Nonprecious Metal Borides: Emerging Electrocatalysts for Hydrogen Production

Abstract

ConspectusThe development of highly active noble-metal-free catalysts for the hydrogen evolution reaction (HER) is the focus of current fundamental research, aiming for a more efficient and economically affordable water-splitting process. While most HER catalysts are studied only at the nanoscale (small particle size and high surface area), metal borides (MBs) are mostly studied in bulk form. This offers a unique opportunity for designing highly efficient and nonprecious HER MBs electrocatalysts based on structure-activity relationships, especially because of their rich compositional and structural diversity.In this Account, we focus on the importance of boron and its substructures in achieving extraordinary HER performances and the importance of using structure-activity relationships to design next-generation MBs electrocatalysts. Studying the Mo-B system, we found that the HER activity of molybdenum borides increases with increasing boron content: from Mo2B (no B-B bonds in the structure, least active) to α-MoB and β-MoB (zigzag boron chains, intermediate activity) and MoB2 (planar graphene-like boron layer, most active). Density functional theory (DFT) calculations have shown that the (001) boron layer in hexagonal MoB2 (α-MoB2) is the most active surface and has similar HER activity behavior like the benchmark Pt(111) surface. However, puckering this flat boron layer to the chair-like configuration (phosphorene-like layer) drastically reduces its activity, thereby making the rhombohedral modification of MoB2 (Mo2B4 or β-MoB2) less active than α-MoB2. This discovery was then further supported by studies of the Mo-W-B system. In fact, the binary WB2, which also contains the puckered boron layer, is also less active than α-MoB2, despite containing the more active transition metal W, which performs better in elemental form than Mo. To design a superior catalyst, the more active W was then stabilized in the hexagonal α-MoB2 structure through the synthesis of α-Mo0.7W0.3B2 ,which indeed proved to be a better HER electrocatalyst than α-MoB2. Using the isoelectronic Cr instead of W led to the α-Cr1-xMoxB2 solid solution, the HER activity of which followed unexpected canonic-like (or volcano-like) behavior that perfectly matched that of the c lattice parameter trend, thereby producing the best catalyst α-Cr0.4Mo0.6B2 that outperformed Pt/C at high current density, thus underscoring the effectiveness of the structure-activity concept in designing highly active catalysts. This concept was further applied to the V-B system, leading to the discovery of an unexpected boron chain dependency of the HER activity that ultimately led to the prediction of new VxBy catalysts and their crystal structures and overpotentials. Finally, reducing the particle sizes of all of these bulk crystalline catalysts is also possible and offers an even greater potential as demonstrated for nanoscale a-MoB2 and VB2. Nevertheless, these crystalline nanomaterials are still highly agglomerated due to the high temperature required for their synthesis, thus the synthesis of highly dispersed MBs is an urgent goal that will enable the fulfillment of their extraordinary potential in the future.