A covalent organic framework integrating naphthalenediimide and triphenylamine units (NT-COF) is presented. Two-dimensional porous nanosheets are packed with a high specific surface area of 1276 m g . Photo/electrochemical measurements reveal the ultrahigh efficient intramolecular charge transfer from the TPA to the NDI and the highly reversible electrochemical reaction in NT-COF. There is a synergetic effect in NT-COF between the reversible electrochemical reaction and intramolecular charge transfer with enhanced solar energy efficiency and an accelerated electrochemical reaction. This synergetic mechanism provides the key basis for direct solar-to-electrochemical energy conversion/storage. With the NT-COF as the cathode materials, a solar Li-ion battery is realized with decreased charge voltage (by 0.5 V), increased discharge voltage (by 0.5 V), and extra 38.7 % battery efficiency.

The electrochemical CO reduction (ECDRR), as a key reaction in artificial photosynthesis to implement renewable energy conversion/storage, has been inhibited by the low efficiency and high costs of the electrocatalysts. Herein, we synthesize a fluorine-doped carbon (FC) catalyst by pyrolyzing commercial BP 2000 with a fluorine source, enabling a highly selective CO -to-CO conversion with a maximum Faradaic efficiency of 90 % at a low overpotential of 510 mV and a small Tafel slope of 81 mV dec , outcompeting current metal-free catalysts. Moreover, the higher partial current density of CO and lower partial current density of H on FC relative to pristine carbon suggest an enhanced inherent activity towards ECDRR as well as a suppressed hydrogen evolution by fluorine doping. Fluorine doping activates the neighbor carbon atoms and facilitates the stabilization of the key intermediate COOH* on the fluorine-doped carbon material, which are also blocked for competing hydrogen evolution, resulting in superior CO -to-CO conversion.

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.

Conspectus Metal–CO2 batteries with CO2 as cathode active species give rise to opportunities to deal with energy and environmental issues simultaneously. This technology is more appealing when CO2 is flexibly reduced to chemicals and fuels driven by surplus electricity because it represents a low-cost and controllable approach to maximized electricity utilization and value-added CO2 utilization. Nonaqueous metal–CO2 batteries exhibited high discharge voltage and capacity with carbon and oxalate as reduction products from CO2 electrochemistry that lacks proton. In contrast, aqueous Zn–CO2 batteries implemented flexible CO2 electrochemistry for more value-added products accompanied by energy storage based on a proton-coupled electron transfer mechanism. In this Account, we have exemplified our recent results in the development of CO2 electrochemistry from nonaqueous Li–CO2 batteries to aqueous Zn–CO2 batteries toward practical value-added CO2 conversion. Aimed at the challengingly limited CO2 electrochemistry and high cost of nonaqueous Li–CO2 batteries, we proposed aqueous Zn–CO2 batteries. Our previous works on nonaqueous Li–CO2 batteries, aqueous Zn–air batteries, and aqueous CO2 reduction electrocatalysts further shed light on battery mechanism, device construction, and electrocatalyst design. For example, bipolar membranes maintain the stability of the basic anolyte and neutral catholyte, as well as the kinetics of ion transport at the same time, forming the device base for aqueous Zn–CO2 batteries. Moreover, in terms of the electrocatalyst catalyzing both discharge and charge reactions on the cathode, the design of multifunctional electrocatalysts is of great importance for not only CO2 electrochemistry but also spontaneous discharge and energy efficiency of aqueous Zn–CO2 batteries. We have explored a series of multifunctional electrocatalyst cathodes, including noble metal, transition metal, and metal-free materials, all of which facilitated CO2 electrochemistry in aqueous Zn–CO2 batteries with value-added carbon-based products. Meanwhile, several operating models for practical complicated situations are presented, such as rechargeable, reversible, dual-model, and solid-state batteries. Zn–CO2 batteries with different models require different design mechanisms for electrocatalyst cathodes. Reversible aqueous Zn–CO2 batteries with HCOOH generation were enabled by electrocatalysts capable of catalyzing the interconversion of CO2 and HCOOH at low overpotentials, rechargeable aqueous Zn–CO2 batteries were allowed by electrocatalysts capable of catalyzing efficient CO2 reduction and O2 evolution, and dual-model aqueous Zn–CO2 batteries were realized by electrocatalysts capable of catalyzing CO2 reduction, water oxidation, and oxygen reduction. Concluding remarks include a summary of recent CO2 electrochemistry in metal–CO2 batteries and a brief discussion of future challenges and opportunities for practical aqueous Zn–CO2 batteries, such as highly reduced products and high production rate.

Conductive metal–organic framework nanowire arrays with Fe doping demonstrate self-adaptable structure adjustment for electrocatalytic oxygen evolution.

As a promising technique for CO2 fixation/utilization and energy conversion/storage, the metal–CO2 battery has been studied to improve its interconversion between CO2 and carbonates/oxalates. Herein, we propose and realize a reversible aqueous Zn–CO2 battery based on the reversible conversion between CO2 and liquid HCOOH on a bifunctional Pd cathode. The 3D porous Pd interconnected nanosheet with enriched edge and pore structure, has a highly electrochemical active surface to facilitate simultaneous selective CO2 reduction and HCOOH oxidation at low overpotentials. The resulting battery has a 1 V charge voltage, a cycling durability over 100 cycles, and a high energy efficiency of 81.2 %. The battery mechanism is proposed as Zn+CO2+2 H++2 OH−↔ ZnO+HCOOH+H2O, through which the reversible conversion between CO2 and liquid HCOOH was realized.

Metal–CO2 batteries represent a promising priority for sustainable energy and the environment. However, CO2 utilization in nonaqueous electrolytes mostly involves difficult CO2 electrochemistry, leading to poor selectivity and limited cycle performance. Herein, an aqueous rechargeable Zn–CO2 electrochemical cell that tunably produced CO fuel gas (90% Faradaic efficiency) during cell discharge (cathodic reaction: CO2 + 2e− + 2H+ → CO + H2O) and O2 during cell charge at ≈2 V (cathodic reaction: H2O → 1/2O2 + 2e− + 2H+), mimicking the separate steps of CO2 fixation and water oxidation during photosynthesis while exhibiting the advantages of high efficiency, tunable products, and operation independent of sunlight is proposed and realized. The cell achieves a remarkable energy efficiency of 68% with fuel generation, providing an alternative for the green, efficient, and safe utilization of CO2 by metal–CO2 batteries.

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