I've had this plant for a few months, actually have no idea what it is (if someone could let me know that would be great). I am getting a lot of dead leaves, although there are also some fresh shoots, more is dying than growing. Any advice on what might be the problem? It's in a hallway so receives no sunlight, I water it a few times a week and have a moisture probe which says the moisture is around 20%-30%.
Your question has the essential clue: a hallway with no sunlight. If you look closely, you will see that the new leaves are significantly larger and lighter than the older ones, which the plant presumably already had when you received it. This is the bonsai’s desperate attempt to have enough foliage to sustain itself via photosynthesis.
The immediate measure is to find a spot with sufficient natural light, although not in full sun right now, that would cause sunburn at the new leaves.
Unfortunately I can’t identify the tree, but that’s the next step to determine its preferred conditions w/r to light, water and fertilizer. If I had to guess, I’d say Chinese elm, but I am by no means sure. This is particularly important, because bonsais are growing in artificially restricted (or artistic) conditions, which therefore must be carefully maintained and managed. Some are even better suited to grow outdoors, depending on species and your location.
We cannot supply complete care instructions within an answer here, but I really recommend the two steps mentioned above. You have a beautiful beautiful bonsai in your home - and as all bonsais it needs diligent care to thrive.