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Limitations and Improvement Strategies for Early-Transition-Metal Nitrides as Competitive Catalysts toward the Oxygen Reduction Reaction Junming Luo, Xinlong Tian, Jianhuang Zeng, Yingwei Li, Huiyu Song, and Shijun Liao* The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China S Supporting Information *

ABSTRACT: The poor catalytic activity of early-transition-metal nitrides has prevented them from being competitive catalysts toward the oxygen reduction reaction (ORR). In the present study, we first explored the limitations for early-transition-metal nitrides as competitive catalysts in the view of O2 dissociation, finding that the limitations include insufficient d electrons (in the case of ScN, TiN, and VN) and unsuitable surface geometric structure (in the case of CrN), both of which can result in no O2 dissociation on early-transition-metal nitrides. Then on the basis of the above knowledge, we took VN as an example and proposed a strategy to enhance its ORR activity by enriching its d electrons through doping with 3d transition metals. The doped VN showed greatly enhanced ORR activity, with Co-doped VN exhibiting the best performance; its ORR activity was close to that of JM 20 wt % Pt/ C. X-ray photoelectron spectroscopy (XPS) clearly revealed that Co doping significantly increased the proportion of V in a lowvalence state. O2 temperature-programmed desorption (O2-TPD) measurements also presented some very important information induced by doping. Our theoretical analysis and experimental studies indicated that early-transition-metal nitrides with insufficient d electrons can be promising ORR catalysts via the strategy of enriching their d electrons through doping elements with rich d electrons. KEYWORDS: fuel cells, early-transition-metal nitrides, oxygen reduction reaction, O2 dissociation, d electrons

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INTRODUCTION

attention, as their performance is comparable to or even superior to that of commercial Pt/C in alkaline media. Early-transition-metal nitrides are known for their tremendous physical properties, including high hardness, high melting points, and high corrosion resistance.18 They are generally prepared by diffusing N atoms into the metal lattices and combine the characteristic properties of covalent solids, ionic crystals, and transition metals.19 Early-transition-metal nitrides have a rock-salt-type (NaCl-type) structure in which both the metal atoms and the N atoms have octahedral coordination (specifically, each metal (N) atom is coordinated by six N (metal) atoms). This makes them more stable than latetransition-metal nitrides, in which the metal is coordinated by fewer N atoms. Indeed, the stabilities of early-transition-metal nitrides were proven by DiSalvo’s group, who conducted systematic research to replace carbon with early-transitionmetal nitrides as catalyst supports.20−26 Their work also indicated that early-transition-metal nitrides have good

Fuel cells are widely regarded as one of the most promising clean energy technologies due to their high energy conversion efficiency, ease of operation, and zero/low emissions. Unfortunately, the commercialization of fuel cells has been hindered by several factors, such as their sluggish ORR rate, their high cost (caused mainly by the use of Pt-based catalysts), and their insufficient stability. One of the strategies to address these challenges is to develop low-Pt catalysts, such as core− shell structured catalysts and Pt-alloy catalysts, to lower the cost but maintain or even surpass the ORR activity of Pt.1−11 There is no doubt that developing low-Pt catalysts is a good strategy in the transition period of fuel cell commercialization. However, due to the scarcity of Pt, Pt-free (or precious-metal-free) catalysts will eventually be needed to support the sustainable commercialization of fuel cells. To get rid of Pt and lower the cost, many researchers have worked on exploiting cheap catalysts with excellent ORR activity. So far, great progress has been made. Some of the most important non-precious-metal catalysts are carbon materials codoped with non-precious metals and N (metal-Nx/C);12−17 they have drawn significant © 2016 American Chemical Society

