Net Energy Balance Assessment for a Coupled Photoelectrochemical Water Splitting Device
Authors: Zhang Xinyi , Schwarze Michael , Schomäcker Reinhard , van De Krol Roel , Abdi Fatwa 
Published: July 2022

Photoelectrochemical (PEC) water splitting is a promising renewable energy technology to produce green hydrogen for the future fossil-fuel-free society. Over the past decade, research on PEC water splitting devices has achieved significant improvements in the demonstrated solar-to-hydrogen (STH) efficiencies. The improved efficiencies have led to the development of large-scale devices [1,2] and the coupling of hydrogen production with the synthesis of valuable chemicals [3,4]. The co-generation approach offers a potential route towards achieving a levelized cost of hydrogen (LCOH) that is competitive with the current market price of hydrogen and increases the overall economic feasibility of the PEC technology.

This study evaluates the potential of co-producing hydrogen and methyl succinic acid (MSA) by coupling the hydrogenation of itaconic acid (IA) into MSA inside a PEC water splitting reactor. We used a PEC device that uses BiVO4 as the top absorber and a silicon solar cell as the bottom absorber, as reported previously [1,5]. To address the feasibility of this approach, a net energy balance assessment is conducted, and the results are compared with the benchmark values for conventional MSA production. We follow the Techno-Economic Assessment & Life Cycle Assessment Guidelines for CO2 Utilization (Version 1.1) which provides a specific protocol for multi-functional PEC devices [6]. Life cycle inventory (LCI) values from the literature and Ecoinvent database [7] are used to construct the target scenarios in Simapro v9.2.0.

Our results show that the energy demand of our PEC device is ca. 3800 MJ/m2, and the most energy intensive components are the photoelectrode (~70%) and the Nafion membrane (8%). Under the base case condition (i.e., STH = 5%, device longevity = 10 years) and when H2 is the only product, a negative net energy balance of ca. -160 MJ/m2/year is obtained. However, with a coupled hydrogenation reaction, a zero net energy balance (i.e., energy breakeven) can already be achieved when only 2% of the produced H2 molecules are converted into MSA (see red circle in Fig. 1a). Figure 1b shows the cumulative energy demand to produce one kg of MSA under a more optimistic scenario, in which the H2-to-MSA conversion efficiency is 0.4. Under this condition, the net energy production is ca. 3500 MJ/m2/year, which translates to a cumulative energy demand of ca. 13 MJ/kg of MSA (see red circle in Fig. 1b). This is much lower compared to MSA produced using conventional hydrogenation methods (i.e., ~90 MJ/kg MSA), which underlines the attractiveness of the coupled PEC approach.

Finally, we analyze the potential for further improvement of the net energy balance. We explore possibilities of replacing device components (e.g., photoelectrode, membrane) and assess the impact to the net energy balance of the device. The result of this optimization study will be presented, and the most effective strategy will be outlined.

Keywords

: water splitting, (photo)electrochemistry, net energy assessment, coupled catalysis, hydrogenation

References

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Figure 1