The efficient interconversion of chemicals and electricity through electrocatalytic processes is central to many renewable-energy initiatives. The sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER)1,2,3,4 has long posed one of the biggest challenges in this field, and electrocatalysts based on expensive platinum-group metals are often required to improve the activity and durability of these reactions. The use of alloying5,6,7, surface strain8,9,10,11 and optimized coordination environments12 has resulted in platinum-based nanocrystals that enable very high ORR activities in acidic media; however, improving the activity of this reaction in alkaline environments remains challenging because of the difficulty in achieving optimized oxygen binding strength on platinum-group metals in the presence of hydroxide. Here we show that PdMo bimetallene—a palladium–molybdenum alloy in the form of a highly curved and sub-nanometre-thick metal nanosheet—is an efficient and stable electrocatalyst for the ORR and the OER in alkaline electrolytes, and shows promising performance as a cathode in Zn–air and Li–air batteries. The thin-sheet structure of PdMo bimetallene enables a large electrochemically active surface area (138.7 square metres per gram of palladium) as well as high atomic utilization, resulting in a mass activity towards the ORR of 16.37 amperes per milligram of palladium at 0.9 volts versus the reversible hydrogen electrode in alkaline electrolytes. This mass activity is 78 times and 327 times higher than those of commercial Pt/C and Pd/C catalysts, respectively, and shows little decay after 30,000 potential cycles. Density functional theory calculations reveal that the alloying effect, the strain effect due to the curved geometry, and the quantum size effect due to the thinness of the sheets tune the electronic structure of the system for optimized oxygen binding. Given the properties and the structure–activity relationships of PdMo metallene, we suggest that other metallene materials could show great promise in energy electrocatalysis.
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The data that support the findings of this study are available from the corresponding author on reasonable request.
The Vienna ab initio simulation package (VASP) used for the DFT calculations is available at https://www.vasp.at. The LAMMPS package used for the classical molecular dynamics simulations is available under a GNU Public License at https://lammps.sandia.gov.
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This work was financially supported by the National Key R&D Program of China (number 2016YFB0100200), the National Natural Science Foundation of China (number 51671003), the Beijing Natural Science Foundation (number JQ18005), the BIC-ESAT project, the China Postdoctoral Science Foundation (number 2017M610022) and the Young Thousand Talents Program. The work at California State University Northridge was supported by the National Science Foundation PREM programme (DMR-1828019). The electron microscopy work used resources of the Center for Functional Nanomaterials, which is a US Department of Energy Office of Science Facility, at Brookhaven National Laboratory under contract number DE-SC0012704.
The authors declare no competing interests.
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Peer review information Nature thanks Gu-Gon Park and Yun Zong for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Characterization of PdMo bimetallene.
a, b, Representative HRTEM images of individual PdMo bimetallene nanosheets after exposure to the electron beam for 1 s (a) and 5 s (b). The observation of ‘holes’ after only a few seconds indicates the ultrathin nature of PdMo bimetallene. c, d, TEM (c) and HRTEM (d) images of the cross-section. e, TEM–EDX spectrum of PdMo bimetallene. f, The k3-weighted χ(k)-function of the EXAFS spectra for PdMo bimetallene (blue), Pd metallene (red) and Pd foil (black). The inset shows the results of EXAFS data fitting for different samples. The Pd–Pd(Mo) distance in the metallene samples is greater than that in the Pd foil, which suggests an expanded lattice for suprathin metallene materials; this is in good agreement with the results of XRD experiments and molecular dynamics simulations. g, h, The flat surface of PdMo bimetallene (g) is transformed to a wavy surface (h) after 400 ps in the molecular dynamics simulation. The inset shows an enlarged view of the centre of the nanosheet. i, Mo 3d core-level XPS spectra of the Pd metallene/C and PdMo bimetallene/C catalysts. j, Cu 2p core-level XPS spectra of PdMo-Cu and the Cu reference. The absence of Cu in the PdMo-Cu trace (blue) indicates there was no galvanic replacement of Mo with Cu2+, confirming that Mo is located inside the metallene. k, The ∆EO of PdMo, PdW, PdCu and PdNi as a function of charge transfer from other metals to Pd. PdMo bimetallene is the most active among all the PdM (M = Cu, Ni, Mo and W) materials tested, owing to the optimal charge transfer from Mo to the surface Pd atoms.