Received: June 8, 2016 Revised: July 26, 2016 Published: August 8, 2016 6165

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off and cooled to room temperature. The remaining NH3 in the furnace was evacuated using a N2 flow, and the synthesized V0.95M0.05N was collected. TiN, VN, V1−xCoxN, and CrN were prepared using the same procedures described above, except that the corresponding parameters (precursor composition, doping concentration, and annealing temperature) were adjusted. Characterization of Catalysts. X-ray diffraction (XRD) was conducted on a TD-3500 powder diffractometer (Tongda, People’s Republic of China) operated at 30 kV and 20 mA, using Cu Kα radiation sources. Transmission electron microscopy (TEM) images, high-resolution TEM (HR-TEM) images, and selected area electron diffraction (SAED) were acquired from a JEM-2100HR microscope (JEOL, Japan). XPS was performed on an Axis Ultra DLD X-ray photoelectron spectrometer employing monochromated Al Kα X-ray sources (hν = 1486.6 eV). O2-TPD measurements were performed on an AutoChem II 2920 chemisorption analyzer (Micromeritics, USA). All samples were first treated at 200 °C for 30 min under a He atmosphere to remove adsorbed water. Then, after the samples had cooled to room temperature, an O2/Ar (2% O2) gas flow of 30 sccm was introduced to absorb on the samples for 1 h. After the remaining O2 was evacuated using a He gas flow, the temperature of the samples was raised at a rate of 20 °C min−1 from room temperature to 550 °C. For comparison, the baseline of VN was recorded after VN was pretreated at 550 °C for 5 h under a He atmosphere. It should be mentioned that here we focused only on the desorption peaks originating from atomic oxygen (dissociated O2). The observation of molecular oxygen desorption requires a low temperature beyond the instrument’s capability. Energy dispersive X-ray (EDX) analysis was carried out with a field-emission scanning electron microscope (Hitachi S-4800). The Brunauer−Emmett−Teller (BET) surface area was measured by nitrogen adsorption− desorption on a TriStar II 3020 gas adsorption analyzer. Evaluation of Catalysts. The ORR performance of the synthesized catalysts in both 0.1 M KOH solution and 0.1 M HClO4 solution was evaluated with electrochemical tests conducted on an electrochemical workstation (Autolab PGSTAT302N, The Netherlands) at room temperature (25 ± 1 °C), using a three-electrode cell with a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA). The cell consisted of a glassy-carbon-based working electrode (GC, 0.196 cm2), a Pt-wire counter electrode, a Ag/ AgCl (3 M NaCl solution) reference electrode for the acidic medium, and a Hg/HgO (1 M KOH solution) reference electrode for the alkaline medium. All potentials are quoted with respect to the reversible hydrogen electrode (RHE). All catalyst-loaded electrodes were prepared as follows. First, a catalyst ink was prepared by ultrasonicating a mixture of 5 mg of the catalyst and 1 mL of 0.25 wt % Nafion ethanol solution for 30 min. Then, 5 and 8 μL of catalyst ink were pipetted onto the working electrode in the RDE tests and rotating ring disk electrode (RRDE) tests, respectively. Finally, the GC electrode was dried under an infrared lamp for 10 min. The RRDE tests (0.247 cm2 GC disk, 0.180 cm2 Pt ring) were conducted to investigate the ORR electron transfer mechanism. The Pt ring was polarized at 0.7 V vs Hg/HgO in the alkaline medium. The electron transfer number (n) was calculated by n = 4Idisk/(Idisk + (Iring/N)), and the H2O2 yield was calculated by H2O2 (%) = (200Iring/N)/(Idisk + (Iring/N)), where Idisk and Iring were the absolute values of the disk current

conductivities; thus, they meet the basic requirement to be electrocatalysts. However, to use early-transition-metal nitrides as catalysts, their electronic properties should be considered as well. Fischer first introduced using the molecular orbital diagram to describe the electronic structure of earlytransition-metal carbides and nitrides.27 The molecular orbital diagram is also very convenient for estimating whether earlytransition-metal carbides and nitrides have a d electronic structure.18,28 For example, a C atom has a 2p2 electronic structure; to guarantee the d electronic structure of earlytransition-metal carbides, the metal in the fourth period should have at least five 3d4s electrons. This means TiC would have no d electronic structure but VC would, and indeed, this has been proven true.18,29,30 Similarly, an N atom has a 2p3 electronic structure; therefore, to guarantee the d electronic structure of early-transition-metal nitrides, the metal in the fourth period should have at least four 3d4s electrons. This means that ScN would have no d electrons but TiN would, and this also has been confirmed.18,31 Since a C atom has one less electron than an N atom, metal carbides have one less d electron than their corresponding nitrides. Although earlytransition-metal carbides and nitrides are very similar, given that the d electrons are so important to metal-containing catalysts, we chose to focus on the latter. In the past decade, many early-transition-metal nitrides have been investigated as ORR catalysts.32−36 It has been found that the ORR activity of single early-transition-metal nitrides is far lower than that of commercial Pt/C catalysts, but the reason for this has barely been discussed or understood. Undoubtedly, becoming acquainted with the limitations of early-transitionmetal nitrides will help us better understand their catalytic behaviors and find the right solutions to improve their ORR activity. Hence, in this work, we first explored the limitations of early-transition-metal nitrides as competitive ORR catalysts from the perspective of O2 dissociation. Inspired by theoretical analysis and experimental results, we chose VN as an example and proposed a corresponding strategy to enhance its ORR activity. We prepared a series of transition-metal-doped VN catalysts and found that the ORR performance of VN could be significantly enhanced by doping with Co, confirming the validity of the strategy. The ORR performance of Co-doped VN in an alkaline medium was almost comparable to that of a commercial Pt/C catalyst. As far as we know, it is one of the best early-transition-metal nitride-based ORR catalysts identified to date.

