I agree with most everything everyone's said, here. Artists tend to be really creative people in general -- an artist learning to write their own story for a comic isn't as difficult as a story-writer learning to draw their own comic... So basically, in asking for an artist to take on your written project without pay, you're requesting an uneven exchange in skills. (the ability to draw being a higher-cost skill than the ability to write.) Because of this, only lower level artists would be attracted to such a project, as the exchange then is then more equal. (Less time, less energy, faster production, and less effort is put into the art. Evening the exchange - but the result is often disappointing.).
How does one get around this uneven exchange? Well, there's three basic routes you can take:
1: 
Pay. Payment, obviously, evens the exchange.
2: 
Create absolutely mind-blowing, drool-worthy story work. So good that artists will be tripping over themselves to get the gig. But, of course, if you're that damned good you probably won't need to even put it into comic format! Go and get yourself published, or hired as a writer for some company if you reach that level of awesome-sauce greatness. Dayum!
3: 
Relations. If you're a good friend of mine, and we work well together, and we help each other improve with our work and creative growth? In that situation, a writer/artist partnership makes a lot of sense. Because in that situation, you offer me something worth more than money: A good, mutually productive working creative peer relationship.
If you look at most successful writer/artist teams, this is exactly how they go about it: They are, first and foremost, friends. Bros. Compadres. In such a situation, both of us are invested in the work, respect one another, and know each other well enough to not let one another give out or give in.
Instead of the writer cracking a whip on the artist and treating them like ungrateful dirt (an abusive writer/artist relationship I've regrettably put up with several times in the past, even from 'friends' 
.).
Instead, they mutually bolster and support each other. Both trust and respect each other enough to edit and improve upon each others work.
Such a partnership is ideal, as it builds up both the writer as well as the artist in regards to emotional, creative, and social support. (albeit it takes time to cultivate such a relationship).
The artist helps build upon writers creative work, bolsters their self-conscious view of their story (Heck, look at these comments: 'I'm nowhere near that level/I'm not good enough yet/ect'. Stop thinking so little of yourself! - try and find someone who's around the same level as you, whom you can team up with, and help build each other up and grow together!)... helps bounce ideas, and helps keep them from getting serious writers block.
And likewise, the writer also offers the artist encouragement, genuine & trusted critique & feedback, and keeps the artist from breaking down out of sheer exhaustion and throwing all of their work into a pit of a burning volcano of doom. 
And when I say friend? I don't mean the kind of friend who doesn't have the heart to tell you you look horrible in that outfit, tip-toeing around your feelings... no. Much to fragile. You need that hardy kind of friend, who will tell you that that dress makes you look like a hippo stuffed into a sausage casing from hell - you're awesome, you deserve to look awesome, and I know you can look awesome - now lets both go and try this again!
Now that I think of it ... A writer/artist partnership like that is a lot like a marriage... LOL. And the project being the child. Project-child suffers greatly upon marital strain, passionless money-based relations, lack of support, bad communication, no mutual respect, unequal partnership, & most dreadful: divorce. (Poor project-child. 
)
Okay, I think I've been rambling too much... Not enough sleep? Not enough coffee! 
Haha!... Heh... Hopefully some of what I said made sense. 
Anyways, good luck!
