Can you use words like "free" and "brave" as nouns?

So the question seems to have become, more generally, "What's a noun?"
That's a good question, and worth answering.

Let me start by mentioning the fact that most English speakers' idea of what noun means comes from grade school

(i.e, what we used to call "grammar school", which is a very ironic term for linguists, since nobody ever learns English grammar in "grammar school" -- it's not taught in Anglophone schools -- and nobody ever studies English grammar after "grammar school").

Consequently, almost all discussion of English grammar conducted by adults is conducted at about the third-grade level, because that's where one finds the audience. This leads to things like looking up grammatical terms in a dictionary, which is about as useful as looking up natural logarithms in a dictionary. Noun is not defined in advance; it's determined by use.

Noun (from Latin nomen 'name') was one of the Partes Orationis 'parts of speech', a categorization from the late Roman Empire that was memorized by every schoolboy learning Latin (which meant every schoolboy) for well over a millennium.

The problem is the usual one with old conceptual machinery that's been sitting around for 1500 years -- parts are rusty and the original use is no longer useful and nobody talks that way any more -- so we have new parts of speech, with their own acronym - POS. The problem is that anybody can see there's more than 8 kinds of word. For sure, noun is one; but there's different kinds of noun. There's countable nouns and mass nouns. There's animate nouns and neuter nouns. There's abstract nouns and concrete nouns. And that's only nouns -- verbs have many more varieties; verbs run everything.

We were taught in grammar school that nouns are words that mean "a person, place, or thing". That sounds really definitive, until you get to nouns like diagonalization, homeostasis, or presentiment.

In fact, this is precisely backward. Nouns are not always persons, places, or things; but persons, places, and things are always nouns. So it's part right -- the easy third-grade part. As for the rest, it depends in part on how precise you want to get -- there are parsers with several hundred POS categories -- and in part what kind of analysis you want to do. Nouns in general are distinguished by their use in sentences. There has to be a noun (or at least a pronoun) in every noun phrase, and there has to be a noun phrase in every English sentence -- though it doesn't have to be audible, it does have to be understood.

Only nouns (and pronouns, which are usually easy to recognize because they're a closed class) can be subjects and objects of verbs in English. Which is to say that

• Anything used as the subject or object of an English sentence can be considered a noun.
  This includes multi-word constituents like subordinate clauses and prepositional phrases.

And this mean that being a noun is not a label stuck to a word in a dictionary, but to a constituent in a sentence. For instance, anything, including direct speech, following He said is a noun in that sentence. Any sentence preceding is a shame is a noun in that sentence. One's an object and the other's a subject, and there's an infinite number of each.

OK, so with an adjective that's got an article, like the free and the brave, you have a fixed phrase that means the free people and the brave people. It's a common construction with a definite article (the) plus some adjective that can describe a person (or a group of people), where the Adjective means 'people who are Adjective' -- the homeless, the depraved, the tall, the college-educated.

In this construction, the meaning is generic, group, and plural; the tall can't mean 'the tall person', the way it can in many languages†, and *a tall doesn't mean anything at all, since the idiom requires a definite article.

†For instance, Latin. The Latin grammarians who coined the original Parts of Speech did not, in fact, notice adjectives. They considered adjectives to be nouns without intrinsic gender, so they could take their gender from a noun they modified. Other than that, they behaved like nouns and took all the same endings, and an adjective that didn't modify a noun was taken to be a noun itself -- longum 'long [neut sg nom]' alone could mean 'a long thing', which the Romans correctly identified as a noun phrase, though only by treating longus, -a, -um as a noun. This is why technical terms like noun have to be related to theories that use them.

In your example, "free" and "brave" are often called "collective adjectives" or "adnouns". The part of speech ascribed to these words is usually "adjective", and they are used in sentences as "nominals". Being nominals, they may function as subjects, objects, complements, etc.

Sometimes people call these words nouns, but in my experience, those people are usually using the word "noun" somewhat loosely, as a shorthand for "nominal". (Some people call gerunds "nouns", too, but I won't go off on that tangent . . .)

However, some adnouns can become "nominalized", i.e., become actual nouns. That appears to be the case in the example that k1eran cited in a comment (from Merriam-Webster):

brave noun
Definition of brave (Entry 3 of 3)
1 [in part borrowed from French, noun derivative of brave brave entry 1] : one with mental or moral strength to face danger, fear, or difficulty : one who is brave (see brave entry 1)
// … none but the brave deserves the fair.
— John Dryden

Of course, different dictionaries might have different opinions on this.