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EXPERIMENTAL SECTION Synthesis of Catalysts. A series of VMN catalysts (M = Ti, Cr, Mn, Fe, Co, Ni, Cu) was prepared by ammino complexation followed by nitridation under an NH3 atmosphere. NH4VO3 and MClx (or M(CH3COO)x) were used as the precursors of V and M, respectively. NH4VO3 and MClx (or M(CH3COO)x) in a V:M atomic ratio of 19:1 were first mixed in aqueous solution, and then NH3 was introduced into the solution for 30 min to induce ammino complexation after a homogeneous solution was obtained. Once this was finished, the solution was dried at 90 °C until a dry precursor was obtained, and then the precursor was ground into a powder. Subsequently, the powdered precursor was placed in a tubular furnace for nitridation under an NH3 flow of 30 sccm using a temperature-programmed procedure: room temperature to 630 °C at 5 °C min−1; 630−650 °C at 1 °C min−1; 650 °C for 2 h. After the procedure was terminated, the furnace was powered 6166

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ACS Catalysis and ring current, respectively, and N was the collection efficiency at the ring electrode (N = 0.37).

eliminated. Therefore, to split the O−O bond, the antibonding orbital of the O2 molecule should receive four electrons from the catalyst. Thus, we suggest that the first precondition for dissociating O2 is that the catalyst should be able to donate four electrons to O2 in the form of a chemisorption bond. In terms of energy, the smaller the energy gap between the antibonding orbital and the donated electrons, the easier the electron donation from the catalyst to O2. According to ionization energy, extremely high excitation energy is needed for one atom to donate four electrons. However, if the active site for O2 adsorption is composed of multiple atoms on the catalyst surface, then the electrons donated per atom will be far fewer, which will lower the excitation energy as well. Thus, the energy barrier for O2 dissociation at adsorption sites composed of multiple atoms will be much lower than that at adsorption sites composed of a single atom. Indeed, the side-on adsorption mode (or bridge adsorption) involving multiple atoms is a more effective pathway for O2 dissociation than the end-on adsorption mode (or Pauling adsorption) involving a single atoms.38 Hence, to lower the energy barrier and achieve more efficient O2 dissociation, the catalyst should have an appropriate surface structure with multiple atoms to realize the side-on adsorption of O2. We therefore suggest this is the second precondition. Do early-transition-metal nitrides meet the aforementioned two preconditions? Here we take early-transition-metal nitrides in the fourth period as examples for our analysis. According to the molecular orbital diagram, ScN, TiN, VN, and CrN have the d0, d1, d2, and d3 configurations, respectively.18 The lack of d electrons makes ScN unsuitable as an ORR catalyst. In the cases of TiN, VN, and CrN, we should further consider their surface structure and the mode of O2 adsorption on their surfaces. The surfaces of early-transition-metal nitrides can be coterminated by metal and N atoms or solely terminated by N atoms or metal atoms.19,39,40 According to molecular orbital theory, all of the electrons in the metal and N atoms fill the linear combination orbital of metal and N atoms in the order of energy level. Since the hybridized d orbital is mainly derived from the metal atoms, the majority of the d electrons in these nitrides are distributed on the metal atoms, and only a few d electrons are distributed on the N atoms. Thus, the N-

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RESULTS AND DISCUSSION 1. Limitations Preventing Early-Transition-Metal Nitrides from Being Competitive ORR Catalysts. It is wellknown that the ORR can proceed through either the fourelectron-transfer mechanism or the two-electron-transfer mechanism and that O2 dissociation only occurs in the former.37 The four-electron-transfer mechanism is much more desirable, as it can maximize the output current and voltage of fuel cells using the same amount of O2. Therefore, a good catalyst should be able to dissociate O2 and initiate a fourelectron process for the ORR. What are the preconditions for the dissociation of O2 molecules? To answer this question, we were guided by the molecular orbital of O2. As presented in Figure 1a, the bonding and antibonding orbitals of O2 are filled

Figure 1. (a) Simple schematic of O2 molecular orbitals, depicting only the bonding and antibonding orbitals. (b) Top view of the (200) surface of early-transition-metal nitrides, with gray balls representing metal atoms and blue balls N atoms.

with six and two electrons, respectively. According to molecular orbital theory, the O−O bond strength is determined by the difference in the numbers of electrons occupying the bonding and antibonding orbitals: the smaller the difference, the weaker the bond. When the electron numbers in these two orbitals are the same, these two orbitals will turn into a nonbonding orbital, implying that the interaction between the two O atoms is

Figure 2. RRDE tests of TiN, VN, and CrN in 0.1 M KOH solution at a rotation speed of 1600 rpm: (a) disk current and ring current; (b) H2O2 yield and electron transfer number, n. 6167