Extended Data Fig. 2 Key characteristics of the synthesis.
a, b, TEM image (a) and corresponding TEM-EDX spectrum (b) of the products obtained under the same reaction conditions as for PdMo bimetallene but in the absence of ascorbic acid. The absence of molybdenum in the as-obtained product suggests that ascorbic acid is crucial for the formation of the PdMo alloy. c, d, TEM images of the products obtained under the same reaction conditions as for PdMo bimetallene but in the absence of Mo(CO)6. This observation confirms the essential role of CO molecules in the synthesis of ultrathin 2D nanostructures. e–h, TEM images of the products obtained under the same reaction conditions as for PdMo bimetallene except for the use of 8 mg Mo(CO)6 (e, f) and 2 mg Mo(CO)6 (g, h). i, j, TEM images of the products obtained under the same reaction conditions as for PdMo bimetallene, but with three times the concentration of metal precursors. The observations suggest that this approach can be used to tune the lateral size of the nanosheet. k, l, TEM image (k) and corresponding TEM–EDX spectrum (l) of the products obtained under the same reaction conditions as for PdMo bimetallene, except for the replacement of Mo(CO)6 with W(CO)6. m, n, Photographs of the colloids from a scale-up (40 times) synthesis of PdMo bimetallene before (m) and after (n) reaction. o, Typical TEM image of PdMo bimetallene from the scale-up synthesis.
Extended Data Fig. 3 Growth mechanism of PdMo bimetallene.
a–h, Typical TEM images of PdMo bimetallene products collected after reaction times of 10 min (a, b), 20 min (c, d), 1 h (e, f) and 12 h (g, h). i, Changes in the atomic composition of PdMo bimetallene, determined by TEM–EDX and ICP–AES. j, UV–visible absorption spectra of PdMo bimetallene obtained at different reaction times. k, Photograph of a cyclohexane dispersion of the as-prepared PdMo bimetallene collected at 12 h. l, Synthetic scheme for the production of PdMo bimetallene. At the early stage of synthesis (10 min), the product contained nanocrystals with dimensions of only a few nanometres, with a definite 2D morphology. From 10 min to 20 min, the size of the nanocrystals gradually increased to tens of nanometres—the ICP–AES and TEM–EDX spectra revealed negligible amounts of molybdenum in these two intermediates, indicating these initial 2D nanocrystals were composed of pure palladium. As the reaction time increased to 60 min, curved nanosheets—with dimensions of hundreds of nanometres—were formed, accompanied by an obvious increase in molybdenum content. This suggests the rapid growth of palladium nanosheets and the slow incorporation of molybdenum during this period. When the reaction time was increased to 12 h, there was no substantial change in the dimensions of the nanosheets, suggesting that most of the palladium precursors were consumed in the first 60 min. The content of molybdenum, however, rapidly increased to around 12%, indicating the incorporation and interdiffusion of molybdenum atoms into the metallene.
Extended Data Fig. 4 Electrochemical properties of various catalysts.
a, b, CVs of commercial Pt/C, Pd metallene/C and PdMo bimetallene/C catalysts in 0.1 M HClO4 (a) and 0.1 M KOH (b). Scan rate, 50 mV s−1; Pt (or Pd) loading, 7.5 mg per cm2 geometric area. c, d, CVs of commercial Pd/C catalyst in c, 0.1 M HClO4 and (c) 0.1 M KOH (d). Scan rate, 50 mV s−1; Pt (or Pd) loading, 15 mg per cm2 geometric area. e–h, CO stripping voltammograms of commercial Pt/C, commercial Pd/C, Pd metallene/C and PdMo bimetallene/C catalysts. The voltammograms were conducted in 0.1 M HClO4. Scan rate, 20 mV s−1. The integrated charge of CO stripping is used to calculate the ECSA of each catalyst. i–k, CVs and Cu stripping voltammograms of commercial Pd/C, Pd metallene/C and PdMo bimetallene/C catalysts before and after 15,000 and 30,000 potential cycles. The CVs were conducted in 0.05 M H2SO4, whereas the Cu stripping voltammograms were conducted in 0.05 M H2SO4 + 2 mM CuSO4. Scan rate, 20 mV s−1. The integrated charge of adsorbed Cu is used to calculate the ECSA of each catalyst before and after durability tests. l–n, CVs of commercial Pd/C, Pd metallene/C, and PdMo bimetallene/C catalysts in N2-saturated 0.1 M HClO4 solution. Scan rate, 50 mV s−1. The integrated charge of oxide reduction is used to calculate the ECSA of each catalyst. o, ORR polarization curves of PdMo bimetallene/C in O2-saturated 0.1 M KOH at different rotation rates. Scan rate, 20 mV s−1. p, The corresponding Levich plots at different applied potentials.
Extended Data Fig. 5 Comparison of the performance of PdMo bimetallene/C with control samples and state-of-the-art catalysts from literature.
a–h, TEM images (a, c, e, g) and ORR curves and CVs (inset) (b, d, f, h) of PtPb nanoplates (a, b), hierarchical PtNi nanowires (h-NWs; c, d), ultrathin PtNi nanowires (u-NWs; e, f), and PVP-capped Pd nanosheets (g, h). All CVs were recorded in N2-saturated 0.1 M KOH at 50 mV s−1, and the ORR polarization curves were recorded in O2-saturated 0.1 M KOH at a scan rate of 20 mV s−1 and a rotation rate of 1,600 rpm. i, The table shows the ORR performance of PdMo bimetallene/C, in comparison with control catalysts in this study and other reported high-performance ORR catalysts in alkaline electrolytes.
