Abstract:

Hydrogen energy is a potential energy storage carrier due to its advantages of cleanliness, high efficiency and renewability. Electrocatalytic water splitting is an ideal method to generate hydrogen, and the slow kinetics of water oxidation, namely, oxygen evolution reaction (OER), greatly restricts its practical application. To reduce the energy consumption required for OER, methanol oxidation reaction (MOR) with a much lower theoretical potential is very promising to replace OER to assist hydrogen generation. The theoretical potential of MOR is only 0.016 V vs. SHE (standard hydrogen electrode), which is much lower than that of OER (1.23 V), and the energy-saving can be about 60% compared to that of traditional water electrolysis. Therefore, using MOR instead of OER to realize methanol electrolysis for hydrogen production is an effective way to reduce energy consumption. An efficient bifunctional catalyst is very important for green hydrogen generation from overall methanol electrolysis. Currently, Pt-based materials are still the best catalyst for hydrogen evolution reaction (HER) and MOR, while they are more challenging in the MOR as they are prone to intermediates poisoning during the catalytic reactions. The introduction of transition metal-based promoters such as phosphides is an effective strategy to promote the catalytic ability for methanol oxidation. Herein, ultrafine Pt nanoparticles with an average particle size of 2.53 nm evenly grown on MoP-NC nanosphere (Pt/MoP-NC) were demonstrated as an efficient electrocatalyst for methanol electrolysis towards hydrogen generation. The introduction of MoP-NC nanospheres support not only restricts the aggregation of Pt, but also improves the catalytic performance and anti-poisoning ability. Specifically, Pt/MoP-NC catalyst exhibited high methanol oxidation performance with a peak current density of 90.7 mA∙cm⁻², which was 3.2 times higher than that of commercial Pt/C catalysts, and good hydrogen evolution reaction performance with a low overpotential of 30 mV to offer 10 mA∙cm⁻² in an acid medium, which was comparable to commercial Pt/C. The assembled Pt/MoP-NC||Pt/MoP-NC electrolyzer showed a cell voltage of 0.67 V at 10 mA∙cm⁻², ca. 1.02 V less than that of the overall water splitting system (1.69 V). The high catalytic ability of Pt/MoP-NC originated from the electronic effect between noble metal active center Pt and the adjacent MoP-NC support with a unique layered porous spherical structure. The partial electron transfer from MoP to Pt can lower the d-energy band center of Pt, which weakened the binding energy between Pt and adsorbed toxic intermediates. In addition, the oxophilic MoP-NC nanospheres can activate water to provide more hydroxyl species and facilitate the oxidative removal of CO intermediates adsorbed on the Pt active sites. The current work might inspire the design and preparation of novel catalyst platforms for methanol electrolysis in hydrogen generation.

Key words: Electronic effect, Methanol electrolysis, Transition metal phosphides, Platinum-based catalyst, Bifunctional catalyst