Energy Related Electrocatalysis

Theoretical design of effective catalysts based on abundant elements for sustainable energy generation. Pt-free catalysts for fuel cell technology.

Development of effective and environment friendly catalysts based on abundant elements for energy and environment related applications is emerging task. Instead of investigation of the well-known and well-studied catalytic materials based on the precious metals, like Pt, Pd, Ru, etc. we are keen to study how to functionalize abundant catalytically inactive or even completely inert nanomaterials and control their catalytic properties (activity, selectivity) via support design and morphology. The exciting example demonstrating credibility of the proposed approach is our recent progress on theoretical prediction of functionalization of the hexagonal boron nitride (h-BN) based nanomaterials for the oxygen reduction reaction (ORR). The ORR is a key process that allows fuel cells to operate. Currently the most efficient catalysts for ORR are based on precious metals, such as platinum. The relatively low efficiency of the known ORR catalysts, voltage losses at the cathode, the high cost and limited resources of platinum prevent the wide use of fuel cells in practical applications. We have demonstrated absolutely novel and fascinating effect: an inert h-BN monolayer can be functionalized by the nitrogen doping or by the metal support and become catalytically active for ORR. The energetics of adsorption of ORR intermediates, such as, O₂, O, OH, OOH, and H₂O on N-doped h-BN monolayer is quite similar to that known for a Pt(111) surface. Analysis of the free energy changes along the ORR pathway allows us to suggest that a N-doped h-BN monolayer can demonstrate catalytic properties for the ORR under the condition that electron transport to the catalytically active center is provided. The Ni(111) support can critically change the chemical and physical properties of defect-free monolayer h-BN, considerably promoting the adsorption of ORR intermediates, and therefore, h-BN/Ni(111) system can be catalytically active for the ORR. The described effect occurs due to the mixing of the dz₂ orbitals of the transition metal support with the N-p_z and B-p_z orbitals of h-BN. Although simple potential-dependent modeling of the energetics of the ORR on h-BN/Ni(111) indicates the limitation of the ORR process due to the large overpotential, our calculations demonstrate principal ability to functionalize inert materials for the ORR and open new ways to design effective precious metal free catalysts based on materials never been considered as catalysts before. These theoretical predictions initiated an intensive experimental investigation of the described phenomena in the group of Professor K. Uosaki (MANA, NIMS, Japan).

Figure: Proton attack of the activated O₂ adsorbed on the N-doped h-BN monolayer (left). Functionalization of monolayer h-BN by the metal support for the oxygen reduction reaction (middle). Free energy diagram for the ORR on h-BN/Ni(111) (right).

Recently we have successfully demonstrated a new approach toward a non-precious metal oxygen reduction catalyst for fuel cells. In collaboration with the group of Professor Kohei Uosaki at the National Institute for Materials Science (NIMS), Tsukuba, Japan we have advocated and successfully demonstrated a theory that when boron nitride (BN), which is originally an insulating material, is placed on a gold surface, it can function as an electrocatalyst for ORR. We have discovered that when BN is placed on a gold surface, its electronic state changes in such a way that BN can function as an oxygen-reduction catalyst. In the group of Prof. Uosaki various types of BN (e.g. nanosheets, nanotubes) were placed on a gold surface, and examined towards their activity for the oxygen-reduction reaction by a rotating disk electrode. They observed a maximum of about 270 mV positive shift for oxygen reduction current to be observed at the gold electrode. On the other hand, no such catalyst activity was observed when a carbon was used as the substrate. Thus, we have demonstrated that BN-gold interaction is a key factor for BN to function as an electrocatalyst for the ORR. Although the new catalyst is still less reactive than platinum, we succeeded in showing an extremely promising direction in the process of searching for and designing a new catalyst material, through the combination of theoretical calculation and experiments. This approach is expected to lead to the future development of materials for an electrode for fuel cells without using platinum.

