Communication Electrocatalytic Synthesis
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Hierarchical Cobalt Phosphide Hollow Nanocages toward Electrocatalytic Ammonia Synthesis under Ambient Pressure and Room Temperature Wenhan Guo, Zibin Liang, Junliang Zhao, Bingjun Zhu, Kunting Cai, Ruqiang Zou,* and Qiang Xu* Nitrogen is the most abundant element in our earth’s atmosphere and is an essential building block of biological molecules like amino acids and nucleotides in living organisms.[1] However, its common form of gaseous dinitrogen cannot be directly utilized unless fixed in the form of ammonia.[2] In nature, this process is conducted by nitrogenases inside a few kinds of microorganisms known as Azotobacter.[3] Efforts to imitate such a process through non-biological methods (known as artificial nitrogen fixation) have led to the establishment of modern ammonia industry and catalyst science.[4–6] Nowadays, ammonia-based nitrogen fertilizers feed more than half of the global population, and it is no exaggeration to say that ammonia industry constitutes the foundation of modern agriculture and human society.[7,8] Modern ammonia synthesis plants are highly energy and capital intensive facilities, which produce around 150 million tons of ammonia per year by the Haber-Bosch process from gaseous dinitrogen and dihydrogen under high temperature (350–600 °C) and pressure (15–40 MPa), consuming 1–2% of the world’s annual energy production.[2,9] Despite the rich nitrogen reservoir in air, the hydrogen feedstock and large amount of energy required to overcome the kinetic barrier are largely dependent on fossil fuels, leading to enormous CO2 emission.[10,11] The major hurdle of ammonia synthesis is the activation and dissociation of dinitrogen molecules. Due to the ultrastrong NN triple bond (with a bond energy of 940.95 kJ mol−1) and absence of permanent dipole, reduction of nitrogen with dihydrogen is very difficult at the cost of huge energy input even with the best known catalysts to date. Consequently, the HaberBosch reactions are generally conducted at high temperature to overcome the kinetic barriers. Due to the exothermic nature of the reaction, such unfavorable conditions for equilibrium result in the low yield of ammonia, which in turn requires high pressure to compensate the disadvantageous equilibrium position. Alternatively, nitrogen fixation via a distinct associative protoncoupled electron transfer (PCET) mechanism could take place under much milder conditions,[11] namely, room temperature and ambient pressure, thus avoiding the deviated equilibrium
Electrochemical nitrogen reduction reaction (NRR) under room temperature and ambient pressure is a promising energy- and environmental-friendly method for ammonia synthesis, which currently highly relies on the energyconsuming Haber-Bosch process with enormous CO2 emissions. This study reports the synthesis of a noble-metal-free CoP hollow nanocage (CoP HNC) catalyst from a metal-organic framework precursor through a layered-doublehydroxide intermediate, and the use as the cathode for electrochemical NRR. The 3D hierarchical nanoparticle–nanosheet–nanocage structure provides rich surface active sites for nitrogen adsorption and reduction. When applied for NRR, CoP HNC exhibits exciting performance with high Faraday efficiency at low overpotentials (7.36% at 0 V vs reversible hydrogen electrode [RHE]), and the ammonia yield rate increases exponentially at more negative potential, –1 reaching 10.78 µg h−1 mg cat at −0.4 V (vs RHE) with good selectivity (no hydrazine produced) under ambient conditions. This noble-metal-free electrocatalyst with promising performance demonstrates the unique potential of transition metal and their compounds in the field of NRR, providing new perspectives to rational catalyst design and mechanism study.
W. Guo, Z. Liang, Dr. B. Zhu, K. Cai, Prof. R. Zou Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, P. R. China E-mail:
[email protected] J. Zhao Department of Energy and Resource Engineering College of Engineering Peking University Beijing 100871, P. R. China Prof. Q. Xu Research Institute of Electrochemical Energy National Institute of Advanced Industrial Science and Technology (AIST) Ikeda, Osaka 563-8577, Japan E-mail:
[email protected] Prof. Q. Xu AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL) Yoshida, Sakyo-ku, Kyoto 606-8501, Japan The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smtd.201800204.
