Hydrogen is one of the most promising energy carriers, especially if produced from renewable energy sources, replacing fossil fuels for both portable and stationary applications [1]. Although hydrogen is one of the most abundant elements on Earth, it does not exist in nature in molecular form. Currently, most of the hydrogen is produced from fossil fuel-based technologies, such as gas reforming. Only 5% of worldwide hydrogen production comes from water electrolysis. The main problem of this process is the high overpotentials necessary to split the water molecule into hydrogen and oxygen and, consequently, the high energy consumption. To overcome this problem, different electrode materials such as Pt, Ni, Co, Ir and Rh, have been tested as cathodes for water electrolyzers.

Pt has high activity and good stability for the hydrogen evolution reaction (HER), but its limited resources and high price prevent its use on industrial scale [2]. Alkaline water electrolysis is a mature technology that typically operates at temperatures around 60 – 80 ºC and uses cathodes based on Ni, Raney Ni and Co [3]. These cathodes are less expensive than Pt, but show good activity (in alkaline media) and corrosion resistance.

A common approach used in electrocatalysis to minimize the utilization of precious metals involves employing an electrocatalyst support material. The support material has several functions, as to increase the catalytic electrochemically surface area, to stabilize and to accommodate the metal particles and also to increase the catalyst’s conductivity [4-6]. Carbon-based supports are the mostly used, as they can offer all the aforementioned properties [6].

The present work combines the advantages of using highly active Pt together with low cost transition metals, like Ni, Co and Cu, with the employment of reduced graphene oxide (rGO) as an efficient carbon-based electrocatalyst support. Thus, Pt and three different PtM alloys supported on rGO, i.e., Pt/rGO, PtCo/rGO, PtCu/rGO and PtNi/rGO, were tested as electrocatalysts for hydrogen production by alkaline water electrolysis. rGO has adequate features to enhance and stabilize the metal particles, with rGO-supported catalysts having recently demonstrated promising results in alkaline media [7].

To prepare the working electrodes, 5 mg of each of the fours electrocatalysts was dispersed in 125 µL of 5 wt.% solution of polyvinylidene fluoride (PVDF, Alfa Aesar) in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich) followed by ultrasonic treatment for ca. 30 min. The working electrodes were prepared by pipetting 80 μL of the corresponding catalytic ink onto a polished glassy carbon (GC) electrode (A = 4 cm²). The electrodes were dried at 80 ºC for 4 hours. A conventional three-electrode setup was used for the electrochemical characterization. The rGO-supported PtM (M = Co, Cu, Ni) electrode was used as working electrode, the counter electrode was a Pt mesh (Johnson Matthey) of 25 cm² area, and a calomel electrode (Hanna Instruments, HI-5412, 3.5 M KCl) was used as the reference. All recorded potentials were converted to RHE scale using the equation E_RHE = E_cal + 0.250 + 0.059 pH.

The evaluation of HER kinetics at the PtM/rGO composites was done by linear scan voltammetry measurements at scan rate of 1 mV s^-1, starting from the open circuit potential up to -0.17 V in 8 M KOH (AnalaR NORMAPUR, 87 wt.%) solutions. Higher current densities were achieved for the PtM/rGO electrocatalysts, in comparison with the non-alloyed material (Pt/rGO). It was also observed that increasing the temperature up to 65 ºC leads to a substantial increase of HER current densities at all electrocatalysts.

Furthermore, long-term stability of the four electrocatalysts’ activity for HER was evaluated by chronoamperometry studies at 25 and 65 ºC, revealing better stability at lower temperature. It is shown that PtM/rGO nanocomposites are good candidates for application as novel electrocatalysts for the HER in alkaline media.

[1] J.A.S.B. Cardoso, B. Šljukić, M. Erdem, C.A.C. Sequeira, D.M.F. Santos, Catalysts 50, 8 (2018).

[2] D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, J.L. Figueiredo, Energy 50, 486 (2013).

[3] D.S.P. Cardoso, L. Amaral, D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, Int. J. Hydrogen Energy 40, 4295 (2015).

[4] M. Martins, B. Šljukić, C.A.C. Sequeira, O. Metin, M. Erdem, T. Şener, D.M.F. Santos, Int. J. Hydrogen Energy 41, 10914 (2016).

[5] R.C.P. Oliveira, V. Vasić, D.M.F. Santos, B. Babić, R. Hercigonja, C.A.C. Sequeira, B. Šljukić, Electrochim. Acta 269, 517 (2018).

[6] E. Antolini, Appl. Catal. B 88, 1 (2009).

[7] M. Martins, B. Šljukić, O. Metin, M. Sevim, C.A.C. Sequeira, T. Şener, D.M.F. Santos, J. Alloys Compd. 718, 204 (2017).