Tandem Cell For Solar Water Splitting

Photoelectrochemical(PEC)water splitting without external bias is a potential solution to the growing energy crisis,since it can convert solar energy into chemical energy directly.Tandem cell is a frequently-used configuration for unassisted overall water splitting,because the advantages of each component are tied together to form a high-efficient integration.PEC water splitting device employing tandem configuration is based on different photoelectrode absorbers,and there are two main models consisting of photoanode/photocathode(PEC/PEC)and photoelectrode/photovoltaic(PEC/PV)tandem cells.One approach for overall water splitting is to use a photocathode and a photoanode connected in series to form a PEC/PEC tandem cell,in which the sunlight can pass through the n-type semiconductor and then the p-type semiconductor.Another feasible pathway for unassisted overall water splitting is to combine the photoelectrodes with an integrated PV device to form a PEC/PV tandem cell,which means the PEC device is supported by the voltage-biased from the PV cell.There is another device for water splitting called PV/Electrolysis cell(PV/EL),without the help of PEC system.The PV/EL cell can also realize the water splitting.The PV/EL system directly connects PV cells with two metal electrodes,and water splitting is conducted in the electrolyzer.This paper focuses on the two main configurations for PEC tandem devices:PEC/PEC and PEC/PV tandem cells.The silicon-based semiconductor materials acting as photocathode and PV cell were separately connected with BiVO4 photoanode to constitute the PEC tandem devices,and they all realized the unbiased overall water splitting.We first combined the BiVO4 photoanode with the heavily-doped silicon nanowires(SiNW-n+).The device reached 0.09 mA/cm2 without additional bias for overall water splitting,and the solar-to-hydrogen(STH)conversion efficiency was 0.12%.Then the tandem p-i-n silicon cell was selected as the photoelectrodes,which means it can directly contact with the electrolyte.After the PEC measurement it is confirmed as a p-type semiconductor.With the same composition of BiVO4 photoanode,the PEC/PEC tandem device was formed by the tandem p-i-n photocathode.Due to the particularity structure of the tandem p-i-n,the photocathode company with built-in PV system,the final PEC/PEC tandem device performance obtained a great help.The photocurrent of the device can reach 0.65 mA/cm2 corresponding STH conversion efficiency of 0.8%.Then the tandem p-i-n was set outside the circuit as a PV cells combined with the BiVO4 in series to form a PEC/PV tandem device.Only the STH conversion efficiency of 0.0738%was gained.To advance the efficiency,we used ordinary commercial silicon cells instead of the tandem p-i-n.Finally we got STH efficiency of 0.74%.The PEC/PV system is relatively easy to obtain better conversion efficiency,because it does not need to consider the energy band matching problem of two photoelectrodes.Only the light absorption of the front and rear components need to be considered.For the structure of PEC/PEC system,the configuration is relatively simple.But it is necessary to consider the band matching and light absorption of the two photoelectrodes,which means it is relatively strict in the selection of materials.This paper shows the improvement in the performance of PEC/PEC devices by applying the tandem p-i-n photocathode into PEC/PEC tandem cell.In the future,through regulating the thickness of i-layer in the p-i-n cell,the PEC performance may get promotion.The application of NiFe LDH catalyst in the water and seawater splitting was investigated in this paper.After comparing the performance of NiFe/Ni foam electrodes and Ni(OH)2/Ni foam electrodes in water and seawater,the two electrode systems of NiFe/Ni foam electrodes were selected.Firstly two electrode system of the NiFe/Ni foam electrodes was combined with two ordinary commercial silicon cells in series.The device realized overall water splitting without bias.After The 1000 s of testing,the device's current density is 4.2 mA/cm2,corresponding STH efficiency of 5.17%.The two electrode system of NiFe/Ni foam electrodes was supported by the bias of the electrochemical workstations for seawater splitting,and the current was considered as corrosion current since the dissolution happened at the anode.Through exploring the two electrode system of the NiFe/Ni foam electrodes applied in water splitting,unbiased overall water splitting was realized.