DOI: 10.1002/smtd.201800204
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positions, harsh reaction conditions, and related potential safety issues. Such process is chosen by nature in microorganisms[12] and also the key for artificial nitrogen fixation via photo catalytic[13–15] or electrocatalytic[16–18] approaches, producing ammonia directly from water and atmospheric nitrogen. Electrochemical synthesis of ammonia via the nitrogen reduction reaction (NRR) is especially appealing considering the following merits: i) The thermodynamic potential for nitrogen reduction is similar to that of hydrogen evolution; ii) Electrochemical processes could be conducted continuously and easily scaled with high throughput and space-time yield; iii) The energy required for electrochemical NRR could be provided by carbon-free renewable power sources like solar panels and wind turbines. However, despite previous efforts in system construction and optimization, an efficient cathode catalyst, as the essential part of electrochemical NRR process, remains to be developed. During the past decade, quite much progress has been made for electrochemical ammonia synthesis at elevated temperatures,[19] while only limited data about feasible electrochemical catalysts for NRR under room temperature were reported up to date,[20,21] most of them based on noble-metal-based systems, typically, Ru,[22,23] Au,[24–26] and Pt.[27] Recently, a few non-noblemetal catalyst systems were proposed to present noticeable electrochemical NRR activity under room temperature and ambient pressure, including Li-incorporated polyimides,[28] amorphous inorganic oxides,[29] and metal-free nitrogen-doped carbons.[30] However, all the reported materials suffer from extremely low ammonia production efficiency and yield due to the much easier kinetics of the competing hydrogen evolution side reaction. On the other hand, compared to the well-studied gas-phase ammonia synthesis, knowledge about aqueous NRR remains much limited, and great gaps between theoretical calculation and experimental data exist from time to time, making it almost impossible to predict and guide the development of new catalysts by computational methods.[11,18] For example, the theoretically promising metallic Mo catalysts only exhibited low experimental activities with Faraday efficiencies below 1%.[31] Such mismatching likely originated from the poor understanding of reaction mechanisms and interfacial processes due to insufficient experimental evidences and numbered catalyst systems. Most parts of the map are still missing, and expanding the scope of exploitable electrocatalysts becomes one of the research emphases in the current stage. First-row transition metals (typically Fe, Co, Ni) and their composites (oxides, sulfides, nitrides, carbides, phosphides, etc.) have attracted significant attention in the field of electrocatalysis.[32–35] Specifically, transition metal phosphides have presented promising performance toward toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).[36–39] Instead of homogeneous metal surface, the coordinatively unsaturated active sites on phosphide surface might be beneficial for the bonding of nitrogen-related intermediates, and their potential in electrochemical NRR should be worthy of exploration. Herein, we developed a novel cobalt phosphide nanocatalyst as a proof-of-concept demonstration toward electrochemical NRR for ammonia synthesis. Cobalt phosphide hollow nano cages (CoP HNC) assembled from ultrathin CoP nanosheets were derived from novel metal-organic framework (MOF) nanocrystals in two steps via solvothermal conversion into layered double hydroxide intermediates, followed by thermo-assisted
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phosphorization. The CoP HNCs electrocatalyst with highly exposed surface active sites and hierarchical structure exhibited encouraging activity toward electrochemical NRR in alkaline media under room temperature and ambient pressure. Complete reduction of N2 into NH3 was confirmed with no N2H4 production, and ammonia synthesis promoted by the CoP HNCs was observed to be highly potential sensitive, showing increasing production rates and decreasing Faraday efficiencies as overpotentials increased, and thus the necessity to balance both factors to achieve optimized catalytic performance. The synthesis process of CoP HNCs is schematically illustrated in Figure 1a. ZIF-67 precursors were first synthesized by a facile room-temperature precipitation method by simply mixing the methanol solutions of Co(NO3)2 and 2-methylimidazole precursors.