favicon doesn't show

Hi,

I'm trying to add a LinkedIn favicon to the secondary menu of my website, but while phone and envelope work, this one remains blank...

I added these codes:

<i class="fa fa-linkedin-in" aria-hidden="true"></i>
<i class="fa fa-linkedin-in"></i>
<i class="far fa-linkedin-in"></i>

And maybe a couple o others too. Could you advise me where I'm going wrong?

thank you,

Piet