Hi,

I worked on VRFB ~5 years ago. The key here is electrolytes (ions and water) crossover through the membrane during VRFB operation. You can literature search to dig out detailed studies on the issue.

I'm not exactly clear on what your set up is, so it's kind of hard to diagnose the issue. Could you show a diagram of your cell set up? I'm trying to wrap my mind around "the feeding side became the anode". Also, when you say anode, do you mean the 4/5 couple, right?

@Vivian Thanks for your help 

@Pavel Well it is hard to explain the set up so I have attached a photo of my experimental set up here. 

Please disregard the load terminal colors this is an old photo. So the left side is the anode and the carbon felt electrode on this side is always dried up after a 10 cycle test. 

I hope to figure this problem out  and I appreciate your help. 

P.S. There are 100ml of Vanadium 3.5 solution in each container. 

So judging by the color of the electrolyte, it looks like you are talking about the 2/3 couple. One possibility that comes to mind is that your felt remains hydrophobic for the duration of the experiment. Typically, these felts, as received, are hydrophobic and do not wet well. On the 4/5 side, you have a strongly oxidizing environment and are running oxidation on the charge, so there the felt should quickly accumulate oxides on the surface and become hydrophilic, so it makes sense that it would remain wet after cycling. On the 2/3 side, you are in the hydrogen-making territory (even though kinetically very little hydrogen is evolved), and there isn't much oxidation to speak of. This might keep the graphite felt oxide free and hydrophobic for the duration of your experiment. This would definitely impact the efficiency, especially since a hydrophobic felt is more prone to channeling, where electrolyte follows a path of least resistance and most of the felt doesn't see much flow. A simple method to test this is to take out the dry felt, pipette some electrolyte on it, and see if it soaks in or beads up. As a fix, you can switch the felts (between the + and - sides), or just reverse the polarity on your cell. At that point, most of the surface oxides on the "wet" felt should remain -- the 2/3 environment is not reducing enough to knock all of them off -- and you should have hydrophilic felt on both sides.

There is also some literature out there on pre-treatment of graphite felts, for example by Maria Skyllas-Kazacos, or Matthew Mench and Tom Zawodzinski at U Tennesee to name a few. Depending on what you are after with your experiments, these may or may not be useful to you.