Thank you very much for bringing up your concern via the community and thanks a lot for the reply from @pauldey which is very helpful.
It's correct that you can use a USB to USB-C cable to charge the PowerCore Speed 20000 PD with a normal wall charger. But it will take forever to charge it up. Therefore, we usually recommend our customer to charge up the battery with a PD wall charger such as MacBook 29W adapter or MacBook Pro 61W or 87W adapters.
I hope this is helpful. If you need any further assistance, you can email us via firstname.lastname@example.org.
All the best to you in 2018!