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ACS Catalysis terminated surfaces are relatively inert and inactive, whereas the metal-terminated surfaces are unstable and tend to bind too strongly with adsorbates due to extremely high surface energy.41 Therefore, surfaces that coterminate with metal and N atoms are generally considered active surfaces for catalysis. The (200) surface of face-centered-cubic (fcc) metal nitrides (and carbides) is particularly interesting from a catalysis point of view because it is the most stable surface and exhibits both metal atoms and N (or C) atoms.42−45 It has been widely chosen to simulate the adsorption of atoms and small molecules on fcc metal nitrides (and carbides). Moreover, the (200) surface of early-transition-metal nitrides is the main peak in XRD patterns. Hence, the (200) surface of nitrides can be considered the most representative adsorption plane, which is also confirmed by our XRD, HR-TEM, and SAED analysis results. As presented in Figure 1b, on the (200) surface, metal atoms are isolated by N atoms; this unique geometric surface ensures that not more than two metal atoms can participate in the adsorption of O2. Therefore, to guarantee a four-electron donation from the catalyst to O2, each metal atom of the earlytransition-metal nitrides should have at least two d electrons if the adsorption site consists of only two metal atoms. This can be confirmed by the theoretical calculation results of Graciani et al.,45 who found that O2 dissociation occurs on neither TiN nor VN when the adsorption site is formed by two V atoms but occurs on VN when the adsorption site is formed by two V atoms and one N atom. Although TiN, VN, and CrN have the d1, d2, and d3 configurations, respectively, due to orbital hybridization the d electrons on the Ti, V, and Cr atoms should be fewer than but close to one, two, and three, respectively. For TiN, the d electron number on each Ti atom is far less than two; thus, the insufficiency of d electrons makes it difficult to dissociate O2. For VN, the d electron number on each V atom is close to but less than two, which is probably why the N atom is necessary for the dissociation of O2, as it can provide the required d electrons. In the case of CrN, since each Cr atom has more than two d electrons, O2 dissociation should be expected if O2 is able to adsorb on two Cr atoms by the side-on mode. However, as shown in Figure 2a, we found that the ORR activity of CrN was close to that of TiN but inferior to that of VN. The results of the RRDE tests showed that the electron transfer number of the ORR on TiN and CrN was close to two, indicating that most of the O2 could not be dissociated on TiN or CrN. In the case of TiN, this can be explained by there being too few d electrons, but this reasoning does not apply to CrN. Thus, we suspect that the geometrical structure of the CrN (200) surface is what impedes O2 dissociation. As mentioned earlier, having two metal atoms participating in the adsorption of O2 is imperative for O2 dissociation on the surface of nitrides. On the (200) surface, this can be realized in two ways: the M−N−M side-on mode or the M−M side-on mode (see Figure 1b). To dissociate O2, the adsorption site should also have a suitable bond length to stretch the O−O bond to a certain degree. The work of Seifitokaldani and his co-workers indicated that the O−O bond adsorbed on the M−M site can be stretched up to 3 Å before breaking.46 According to their results, the 2.934 Å bond length of Cr−Cr, as shown in Table 1, is not long enough to split the O−O bond, suggesting that it is very likely that O2 cannot be dissociated on the CrN (200) surface via the Cr−Cr side-on mode. Conversely, the 4.149 Å bond length of Cr−N−Cr is long enough to split the O−O

Table 1. M−M and M−N−M Bond Lengths in TiN, VN, and CrN bond length (Å) nitride

M−M

M−N−M

TiN (PDF #38-1420) VN (PDF #35-0768) CrN (PDF #65-2899)

3.000 2.927 2.934

4.242 4.139 4.149

bond. Nevertheless, the nearly 90% H2O2 yield (see Figure 2b) implies that O2 cannot be dissociated through this mode either. It is probable that O2 cannot be dissociated on the CrN (200) surface through the Cr−N−Cr side-on mode because the long distance between the two Cr atoms or the presence of N atoms makes this adsorption mode unavailable. To verify whether O2 dissociation in this situation could, theoretically, occur, we built a simple model (see Figure 3) that assumes O2

Figure 3. Simulation of the side-on adsorption of O2 on the axis of the (200) surface of TiN, VN, and CrN. The lattice constant values, a, are from PDF# 38-1420, PDF# 35-0768, and PDF# 65-2899, dO2 is the diameter of O2, rO and rM are the atomic radii of O and the metal atoms, respectively, and j is the adjacent metal distance.

is able to adsorb on the ideal CrN (200) surface through the Cr−N−Cr side-on mode and then used the parameter h that was calculated on the basis of the mathematical principles of the tangent circle and the right-angle triangle to testify whether the assumption stands correct. The only factor hindering the simultaneous adsorption of O2 on the two metal atoms is the positional conflict between the O atoms and the N atoms; thus, on an ideal (200) surface, we can set the sum value of the atomic radii of O and N atoms (1.4 Å) as the threshold value of h. As long as h ≥ 1.4 Å, the positional conflict will not occur. As presented in Figure 3, although the h value of CrN is the lowest, it is still higher than 1.4 Å. Therefore, on an ideal (200) surface, O2 is able to form the Cr−N−Cr side-on adsorption mode irrespective of the large distance between the two Cr atoms. Nevertheless, it should be mentioned that rippled surface relaxation has been identified as a common phenomenon in metal nitrides.39,45,47,48 On a relaxed surface, metal atoms move inward while N atoms move outward with respect to their position on an ideal surface. Thus, the threshold value of h on a relaxed surface should also take into consideration the inward distance of the metal atoms (ΔhM) and the outward distance of the N atoms (ΔhN). That is to say, the threshold value of h on a relaxed surface should be determined by 1.4 + ΔhM + ΔhN. If the value of ΔhM + ΔhN is large enough, then the h values of TiN, VN, and CrN, especially CrN, may be lower than the threshold value. Joly et al.47 found that N atoms were 0.221 Å above Cr atoms in the outermost layer of CrN (ΔhM + ΔhN = 6168

