Design Principle and Loss Engineering for Photovoltaic-Electrolysis Cell System

Abstract

The effects of exchange current density, Tafel slope, system resistance, electrode area, light intensity, and solar cell efficiency were systematically decoupled at the converter-assisted photovoltaic-water electrolysis system. This allows key determinants of overall efficiency to be identified. On the basis of this model, 26.5% single-junction GaAs solar cell was combined with a membrane-electrode-assembled electrolysis cell (EC) using the dc/dc converting technology. As a result, we have achieved a solar-to-hydrogen conversion efficiency of 20.6% on a prototype scale and demonstrated light intensity tracking optimization to maintain high efficiency. We believe that this study will provide design principles for combining solar cells, ECs, and new catalysts and can be generalized to other solar conversion chemical devices while minimizing their power loss during the conversion of electrical energy into fuel.

Figure 1

Design principle of the PV-Conv-EC system based on an independent PV, the EC performance, and the existence of a converter. (a) Hydrogen power per square centimeter (p_H2) and kinetic loss per square centimeter (p_kin) at a given current density–voltage (j–V) curve of the PV and EC. The intersection between the PV and EC j–V curve has a lower voltage and a higher current density than the p_PV,max point. (b) p_H2 and p_kin after the dc/dc converter assistance on (a). The extra p_H2 can be achieved by p_PV,max utilization. (c) p_H2 and p_kin of the PV-Conv-EC system at each p_PV,max and the overpotential of reaching the 10 mA cm^–2 (η_10mA) condition. Extra p_H2 gain by the application of the converter is also displayed. (d) p_H2 of PV–EC and the PV-Conv-EC system depending on the EC performance (Tafel slope and η_10mA) at 30% PV efficiency.

Scheme 1. Schematic of the System That Is Based on Two Series-Connected Single-Junction GaAs PVs Equipped with a dc/dc Converter and an MEA EC

The electrolyte was continuously circulated only at the anode compartment of the EC.

Figure 2

Electrochemical analysis of the electrode material and the MEA EC. (a) Tafel slope of each cathode and anode and the MEA EC system. The sum of the Tafel slope and the overpotential of each electrode is similar to the MEA EC. (b) j–V curve of the MEA EC at different electrolyte resistances controlled by the distance between the cathode and anode. The inset displays the similarity of the j–V curve at each resistance after iR compensation.

Figure 3

solar-to-hydrogen energy conversion process of the PV-Conv-EC system at different PV illumination areas. (a) Diagram indicates circuit of the PV-Conv-EC system. Maximum power of PV (p_PV,max) can be provided to the EC with the optimum duty input to the dc/dc converter, which is called MPPT. η_convp_PV,max is distributed to the p_H2 and p_kin. (b) Predicted STH efficiency based on power–voltage curve and I–V curve, assuming 100% of p_PV,max consumption on the EC. The filled circle and hollow circle represent the p_PV,max point and the point where the EC consumes p_PV,max at each surface area of the PV. MPPT can be achieved by inputting the theoretical duty (D^T), which is the ratio between the voltage of the EC that consumes p_PV,max (V_EC,MPPT) and the voltage at the p_PV,max (V_PV,max). (c) Current density and STH efficiency under chopped illumination on the PV-Conv-EC system under optimized duty (D^O). The recovery of current density after each chopping cycle indicates the stability of the system. (d) Converter efficiency (η_conv) and (V_EC/V_PV) (1/D) at each input duty (D) under various A_PV conditions. The range of D was D^O ± 0.02 to show the duty dependency near the MPPT region. (e) Converter efficiency (η_conv) and (V_EC/V_PV) (1/D) with a wide range of input duties (D) with A_PV = A_EC = 6 cm² configuration under 100 mW cm^–2 light irradiance. The η_conv and (V_EC/V_PV) (1/D) values are almost consistent throughout the D range.

Figure 4

solar-to-hydrogen energy conversion process of the PV-Conv-EC system at different solar power densities. (a) Prediction of STH and D^T based on the I–V curve of PV and EC. (b) STH efficiencies under light-chopped illumination at D^O. The STH efficiencies under each condition are converted from each current and solar power density. (c) Current density measured at the EC for 4000 s at 30 mW cm^–2 solar power density. The current density was converted into STH with 1.23 V and light power density.