Where is the chamber located? In the tank? Also, looks like you have the water flow going up the chamber. If that's the case, it won't do much good as CO2 rises in the water, and the flow will expel it from the chamber with little to no affect. Now if the water was going down, and you had the airstone in the middle to upper part of the chamber, the rising CO2 will fight the current, thus dissolving. you can also put another hole at the top of the chamber and have an airline going back to the powerhead venturi, to keep excess CO2 from building up in the chamber.
What I would do is your upper option to the airstone, but have the airstone at the top of the chamber. Then have the powerhead output going into the top of the chamber, so water flow goes down. Then have an air valve at the top connected to the venturi of the powerhead to remove any excess CO2 buildup and put back into the powerhead.