PDF Publication Title:
Text from PDF Page: 002
ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20287-w The increasing demand for clean and renewable energy has stimulated the development of many important technologies for simultaneous conversion and storage of intermittent solar energy1–4. Among them, solar-driven photoelectrochemical (PEC) water splitting to produce hydrogen5–7 and reduction of carbon dioxide for the production of fuels8,9 have received considerable research interests due to the promises for harvesting solar energy and storing it as chemical energy10,11. However, practical PEC technol- ogies are impeded by several key challenges2,10,12,13, including the sluggish kinetics of fuel-producing reactions that require efficient and robust electrocatalysts, the poor stability of many photoelectrode materials under PEC conditions, and the need for additional fuel cell devices to regenerate electricity from solar fuels. Recently, solar flow batteries (SFBs)14–18 that monolithically integrate photovoltaics (PVs) or regenerative PEC cells and redox flow batteries (RFBs)19,20 have emerged as an alternative approach to avoid the aforementioned issues of conventional PEC devices yet achieve the same function of harvesting and storing solar energy into chemical energy. In SFBs, redox couples with facile kinetics are used to store and release solar energy as electricity under mild electrochemical conditions. This eliminates the need for electrocatalysts and separate fuel cell devices and could relax the stability requirement for photoelectrode materials. In order to achieve high-performance SFBs, efficient solar cells21–25 are needed. III–V semiconductor materials are com- monly used for high-efficiency PV applications due to their direct bandgap, high absorptivity for sunlight, high electron mobility, and well-controlled crystal growth21,26,27. The integration of a triple-junction III–V (InGaP/GaAs/Ge) photoelectrode with aqueous organic 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO) and methyl viologen redox couples in an SFB device yielded a round-trip solar-to-output electricity efficiency (SOEE) of 14.1% for SFBs15. Note that the SOEE of an integrated SFB device is calculated using the following equation14: using p-type GaAs substrates. However, n-type GaAs substrates are less costly due to the easier fabrication proces34,35. Further- more, it is desirable to harvest photons at the epitaxial n–p junction side, but immerse the protected substrate side of the photoelectrodes in contact with electrolytes for electrochemical reactions6,32. These design constraints mean that, in order to enable efficient, stable, and practical SFB devices, we need to design unconventional back-illuminated SJ GaAs solar cells based on n-type GaAs substrates to achieve high carrier collection efficiency and reduced production cost at the same time. In this work, we present an efficient and stable SFB based on a back-illuminated SJ-GaAs photoanodes with an unusual n–p–n sandwich structure using n-GaAs substrates that are integrated with robust bis ((3-trimethylammonio)propyl)-ferrocene dichloride (BTMAP-Fc), bis (3-trimethylammonio)propyl violo- gen tetrachloride (BTMAP-Vi), and N-methyl-2,2,6,6-tetra- methylpiperidin-1-oxyl (NMe-TEMPO) redox couples in neutral pH electrolytes. To optimize the SOEE of the SFB, we carried out numerical simulations to find the relationship between SOEE and the operating state-of-charge (SOC) range. The highly efficient SJ-GaAs photoelectrode, good potential match, and rational operating condition engineering led to an average SOEE of 13.3% by using the BTMAP-Vi/BTMAP-Fc redox couples. Furthermore, the robust neutral pH aqueous redox couples used and the TiO2 thin film protecting the GaAs substrate from photocorrosion significantly extended the lifetime of the III–V photoelectrode based SFB to more than 400 h (over 150 cycles). Based on a more refined voltage matching analysis of the actual performance of the SJ-GaAs photoelectrode, we further developed another SFB device with better matched BTMAP-Fc and NMe-TEMPO redox couples and demonstrated an average SOEE of 15.4%. This work further reveals insights on how to effectively utilize solar cells that have higher photocurrent densities but lower photovoltages for high- performance SFBs toward practical applications. Results Design of the SJ-GaAs solar cell. In order