A fuel cell is a device that converts chemical energy into usable electricity. It is the beating heart of many modern utilities, from remote weather stations to fuel cell-powered submarines. One of the more popular fuel cells is the proton exchange membrane fuel cell (PEMFC). The technology that enables the fuel cell is called a proton exchange membrane, or PEM.
PEM acts like the bouncer guarding the highway inside a fuel cell. Its job is to let positively charged particles or protons pass from one side to the other, creating electricity and stopping other unwanted molecules, like methanol, from sneaking across. If methanol gets through, it makes the fuel cell less efficient and even damaging it. Right now, a common material for PEMs is called Nafion, but it's pretty expensive to make, creates hazardous waste, and isn't perfect at blocking methanol, especially at higher temperatures. Researchers are searching for better, more sustainable alternatives to Nafion.
A team of scientists from the Indian Institute of Science Education and Research (IISER) Bhopal has been exploring the potential of natural materials to replace nafion. They've been experimenting with making bio-based PEMs made of materials extracted from living things. Bio-based materials are usually abundant, cheaper, biodegradable, and do not pollute.
They set out to create a new kind of PEM using a mix of three natural ingredients: sulfonated poly(vinyl alcohol) (SPVA), chitosan (CH), and cellulose nanocrystals (CNC). SPVA is a modified version of a common polyvinyl polymer treated to help protons move. Chitosan comes from things like shrimp shells and has properties that can help protons travel and is compatible with SPVA. Cellulose nanocrystals are tiny, strong rod-like particles extracted from plants, like wood pulp or paper, adding strength and structure to the PEM.
The researchers combined these three materials to see if they could make a super-membrane. They found a blend with 90% SPVA and 10% chitosan worked best. This blend was better than just SPVA alone because the different parts of SPVA and chitosan (acidic groups from SPVA and basic groups from chitosan, plus lots of hydroxyl groups from both) interacted well, creating a smoother highway for protons through hydrogen bonding and other attractions.
Next, they took this best-performing blend and added the tiny cellulose nanocrystals, trying out different amounts (1%, 3%, and 5% of the total weight). They mixed everything in a liquid and then cast it into thin films. They then put these new composite membranes through a series of tests to see how well they performed.
The researchers found that adding the cellulose nanocrystals made the membranes even better. The tiny CNC rods mixed well, interacting strongly with the SPVA and chitosan polymers. The CNCs have chemical groups on their surface, like sulfate and hydroxyl groups, that also help protons move. So, adding them was like adding more lanes and helpful guides to the proton highway, making proton transfer even faster. This is partly explained by something called the Grotthuss mechanism, where protons don't just travel directly but hop from one water molecule to another in a network, and the CNCs helped create a better network for this hopping.
But the CNCs did more than just help protons. They also made it harder for methanol to get through. The CNCs create a tiny maze or an obstacle course inside the membrane. Methanol molecules trying to sneak across had to navigate around these tiny rods, slowing them down significantly. This is called increasing the tortuosity of the membrane. The best result for blocking methanol came from the membrane with 3% CNCs. This membrane significantly reduced methanol permeability compared to the SPVA-only membrane and the SPVA/chitosan blend.
The CNCs also made the membranes much more rigid and more stable. When they pulled on the membranes, the ones with CNCs were much stronger, with the best over 80% stronger than the SPVA/chitosan blend and over 100% stronger than the SPVA-only membrane. This increased strength comes from the CNCs reinforcing the polymer blend, like adding rebar to concrete. They also found that the composite membranes could handle higher temperatures better before starting to degrade, which is essential for fuel cell operation.
The balance between letting protons through easily and blocking methanol is measured by membrane selectivity. The membrane with 3% CNCs had the highest selectivity among all the ones they tested, meaning it did the best job overall at its two main tasks. The researchers noted that while their new membranes performed very well, especially at room temperature, their stability at very high temperatures and humidity might be further improved by carefully adding chemical crosslinking between the polymer chains, which they didn't focus on in this study. However, achieving good performance without initial chemical crosslinking is a notable step forward, as crosslinking can sometimes reduce proton conductivity.
By successfully creating high-performing PEMs from sustainable, bio-based materials like modified plant and shell components, these researchers are paving the way for cheaper, more environmentally friendly fuel cells. These fuel cells could power our portable devices more efficiently and sustainably in the future. It's a step towards cleaner energy technology, showing how materials from nature can be engineered to solve complex problems and help build a greener world.
This research article was written with the help of generative AI and edited by an editor at Research Matters.