For application icons, usually shown in browser tabs or bookmarks, use the <tc:metaLink> tag. It will be rendered as an HTML <link> tag inside the <head> tag.
<tc:metaLink>
<link>
<head>
There are different formats for different devices possible. Especially for mobile devices there are a lot of possibilities. In the MDN Web Docs you may find further information.
In this demo it is used in the template file main.xhtml. Example:
main.xhtml
<tc:metaLink href="#{request.contextPath}/image/feather.ico" rel="shortcut icon"/> <tc:metaLink href="#{request.contextPath}/image/feather.png" rel="apple-touch-icon"/>