[40,41] The phase purity of the as-prepared ZIF-67 is confirmed by powder X-ray diffraction (PXRD) patterns (Figure S1, Supporting Information), and scanning electron microscopy (SEM) images reveal that the products are nanocrystals with regular rhombic dodecahedral shape and an average size of ≈700 nm (Figure S2, Supporting Information). The ZIF nanocrystals were then subjected to solvothermal treatment in the presence of Co2+ in ethanol, and converted into cobalt layered double hydroxides (Co-LDH) hollow nanocages, confirmed by XRD (Figure S3, Supporting Information).[42] The diffraction peaks at 12.4°, 24.9°, 32.7°, and 58.3° match well with the (003), (006), (012), and (110) diffractions of a typical Co-LDH material. Transmission electron microscope (TEM) image shows the structure evolution of the solid nanocrystals into hollow nanocages assembled from ultrathin cobalt layered hydroxide nanosheets with continuous and smooth surfaces (Figure S4, Supporting Information), while the polyhedron morphology is well preserved. The Co-LDH nanocages were further converted into the final CoP HNC via thermal-assisted phosphorization by NaH2PO2 at 350 °C, and the pink intermediates turned into black phosphide powders. Figure 1b shows the XRD pattern of CoP HNC. The XRD pattern of the CoP HNCs matches well with CoP (PDF#29-0497), and the weak diffraction intensity and obvious peak broadening indicate the ultrasmall size of the CoP nanoparticles. Helium ion microscopy (HIM) was applied to study the surface morphology of CoP HNC. Compared with conventional SEM, HIM can provide better stereoscopic depth of field and higher resolution toward surface fine structures. From the large-area HIM image in Figure 1c, it can be seen that the CoP HNC presents polyhedron morphology with rough surfaces, similar to its LDH precursor. Occasionally found broken shells indicate the CoP HNC to be hollow inside (Figure S5, Supporting Information). Close inspection of a single particle at higher magnification (Figure S6, Supporting Information) more clearly shows that the particles consist of wrapped ultrathin nanosheets. TEM images further reveal the hierarchical structure of the CoP HNC. The large-area TEM image in Figure 1d clearly confirms the large particles under HIM to be hollow nanocages with thin shells, assembled from ultrathin CoP nanosheets with thickness below 10 nm. Instead of the smooth surfaces of Co-LDH nanosheets, in-plane pores are frequently found on the CoP nanosheets (Figure S7, Supporting Information). Figure 1e gives a magnified image of the edge of one nanocage, showing that the nanosheets were
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X-ray photoelectron spectroscopy (XPS) was further carried out to determine the surface state of the CoP HNC catalyst (Figure 2). As shown in Figure 2a, the highresolution Co 2P3/2 spectrum of CoP HNC can be deconvoluted into several peaks at 778.57, 781.58, and 783.16 eV, corresponding to CoP, oxidized cobalt species and cobalt hydroxides, respectively.[43] The CoP peak is very close to that of Co metal (≈778.2 eV), indicating its near-metallic nature, while the positively shifted peak position implies the partial charge transfer from cobalt to phosphorus. The high-resolution P 2p spectrum of CoP HNC (Figure 2b) consists of component peaks at 129.25 and 129.94 eV, attributing to P 2p3/2 and P 2p1/2 in cobalt phosphide, respectively, together with peaks at 131.72 and 134.37 eV corresponding to oxidized phosphorus species of higher valence. The existence of oxidized cobalt and phosphorus peaks is due to the exposure of highly active phosphide surface to atmospheric oxygen and moisture.[39] Electrochemical NRR tests are carried out to determine the nitrogen reduction ability of CoP HNC catalyst. Reactions are conducted in a custom-made gas-tight two-compartment H-typed electrolysis cell with circulating water bath at the cathode compartment for temperature control, while the two compartments are separated by a proton-conductive Nafion membrane. An additional adsorption bottle filled with diluted sulfuric acid as adsorbent is connected to the end of reactor to collect any ammonia undissolved in the electrolyte. Constant N2 gas flow is bubbled into the aqueous electrolyte as reaction feedstock in the cathode compartment at ambient pressure and room temperature, and alkaline electrolyte (1.0 m KOH) is chosen to suppress the HER side Figure 1. a) Schematic for the synthesis procedure of CoP HNC; b) XRD, c) HIM image and d) TEM image of CoP HNC; e) HRTEM image of an edge of the nanosheet in CoP HNC. White reaction. During reaction, N2 is reduced with dotted circles show CoP nanoparticles with different lattice orientations. Inserted shows a magthe assistant of electrons transferred from nified region with typical CoP lattice fringes. the cathode electrode loaded with CoP HNC electrocatalyst and protons provided from the H2O molecules in aqueous electrolyte, while OER takes place at assembled from accumulated ultrasmall (
