As the global energy system shifts toward renewables, one challenge remains central: how to store energy when the sun is not shining and the wind is not blowing. While batteries, hydrogen, and synthetic fuels dominate the discussion, a lesser-known but highly promising concept is emerging from chemistry: Power-to-Polychlorides.

This approach can be understood as a new branch of Power-to-X, where renewable electricity is converted into a chemical form that can store energy indirectly—by enabling flexible industrial demand and stabilising the grid.

 

From Chlorine Problem to Energy Opportunity

Chlorine is one of the most important industrial chemicals worldwide. It is essential for water purification, pharmaceuticals, electronics, and materials production. However, its production via chlor-alkali electrolysis is extremely energy-intensive and accounts for a significant share of industrial electricity demand.

At the same time, chlorine is difficult and risky to store in large quantities because it is a toxic gas. As a result, production must closely follow demand—making it inflexible and poorly suited to fluctuating renewable energy supply.

This is where the Power-to-Polychlorides concept comes in.

 

The Breakthrough: Storing Chlorine in Ionic Liquids

Recent research shows that chlorine can be safely and efficiently stored in ionic liquids—special salts that are liquid at low temperatures. Instead of storing chlorine as a gas, it is chemically bound within these liquids in the form of polychloride ions.

In simple terms:

  • A chloride ion (Cl⁻) binds one or more chlorine molecules (Cl₂)

  • This forms stable structures such as Cl₃⁻ and larger polychloride complexes

  • These are dissolved in ionic liquids, creating a safe, low-volatility storage medium

 

The result is a liquid chlorine storage system that:

  • Can store large amounts of chlorine

  • Releases chlorine when mildly heated

  • Is significantly safer than pressurised gas storage

  • Can be reused multiple times

This transforms chlorine from a difficult-to-handle gas into a manageable, transportable and scalable storage medium.

 

Power-to-Polychlorides: A New Power-to-X Pathway

The key innovation is not just chemical—it is systemic.

By combining chlorine production with this new storage method, the process becomes flexible:

  • When renewable electricity is abundant

    → Chlorine production is ramped up

    → Chlorine is stored in ionic liquids (charging the system)

  • When electricity is scarce

    → Production is reduced or paused

    → Stored chlorine continues to supply industrial processes

This mechanism acts like a virtual energy storage system, similar to pumped hydro—but instead of storing energy as water at height, it is stored as chemical potential in polychlorides.

Importantly, no electricity is fed back into the grid. Instead, the system stabilises the grid by shifting electricity demand over time.

This is the essence of Power-to-Polychlorides: Converting renewable electricity into flexible industrial demand via chemical storage.

 

Why This Matters for Power-to-X

Power-to-X is about converting renewable electricity into useful products—fuels, chemicals, or heat. Power-to-Polychlorides expands this concept by adding a new dimension:

  • It links electricity and chemical industry processes

  • It enables sector coupling between energy and materials

  • It provides large-scale, long-duration storage via demand flexibility

  • It leverages existing industrial infrastructure

Even more, the energy density of such systems could reach levels comparable to large pumped storage facilities—highlighting their potential relevance for grid-scale applications.

 

Beyond Storage: More Efficient Chemistry

An additional advantage lies in the chemistry itself. Chlorine stored in polychloride form is more reactive (“pre-activated”), allowing certain industrial reactions to proceed:

  • At lower temperatures

  • Without catalysts

  • With fewer by-products

This could reduce both energy consumption and emissions in chemical manufacturing.

 

A Step Toward a Defossilised Industry

The concept of Power-to-Polychlorides demonstrates how existing industrial processes can become part of the energy transition—not just consumers of electricity, but active participants in balancing renewable systems.

By enabling flexible operation of one of the most energy-intensive chemical processes, this approach could:

  • Improve grid stability

  • Reduce reliance on fossil-based electricity

  • Accelerate the integration of renewable energy

It is a reminder that the future of Power-to-X is not limited to fuels like hydrogen or e-fuels—but includes innovative chemical pathways that connect energy and industry in new ways.

 

Source

Sebastian Hasenstab-Riedel: “Chlor als Energiespeicher”, Spektrum der Wissenschaft, 03/2026