Silicon nanowires' based photoanode for hydrogen evolution

https://doi.org/10.1016/j.ijhydene.2018.04.244Get rights and content

Highlights

  • Newly designed (photo)electrochemical cell with photovoltaic panels as energy source.

  • The specially prepared Si-nanowires deposited on Mo served as a photo-anode.

  • The optimal bias of experiments at 0.3 V guaranteed a massive hydrogen evolution.

  • Evolving hydrogen formed Pd or Ag nanoparticles by reduction of metal salts.

  • Nanoparticles progressed towards fractal structures caused by diffusion processes.

Abstract

The solar driven photo-electrolysis of water for hydrogen and nanoparticles formation from electrolyte with corresponding metal salts was investigated. The photoanode consisted of a thin layer of silicon nanowires deposited on molybdenum and a platinum wire as the cathode was used. The prepared photo-anodes were stable up to 0.3 V against the saturated Ag/AgCl electrode without any signs of gaseous oxygen evolution. This photo-electrochemical system enhanced the nanoparticle formation during the illumination phase accompanied with lowering the external potential needed. The present results show that hydrogen in statu nascendi is formed during illumination of photoanode, which causes the formation of nanoparticles by reduction of metal salts. During the photo drive electrolysis, hydrogen is stored in/on forming nanoparticles. The size of nanoparticles can be determined by time and an external bias value inserted between the working and platinum electrodes in a classical three electrode system. It was confirmed that the formation of nanoparticles was limited by diffusional processes of hydrogen in statu nascendi in electrolyte, which is in agreement with literature.

Introduction

Energy and environmental issues at a global level belong to the most important topics. It is indispensable to construct clean energy systems in order to solve the issues. Hydrogen will play an important role in such systems due to the fact that it is the ultimate clean energy and it can be used in fuel cells. The use of solar energy for electricity generation and the use of this electricity for hydrogen production by alkaline water electrolysis promises to be a truly sustainable scheme for hydrogen economy [1], [2], [3].

There are several ways of solar hydrogen production and storage [4], [5], [6], [7]. One of them is the photocatalytic water splitting process, in which the photon energy is converted into chemical energy accompanied with a largely positive change in the Gibbs free energy through water splitting [8], [9], [10]. This reaction is similar to photosynthesis used by green plants because these are uphill reactions. Therefore, photocatalytic water splitting is regarded as an artificial photosynthesis and is an attractive and challenging theme in chemistry.

During the past 40 years, various photocatalyst materials have been developed to split water into H2 and O2 under UV and visible light illumination. However, efficient materials for water splitting under visible light irradiation have not been found yet. Nevertheless, new photocatalyst materials for water splitting have recently been discovered one after another. One of the perspective material is the nanocrystalline silicon, which is a surface-nanostructured Si with an extremely efficient light absorption capability [11], [12] and, regrettably, with intense charge recombination and low electrochemical stability. Nevertheless, the photocatalytic water splitting is still a challenging reaction even though the research history is long. Moreover, hydrogen produced by this process can be either stored in metal hydride or carbon nanotubes, polymers and chemical complexes for subsequent utilization [13], [14], [15].

For that reason, this work was focused on evolution of hydrogen in a newly designed (photo)electrochemical cell in which two photovoltaic panels served as the source of electric energy for long-term experiments. Specially prepared Si nanowires on molybdenum were employed as one of electrodes [16]. Subsequently, utilization of evolving hydrogen for preparation of nanoparticle catalysts together with its storage in Palladium were also tested. This method is unique since the preparation of nanoparticles can be carried out continuously without an addition of further reducing agents.

Section snippets

Preparation of Si nanowires

Si nanowires were grown on molybdenum (0.5 mm, 99.9 w/w%, Aldrich) and iron substrates (0.5 mm, 99.9w/w%, Aldrich) in a quartz tube placed in an oven. Prior to the deposition, 2 nm thick gold layers were sputtered on the substrate. First of all, the tube was evacuated using a turbo station unit (TC110 Pfeiffer Vacuum) to reach pressure lower than 5.0 10−4 Pa. Subsequently, the temperature in the oven was increased up to 500 °C and after reaching that value, silane SiH4 was allowed to enter the

Layers preparation

The deposition of silane on molybdenum, which yielded thick brown films, was applied for the electrode preparation based on Si nanowires on Mo substrate with 2 nm thick gold layers. The used preparation system provided the growth of Si nanowires with a uniform thickness profile. EDX analysis (not shown) revealed that nanowires are composed of silicon. According to the Scanning Electron Microscopy (SEM) photographs (Fig. 2a), a large part of prepared nanowires possessed the length of more than

Conclusion

The system for evolution of hydrogen by a photo-electrochemical reaction and its utilisation in preparation of nanoparticles was successfully tested. The hydrogen evolution took place at the surface of specially designed photoanode based on silicon-nanowires deposited on Mo. The photovoltaic cell was employed as the only source of electrical energy for the electrochemical cell. The electrochemical results verified the photo-electrochemical stability of the system up to 0.6 V bias against the

Acknowledgment

The support of Grant Agency of the Czech Republic (grant No. 15-14228S) and the Operational Programme Research, Development and Education of the Ministry of Education, Youth and Sports of the Czech Republic (listed as “ÚCHP Mobilita”), reg. number – CZ.02.2.69/0.0/0.0/16_027/0007931 is gratefully acknowledged.

References (24)

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