This review summarizes recent advances in photoelectrochemical energy storage materials and related devices for direct solar to electrochemical energy storage. Design principles, challenges and future developments are specifically highlighted.

Photocatalytic water oxidation is a key half-reaction for various solar-to-fuel conversion systems but requires simultaneous water affinity and hole accumulation at the photocatalytic site. Here, we present the rational design and synthesis of an ionic-type covalent organic framework (COF) named tetraphenylporphyrin cobalt and cobalt bipyridine complex (CoTPP-CoBpy ₃ ) COF, combining cobalt porphyrin and cobalt bipyridine building blocks as a photocatalyst for water oxidation. The good dispersibility of porous large-size (>2 micrometers) COF nanosheets (≈1.45 nanometers) facilitates local water collection; the ultrafast triplet-state charge transfer (1.8 picoseconds) and prolonged charge separation (1.2 nanoseconds) further contribute to the efficient accumulation of holes in the CoTPP moiety, leading to a photocatalytic dioxygen production rate of 7323 micromoles per gram per hour. Moreover, we have identified an end-on superoxide radical (O ₂ ^· ) intermediate at the active site of the CoTPP moiety and proposed an electron-intermediate cascade mechanism that elucidates the synergistic coupling of electron relay (S ₁ -T ₁ -T ₁ ′) and intermediate evolution during the photocatalytic process.

Solar-to-electrochemical energy storage in solar batteries is an important solar utilization technology alongside solar-to-electricity (solar cell) and solar-to-fuel (photocatalysis cell) conversion. Integrated solar batteries that integrate photoelectrodes with redox-electrodes realize indirect solar energy storage based on dual energy matching (photo-carriers and redox couples) and two distinct processes (electricity generation and redox reaction). In contrast, the emerging coupled solar batteries allow direct solar energy storage via a photo-coupled ion transfer at photoelectrochemical storage electrode materials with both light harvesting and redox activity. However, issues of rapid charge recombination of these photoelectrochemical storage materials and misaligned band energy of the devices have resulted in a limited efficiency that hampers the development of coupled solar batteries. In this review, we describe how photoelectrochemical storage materials and coupled solar batteries can be designed to promote the coupling between photogenerated charges and redox reactions for high efficiency. We discuss the characteristics of recent photoelectrochemical storage materials in coupling basic functions such as light harvesting and redox activity, along with new approaches to promote charge separation. We also assess the mechanisms and limitations of current device design (e.g., photo-assisted and photo-charged solar batteries), based on which opportunities for a new device with matched band alignments are proposed. Finally, we translate these mechanistic insights and key metrics into standardized protocols for future modifications, innovations, and ultimately, commercialization of coupled solar batteries.

A charge-separated and redox-active C ₆₀ @porous organic cage (C ₆₀ @POC) compound was synthesized to act as the cathode in a photo-assisted Li-organic battery, which enables efficient solar to electrochemical energy conversion and storage.

Stable lithium metal negative electrodes are desirable to produce high-energy batteries. However, when practical testing conditions are applied, lithium metal is unstable during battery cycling. Here, we propose poly(2-hydroxyethyl acrylate-co-sodium benzenesulfonate) (PHS) as negative electrode protective layer. The PHS contains soft poly (2-hydroxyethyl acrylate) and poly(sodium p-styrene sulfonate), which improve electrode flexibility, connection with the Cu current collector and transport of Li ions. Transmission electron cryomicroscopy measurements reveal that PHS induces the formation of a solid electrolyte interphase with a fluorinated rigid and crystalline internal structure. Furthermore, theoretical calculations suggest that the -SO₃^- group of poly(sodium p-styrene sulfonate) promotes Li-ion motion towards interchain migration through cation-dipole interaction, thus, enabling uniform Li-ion diffusion. Electrochemical measurements of Li | |PHS-coated-Cu coin cells demonstrate an average Coulombic efficiency of 99.46% at 1 mA/cm², 6 mAh/cm² and 25 °C. Moreover, when the PHS-coated Li metal negative electrode is paired with a high-areal-capacity LiNi_0.83Co_0.11Mn_0.06O₂-based positive electrode in multi-layer pouch cell configuration, the battery delivers an initial capacity of 6.86 Ah (corresponding to a specific energy of 489.7 Wh/kg) and, a 91.1% discharge capacity retention after 150 cycles at 2.5 mA/cm², 25 °C and 172 kPa.