(Invited) Electro- and Photoelectro- Catalysts Obtained by Pyrolysis of Bimetallic Amorphous Metal-Organic Frameworks

Abstract

The rising world population and industrialization are predicted to provoke a global power demand of 28 TW and a CO2 emission of 45 Gt by 2040.[1] Proton exchange membranes fuel cells (PEMFCs) and electrochemical water splitting have attracted wide attention to face with the demand of energy, the dwindling of fossil fuels and the deterioration of the environment. In PEMFCs, hydrogen is oxidized at the anode (hydrogen oxidation reaction = HOR) and oxygen is reduced at the cathode (oxygen reduction reaction = ORR), with water as the major product. In the opposite process, the hydrolysis of water produces hydrogen and oxygen at the cathode (hydrogen evolution reaction = HER) and anode (oxygen evolution reaction = OER), respectively.[2] Under operating conditions, both efficiency and conversion rate are hindered by activation barriers, which result in overpotentials. Platinum group metals (PGM) and their derivatives have shown high catalytic capacity. However, the industrial applications of these catalysts are restricted by their scarcity and cost. Consequently, efforts have been made to design alternative, cheap and active catalysts to make the hydrogen economy closer to reality.[3] The present study describes an approach toward the general synthesis of bimetallic amorphous metal-organic frameworks (aMOFs) to subsequently prepare mono- or multimetallic electro- and photoelectro- catalysts. It is worth mentioning that aMOFs are starting to emerge as alternative materials, beyond the dictatorial domain of crystalline MOFs. The aforementioned approach is based on merging our liquid-liquid interface synthesis method,[4] together with the metalloligand approach.[5] As prove of concept, we have synthesized and characterized a family of bimetallic aMOFs: NEU-5 (= [Zn(FeTpyCOOH)(PF6)2]n), NEU-6 (= [Zn(Ru(terpy*)2)(PF6)2]n), NEU-7 (= [Fe(Ru(terpy*)2)(PF6)2]n) and NEU-8 (= [Ti(Ru(terpy*)2)(PF6)2]n). Electro- and photoelectro- catalysts based on the controlled pyrolysis of these frameworks have been subsequently developed for efficient catalysis toward HER, OER and ORR in both acidic and alkaline media. Thus, functional Fe2P@PNDCN, RuP@PNDCN, Fe3O4/RuO2@NEU-7 and Ru2O/TiN/TiO2@NEU-8 have been fabricated by controlled pyrolysis of NEU-5, NEU-6, NEU-7 and NEU-8, respectively. As a catalyst electrode for ORR, the half-wave potentials of Fe2P@PNDCN have been 0.78, 0.78 and 0.91 V in 0.5 M H2SO4, 0.1 M HClO4 and 1 M KOH, respectively. The as-obtained RuP@PNDCN has exhibited excellent HER activity. RuP@PNDCN has shown a very low overpotential of 65 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4. RuP@PNDCN have been also active toward the HER in 1 M KOH, displaying a low overpotential of 74 mV at a catalytic current density of 10 mA cm-2. Fe3O4/RuO2@NEU-7 has exhibited outstanding electrocatalytic activity upon the OER with overpotentials of only 250 and 450 mV to reach 10 mA cm-2 current density in 1 M KOH and 0.5 M H2SO4, respectively. In acid conditions at a catalytic current density of 10 mA cm-2, while as-synthesized Ru2O/TiN/TiO2@NEU-8 has exhibited an overpotential of 67 mV, Ru2O/TiN/TiO2@NEU-8 excited under blue light irradiation has promoted HER and displayed an ultralow overpotential of 33 mV.[2] Therefore, this work presents a synthesis strategy to develop bimetallic aMOFs, which can be subsequently pyrolyzed to fabricate electro- and photoelectro- catalysts. We anticipate that great improvements in the field of catalysis will be achieved by the rational synthesis of new bimetallic aMOFs.