Finely ground powders were examined with a Rigaku Ultima VI powder X-ray diffractometer (PXRD) with Cu K radiation (Kα1, λ = 1.5406 Å and Kα2, λ = 1.5444 Å). Crystal structures of the oxide and the resulting nitride were confirmed by PXRD profiles using the GSAS package. The nitride crystalline domain size can be estimated from a Rietveld fit of the Lorentzian function, as discussed previously in the synthesis of metal (oxy)nitrides from Zn-containing oxides.34 Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were performed with a LEO-1550 field emission SEM (FSEM). Approximately 2 mg of CrN was dispersed in 2 mL of ethanol. The solution was sonicated for 30 min, and then one drop of the solution was transferred by glass pipet onto a TEM copper grid (3.05 mm holey carbon, Sigma-Aldrich). Transmission electron
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RESULTS AND DISCUSSION Synthesis of Mesoporous CrN. Mesoporous CrN was synthesized using a method similar to one we reported previously.35 The XRD patterns for the washed nitride product of the ammonolysis of K2Cr2O7 at 700 °C for 8 h, CrN, can be seen in Figure 1a; this figure confirms the phase purity of the ammonolysis product. The CrN crystallizes in the space group
Figure 1. (a) PXRD patterns for CrN prepared by ammonolysis of K2Cr2O7 at 700 °C for 8 h, Pt/CrN, and Pt/C catalysts and (b) Rietveld refinement of the CrN. 1784
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crystalline oxide grains.34 The electrical conductivity of mesoporous CrN powder compressed at 35 bar was 54 S/ cm, which is about 1 order of magnitude higher than the reported conductivity of carbon-based supports under similar conditions (∼4 S/cm).40 A reported value for bulk, rocksalt CrN electrical conductivity is 1.1 × 103 S/cm.41 Thus, the conductivity of the compressed mesoporous CrN powder is approximately 2 orders of magnitude lower than that of the bulk material, presumably due to both the porosity and the weak particle−particle contacts at low pressure. Formation Mechanism of Mesoporous CrN. The mechanism of formation of mesoporous CrN is similar to the case of mesoporous VN.35 Under ammonia flow at 700 °C, the formation of KOH or KNH2 from potassium oxide is thermodynamically possible. Other compounds such as mixed hydroxides and amides or imides may also be produced during the heating and cooling processes. In any case, these byproducts are either solids or liquids at the synthesis temperature and apparently sublime slowly during the reaction or wash out using DI water after the reaction. The observed mesoporosity in CrN results from phase separation of the nitride from the other byproducts. We hypothesize that the surface energies and diffusion constants of the relevant phases allow such separation to proceed at the mesoscale. In addition, the sintering rate of the nitride is slow enough that the porosity is not eliminated during the heating period, because the diffusion rates in nitrides are so low. This mechanism is similar to that seen in the formation of porous Vycor, where spinodal decomposition of aborosilicate glass forms two phases that are separated on the nanoscale.42 An ordered mesoporous CrN, made using a hard silica template, with comparable high surface area (78 m2/g) was previously reported by Shi et al.33 In their work, a wide range of pore-size distribution was observed (from 5 to 25 nm), which is also comparable to our CrN product. However, the present method does not require the use of a template and its subsequent removal. Mesoporous CrN as Catalyst Support. Pt particles are deposited on mesoporous CrN and carbon black through a polyol process. Figure 1a shows the XRD patterns of the Pt/ CrN and Pt/C electrocatalysts in addition to the XRD patterns of the mesoporous CrN that was synthesized and used as the catalyst support. All of the diffraction peaks of mesoporous CrN are also observed in the Pt/CrN catalyst. The Pt particles crystallize in the space group Fm3̅m with refined lattice parameter a = 3.9311(4) Å. The diffraction peaks of Pt in the Pt/C system are almost identical to those of Pt on CrN. The weak diffraction peak observed at 2Θ ≈ 24° for Pt/C is attributed to the carbon black. The size and morphology of the Pt/CrN catalyst can be observed in Figure 2c and d. The average size of the Pt particles in the Pt/CrN catalyst is 4.5 nm, and the average size of the Pt particles in the Pt/C catalyst that was used for comparison was 4.2 nm. The difference in average Pt particle size between the two catalysts was not very large, indicating that average Pt particle size was relatively unaffected by which catalyst support was used. Pt particles are uniformly dispersed on mesoporous CrN with little obvious aggregation and have a narrow size distribution. The cyclic voltammograms of the mesoporous CrN seen in Figure 4 show that no redox peaks associated with CrN are observed when cycling between −0.8 and 1.2 V (20 mV/s vs SCE) for 10 cycles, indicating that the CrN is electrochemically stable, at least on the minutes time scale.
Fm3m ̅ with a refined lattice parameter of a = 4.1430(2) Å as shown in Figure 1b. The CrN products from ammonolysis at 700 °C show relatively broad diffraction peaks due to the small crystalline domain sizes. The EDX result of the CrN product shows no K peak, which suggests the possible K-containing byproducts have been washed out by DI water. The PXRD refinement of the product obtained shows that the average nitride crystalline domain size is 29 nm. SEM and TEM images were used to observe the surface morphology of the binary nitride products. Figure 2a and b shows SEM images of the CrN, with pores on the scale of 5−10 nm.
Figure 2. SEM (a,b) of the CrN prepared by ammonolysis of K2Cr2O7 at 700 °C for 8 h, and TEM (c,d) of the Pt/CrN catalyst: darker spots are Pt.
Chemical, Surface Area, and Conductivity Properties. It is expected that nitrides produced from the ammonolysis of oxides contain residual amounts of oxygen.21,39 The ammonolysis products of single phase CrN show that the CrN contained