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ACS Catalysis 0.221 Å). Gauthier et al.48 found that N atoms were 0.168 Å above V atoms in the outermost layer of VN (ΔhM + ΔhN = 0.168 Å). Therefore, for VN, the threshold value of h can reach 1.568 Å, which is lower than 1.574 Å, while for CrN, the threshold value of h can reach 1.621 Å, which is higher than 1.493 Å. This suggests that the positional conflict between the N and O atoms will not occur in VN but will occur in CrN, verifying that the latter is unavailable for O2 to form the Cr− N−Cr side-on adsorption mode on a relaxed CrN (200) surface. Therefore, as we expected, it is very likely that O2 dissociation on CrN is limited by its unsuitable surface geometric structure. In summary, on the basis of the above analysis and our experimental results, we suggest that the poor ORR activity of the early-transition-metal nitrides is probably caused by limitations on O2 dissociation in this type of catalyst, which fall into two categories: insufficient d electrons and unsuitable surface geometric structure. For ScN, TiN, and VN, the insufficiency of d electrons in the metal atoms limits O2 dissociation, whereas for CrN, an unsuitable surface geometric structure is probably the main limitation on O2 dissociation. Hence, for early-transition-metal nitrides to be competitive ORR catalysts, both of these limitations should be removed. 2. Doping as a Strategy To Enrich the d Electrons of VN. Since it is difficult to tune the surface structure of nitrides, we selected early-transition-metal nitrides that are limited only by insufficient d electrons, to see whether their ORR activity could be enhanced. Enriching their d electrons should be a good strategy to enhance their ORR activity. As shown in Figure 2b, the electron transfer number of VN was about 3.2, indicating that VN has a suitable (200) surface structure for O2 dissociation. We therefore chose VN and proposed a strategy to enrich its d electrons by doping it with Cr, Mn, Fe, Co, Ni, or Cu, each of which has more d electrons than V. For comparison, we also doped VN with Ti, which has one less d electron than V. Figure 4 shows the XRD patterns of the synthesized VN and V0.95M0.05Ns. It can be seen that the pure VN and the V0.95M0.05Ns had identical peaks that corresponded with the fcc structure of VN (JCPDS No. 35-0768). This suggests that VN was able to retain its fcc structure after having 5 atom % M doped into its lattice. In addition, the (200) surface is the major

peak in both VN and V0.95M0.05Ns. Figure 5 presents TEM images of VN and V0.95M0.05Ns. It can be seen that the synthesized VN and V0.95M0.05Ns were aggregated nanoparticles, and the particle size of V0.95Ti0.05N was smaller than that of the other V0.95M0.05Ns. The HR-TEM and SAED images presented in Figure 5a indicate that VN nanoparticles have a well-defined (200) surface, which is quite consistent with the XRD patterns. The EDX results listed in Table S1 in the Supporting Information show that the real M/V ratio of the prepared V0.95M0.05Ns was close to the ratio in the recipe. The ORR performance of VN and V0.95M0.05Ns in 0.1 M HClO4 solution and 0.1 M KOH solution are shown in Figure 6. In the 0.1 M HClO4 solution, only V0.95Co0.05N showed significantly enhanced ORR activity, although the activity was still far below that of commercial JM 20 wt % Pt/C. Unfortunately, the ORR activity of V0.95Co0.05N showed an evident decrease after 10 cycles (see Figure S1a in the Supporting Information). The materials doped with other transition metals did not exhibit any obviously improved activity. However, the situation was quite different in the 0.1 M KOH solution. As shown in Figure 6b, the ORR activities of V0.95Ti0.05N, V0.95Cr0.05N, V0.95Fe0.05N, and V0.95Ni0.05N did not show much improvement with respect to VN but V0.95Mn0.05N, V0.95Co0.05N, and V0.95Cu0.05N showed obviously improved ORR activity in comparison to VN. Co-doped VN demonstrated the best ORR activity among the doped nitride materials, with the activity of V0.95Co0.05N approaching that of JM 20 wt % Pt/C; the half-wave potential gap between them was less than 100 mV. In addition, the activity of V0.95Co0.05N declined only slightly after 10 cycles, whereas degradation was evident for V0.95Mn0.05N and V0.95Cu0.05N (see Figure S1b); the stability test shows that V0.95Co0.05N can maintain ∼85% ORR activity during 30000 s (see Figure S1c). We also investigated the effects of the Co doping amount and the nitridation temperature on the ORR activity of VCoN and found that the optimal doping amount was 5 atom % and the optimal nitridation temperature was 650 °C (see Figure S2 in the Supporting Information). Interestingly, it can be noted that V0.95M0.05Ns were more active in alkaline solution than in acid solution. Actually, it is widely recognized that the non-platinum catalysts generally exhibited ORR activity in alkaline solution superior to that in acid solution. According to the literature, the ORR proceeds through different pathways in alkaline solution and in acid solution.49,50 We believe that the elementary reactions in alkaline solution may be more beneficial to the adsorption, activation, or adsorption of oxygen species in comparison to those in acid solution, and that is probably the reason ORR catalysts are generally more active in alkaline media than in acid media. It should be pointed out that the ORR performance of V0.95Co0.05N is exciting, as it is virtually competitive with the performance of JM 20 wt % Pt/C in an alkaline medium; indeed, it is, to date, one of the best reported nitride-based catalysts for the ORR. To figure out why V0.95Co0.05N had a much better ORR activity than VN, we conducted RRDE tests to compare the electron transfer mechanisms in VN and V0.95Co0.05N. As shown in Figure 7a, V0.95Co0.05N had a much lower ring current and a much higher disk current than VN. The calculated results in Figure 7b show that the electron transfer number for the ORR on VN was about 3.2, indicating that the ORR pathway was a combination of two-electron- and four-electron-transfer mechanisms. The H2O2 yield suggests

