Every second, the sun generates more energy than humanity uses in a year. Wind turbines can spin at night. The challenge isn't making clean electricity. It's storing enough of it to power homes and cities when the sun sets and the wind dies down.

Redox flow batteries have emerged as a leading contender for solving this problem. Unlike the lithium-ion batteries in phones and electric vehicles, flow batteries store energy in chemical form in liquid tanks outside the battery cell itself. This gives them a crucial advantage: the amount of energy they can store scales independently from their power output. A utility company can add more storage tanks without rebuilding the battery hardware, making flow batteries potentially cheaper and longer-lasting for grid-scale storage.

But flow batteries have a weakness. Inside the battery sits a thin membrane, barely thicker than a human hair, that separates two liquids containing different chemical compounds. This membrane has to do something that seems contradictory: allow certain ions to pass through quickly while blocking larger molecules. Get this balance wrong, and the battery becomes inefficient or degrades rapidly.

Researchers have now engineered a membrane that breaks what appeared to be a fundamental tradeoff, achieving both high conductivity and superior selectivity simultaneously. The material could accelerate the deployment of next-generation flow batteries for long-duration energy storage.

The Membrane Problem

Existing ion-exchange membranes used in flow batteries face what engineers call the conductivity selectivity tradeoff. A membrane needs to rapidly transport positive ions like potassium while simultaneously blocking the larger redox-active molecules that store the energy. This is like asking a door to let people through quickly while preventing trucks from entering.

Commercial membranes like Nafion, made from perfluorinated compounds, perform reasonably well but come with serious drawbacks. They cost roughly $500 per square meter. Manufacturing them requires persistent organic pollutants, creating environmental concerns. In alkaline flow batteries, which show promise for cost-effective energy storage, they transport hydroxide ions poorly.

The alternative membranes that researchers have developed from hydrocarbon polymers offer advantages. They're cheaper and more sustainable. Yet they suffer from dense, poorly connected water channels inside their structure. This low free volume makes them sluggish at conducting ions, limiting battery performance.

"Traditional sulfonated polymers usually are made from linear, flexible polymer chains, which tend to pack efficiently with low free volume," the research team noted. This creates water channels that are disconnected and tortuous. Ions take inefficient paths through the material, slowing everything down.

The Design Innovation

The key insight was to introduce rigidity into the polymer backbone in a way that prevents efficient packing. The team incorporated triptycene, a three-dimensional molecular structure shaped like a cage. This unit acts like a molecular wedge, forcing the polymer chains into twisted, contorted configurations that can't align neatly.

The result resembles the difference between throwing steel rods into a box versus meticulously stacking them. The rod pile leaves gaps and irregular spaces. The stacked rods fit tightly with minimal empty space. The team wanted the messy pile.

By adding triptycene and carefully controlling the degree of sulfonation (the process of adding sulfonic acid groups that enable ion transport), they created a polymer with intrinsic microporosity. The material contains subnanometer-sized pores, smaller than a millionth of a millimeter, distributed throughout its structure. Molecular simulations revealed something remarkable: these pores formed interconnected water channels that grew more connected as the sulfonation increased.

Most importantly, the water channels had the right shape. Molecular modeling showed they were narrow enough to exclude larger redox-active molecules like ferrocyanide, which can be several nanometers across, but wide enough to permit the rapid transit of small ions.

A Dual Ion Transport System

The design produced an unexpected bonus. In alkaline solutions, the membranes exhibited what researchers call dual ion transport. Small positive ions like potassium moved rapidly through the channels via normal electrostatic interactions with the negatively charged sulfonic acid groups. But hydroxide ions, which are negatively charged, also moved through the membrane quickly.

This was unusual. Hydroxide ions shouldn't easily traverse a negatively charged membrane; similar charges repel. Yet the hydroxide ions found pathways through hydrogen bonding networks within the water channels. This process, called the Grotthuss mechanism, involves hydroxide ions hopping from one water molecule to the next.

"The high KOH conductivity and high OH transference number of sPEEK-Trip-1.55 facilitated high-capacity utilization and energy efficiency even at extreme current densities," the team reported. This dual transport was crucial. In alkaline flow batteries, both cations and anions contribute to moving charge. Having both move quickly simultaneously meant higher power density and efficiency than existing membranes.

Performance Breakthroughs

The team tested their membranes in three different redox flow battery chemistries.

In neutral pH batteries using viologen and ferrocyanide, the membrane achieved energy efficiencies above 80 percent. This surpassed commercial alternatives while preventing crossover of the active molecules that could degrade battery performance.

In alkaline organic batteries combining quinone and ferrocyanide compounds, the results proved striking. The membrane allowed operation at current densities exceeding 500 milliamperes per square centimeter. For comparison, many existing membranes struggle above 200. The peak power density reached 560 milliwatts per square centimeter.

Most impressively, when tested in alkaline zinc-iron flow batteries—a chemistry that shows particular commercial promise—the membrane delivered a peak power density of 2.5 watts per square centimeter. That's roughly three times better than conventional membranes tested under identical conditions. The battery ran stably for over a month at high current densities, including extended cycling at 700 milliamperes per square centimeter.

These aren't incremental improvements. The performance exceeded the so-called upper bound limit established by decades of membrane development. Scientists had long assumed you couldn't exceed this boundary without sacrificing selectivity. This material did.

Why This Matters

Flow batteries could become the backbone of renewable energy infrastructure. Solar and wind generate electricity intermittently. Utilities and grid operators need ways to store that power for hours or days. Lithium-ion batteries excel at quick response but degrade quickly over thousands of charge cycles. Flow batteries can cycle tens of thousands of times with minimal degradation, and their long-term cost per kilowatt-hour can be lower than alternatives.

The membranes represent a breakthrough because they address the fundamental physics that has limited flow battery performance. Faster ion transport means lower resistance, which means less energy wasted as heat during charging and discharging. Better selectivity means the active chemicals don't crossover and degrade, extending battery life. Getting both simultaneously opens new possibilities.

The material shows another advantage: simplicity of manufacture. The synthesis uses established chemistry and produces a stable polymer solution that can be cast into membranes using standard industrial techniques. Unlike cutting-edge ceramic or zeolite membranes, this material could potentially scale to commercial production without revolutionary equipment.

Remaining Challenges

The research team was candid about limitations. The membranes degrade in extremely oxidizing conditions and may not work with vanadium chemistry, which uses highly oxidizing vanadium pentoxide. The zinc battery results showed some signs of side reactions over very long cycling periods.

These aren't fatal flaws. The membranes excel precisely in the emerging battery chemistries that avoid extreme conditions. Organic molecules, quinones, and zinc-iron combinations represent the next generation of flow batteries that show commercial potential without requiring exotic materials.

A Path Forward

The membrane concept extends beyond flow batteries. The design principles could apply to fuel cells, water electrolyzers, and electrochemical separations. The core idea of using structural rigidity to maintain microporosity while enabling selective transport represents a different way of thinking about ion-conducting materials.

Large scale manufacturing is underway. The team has demonstrated these membranes in kilowatt-scale battery stacks and plans further scale-up testing. If manufacturing proves economically viable at scale, these materials could enable the transition to renewable energy by finally solving the century-old problem of efficient energy storage.

The fundamental tradeoff between conductivity and selectivity, which has constrained membrane design for decades, may finally be broken.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1016/j.joule.2024.11.012