Abstract
In an attempt to mitigate the effects of climate changes, modern societies are facing a revolution in energy supply. Future economy will be increasingly dominated by carbon-free and renewable energy sources. In this context, hydrogen will play a fundamental role in the energy transition from fossil to green sources. It offers the possibility to smooth the intermittency typical of many renewable energy sources and it constitutes an ideal complement for batteries in the transport sector. Hydrogen can be easily produced employing water electrolysis, stored in large amounts and subsequently re-electrified in fuel cells. Efficient water electrolysis, however, makes use of noble metal electrocatalysts to lower the overpotential required for the Hydrogen Evolution Reaction (HER) to take place. Due to the limited availability and consequent high cost of noble metals, research is currently focusing on the development of low-cost alternative catalytic materials. One of the most studied electrocatalytic compounds are transition metal phosphides [1]. These materials present remarkable advantages over commonly used Pt based catalysts: comparable performances, lower cost, high abundancy and ease of manufacturing.
In the present work, transition metal phosphides were fabricated through a simple and costless codeposition-annealing process. Amorphous Ni-P and Co-P alloys were codeposited with red phosphorus particles and subsequently annealed [2]. The annealing step was applied to promote the formation of intermetallics through interdiffusion between pure phosphorus particles and the metallic matrix. As a result, different phase pure metal phosphides were obtained. The most important advantage of this methodology is the possibility to overcome the compositional limit typical of electrodeposited phosphorus-based alloys. Elemental phosphorus codeposition allows to reach P concentrations higher than 50 % at., whereas simple Ni-P solid solution electrodeposition is compositionally limited to 25 % at. Enhancing P content is fundamental to obtain phosphorus rich compounds like Ni2P and Co2P, which are characterized by high catalytic activities for HER [3]. Furthermore, the technique allows to confine elemental P inside the coating during the annealing step. This peculiarity strongly optimizes material usage and it limits the presence of gaseous phosphorus, which is typical of many phosphorization processes used in literature to obtain metal phosphides [4] and it can result into the formation of the unstable white P allotrope.
Obtained nickel and cobalt phosphides were characterized with different techniques (SEM, XRD, EDS) to assess their morphology, phase structure and chemical composition. A special emphasis was placed on XPS characterization. Thanks to its ability to analyze the first superficial layers of the coatings, XPS was employed to determine the chemical state of the elements present on the surface of the electrocatalysts. Being electrocatalysis an interfacial phenomenon, this allowed to link material surface properties with observed electrocatalytic performances.
The electrocatalytic HER activity and stability of the different transition metal phosphides were tested in 0.5 M H2SO4. In all cases, remarkable results were obtained, with the lowest overpotential obtained in the case of Co-P/P codeposition and subsequent annealing at 290 °C: 62 mV vs. RHE at a current density of 10 mA/cm2 in 0.5 M H2SO4 solution. This result is in line with the literature available on cobalt phosphides as electrocatalysts [5].
[1] P. Xiao et al., Adv. Energy Mater. 5, 1500985 (2015)
[2] R. Bernasconi et al., ACS Appl. Energy Mater. 3(7), 6525–6535 (2020)
[3] A. R. J. Kucernak et al., J. Mater. Chem. A 2, 17435 (2014)
[4] X. Wang et al., Angew. Chem. 54(28), 8188 (2015)
[5] Saadi et al., J. Phys. Chem. C 118(50), 29294–29300 (2014)