to center a button in an ionic application, you simply use this code

<ion-buttons text-center>
<button ion-button type="submit">Submit</button>
</ion-buttons>

if you only use the <button> tag alone, , it will not work. you have to wrap it in a <ion-buttons> tag, for example, the following examples will not center a button

<button ion-button text-center type="submit">Submit</button>