Such thing as bike pedal power storage?

Possible to create? Sure. Worth it? Not so much.

You'll have to get the power to fill up your storage from somewhere - and then get it back out onto the wheel. The processes for that are usually horrendously ineffective, so you'll lose maybe half the energy in the process. Plus the added weight for the setup, and it's not worth it any more.

What can work is electrical assist, where you get the power from an electric outlet (and ultimately from any power station) and store it in a battery. Recuperation (i.e. storing energy e.g. from braking back into the battery) on a bike doesn't really work. Again, the effort involved here outweighs the benefits.

Buy an e-bike.

Any sort of energy storage device would add weight to the bike. When you are going uphill, weight is your enemy. As well, that sort of device would make pedalling on flat roads much more work.

It is theoretically possible, using devices such as flywheels, generators and batteries, or compressors and compressed air engines.However all of them would weigh so much that the extra effort needed to carry them around attached to a bicycle would negate any benefits.

It doesn't really work.

Pedalling harder on the flat to get it easier uphill only moves the effort around. And since you lose some both putting the energy in as well as when taking it out, the overall effort becomes bigger.

It's more energy efficient to have a geared bike, use the gears right and climb at an effort level you can manage.

What would work is if you have an e-bike with regen braking.

That means collecting energy when you brake, which later can be used to power you forward.

That works b/c the energy collected from braking would otherwise only have been wasted. As opposed from if you were to charge the bike through pedalling.

There's flywheel technology. Here's but one of several videos on this subject. Though this person was using it on a single speed bicycle I've seen this put on multi geared bicycles also.

https://youtu.be/-VF7j8mHw9k

There's also regen systems that help recharge a battery on ebikes. Some of these systems use the motor like a generator to place charge back into the battery when pedaling only, using gas only if it's a hybrid, coasting or braking.

If the hills are really steep like a 25%+ grade, I recommend using a 22:34 ratio on a pedal only bicycle.

There's also gas and/or electric assist systems. I use a mid drive system on my gas bike that shifts gears automatically.

https://youtu.be/pGDGIIdn--s