engleski

Conducting Polymers as Pattering Templates for the Deposition of Nickel as a Hydrogen Evolution Electrocatalyst

Hydrogen, the most abundant element on earth, is the cleanest, sustainable, simplest and ideal fuel. Therefore, hydrogen is increasingly considered as the fuel of the future. The electrolysis of water represents the only renewable and fully environmentally friendly process of hydrogen production, without co-generation of a green-house gas, CO2. Industrial water electrolysis is currently carried out using a liquid alkaline electrolyte. However, the use of solid polymer electrolyte membrane (PEM)– type generators based on the fuel-cell technology to produce hydrogen from demineralized water would offer a number of advantages compared to the classical alkaline process, especially for residential and small scale applications based on the renewable energy sources (solar, wind, hydro). However, one of the main obstacles associated with the large-scale commercial application of the PEM hydrogen generator is related to high investment costs, mainly due to the use of noble metals as catalyst materials for the hydrogen evolution reaction (HER)[1]. Therefore, there is a major need to develop new active, efficient, stable, and cheap electrocatalysts for water splitting in the PEM hydrogen generator, which would offer low overpotentials for the HER at high current densities. We have investigated a possibility of using conducting polymers (CPs), polypyrrole (PPY) and polyaniline (PANI), as a catalyst pattering matrix for the development of Ni-based HER catalysts. The mechanisms and kinetics of the HER in an acidic medium has been investigated using an electrochemical linear polarization (LP) and impedance spectroscopy technique (EIS), while the morphology and structure of the catalyst has been studied using a scanning electron microscopy (SEM). It has been shown that pattering a glassy carbon electrode surface with CPs offers a convenient method of increasing the surface area of the nickel electrocatalytic coating. The influence of a CP matrix thickness and Ni loading on the electrode overall HER electrocatalytic activity has been investigated. IN summary, it has been demonstrated that Ni/CP electrocatalysts offer significantly higher HER electrocatalytic activity when compared to a classical nickel layer electrodeposited on a copper substrate (Fig.1)[2]. This has been explained on the basis of the increased surface roughness (area) of the Ni/CP coatings. It has been demonstrated that the morphology of the electrodeposited CP matrix greatly influences the overall electrocatalytic activity of the Ni catalyst layer. The PANI matrix has been found to offer significantly higher porosity compared to the PPY matrix, which enables a higher Ni surface area to be obtained per amount of nickel loaded. This yields a higher overall HER electrocatalytic activity and a highly porous Ni-layer structure suitable for the design of PEM-type membrane-electrode-assemblies. The HER electrocatalytic activity of Ni / PPY coatings greatly depends on the method of the catalytic layer preparation, and the loading of PPY and Ni. The highest activity has been obtained for short PPY deposition times and long Ni deposition times. The difference has been explained on the basis of the PPY layer porosity and amount (i.e. surface area) of Ni created. It has also been shown that the co-deposition method (simultaneous deposition of Ni and PPY) offers a significant increase in the Ni dispersion and thus increased electrocatalytic activity. Two surface regions of the Ni layer have been found to be active in the HER: Ni islands directly exposed to the electrolyte solution (1^st region), which are connected to the GC substrate through Ni ‘ stalks’ embedded into a PPY matrix (2^nd region). It has been postulated that the mass-transfer limitations located at the Ni/PPY interface (in the interior Ni / PPY layer - 2^nd region) are responsible for the decreased HER rate at high overpotentials. The results on Ni/PANI electrocatalysts have also revealed that the overall HER activity of these catalysts depends on both the thickness of the PANI matrix and Ni loading, with the thickness of the PANI matrix as a prevailing factor. The highest HER electrocatalytic activity has been found in a case of a thick PANI matrix and low Ni loading, while a catalyst with a thin PANI matrix and low Ni loading has offered the lowest HER electrocatalytic activity. The HER on Ni/PANI catalysts has been found to be entirely kinetically controlled in the whole overpotential region studied, with the absence of mass transfer limitations as on Ni/PPY electrocatalysts. This is due to the large difference in the 3D structure of the two catalysts. In overall, the two best Ni/CP catalysts from each group have been found to offer very comparable HER electrocatalytic activity at low overpotentials, relevant to the operation of PEM hydrogen generators. Only at high overpotentials the Ni catalyst with a PANI matrix has been found to offer a higher activity, which is due to the favorable 3D porous structure of the Ni/PANI layer. Figure 1: HER LP curves in 0.5M H_2SO_4 for a Ni catalyst layer on a PANI/GC and Cu substrate. Inset: HER Nyquist plot for the two investigated catalysts recorded at overpotential -0.3 V. 1.M.Kondoh, N.Yokoyama, C.Inazumi, S.Maezawa, N.Fujiwara, Y.Nishimura, K.Oguro, H.Takenaka, J.NewMat.Electrochem.Sys. 3(2000)61. 2.E.Navarro-Flores, Z.Chong and S.Omanovic, J.Mol.Catal.A: Chemical, 226(2005)179.

Conducting polymers; Electrocrystallization; Electrocatalysis; Nickel catalyst; HER.

nije evidentirano