Investigation of 2D Boridene from First Principles and Experiments

Recently, a 2D metal boride - boridene - has been experimentally realized in the form of single-layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2. The chemical formula of the boridene was suggested to be Mo4/3B2-xTz, where Tz denotes surface terminations. Here, the termination composition and material properties of Mo4/3B2-xTz from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo4/3B2-xTz is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo1.33B1.9O0.3(OH)1.5F0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo4/3B2-xTz shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.Funding agencies: The Knut and Alice Wallenberg Foundation (KAW 2020.0033), The Swedish Foundation for Strategic Research (EM16-0004 and ARC19-0026), The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No 2009 00971), The Swedish Research Council (no. 2018-03927 and 2019-04233). The calculations were carried out using supercomputer resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC) and the PDC Center for high-performance computing partially funded by the Swedish Research Council through grant agreement no. 2018-05973.</p

Fundamental insight into enhanced activity of Pd/CeO2 thin films in hydrogen oxidation reaction in alkaline media

Palladium supported on ceria (Pd/CeO2) has recently raised strong interest as an alternative catalyst to platinum on the anode electrode in anion exchange membrane fuel cells. Herein, we provide new insight into the enhanced activity of Pd/CeO2 in hydrogen oxidation reaction (HOR) in alkaline media. Using well-defined model thin films, we show that Pd/CeO2 thin films lead to enhanced activity in HOR compared to pure Pd thin films. In situ characterization using electrochemical quartz crystal microbalance provide in-depth understanding of the role of CeO2. CeO2 leads to fundamental differences in adsorption and absorption of key reaction intermediates during HOR. In combination with characterization and theoretical calculations, Pd atoms embedded in CeO2 are shown to be present on the prepared thin films and active for hydrogen activation but are not able to bind CO during CO-stripping characterization. Finally, an estimation of the source of hydroxyl intermediates provided by CeO2 - which could be directly participating in the reaction - is presented herein. Fundamental understanding of the Pd-CeO2 interface in HOR opens new ways to reduce the amount of noble metals in alkaline fuel cells