Identifying Catalytic Active Sites of Trimolybdenum Phosphide (Mo₃P) for Electrochemical Hydrogen Evolution

Abstract

candidates to replace precious metal catalysts owing to the unique electronic properties of their edge structures. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] However, the basal plane of these catalysts containing substantial amount of their surface structure remains nearly inactive making these catalysts inefficient, especially for practical applications. [26][27][28][29] Therefore, designing and developing a new class of nonprecious metal catalysts with an increased intrinsic activity that concurrently hold a high number of active sites still remain as a challenging task in the field of electrocatalysis.Here, we have found nanostructured trimolybdenum phosphide (Mo 3 P) as a novel material for solid-state electrocatalytic reactions owing to its high density of active sites with outstanding electronic properties. We have tested the performance of this catalyst in the electrochemical hydrogen evolution reaction (HER) and compared with Pt, known as the best HER catalyst. Cyclic voltammetry (CV) and in situ differential electrochemical mass spectroscopy (DEMS) results illustrate onset potential of as low as 21 mV that is the closest value to Pt yet reported. [30] The onset potential is also seven, four, and three times lower than other recently developed nonprecious metal catalysts, i.e., MoS 2 , Mo 2 C, and molybdenum phosphide (MoP) nanoflakes (NFs), respectively. [31,32] The turnover frequency (TOF) measurements, actual activity of surface atoms, at 150 mV overpotential also offer 21.5-fold activity improvement for the Mo 3 P catalyst than that of MoS 2 NFs. Atomic-scale characterization Solid-state electrocatalysis plays a crucial role in the development of renewable energy to reshape current and future energy needs. However, finding an inexpensive and highly active catalyst to replace precious metals remains a big challenge for this technology. Here, tri-molybdenum phosphide (Mo 3 P) is found as a promising nonprecious metal and earthabundant candidate with outstanding catalytic properties that can be used for electrocatalytic processes. The catalytic performance of Mo 3 P nanoparticles is tested in the hydrogen evolution reaction (HER). The results indicate an onset potential of as low as 21 mV, H 2 formation rate, and exchange current density of 214.7 µmol s −1 g −1 cat (at only 100 mV overpotential) and 279.07 µA cm −2 , respectively, which are among the closest values yet observed to platinum. Combined atomic-scale characterizations and computational studies confirm that high density of molybdenum (Mo) active sites at the surface with superior intrinsic electronic properties are mainly responsible for the remarkable HER performance. The density functional theory calculation results also confirm that the exceptional performance of Mo 3 P is due to neutral Gibbs free energy (ΔG H *) of the hydrogen (H) adsorption at above 1/2 monolayer (ML) coverage of the (110) surface, exceeding the performance of existing non-noble metal catalysts for HER.

Multifunctional Transition Metal‐Based Phosphides in Energy‐Related Electrocatalysis

“…i) HER volcano plot obtained from plotting exchange current density concerning Gibbs free energy of adsorbed hydrogen on various samples. Reproduced with permission . Copyright 2019, John Wiley and Sons.…”

“…Nevertheless, when we jump to the conclusion that lower M/P stoichiometric ratios in TMPs are better for HER, this issue becomes controversial again. More recently, Asadi and co‐workers reported that tri‐molybdenum phosphide (Mo 3 P) nanoparticles presented a better HER activity than that of phosphorus‐rich MoP from practical experiment and theoretical calculation . Typically, Mo 3 P adopted the tetragonal structure with each Mo atom is coordinated by either two or four P atoms, while MoP crystallized in hexagonal structure with Mo six‐coordinated by P atoms.…”

“…More recently, Asadi and co-workers reported that tri-molybdenum phosphide (Mo 3 P) nanoparticles presented a better HER activity than that of phosphorus-rich MoP from practical experiment and theoretical calculation. [134] Typically, Mo 3 P adopted the tetragonal structure with each Mo atom is coordinated by either two or four P atoms, while MoP crystallized in hexagonal structure with Mo six-coordinated by P atoms. Caused by variation in structure, in Figure 9g, Mo 3 P nanoflakes displayed remarkable electrocatalytic HER behavior with requiring 69 mV overpotential to support a current density of 10 mA cm −2 , superior to that of MoP counterpart (130 mV@10 mA cm −2 ).…”

Kinetically Stable Oxide Overlayers on Mo3P Nanoparticles Enabling Lithium–Air Batteries with Low Overpotentials and Long Cycle Life

“…Recently, we have demonstrated that trimolybdenum phosphide (Mo 3 P) nanoparticles can be a promising catalyst candidate, owing to its unique structure; its surface provides a high density of molybdenum (Mo) active sites with special electronic properties, that is, low work function and high density of d‐orbital electrons at Fermi energy, which can be promising for electrocatalysis. [ 21 ] The important role of surface reconstructions in catalysis, which renders the surface region of the catalyst atomically and geometrically distinct relative to the bulk, has been pointed out. [ 22,23 ] A special type of surface reconstructions that can happen in an oxygen‐rich environment is the formation of ultrathin oxide overlayers.…”

Atomic-Level Functionalized Graphdiyne for Electrocatalysis Applications

“…The ORR is of pivotal significance for fuel cells and metal-air batteries. Similarly, the HER through electrocatalytic water splitting is considered as a promising approach to the production of clean hydrogen [112][113][114]. Recently, several research groups have studied the metal-atom-anchored GDY catalysts for HER electrocatalysis applications with the support of theoretical calculations.…”