Figure 4. XRD patterns of VN and V0.95M0.05Ns annealed at 650 °C for 2 h. 6169

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Figure 5. TEM images of VN and V0.95M0.05Ns annealed at 650 °C for 2 h.

Figure 6. Linear sweep voltammetry curves of VN and V0.95M0.05Ns, calculated by subtracting N2-saturated solution from O2-saturated solution at a rotation speed of 1600 rpm: (a) in 0.1 M HClO4 solution; (b) in 0.1 M KOH solution.

Figure 7. RRDE tests of VN and V0.95Co0.05N annealed at 650 °C in 0.1 M KOH solution at a rotation speed of 1600 rpm: (a) disk current and ring current; (b) H2O2 yield and electron transfer number, n.

Cu into the VN lattice resulted in lattice extension, it was difficult to pinpoint a direct connection between the ORR activity and the lattice distortion trend in the V0.95M0.05Ns. We therefore turned our attention to the doping effects on the electronic structure of VN. The surface compositions of the VN and V0.95M0.05Ns measured by XPS are given in Table S2 in the Supporting Information. It is found that the actual M/V ratios in all V0.95M0.05N samples are higher than their recipe values, revealing the surface enrichment of doping metals in V0.95M0.05N nanoparticles. The XPS spectra of M 2p and N 1s in V0.95M0.05Ns are presented in Figures S4 and S5 in the Supporting Information, respectively. Here we mainly focus on how doping affected the valence states of the V atoms. As

that about 40% of the O2 could not be dissociated on VN. For V0.95Co0.05N, the electron transfer number of the ORR was above 3.8 and the H2O2 yield was lower than 10%, indicating that less than 10% of O2 could not be dissociated on V0.95Co0.05N. These results confirm that O2 dissociation was more favored on V0.95Co0.05N than on VN, which explains why V0.95Co0.05N had much better ORR activity than VN. To determine why V0.95Co0.05N had better ORR activity than the other V0.95M0.05Ns and how V0.95Co0.05N facilitated O2 dissociation, we investigated the doping effects on the geometric and electronic structures of VN; Figure S3 in the Supporting Information shows the effects on the former. Although we found that doping Ti into the VN lattice resulted in lattice contraction and that doping Cr, Mn, Fe, Co, Ni, and 6170

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Figure 8. (a) XPS spectra of V 2p in VN and V0.95M0.05Ns. (b) Proportion of V 2p3/2 in VN and V0.95M0.05Ns. (c) N-determined competitive mechanism.

shown in Figure 8a, the V 2p2/3 spectra fit into three valence states: V1 (V in VN, 514.3 eV), V2 (V3+ in V2O3, 515.6 eV), and V3 (V5+ in V2O5, 517.4 eV).51 Since an N atom has a lower electronegativity than an O atom, the oxidation state of V1 was below three, indicating that there were more than two d electrons in the V1 state. According to the oxidation state, there were two and zero d electrons in V2 and V3, respectively. The large proportion of V3 in VN suggests that the average number of d electrons for the V atoms was indeed less than two. The spectra clearly indicate a much higher proportion of V1 in V0.95Co0.05N and V0.95Ni0.05N and a much lower proportion of V1 in V0.95Cu0.05N in comparison with VN. To better compare the proportional variations induced by doping with the various metals, we calculated the proportions of V1, V2, and V3 on the basis of their corresponding peak areas in the V 2p2/3 spectra. As presented in Figure 8b, in comparison with VN, V0.95Cu0.05N had a much lower proportion of V1 while V0.95Ti0.05N, V0.95Co0.05N, and V0.95Ni0.05N had a much higher proportion of V1. The higher proportion of V1 in V0.95Ti0.05N than in VN was unexpected because Ti has fewer d electrons than V, but it was reasonable given the existence of Ti4+, as shown in Figure S4a. The proportions of V2 in V0.95Cu0.05N and VN were the same, while the proportion in the other V0.95M0.05Ns was lower than in VN. The proportion of V3 in V0.95Co0.05N and VN was the same, while its proportions in the other V0.95M0.05Ns were higher than in VN. After a comprehensive comparison of the three valence states of V, we can confirm that the average