Figure: Modification of the electronic structure of h-BN monolayer after its deposition on the Au(111) surface (top); adsorption of O₂ on the terrace of h-BN/Au(111) and free energy diagram for the ORR on h-BN/Au(111) (middle); adsorption of O₂ at the edge of the small boron nitride island on Au(111) surface.

Figure: The current response due to the oxygen reduction reaction as a function of potential observed by the rotating disk electrode in the oxygen-saturated sulfuric acid aqueous solution: (i) gold electrode; (ii) gold-BN nanotube electrode; (iii) gold-BN nanosheet electrode; (iv) carbon electrode; (v) carbon-BN nanosheet electrode.

We have proposed a simple concept for obtaining catalysts from inert and hence stable materials by forming their heterojunctions, namely, covering inert Au with corrugated two-dimensional carbon−nitrogen-based (2DCN) porous frameworks. It shows more than 10 times better activity for HER than for the pure Au surface, and it also demonstrates the high catalytic activity for the ORR via an effective 4-electron reduction mechanism, which is different from the usual 2-electron reduction typical for ORR on Au surfaces. This activity induced by formation of a heterojunction was analyzed by a conjugation of computational and experimental methods and found to originate from alternative efficient reaction pathways at heterojunction. It provides not only the method for creating active surface but also the knowledge on elementary steps of such complicated multielectron transfer reactions, thereby leading to intriguing strategies for developing energy conversion reactions based on materials which had never been considered as catalysts before.

Figure: Promotion of the 4-electron ORR pathway at heterojunction of 2DCN and inert Au surface.

Relevant publications

• • A. Lyalin, K. Uosaki, and T. Taketsugu, Oxygen Reduction Reaction Catalyzed by Small Gold Cluster on h-BN/Au(111) Support, Electrocatalysis, (2017).

  • K. Sakaushi, A. Lyalin, S. Tominaka, T. Taketsugu, and K. Uosaki, Two-Dimensional Corrugated Porous Carbon-, Nitrogen-Framework/Metal Heterojunction for Efficient Multi-Electron Transfer Processes with Controlled Kinetics, ACS Nano, 11, 1770–1779 (2017).

  • K. Uosaki, G. Elumalai, H. C. Dinh, A. Lyalin, T. Taketsugu, and H. Noguchi, Highly Efficient Electrochemical Hydrogen Evolution Reaction at Insulating Boron Nitride Nanosheet on Inert Gold Substrate, Scientific Reports 6, 32217 (2016).

  • A. Lyalin, M. Gao, and T. Taketsugu, When Inert Becomes Active: A Fascinating Route for Catalyst Design, The Chemical Record 16, 2324–2337 (2016).

  • M. Gao, M. Adachi, A. Lyalin, and T. Taketsugu, Long Range Functionalization of h-BN Monolayer by Carbon Doping, J. Phys. Chem. C 120, 15993-16001 (2016).

  • G. Elumalai, H. Noguchi, A. Lyalin, T. Taketsugu, and K. Uosaki, Gold Nanoparticle Decoration of Insulating Boron Nitride Nanosheet on Inert Gold Electrode Towards an Efficient Electrocatalyst for the Reduction of Oxygen to Water, Electrochemistry Communications 6, 53-57 (2016).

  • K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction: theoretical suggestion and experimental proof, J. Am. Chem. Soc. (Communication), 136, 6542−6545 (2014).

  • A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Adsorption and catalytic activation of the molecular oxygen on the metal supported h-BN, Topics in Catalysis, 57, 1032-1041 (2014).

  • A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Functionalization of monolayer h‑BN by a metal support for the oxygen reduction reaction, J. Phys. Chem. C 117, 21359-21370 (2013).

  • A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Theoretical predictions for hexagonal BN based nanomaterials as electrocatalysts for the oxygen reduction reaction, Phys. Chem. Chem. Phys. 15, 2809–2820 (2013).