Introduction

Experimental

Results

Discussion

Conclusions

References

Abstract

A solid-state photoelectrochemical (SSPEC) cell is an attractive approach for solar water splitting, especially when it comes to monolithic device design. In a SSPEC cell the electrodes distance is minimized, while the use of polymer-based membranes alleviates the need for liquid electrolytes, and at the same time they can separate the anode from the cathode. In this work, we have made and tested, firstly, a SSPEC cell with a Pt/C electrocatalyst as the cathode electrode, under purely gaseous conditions. The anode was supplied with air of 80% relative humidity (RH) and the cathode with argon. Secondly, we replaced the Pt/C cathode with a photocathode consisting of 2D photocatalytic g-C3N4, which was placed in tandem with the photoanode (tandem-SSPEC). The tandem configuration showed a three-fold enhancement in the obtained photovoltage and a steady-state photocurrent density. The mechanism of operation is discussed in view of recent advances in surface proton conduction in absorbed water layers. The presented SSPEC cell is based on earth-abundant materials and provides a way towards systems of artificial photosynthesis, especially for areas where water sources are scarce and electrical grid infrastructure is limited or nonexistent. The only requirements to make hydrogen are humidity and sunlight.

Introduction

A fundamental societal concern is the impact of climate change on water quality and quantity [1]. It is also expected that the duration, magnitude and frequency of droughts on a worldwide basis will increase due to climate changes [2]. Water electrolysis is already an important and promising method for hydrogen generation and in the near future, clean water supplies will become a vital concern for this technology. Devices that can directly convert the solar energy to fuels, such as hydrogen, are highly desirable because the solar energy is absorbed and stored in the form of fuels in a single step [3e5]. Such “artificial photosynthesis” (AP) devices hold a great promise by mimicking nature in order to perform environmentally sustainable reactions, such as splitting water into O2 and H2, and CO2 fixation to sugars [6,7]. The device design is also very important and has an impact on the overall efficiency and cost [8]. Monolithic device designs, where the photoabsorbers are integrated together with the electrocatalysts, have certain advantages such as reduced area requirements, less frames and connections, as well as reduced footprint [9,10]. In our previous work [11], we demonstrated a monolithic solid-state photoelectrochemical (SSPEC) water electrolysis cell, in which the photoanode and cathode electrodes were attached to opposite sides of a Nafion-based polymer electrolyte membrane. The role of the membrane is to separate the evolved gaseous products, support and minimize the distance between the electrodes (down to a few mm) and provide protonic conductivity. The monolithic SSPEC cell was operated under asymmetric conditions i.e. 0.1 M Na2SO4 solution at the anode and gaseous Ar or air in the cathode compartment. There are a few more other works in the literature inspired by a polymer exchange membrane, PEM-like approach, where the photoanode is fed with a water stream [12,13]. Iwu et al. were the first to photoelectrolyze water vapor in an all-solid-state photoelectrochemical (PEC) cell [14], while Georgieva et al. have employed an all-solid-state PEC cell for the decomposition of organic vapors under UV and visible illumination [15]. Recently, the possibility of steam electrolysis in a high temperature PEC cell has been demonstrated by Brunauer and co-workers [16].