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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20287-w Indeed, when the LSV curve of the SJ-GaAs photoanode is used for the SOEE-E0cell simulation (red curve in Fig. 5a), the predicted optimized E0cell is shifted to 0.59 V, which is significantly lower than the optimized E0cell of 0.74 V predicted using the J–V curve of the solid-state GaAs cell (blue curve in Fig. 5a). Therefore, the E0cell of the BTMAP-Vi/Fc redox couples was actually too high for the actual J–V performance of the GaAs photoanode. The simulated SOEEins-SOC curves for further comparison of the charging behavior by using the SJ-GaAs SFB with the E0cell of 0.46, 0.56, and 0.66V (Supplementary Fig. 12) revealed more uniformly higher SOEEins values across various SOC levels by key factors contributing to the efficiency and stability of this record-holding SFB device: First, the high efficiency of the back- illuminated SJ-GaAs photoelectrode with an unusual n–p–n sandwich design that is friendly for incorporation into liquid cells and neutral aqueous RFB electrolyte with robust BTMAP-Fc/Vi redox couples. Further, the effective protection of GaAs with ALD-TiO2 coating in the more friendly neutral aqueous elec- trolyte. Most importantly, the rationally optimized potential matching and operation conditions of the SFB device. In addition to the efficiency and stability, the solar charging photocurrent density is also a valuable metric for SFBs but has received much less attention so far. Because the operating current density of a typical RFB is usually quite high (>50 mA cm−2), higher photocurrent density of the photoelectrode would result in higher redox couple concentrations (thus higher energy storage capacity) in SFBs and less geometrical area mismatch between the photoelectrode and RFB electrode in SFBs, which in turn could facilitate device engineering and lower the fabrication cost for practical SFB devices. In this regard, high solar charging photo- current densities could be beneficial for practical SFBs, and there- fore, a fruitful future direction could be investigating high- performance SJ solar cells21,22 that can deliver high photocurrent densities but with relatively low photovoltages56. The current study also reveals the challenges that need to be addressed to achieve even higher SOEE and SPUR using this type of photoelectrodes: improving the fill factor under high photocurrent density condi- tions by optimizing the concentration and flow rate of the elec- trolytes, enhancing redox couple kinetics, and refining photoelectrode engineering48, and the need for a repertoire of diverse and robust redox couples19 with closely spaced formal potential values for more precise voltage matching56. In summary, we demonstrated a high performance and long lifetime integrated SFB system with a back-illuminated SJ GaAs photoelectrode and robust aqueous organic redox couples. The average SOEE of 15.4% using the BTMAP-Fc/NMe-TEMPO redox couples sets a new record for the SFBs with SJ photoelec- trodes. Compared with the previous SFB based on III–V triple- junction solar cells, the fabrication cost of the integrated SFBs in this work is significantly reduced and the lifetime is much longer, yet without sacrificing SOEE performance. More importantly, the rational potential matching simulation and comprehensive operating condition optimization enabled an excellent SOEE and SPUR that surpass the previous III–V triple-junction based SFB. The device operation optimization methods developed in this work can also serve as a general strategy for improving the per- formance of other integrated solar energy conversion and storage devices3,4. Compared with separate solar cell + battery devices, integrated SFBs made of cost-effective solar cells could have the benefits of lower cost due to the saving in the expensive max- imum power point tracking and DC–DC conversion electro- nics3,57, potentially higher efficiency, and convenient integrated thermal management in a compact device14,15,17,56,58,59. Our results not only demonstrate a new high-performance SFB but also shed new insights on how to design highly efficient SFBs based on more practical SJ solar cells that often have higher photocurrent densities but relatively lower photovoltages (than tandem solar cells). The success in further developing SFBs could enable practical off-grid electrification applications, such as solar home systems56,57,60. Methods Fabrication of SJ-GaAs cell and photoelectrode. The n-on-p configuration GaAs SJ cells were fabricated in a low-pressure metal–organic chemical vapor deposition (MOCVD) system (EMCORE D180, Agnitron Technology) at 615 °C. A 200-μm- thick n-GaAs substrate was diced into 10 mm × 10 mm square pieces, cleaned, followed by the deposition of a 200 nm of Ni/Au layer as the back metal contact. using the E0 of 0.56 V. In light of this, we further studied a RFB cell Me 50–52 using the BTMAP-Fc and N -TEMPO redox couples in catholyte/anolyte that could deliver a better matched E0cell of 0.558 V as demonstrated in Fig. 5b. An impressive average SOEE of 15.4% for 10 cycles of SFB cycling by using the BTMAP-Fc and NMe-TEMPO redox couples (Fig. 5c), and the average CE and VE of 96.76% and 97.19% can be obtained, respectively (Fig. 5d). Unfortunately, the rather fast capacity decay of the RFB built with these redox couples (Supplementary Fig. 13) prevented us from demonstrating long-term cycling. Hopefully, this issue can be solved by investigating the capacity decay mechanism of the RFB, or by developing other suitable robust redox couples with similarly targeted potentials in future work. Discussion Due to the rational E0cell matching and operating condition opti- mization, this further improved SJ-GaAs SFB achieved an average SOEE of 15.4% and a SPUR of 67.6% (based on a PCE of 22.78% for the SJ-GaAs solar cell), which are the highest among all the SFBs with SJ photoelectrode reported so far17,48,53,54. This SOEE is even higher than that of the previously reported triple-junction III–V SFB despite the higher PCE of 26.1% for the III–V tandem cell15 due to the lower SPUR (54.0%) achieved there than the current SJ-GaAs SFB. This is because the triple junction cell was not integrated with redox couples with the best matched E0cell (predicted to be 1.72V, as shown in Supplementary Fig. 14), which is limited practically by the thermodynamic water splitting potential in aqueous electrolyte (1.23 V). That SFB also did not operate in the optimal SOC range. These results and analyses show that comprehensive potential matching simulation provides the better procedure to design and charge the integrated SFB device, which is the most critical factor to improve both the SOEE and SPUR for a general integrated SFB system. Such improved analyses enabled us to achieve a better SFB performance using an SJ photoelectrode than what was previously achieved using a much more expensive triple junction photoelectrode15. The cost of SJ-GaAs cells can also be further reduced by utilizing epitaxial lift-off fabrication approach30,32,41 or thin-film GaAs solar cells55 in the future. Furthermore, we summarize the SFB performance in repre- sentative previous reports in comparison with that presented herein in Fig. 6. Several key parameters are compared: SOEE (horizontal axis), the current density of the photoelectrode (ver- tical axis), demonstrated cycling lifetime (the radius of the cir- cles). The solar cell structure of each work is marked by the symbols of the red triangle (for single junction) and green pen- tagon (for tandem junction), individually. The photoelectrodes, redox couples, and the corresponding energy capacity of SFB are displayed near each work. The pH of the electrolytes is also marked with the color of the data symbol. It can be clearly seen that the SJ-GaAs SFB device demonstrated in this work features the largest photocurrent density, outstanding continuous opera- tion time, and one of the highest SOEEs (the highest SOEE among all the SJ photoelectrode based SFBs). There are several 8 NATURE COMMUNICATIONS | (2021)12:156 | https://doi.org/10.1038/s41467-020-20287-w | www.nature.com/naturecommunicationsPDF Image | flow battery enabled single-junction GaAs photoelectrode
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