to achieve a good operating potential match between the photoelectrode and aqueous redox couples, we first fabricated and investigated the SJ-GaAs solar cells with an unusual “reversed” n–p–n sandwiched layer stacking with cost-effective n-GaAs substrates. As illustrated in Fig. 1a from bottom to top, the SJ-GaAs solar cell was fabricated by growing a p- on-n tunnel diode on an n-type GaAs substrate followed by an n (emitter)-on-p (base) active junction (see “Methods” for complete fabrication details) We use n-type GaAs substrates for the photo- electrodes due to several advantages over the p-type substrates that are commonly used commercially: n-type GaAs has a lower surface recombination velocity than that of p-GaAs36 and n-GaAs sub- strates are more affordable due to fabrication challenges in the p- type doping process35. However, a higher carrier collection effi- ciency can be realized in GaAs solar cells with the epitaxial growth of n (emitter)-on-p (base), instead of p (emitter)-on-n (base), because the electron diffusion length of the p-type base is much longer than the hole diffusion length of the n-type base30,33. In order to achieve both better device efficiency and lower cost, here we use an unusual “reversed” layer architecture with an n–p–n sandwich geometry, i.e., both the light-harvesting window and photoelectrode/ electrolyte interface using n-type GaAs. Furthermore, we need a tunnel diode (Te doped-GaAs/C doped-AlGaAs) between the n- type substrate and the p–n junction to act as an Ohmic resistor with the purpose of minimizing dopant diffusion37,38. Compared with multijunction III–V photoelectrodes that exhibit high photovoltages (>2.0 V), the SJ-GaAs photoelectrodes feature lower photovoltages (0.9–1.1 V) but higher photocurrents (>21 mA cm−2)21,22,39. The current density–voltage (J–V) E SOEE 1⁄4 electrical;out 1⁄4 Eillumination R Iout Vout dt R ; ð1Þ SAdt where the Eillumination is the incident solar energy, Eelectrical,out is the output electrical energy from discharging the SFB, Iout and Vout are the output current and voltage during discharge, respectively, S is the total incident solar irradiance, and A is the area of the light-harvesting window of the photoelectrode. Despite the high SOEE and the conceptual advance, this III–V based SFB suffers from several shortcomings. First, the maximum power point voltage (VMPP) of the tandem photoelectrode is much higher than the cell potential of SFB (E0cell which is determined by the formal potential difference between the anolyte and catholyte redox couples, i.e., E0cell = E0anolyte − E0catholyte), therefore, a large portion of the high photovoltage is not uti- lized15. Second, the fabrication cost of triple-junction III–V solar cells is too high for practical applications. Third, the Ge bottom cell in the photoelectrode is prone to photocorrosion in aqueous solutions, which has limited the lifetime of the SFB device15. On the other hand, GaAs (Eg = 1.42 eV) solar cells are the most likely to reach the Shockley–Queisser limit22,28,29 due to its optimal bandgap and hold the record power conversion efficiency (PCE) of 29.1% for single-junction (SJ) solar cells21. In addition, the open-circuit voltage (VOC) of SJ-GaAs cells, usually between 0.9–1.1 V22, is within the optimal voltage matching range for many aqueous RFBs. Even though GaAs solar cells are still more expensive than silicon solar cells, these attributes make SJ-GaAs photoelectrodes potentially promising for high-performance SFBs, yet they have not been exploited for SFBs. The common GaAs PV cells30, as well as (tandem) PEC photoelectrodes based on III-V materials31,32, often adopt an n (emitter)-on-p (base) device structure, which has better carrier collection efficiency30,33, 2 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
PDF Search Title:
flow battery enabled single-junction GaAs photoelectrodeOriginal File Name Searched:
s41467-020-20287-w.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Salt water flow battery technology with low cost and great energy density that can be used for power storage and thermal storage. Let us de-risk your production using our license. Our aqueous flow battery is less cost than Tesla Megapack and available faster. Redox flow battery. No membrane needed like with Vanadium, or Bromine. Salgenx flow battery
| CONTACT TEL: 608-238-6001 Email: greg@salgenx.com | RSS | AMP |