Convince my supervisor for me to switch my research field to his major area halfway in my PhD I am going to finish the 2nd year of my PhD and have been working in a field which is not the major research area of my PI. He promised there will be industrial involvement and collaboration with other professors so I agreed to work on this field, thought I have only a little bit background in the area. However all of these promises have just not been fulfilled (I did try to find collaborators myself as well). As nobody in my group is working in this field and my PI is not particularly knowledgeable about it, I feel isolated and helpless. Meanwhile, I am attracted to a field in which my PI is an expert, and several of my colleagues actively work on it. 

I have mentioned this problem to my PI and he agreed I can get trained about the field which I am interested in, while continuing to work on the field I originally worked on. Recently I have found my 2-year results within my original field of study are invalid, and there are fundamental errors in it.

After learning the new field in which my group is much more active and becoming knowledgeable on it, I really feel passionate and motivated about this. It is a more difficult field but I am determined to spend time on it. As I am halfway into my PhD, how should I convince my supervisor that I should switch my research field to an area in which our group is more active and knowledgeable, while abandoning my initial field completely? I am not trying to avoid solving the problem I faced in my initial field and would not like my PI to feel that is the case. I just think working on my initial field here in my group is pointless as nobody is knowledgeable on it, nobody knows what I am doing so nobody is able to give me genuine comments, and most importantly, I am not enthusiastic about it. However I know my supervisor would like to develop this field as it is more commercially viable. I need to admit I am not genius in this field and can't just work on my own without any proper supervision.  

Any opinion is welcome. 

PS: I would not like to change my supervisor. This would be the last resort.
Overall, I think your best argument is switching fields would be better for both you and your supervisor because you're likely to be more productive. But remember that your PhD is yours, and a good supervisor shouldn't have you do work you're not passionate about.

Maintaining motivation in a PhD is maybe half of the job. And doing research by yourself can be pretty demotivating—because, indeed, you can argue it's pointless. Arguably, part of the supervisor's job is to deal with that.

You need to decide whether the answer is to try harder or whether that's a waste of time. If further mistakes are likely, for instance, because you're unsupervised, so that the project is likely to fail, stopping might be a good idea.

Furthermore, you accepted your topic assuming collaborations would arise. These things are often intrinsically uncertain; but you can use this as an argument.


  However I know my supervisor would like to develop this field as it is more commercial viable.


First let me answer to you.

Since you're investing years of your life for research to be published under your name, your PhD should serve your goals.

This has to be balanced with external constraints such as funding. But unless you're being paid by a project forcing you to work on your old topic, constraints are only up to your supervisor.

And especially, even if he's interested in developing a new field, that doesn't mean he has to force you to work on it.

On the other hand, a PhD is also a collaboration where both parties have interest. Talking to your supervisor, again, I'd emphasize that you're likely to be more productive on the field you want to switch to, than on the field you're working on right now.

You also have alternatives to convincing him to switch right now. Since you've already spent time on your current project, and your supervisor suggests you to work on both fields, you might want to give it a try. Try to negotiate focusing on the new field for a while and revisit the issue. At that point, if you're actually making more progress (even just in learning), you might still want to switch and have a stronger case because you're making progress. Or you might realize that after all the old field was not so bad (as they say, "the grass is always greener on the other side of the fence") and decide to work on both projects.

Relatedly, even though that's not your core question: If you have a short time limits on your PhD (3-4 years), you have a bigger problem and more motivation to get something out of your current project if possible. But if you have a more open-ended PhD (5-7 years as common in the US), changing topics after 2 years shouldn't be a big problem.