# A water-soluble copolymer for storage and electron conversion in photocatalytic on-demand hydrogen evolution

## TL;DR

A water-soluble polymer can store solar energy and release it as hydrogen on demand, offering a sustainable energy storage solution.

## Contribution

A recyclable, water-soluble copolymer enables efficient solar energy storage and on-demand hydrogen production.

## Key findings

- The system achieves over 80% charging efficiency under visible light.
- Stored electrons can be used for hydrogen evolution with up to 72% efficiency.
- The polymer allows multiple cycles of charging, storage, and catalysis without isolation.

## Abstract

Cost- and energy-efficient long-term storage of excess solar energy remains a major bottleneck in the transition to a sustainable society. Here, we present a water-soluble redox-active copolymer containing viologen moieties that can be charged with electrons upon visible light irradiation using a tris[4,4’-bis(tert-butyl)−2,2’-bipyridine]ruthenium(II) complex as chromophore. In the presence of a sacrificial donor, the system achieves charging efficiencies above 80% and fully maintains this state for several days. Subsequent acidification and the addition of various catalysts enable on-demand usage of the stored electrons for proton reduction to hydrogen with up to 72% efficiency. The system further demonstrates reversibility via a simple pH switch, allowing multiple charging, storage, and catalysis cycles without time-consuming polymer isolation. The present study presents a direct on-demand hydrogen evolution method through discharging of a water-soluble polymer that functions as a temporary energy and electron storage material.

The study presents a recyclable polymer system that stores solar energy and releases it as hydrogen on demand, offering an efficient and sustainable route for renewable energy storage and fuel generation.