valence states of V in V0.95M0.05N followed the order V 0.95 Co 0.05 N < V 0.95 Ni 0.05 N < VN ≈ V 0.95 Ti 0.05 N < V0.95Cr0.05N ≈ V0.95Fe0.05N < V0.95Mn0.05N < V0.95Cu0.05N. These results indicate that doping metals with rich d electrons, such as Co and Ni, enriched the d electrons of the V atoms in VN significantly, whereas doping metals with half-filled and fully filled electronic structures, such as Mn and Cu, clearly decreased the d electrons of the V atoms in VN. However, according to the electronegativity of V and the doping metals, it was not reasonable for V to directly receive electrons from the doping metals, as most of the doping metals (except Ti and Mn) have electronegativity stronger than that of V. Since N has a much stronger electronegativity than V or M, it would have been more reasonable for N to gain electrons from both V and M. N would have gained fewer electrons from V if it could gain more electrons from M, and vice versa. Therefore, we suggest that the variations in the valence states of V in V0.95M0.05Ns may have been induced by an N-determined competitive mechanism, as presented in Figure 8c. When M = Cu, it is much harder for N to gain electrons from Cu than from V, since the fully filled d orbital of Cu is very stable; thus, N will take more electrons from V, resulting in further depletion of the d electrons from the V atoms. For M = Cr, Mn, Fe, there will be an encounter with the half-filled d orbital if N takes three electrons from each of these metals; thus, it will be a little harder for N to gain electrons from these metals than from V. Hence, V atoms will again be deprived of d electrons. The 6171

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V0.95Mn0.05N, and V0.95Cu0.05N also showed three strong peaks in the same temperature area. Generally, the desorption temperature is proportional to the adsorption strength. In comparison to VN, the three strong desorption peaks of V0.95Co0.05N and V0.95Mn0.05N showed a slightly positive shift, while those of V0.95Cu0.05N showed an evidently negative shift, indicating that doping with Co or Mn strengthened the adsorption of oxygen on the catalyst whereas doping with Cu weakened it. Moreover, the peak height ratio of T3/T1 in V0.95Co0.05N was much higher than that in VN, while in V0.95Cu0.05N it was much lower than that in VN, indicating that Co doping resulted in stronger adsorption centers on VN, while Cu doping had the opposite effect. That is to say, the reactivity of the inherent O2 adsorption sites of VN was enhanced by Co doping but reduced by Cu doping. Interestingly, this was quite consistent with the XPS results. It should be pointed out that the O2-TPD curves of V0.95Co0.05N, V0.95Mn0.05N, and V0.95Cu0.05N showed new peaks that did not appear in the curves for VN. This result confirms that V0.95Co0.05N, V0.95Mn0.05N, and V0.95Cu0.05N did create new O2 adsorption sites. In comparison to the strong peaks at 200−350 °C, the new peaks had a higher desorption temperature, implying that the reactivity of the newly formed O2 adsorption sites was higher than that of the inherent O2 adsorption sites of VN. This explains why V0.95Mn0.05N and V0.95Cu0.05N showed better activity than VN even though doping with Mn or Cu lowered the d electrons of V. Mn and Cu have more electrons than V, and due to their stable electronic structures, Mn and Cu tend to retain most of their electrons. Therefore, it is understandable that newly formed O2 adsorption sites involving Mn or Cu atoms would have had a higher reactivity than the inherent O2 adsorption sites of VN. Although N tends to gain more electrons from Co than from V, the abundance of d electrons in Co would explain Co-involved new adsorption sites having higher reactivity than the inherent O2 adsorption sites of VN. Thus, the remarkably enhanced activity of V0.95Co0.05N is attributable not only to an increase in the d electrons on V atoms but also to new O2 adsorption sites. According to the results from XPS and O2-TPD, the effects of Co doping are reflected in two ways: (i) enrichment of the d electrons on the V atoms and (ii) the formation of new O2 adsorption sites with high reactivity. The replacement of part of the V atoms by Co atoms is itself a mechanism for enriching the d electrons of VN. Moreover, Co doping can significantly increase the d electrons on the V atoms. Hence, Co doping dually enhances the d electrons of VN, which explains why V0.95Co0.05N outperformed the other V0.95M0.05Ns. As mentioned earlier, due to the lack of sufficient d electrons on V atoms, O2 dissociation on VN can only occur at the adsorption site formed by two V atoms and one N atom. It is possible that the enhancement of d electrons on the V atoms and the new O2 adsorption sites, both induced by Co doping, increased the number of active sites for O2 dissociation, which is very likely why O2 dissociation was more favored on V0.95Co0.05N than on VN. In summary, we have greatly enhanced the ORR activity of VN by doping with Co, which confirms that enriching the d electrons via doping elements with rich d electrons is a good strategy to enhance the ORR activity of early-transition-metal nitrides limited by insufficient d electrons. It should be mentioned that this strategy works not only for VN but also for TiN. Our group has also found another successful case in Ni-doped TiN,52 further verifying the validity of this strategy.