Extended Data Fig. 6 ORR performance of various catalysts in acidic media.
a–c, ORR polarization curves (a), corresponding Tafel plots (b) and mass- and specific activities (c) of commercial Pd/C, commercial Pt/C, Pd metallene/C and PdMo bimetallene/C catalysts. The dashed line in c and the purple square in b represent the 2020 technical target for mass activity set by the US Department of Energy. d, ORR polarization curves of PdMo bimetallene/C in O2-saturated 0.1 M HClO4 at different rotation rates. e, The corresponding Levich plots at different applied potentials. The ORR polarization curves were recorded in O2-saturated 0.1 M HClO4 at a scan rate of 20 mV s−1. The error bars in c represent the standard deviations of at least three independent measurements of the same sample. The mass activity of PdMo bimetallene/C reaches 0.66 A per mg PGMs at 0.9 V versus RHE, around 3 and 10 times higher than those of commercial Pt/C and Pd/C, respectively, also exceeding the 2020 technical target set by the US Department of Energy (0.44 A per mg PGMs, indicated by the purple square in b. f, ORR polarization curves of PdMo bimetallene/C before and after AST for 1,000 cycles in O2-saturated 0.1 M HClO4.
Extended Data Fig. 7 Morphology changes of various catalysts after AST.
a–l, Representative TEM images of commercial Pt/C catalyst (a–d), commercial Pd/C catalyst (e–h), and PdMo bimetallene/C catalyst (i–l) before (a, c, e, g, i, k) and after (b, d, f, h, j, l) 30,000 electrochemical cycles between 0.6 V and 1.0 V versus RHE, in O2-saturated 0.1 M KOH at a scan rate of 50 mV s−1.
Extended Data Fig. 8 OER electrocatalysis and performance of the Zn–air battery.
a, OER polarization curves of various catalysts in O2-saturated 0.1 M KOH at a scan rate of 5 mV s−1. b, OER polarization curves of PdMo bimetallene and commercial IrO2 before and after 500 or 1,000 cycles by cycling between 1.0 and 1.8 V versus RHE at a scan rate of 20 mV s−1. c, Scheme of the aqueous Zn–air battery. d, LED panel (2.5–3 V) powered by two series-connected Zn–air batteries. e, An open-circuit voltage of 1.483 V was obtained for aqueous Zn–air batteries with PdMo bimetallene as air cathode. f, g, Polarization and corresponding power density curves (f) and consumed Zn mass-normalized specific capacities (g) of Zn–air batteries with PdMo bimetallene/C and IrO2 + Pt/C as the cathodic catalysts at a current density of 20 mA cm−2. h, Discharge curves of two Zn–air batteries at current densities of 10, 20, 50 and 100 mA cm−2. i, Charge–discharge profiles of two Zn–air batteries at a current density of 10 mA cm−2 with each cycle being 20 min, corresponding to a shallow discharge depth. j, Galvanostatic charge–discharge cycling of a Zn–air battery using PdMo bimetallene/C as the air cathode at a charge density of 10 mA cm−2 with each cycle being 40 h, corresponding to a DOD of 51.0% (Methods; Zn wire as reference electrode). k, Full discharge profile (voltage versus hours of discharge) for a Zn–air battery using PdMo bimetallene/C as the air cathode at a current density of 10 mA cm−2. l, m, Cycling stability of a Zn–air battery using PdMo bimetallene/C as the air cathode operating at current densities of 50 mA cm−2 (l) and 75 mA cm−2 (m) with each cycle being 4 h (Zn wire as reference electrode). n, TEM image of PdMo bimetallene/C after an operation period as the cathode of a Zn–air battery of 500 h.
Extended Data Fig. 9 Performance of the Li–air battery.
a, Schematic of the configuration of the aprotic Li–air battery. A carbon paper coated with PdMo bimetallene/C, commercial Pt/C, RuO2/Ketjen black or graphite carbon, a lithium plate, and 1 M LiTFSI in TEGDME were used as the cathodes, anode and electrolyte, respectively. b, The first discharge–charge profiles of Li–air batteries with various air electrodes recorded at a current density of 200 mA g−1 and a limited capacity of 1,000 mAh g−1. c, The cycling performance of Li–air batteries with various air electrodes at a current density of 100 mA g−1 and a limited capacity of 500 mAh g−1.
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Luo, M., Zhao, Z., Zhang, Y. et al. PdMo bimetallene for oxygen reduction catalysis. Nature 574, 81–85 (2019). https://doi.org/10.1038/s41586-019-1603-7
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