average valence state of V in V0.95Mn0.05N was higher than that in V0.95Cr0.05N and V0.95Fe0.05N because Mn (3d54s2) has a more stable electronic structure than either Cr (3d54s1) or Fe (3d64s2), but it should have been lower than that of V0.95Cu0.05N, since a fully filled orbital is more stable than a half-filled orbital. For M = Ti, although it is easier for N to gain electrons from Ti than from V, the average valence state of V in V0.95Ti0.05N was barely affected because V has one more electron than Ti. In the cases of M = Co, Ni, each of those metals has many more electrons than V, and there will be no encounter with the half-filled d orbital if N takes three electrons from them. Therefore, N gained more electrons from them, resulting in significant d electron enrichment on the V atoms. According to the XPS results, doping with either Co or Ni increased the number of d electrons of V in VN. However, Figures S3a and S4f in the Supporting Information show that part of the Ni did not blend into the VN lattice but instead existed as Ni3N and metallic Ni, which was probably why the ORR activity of V0.95Ni0.05N did not show much enhancement over that of VN. In contrast, we found that doping with Co significantly enriched the d electrons of V atoms, which was likely the main reason for V0.95Co0.05N showing greatly enhanced ORR activity in comparison with that of VN. Apart from enriching the d electrons, doping may also have created a new type of active site for O2 adsorption. Hence, there is a possibility that the remarkably enhanced activity of V0.95Co0.05N may also be attributed to newly formed adsorption sites. We therefore conducted O2-TPD tests to verify whether the new adsorption sites existed. In addition, to explain why V0.95Mn0.05N and V0.95Cu0.05N showed better activity than VN, even though doping with Mn or Cu lowered the number of d electrons on the V atoms, we conducted O2-TPD tests of V0.95Mn0.05N and V0.95Cu0.05N as well. As shown in Figure 9, in comparison to the flat baseline of VN, the O2-TPD curve of VN presented three strong and

Figure 9. O2-TPD curves of VN, V0.95Co0.05N, V0.95Mn0.05N, and V0.95Cu0.05N.

differentiable peaks at temperatures of 200−350 °C. The XRD pattern of VN after the O2-TPD test (Figure S6 in the Supporting Information) suggests that these peaks were not derived from the self-decomposition of VN. Therefore, we can confirm that these peaks were derived from desorption of atomic oxygen on VN and there were three types of atomic oxygen on VN. The O 2 -TPD curves of V0.95 Co0.05 N, 6172

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CONCLUSIONS We first analyzed and explored, theoretically and experimentally, the possible limitations preventing early-transition-metal nitrides from being competitive catalysts toward the ORR in terms of O2 dissociation, and we suggested that insufficient d electrons may be the main limitation for O2 dissociation on ScN, TiN, and VN, whereas an unsuitable surface geometric structure may be the main limitation for O2 dissociation on CrN. Thereafter, we took VN as an example and proposed a strategy to enhance its ORR activity by enriching its d electrons through doping with transition metals in the fourth period. It was found that Co-doped VN exhibited the most enhanced ORR activity, achieving a performance close to that of JM 20 wt % Pt/C. From XPS and O2-TPD analysis, we found that Co doping had two effectssignificant enhancement of the d electrons on V atoms and the formation of new O2 adsorption sites with high reactivityboth of which facilitate O 2 dissociation, therefore resulting in the remarkably enhanced ORR activity of Co-doped VN. Our study confirms that, for early-transition-metal nitrides with insufficient d electrons, enriching their d electrons is a good strategy to enhance their ORR activity and make them competitive catalysts, a finding that may be helpful in designing and preparing low-cost nitridebased catalysts with high ORR performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01618. Stability tests, effects of Co doping amount and annealing temperature on ORR activity, doping effects on the geometric structure, atomic contents in XPS analysis, XPS spectra of M2p and N 1s, XRD patterns after O2-TPD tests, and BET surface area of the catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S.L.: e-mail, [email protected]; fax, +86 20 87113586. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (NSFC Project Nos. 21276098, 21476088, 51302091, U1301245), Natural Science Foundation of Guangdong Province (Project No. 2015A030312007), and Educational Commission of Guangdong Province (Project No. 2013CXZDA003).

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